Magnetic Reconnection in Plasmas

27 Jul 2015, 09:00 → 21 Aug 2015, 18:00 Europe/Stockholm

132:028 (Nordita, Stockholm)

Andris Vaivads (Swedish Institute of Space Physics), Stefano Markidis (KTH Royal Institute of Technology), Yuri Khotyaintsev (Swedish Institute of Space Physics), Åke Nordlund (Niels Bohr Institute)

Photos from the program - album 1, album 2

Venue

Nordita, Stockholm, Sweden

Scope

Magnetic reconnection is a fundamental multi-­scale plasma process responsible for plasma transport, plasma heating and acceleration of energetic particles in many astrophysical environments, ranging from planetary magnetospheres and solar wind to solar flares, accretion disk corona, and other astrophysical plasmas. The major goal of this program is to increase the knowledge about the magnetic reconnection process in astrophysical plasma environments based on synergies between the studies of magnetic reconnection remotely, in situ, in numerical simulations and in laboratories. In situ data from the recent ESA Cluster, NASA Themis and upcoming MMS (to be launched in March 2015) missions will result by autumn 2015 in unprecedented amount of data on multi-­scale properties of magnetic reconnection from all the characteristic plasma scales -­ from electron up to fluid scales. Numerical simulations have reached maturity that kinetic scale processes for realistic mass ratios can be simulated in high detail in three dimensional geometries allowing for a direct comparison of observational data and simulation results. There are similarly large development in reconnection studies based on remote and laboratory observations. Some of the fundamental questions to be addressed are particle acceleration in magnetic reconnection, magnetic reconnection onset and turbulent reconnection.

Reconnection workshop, 10-14 August 2015

An open workshop on the magnetic reconnection consisting of invited keynote lectures, contributed talks and posters.

Invited Speakers

• Rumi Nakamura, Space Research Institute, Austrian Academy of Sciences
• Merav Opher, Boston University
• Masahiro Hoshino, Univeristy of Tokyo
• Homa Karimabadi, University of California at San Diego
• Nicolas Aunai, LPP
• Laila Andersson, University of Colorado/LASP
• Jörg Büchner, Max Planck Inst. f. Solar System Research
• Paul Cassak, West Virginia University
• Li-Jen Chen, University of New Hampshire
• Andrey Divin, Swedish Institute of Space Physics
• Gian Luca Delzanno, Los Alamos National Laboratory
• Jonathan Eastwood, Imperial College London
• Stefan Eriksson, University of Colorado/LASP
• Klaus Galsgaard, Niels Borh Institute, University of Copenhagen
• Martin Goldman, University of Colorado
• Daniel Graham, Swedish Institute of Space Physics
• Maria Hamrin, Umeå University
• Heli Hietala, Imperial College London
• Huishan Fu, Beihang University
• Ivo Furno, EPFL
• Tomas Karlsson, KTH
• Alexander Lazarian, University of Wisconsin-Madison
• Quanming Lu, University of Science and Technology of China
• Vyacheslav Olshevsky, KU Leuven, University of Leuven
• Nikolai Pogorelov, University of Alabama
• Minna Palmrooth, FMI
• Alessandro Retinò, LPP
• Paolo Ricci, EPFL
• Sergio Servidio, Department of Physics, University of Calabria
• Mikhail Sitnov, JHU/APL
• Jiayong Zhong, Beijing Normal University

School on data analysis and numerical simulations of reconnection, 5-10 August 2015

The school is aimed for junior researchers, where each day overview lectures are followed by hands on work with multi-spacecraft and numerical simulation data.

Extended stay program, 27 July - 10 August & 15-21 August 2015

This part consists of extended workshop-like work among the scientists on the chosen topics of interest. There are a series of seminars and overview lectures during this period. Due to space restrictions, the total number of participants is limited, we expect to be able to cover the travel and lodging support to selected participants (Nordita provides a limited number of rooms in the Stockholm apartment hotel BizApartments free of charge for accepted participants). A minimum stay of one working week is required. The deadline of applications to extended program has passed.

Travel support

PhD students and young Postdoc fellows are eligible for travel grants to participate in the program. If you are interested in such a grant, please mark the corresponding field in the application form, briefly summarize your interest in the program in the comments field, and indicate an estimation of your expected travel expenses. Since only a limited number of grants is available, decision concerning the grants will be made on a case-by-case basis and you will be notified shortly after the application deadline.

Schedule

Time	Activity	Topic
July 27 - August 10	Program	Selected topics based on scientists participating
August 5 - 10	School	Satellite in situ and numerical simulation data analysis
August 10 - 14	Workshop	Magnetic reconnection
August 15 - August 21	Program	Selected topics based on scientists participating

______________

[Timetable - available from start of the program]

Sponsored by:

document

• GEM-Results-iPIC3D.zip
• Kinetic-iPIC3D-Nordita.zip
• MHD-iPIC3D-Nordita.zip
• PlasmoidChain.zip

Slides

• HPCmarkidisNORDITA.pdf
• Kin-iPIC3D-Leuven.pdf
• Kin-iPIC3D-Nordita.pdf
• Markidis-Nordita.pdf
• Nordita_Hoshino_2015.pdf
• nordita_jul2015.pdf
• Noridta2015_Divin2015_presentation.pdf
• Presentation-Bourdin-short.pdf
• THOR_Vaivads_Nordita.pdf

Monday, 27 July

15:30 → 16:30 Nordita Program Presentation

Tuesday, 28 July

09:30 → 10:15 Evolution of the magnetic topology due to reconnection in a 3D MHD corona above an active region - Philippe Bourdin

Title: Evolution of the magnetic topology due to reconnection in a 3D
MHD corona above an active region

Abstract: From observations we know that magnetic features, like flux
tubes, emerge from the photosphere and subsequently rise through the
solar atmosphere. This causes global reconfiguration of the magnetic
topology and hence requires reconnection. Still it is debated, how fast
this reconnection may happen and if this leads to strong intermittent
heating (e.g. nanoflares) or if this process is more consistent with slow
magnetic diffusion and dissipation. In our model, the apex of an
evolved coronal loop raises with about 2 km/s, which has also been
observed for loops just emerging from the chromosphere into the
corona. With our 3D MHD model, that is driven by photospheric
magnetic-field braiding and reproduces coronal observations, we are
able to investigate the magnetic topology reconfiguration process. We
will show how the plasma flow dynamics are influenced by rising
magnetic structures and how that compares to statistical Doppler shift
observations. Within our model we find slow Ohmic dissipation of
current density structures that

10:30 → 12:00 Discussion Session

Wednesday, 29 July

10:30 → 12:00 Discussion Session

Thursday, 30 July

10:30 → 12:00 Discussion Session

Friday, 31 July

10:30 → 12:00 Discussion Session

14:30 → 15:30 Future beam experiments in the magnetosphere: how do we get the charge off the spacecraft?

Monday, 3 August

10:30 → 12:00 Discussion Session

Tuesday, 4 August

10:30 → 12:00 Discussion Session

Wednesday, 5 August

09:30 → 11:00 School Space: Data formats, reading, plotting.

Link to the lecture material

https://github.com/irfu/Nordita_Magnetic_Reconnection_School_2015

11:30 → 13:00 School Space: Single spacecraft methods for boundaries

14:00 → 15:30 School Space: Fluid equations. Walén test. Generalized Ohms Law.

16:00 → 17:30 School Space: Multi-spacecraft methods.

Thursday, 6 August

09:30 → 11:00 School Space: Multi-spacecraft methods. Curlometer, gradients.

11:30 → 13:00 School Space: Distribution functions.

14:00 → 15:30 School Space: Plasma waves. Spectra, interferometry.

16:00 → 17:30 School Space: Work with MMS data.

Friday, 7 August

09:30 → 12:00 Fluid Modeling of Reconnection

12:00 → 14:00 BBQ Lunch

14:00 → 14:30 HPC for Magnetic reconnection

14:30 → 16:00 Visit to the PDC Computing Center and Beskow Supercomputer

Saturday, 8 August

09:30 → 12:30 Kinetic Modeling of Reconnection

12:30 → 16:30 Lunch and Visit to VASA Museum

Monday, 10 August

08:30 → 09:30 Registration

09:30 → 10:40 Morning I

Convener: Dr Stefano Markidis (KTH)

09:30 Welcome and Introduction

09:55 Electron-scale Dissipation near the X-line During Magnetic Reconnection and the Upcoming FLARE (Facility for Laboratory Reconnection Experiments) Device

Despite its disruptive influences on the large-scale structures of space and solar plasmas, the crucial topological changes and associated dissipation during magnetic reconnection take place only near an X-line within thin singular layers on the electron scales. While ion dissipation layers have been frequently detected, the existence of election layers near the X-line and the associated dissipation structures and mechanisms are still an open question, and will be a main subject of the MMS mission. In this presentation, we will revisit the significant discrepancy on the thickness of the electron dissipation layer measured in the MRX experiment [1,2] and in the PIC simulation [3,4]. Although the long wave-length, electromagnetic lower-hybrid drift instabilities has been ruled out as a candidate to explain the discrepancy [5], other two candidates have emerged as possible explanations: (a) 3D flux- rope-like magnetic structures [6] and (b) micro-instabilities at frequencies higher than the lower-hybrid frequency including Debye-scale electrostatic turbulence [7]. Discussions will also include additional results from experimental evidence of electron heating through non- classical mechanisms near the X-line [8], as well as the relevant space observations. We will also present the construction status of the upcoming FLARE device [9] which is designed to access new regimes of magnetic reconnection involving multiple X-lines. The prospective research topics on FLARE will be discussed in relation to the dynamics of the Earth’s magnetosphere. [1] Y. Ren et al., Phys. Rev. Lett. 101, 085003 (2008) [2] H. Ji et al., Geophys. Res. Lett. 35, L13106 (2008) [3] S. Dorfman et al., Phys. Plasmas 15, 102107 (2008) [4] V. Roytershteyn et al., Phys. Plasmas 17, 055706 (2010) [5] V. Roytershteyn et al., Phys. Plasmas 20, 055705 (2013) [6] S. Dorfman et al., Geophys. Res. Lett. 40, 233 (2013); N. Jain, et al., Phys. Plasmas 20, 112101 (2013) [7] J. Jara-Almonte et al., Phys. Plasmas 21, 032114 (2014) [8] J. Yoo et al., Phys. Rev. Lett. 110, 215007 (2013); Phys. Rev. Lett. 113, 095002 (2014) [9] H. Ji and W. Daughton, Phys. Plasmas 18, 111207 (2011)

Speaker: Prof. Hantao Ji (Princeton University)

Slides Nordita2015_Ji.pdf

10:40 → 11:10 Coffe break

11:10 → 12:25 Pre-noon I

Convener: Dr Gian Luca Delzanno (LANL)

11:10 Advancing the understanding of the plasma dynamics through a rigorous Verification and Validation procedure: a practical example

The methodology used to assess the reliability of numerical simulation codes constitutes the Verification and Validation (V&V) procedure. V&V is composed by two separate tasks: the verification process, which is a mathematical issue targeted to assess that the physical model is correctly solved by the numerical code, and the validation, which determines the consistency of the code results, and therefore of the physical model, with experimental data. In the present work, a rigorous V&V procedure is presented and applied showing, through a practical example, how it can advance our physics understanding of plasma turbulence. Bridging the gap between plasma physics and other scientific domains, in particular the computational fluid dynamics community, a rigorous methodology for the verification of a plasma simulation code is presented, based on the method of manufactured solution and Roache's grid converge index. This methodology assesses that the model equations are correctly solved, within the order of accuracy of the numerical scheme, and provides a rigorous estimate of the uncertainty affecting the numerical results. Two- dimensional and three-dimensional verified simulations of the basic plasma physics experiment TORPEX are then performed, and rigorously validated against the experimental data. The validation procedure allows progress in the understanding of the turbulent dynamics in TORPEX, by pinpointing the presence of a turbulent regime transition, due to the competition between the resistive and ideal interchange instabilities.

Speaker: Prof. Paolo Ricci (CRPP, EPFL)

Slides Nordita2015_Ricci.pdf

11:35 Properties of electron pressure anisotropy in the extended electron diffusion region

Electron diffusion region (EDR) is regarded as the key region of collisionless magnetic reconnection. Electrons are unmagnetized inside the EDR, and magnetic field lines are detached from plasma, allowing fast conversion of magnetic energy into energy of plasma. Large-scale kinetic simulations are required for understanding the EDR structure. In this study, two- dimensional Particle-in-Cell (PIC) simulations of collisionless magnetic reconnection are utilized to study electron pressure behaviour in the X-line vicinity. The electron pressure anisotropy is generated by demagnetization of particles on the electron inertial scales, combined with the acceleration by reconnection electric field. Next, the properties of the external EDR are determined by strong pressure anisotropy of plasma upstream of the EDR. The competition between these two processes is responsible for the two-scale structure along the outflow direction, where electrons form a thin jet and the overshoot of the (div Pe)y term exists. We conclude that the anisotropy of the inflow electron population is an important parameter controlling the EDR properties and accordingly the EDR scalings, which in turn may help identifying the EDR in spacecraft observations.

Speaker: Dr Andrey Divin (St. Petersburg State University)

Slides Nordita2015_Divin.pdf

12:00 Non-diffusive transport of suprathermal ions in turbulent plasmas

Suprathermal ions with energies greater than the background plasma species are frequently observed during magnetic reconnection in astrophysical and laboratory plasmas. Understanding the interaction between turbulence and suprathermal ions is a fundamental problem to shed light on the physics governing many plasma phenomena, from particle dropouts during impulsive solar energetic events to transport of alpha particles in fusion reactors. Advances are hampered by difficulties in diagnosing distant astrophysical plasmas as well as fusion-grade plasmas. We report on experimental, numerical and theoretical investigations in the laboratory device TORPEX, which permits full characterization of suprathermal ion dynamics and turbulent structures. TORPEX is a toroidal device in which turbulent structures are intermittently generated and propagate across a confining magnetic fields. Suprathermal ions are locally injected using a miniaturized source and detected using grid-energy analyzers. By combining unprecedented 3D measurements and first-principle numerical simulations, we show unambiguous evidence for the existence of sub-diffusive and super-diffusive suprathermal ion transport regimes. We identify the mechanisms responsible for the non-diffusive transport and show that the transport character is determined by the interaction of the ion orbits with the turbulent structures, and is strongly affected by the ratio of the ion energy to the background plasma temperature. Furthermore, time-resolved cross-field dynamics reveal that suprathermal ions experiencing super-diffusive transport are associated with bursty displacement events. This work links measurements usually inaccessible in fusion and astrophysical plasmas, namely energy resolved 3D time-averaged measurements, with Eulerian time-resolved measurements, which are often the only accessible ones in such systems.

Speaker: Mr Ivo Furno (EPFL-CRPP)

Slides Nordita2015_Furno.pdf

12:25 → 14:00 Lunch

14:00 → 15:15 Post-noon I

Convener: Dr Stefan Eriksson (LASP - University of Colorado)

14:00 Magnetic Reconnection in Plasma Turbulence: from MHD to Vlasov Models

Systematic analysis of numerical simulations of two-dimensional magnetohydrodynamic (2D MHD) turbulence reveals the presence of a large number of X-type neutral points, where magnetic reconnection locally occurs [1]. The associated reconnection rates are distributed over a wide range of values and scales, exhibiting a good agreement with existing theories of magnetic reconnection. This scenario has been further inspected in the contest of Hall MHD and Hybrid-Vlasov models [2]. In particular, kinetic simulations suggest that these inhomogeneous magnetic structures may be the important sites of processes such as heating, temperature anisotropy, and particle acceleration - phenomena commonly observed in nature. This new perspective on reconnection is relevant for both astrophysical and laboratory systems, where plasma is generally in a fully turbulent regime. [1] S. Servidio et al., Phys. Rev. Lett. 102, 115003 (2009) [2] S. Servidio et al. Phys. Rev. Lett. 108, 045001 (2012)

Speaker: Dr Sergio Servidio (University of Calabria)

Slides Nordita2015_Servidio.pdf

14:25 Lower hybrid waves in magnetic reconnection regions

Lower hybrid waves in magnetic reconnection regions Lower hybrid waves are strong plasma waves that are often excited within thin boundaries that form in the vicinity of magnetic reconnection regions. These waves can be excited by sharp gradients in the density that can form along the separatrices or at the front of reconnection jets. The wave length scale is on the order of the electron gyroradius, which makes it hard to study them in detail. In this study, we present a new single spacecraft method that correlates the wave electric and magnetic fields to decide the velocity of the waves in the plane perpendicular to the background magnetic field. From the velocity, the length scale and electrostatic potential can be inferred. We apply this method to magnetic reconnection events in different regions of the magnetosphere. We investigate if the waves can have impact on the magnetic reconnection process or serve as a diagnostic tool, providing additional knowledge about the magnetic reconnection region.

Speaker: Ms Cecilia Norgren (Swedish Institute of Space Physics)

Slides Nordita2015_Norgren.pdf

14:50 Whistler-wave hypothesis for magnetic reconnection

A wave-driven scenario of magnetic reconnection is presented, the whistler-wave hypothesis, to explain the triggering mechanism of the reconnection in space and astrophysical plasmas. Magnetic reconnection releases a huge amount of energy on a short time scale, and is believed to be in operation in the phenomena of auroral substorms, flares, and coronal mass ejections. The hypothesis is motivated by the recent reports on the whistler wave emission from the reconnecting region both from the observation (Eastwood et al., Phys. Rev. Lett. 102, 035001, 2009) and from the particle-in-cell simulation (Goldman et al., Phys. Rev. Lett., 112, 145002, 2014). The whistler waves in our hypothesis are regarded as a precursor of the reconnection, and play a central role in the reconnection as sketched by the following scenario: (1) Whistler waves are excited prior to the reconnection onset by a macro- or micro-instability due to the enhanced inhomogeneity or the presence of non-thermal components, respectively; (2) The whistler waves in turn scatter particles (e.g., pitch-angle scattering), particularly in favor of electron scattering, and re-distribute the free energy back to the particle thermal population; (3) The wave-particle scattering is so effective that it contributes to a substantial amount of anomalous resistivity; (4) The electric field of the resistivity origin is strong enough to violate the frozen-in condition for the magnetic field on the electron gyro-scale (cf. the generalized Ohm's law); (5) And finally, the reconnection sets on, and remaining whistler waves escape, carrying the energy away from the reconnecting site. The central question in the study of the reconnection under the whistler-wave hypothesis is the causality, that is, if the wave triggers the reconnection, or vice versa. According to the hypothesis, the predictability of the reconnection depends on the detailed knowledge on the condition under which the whistler waves are excited and scatter the particles effectively. The heliospheric plasma group at Space Research Institute in Graz, Austria, is now working on the whistler-wave hypothesis with two independent (but complementary) approaches: analysis of spacecraft data for the Cluster and MMS missions and particle-in-cell simulations. We report our recent results on the causality between the waves and the reconnection, the wave dispersion relations, the four-dimensional energy spectra in the Fourier domain (spanning the wavevectors and the frequencies), and the velocity distribution functions.

Speaker: Dr Narita Yasuhito (Austrian Academy of Sciences)

Slides Nordita2015_Narita.pdf

15:15 → 15:45 Coffee break

15:45 → 17:00 Afternoon I

Convener: Dr Paul Cassak (West Virginia University)

15:45 How to find magnetic nulls and reconstruct field topology with MMS data?

In this study, we apply a new method—the first-order Taylor expansion (FOTE)—to find magnetic nulls and reconstruct magnetic field topology, in order to use it with the data from the forth-coming MMS mission. We compare this method with the previously used Poincare index (PI), and find that they are generally consistent, except that the PI method can only find a null inside the spacecraft (SC) tetrahedron, while the FOTE method can find a null both inside and outside the tetrahedron and also deduce its drift velocity. In addition, the FOTE method can (1) avoid limitations of the PI method such as data resolution, instrument uncertainty (Bz offset), and SC separation; (2) identify 3D null types (A, B, As, and Bs) and determine whether these types can degenerate into 2D (X and O); (3) reconstruct the magnetic field topology. We quantitively test the accuracy of FOTE in positioning magnetic nulls and reconstructing field topology, by using the data from 3D kinetic simulations. The influences of SC separation (0.05~1 di) and null-SC distance (0~1 di) on the accuracy are both considered. We find that: (1) for an isolated null, the method is accurate when the SC separation is smaller than 1 di, and the null-SC distance is smaller than 0.25~0.5 di; (2) for a null pair, the accuracy is same as in the isolated-null situation, except at the separator line, where the field is nonlinear. We define a parameter in terms of the eigenvalues of the null to quantify the quality of our method—the smaller this parameter the better the results. Comparing to the previously used one, this parameter is more relevant for null identification. Using the new method, we reconstruct the magnetic field topology around a radial- type null and a spiral-type null, and find that the topologies are well consistent with those predicted in theory. We therefore suggest using this method to find magnetic nulls and reconstruct field topology with four-point measurements, particularly from Cluster and the forth-coming MMS mission. For the MMS mission, this null-finding algorithm can be used to trigger its burst-mode measurements.

Speaker: Dr Huishan Fu (Beihang University)

Slides Nordita2015_Fu.pdf

16:10 Statistics and accuracy of magnetic null identification

Regions with vanishing magnetic field, also referred to as magnetic nulls, are of high interest in plasma physics. Near magnetic nulls particles become unmagnetized and can by interacting with electric fields be accelerated up to high energies. Magnetic nulls have been observed and studied before using in-situ observations for selected events. Here we present the first statistical study of magnetic nulls in the Earth's nightside magnetosphere. In addition we study how local disturbances in the magnetic field can affect the null type identification and we present a method to estimate the reliability of the null type identification. We study the magnetic nulls using full resolution data from the Cluster spacecraft when their maximum separation are less than one ion inertial length. This is fulfilled in 2003 when the spacecraft separation was approximately 200 km. The magnetic nulls are found using two methods: Poincaré index and Taylor Expansion. The use of Taylor Expansion allows us to find nulls outside the spacecraft tetrahedron volume. All together 25 time intervals containing magnetic nulls are found. We find most of the nulls in the magnetopause current sheet, but a few of them are found in the tail current sheet. We find the currents associated with each null and classify the types of the nulls. We present a detailed analysis of two typical examples of the observed nulls. The first one is from August 6, 2003 00:45:40.02 - 00:45:41.02 UT and the second is from October 28, 2003 04:16:01.0 - 04:16:02.0 UT. In both examples the spacecraft are in the magnetotail and observe magnetic nulls crossing the volume surrounding the spacecraft. In one of the events the type of the magnetic null is changed for a short moment during the crossing. We interpret this jump as being due to local disturbances in the magnetic field. We present a general method of how to estimate the effect local disturbances have on the null type identification. The obtained results are highly relevant for MMS studies of reconnection diffusion regions.

Speaker: Elin Eriksson (Swedish Institute of Space Physics, Uppsala, Sweden and Uppsala University, Department of Physics and Astronomy, Uppsala, Sweden)

Slides NorditaPres.pdf

16:35 Magnetic null points in the three-dimensional kinetic simulations of space plasma

Fully kinetic electromagnetic particle-in-cell code iPic3D is used to model magnetic reconnection in the variety of plasma configurations. We apply Poincare index technique to locate and identify the topological characteristics of the magnetic null points in different three- dimensional simulations. The relevance of magnetic nulls to energy dissipation, turbulence and plasma instabilities is studied in the scenarios of: turbulent dissipation of an unstable plasma configuration, Lunar Magnetic Anomalies, planetary mini-magnetospheres, symmetric and asymmetric 3D Harris sheet configurations. In particular, we found out that magnetic nulls of the spiral topological type associated with the magnetic islands and flux ropes play more important role in energy dissipation than the radial nulls. This finding is in accordance with recent MHD simulations and in situ observations of Cluster spacecraft.

Speaker: Dr Vyacheslav Olshevsky (KU Leuven)

Slides Nordita2015_Olshevsky.pdf

17:00 → 18:30 Poster Presentations

Tuesday, 11 August

09:00 → 10:40 Morning II

Convener: Dr Andris Vaivads (IRF - Uppsala)

09:00 Magnetic reconnection as the driver of eruptions in an emerging magnetic flux tube

Magnetic flux emergence is a very important process in renewing the magnetic field of the solar corona. As new magnetic flux emergence from below the photosphere and reaches in to the corona, the situation for magnetic reconnection are setup in the battle for space between the old coronal magnetic field and the new emerging magnetic field. Numerical MHD experiments investigate this with a simple initial conditions containing the basic components for this situation is used to investigate the different types of magnetic reconnection that takes place during this process. A number of internal magnetic reconnection events in the emerged flux tube initiate eruptions that propels plasma clouds upwards in the coronal domain. In this talk we discuss the different reconnection scenarios that take place in this experiment.

Speaker: Prof. Klaus Galsgaard (Niels Bohr Institute - University of Copenhagen)

Slides Nordita2015_Galsgaard.pdf

09:30 Multi-point observations of reconnection signatures in the Earth's magnetotail

In the Earth's magnetotail, there are two preferred sites for the magnetic reconnection. One is in the distant tail usually beyond the lunar orbit, and is considered to be semi-permanently present. The other is in the near tail at a distance of a few tens of Earth radius (Re), where the magnetic reconnection initially involves closed-field lines in the plasma sheet and hence affected strongly by internal processes in addition to the external drivers. Important consequence of the magnetotail reconnection is the narrow fast plasma jets (known as bursty bulk flows), which provide the major contribution to energy and mass transport in the magnetotail. Interaction with the reconnection jets moving Earthward and the Earth's dipole field lead to acceleration of particles, formation of the field-aligned current system, and associated auroral precipitation, and modifies the near-Earth field configuration. In this way the near-Earth's magnetotail reconnection has also large-scale consequences as manifested during substorms. This presentation high-lights observations of the thin current sheets during near- Earth's magnetotail reconnection and the reconnection jet evolution obtained from multi-point measurements by Cluster and THEMIS and addresses expected observations with the newly launched MMS. Depending on the spacecraft configuration, processes relevant to reconnection with different spatial/temporal scales have been observed by the spacecraft. Characteristics of the Hall-current in the ion diffusion region and a 3D nature of the localized magnetic structures in the reconnection region without guide field and with guide field are presented. Larger scale processes associated with the braking of the fast flow in the near Earth magnetosphere and formation of thin-current sheet that leads to onset of reconnection will be also discussed.

Speaker: Dr Rumi Nakamura (IWF/OEAW)

Slides Nordita2015_Nakamura.pdf

10:15 Instabilities and wave-particle interactions associated with magnetic reconnection

Magnetic reconnection enables the accelerating and heating of electrons, leading to the formation of electron distributions, which can be unstable to a variety of instabilities. For instance, magnetic reconnection can accelerate electrons to form beams and counter-streaming electron populations, as well as loss cone distributions. Additionally, at Earth’s magnetopause magnetospheric and magnetosheath electron populations can mix, leading to unstable electron distributions, which differ from symmetric reconnection at Earth’s magnetotail. Characterizing the resulting waves and instabilities is crucial for understanding magnetic reconnection at electron spatial scales. We investigate the waves and instabilities associated with asymmetric magnetic reconnection at Earth’s magnetopause. Both electrostatic and electromagnetic waves are observed. We investigate how these waves are generated, and their subsequent effects on particle scattering and heating.

Speaker: Dr Daniel Graham (Swedish Institute of Space Physics)

Slides Nordita2015_Graham.pdf

10:40 → 11:10 Coffe break

11:10 → 12:25 Pre-noon II

Convener: Dr Andrey Divin (St. Petersburg State University)

11:10 Localized strong energy conversion regions and their use for identifying reconnection sites

In the vicinity of magnetic reconnection, magnetic energy is transferred into kinetic energy. Such a region hence corresponds to a electrical load and it should manifest itself as large and positive values of the power density, E.J>>0, where E and J are the electric field and the current density, respectively. From simple theoretical arguments we find that an event with E.J>~20pW/m^3 in the Earth's magnetotail (X<-10RE and |Y|<10R_E) is likely to be associated with reconnection. Using Cluster plasma sheet data from 2001--2004 we confirm that events with E.J>~20pW/m^3 indeed often are associated with reconnection. The power density can easily be computed from multi-spacecraft data, and we argue that the power density is a powerful tool for identifying possible reconnection events from large sets of observed data, e.g., from the Cluster and MMS missions.

Speaker: Maria Hamrin (Umeå University)

Slides Nordita2015_Hamrin.pdf

11:35 Ion temperature anisotropies in magnetotail reconnection jets

Magnetic reconnection redistributes energy by releasing magnetic energy into plasma kinetic energy - high speed bulk flows, heating, and particle acceleration. A significant portion of the energy released by magnetotail reconnection appears to go into ion heating, and the heating is anisotropic with the plasma temperature parallel to the magnetic field generally increasing more than the perpendicular temperature. Simulations and theory indicate that this temperature anisotropy can balance part of the magnetic tension force that accelerates the jet, and may even exceed it leading to firehose instability. We examine ARTEMIS dual-spacecraft observations of a long-duration magnetotail exhaust generated by anti-parallel reconnection in conjunction with Particle-In-Cell simulations, showing spatial variations in the anisotropy across the outflow far downstream (>100 ion inertial lengths) of the X-line. A consistent pattern is found in both the spacecraft data and the simulations: whilst the total temperature across the exhaust is rather constant, near the boundaries the parallel temperature dominates. The plasma is well-above the firehose threshold in portions of the exhaust, suggesting that the drive for the instability is strong and the instability is too weak to relax the anisotropy. In contrast, the perpendicular temperature dominates at the mid-plane, indicating that (1) the increase in perpendicular heating is not simply the result of scattering, and (2) despite the large distance to the X-line, particles undergo Speiser-like motion. We also analyse the characteristics of the particle distributions leading to these anisotropies at different distances from the mid-plane.

Speaker: Heli Hietala (Imperial College London, UK)

Slides Nordita2015_Hietala.pdf

12:00 Wave activities and their roles in collisionless magnetic reconnection

Understanding the wave properties in magnetic reconnection is very important in collisionless plasmas. The waves can transport the momentum and energy between the different species, resulting in the anomalous magnetic dissipation, particle heating, and the formation of non-thermal particles. Observations in the Earth’s magnetosphere and laboratory experiment have shown that the wave activities are significantly enhanced in a broad range of frequency around the separatrices and the x-line. The waves are recognized as lower hybrid waves, Langmuir waves, electrostatic solitary waves (ESWs), and whistler waves. However, it is difficult from observation alone to identify the generation mechanisms of the waves and their roles in magnetic reconnection. We have performed large-scale particle-in-cell (PIC) simulations with adaptive mesh refinement (AMR) in 2D and 3D systems. Our simulations have shown that the waves in the separatrix regions can be explained by beam-driven instabilities while shear-driven modes are dominant around the x-line. In both the regions, the electrons are strongly heated due to the waves. In particular, the waves around the x-line are responsible for the magnetic dissipation which drives a collisionless reconnection. In this presentation, we will show recent results of large-scale PIC simulations of anti-parallel collisionless reconnection, and clarify the generation mechanism of the waves and their roles in reconnection.

Speaker: Dr Keizo Fujimoto (National Astronomical Observatory of Japan)

Slides Nordita2015_Fujimoto.pdf

12:25 → 14:00 Lunch

14:00 → 15:15 Post-noon II

Convener: Prof. Huishan Fu (Space Science Institute, Beihang University)

14:00 Tracing the flow of energy through magnetotail reconnection

Magnetic reconnection is one of the most important processes at work in space plasma environments, controlling energy storage, transport and release. In particular, it is crucial to the physics of the solar wind - magnetosphere interaction because it can explosively release stored energy, transforming it into different forms in the reconnection outflow. Magnetotail reconnection can be divided into three interacting regions: the diffusion region in the vicinity of the X-line, the exhaust, and the dipolarization front, where the exhaust leading edge interacts with the pre- existing plasma. Here we review and discuss recent work examining aspects of energy transfer and partition in different parts of the magnetotail reconnection process. The required change in field topology takes place in the electron diffusion region, enveloped by the larger ion diffusion region. Analysis of Cluster data has revealed that the partition of energy coming out of the diffusion region into the exhaust appears to be, on average, dominated by ion enthalpy flux, although there is also a large and significant Poynting flux which in localized regions may in fact dominate. Electron heating also occurs in the ion diffusion region; this heating is anisotropic and limited by various instabilities which may play an important role in mediating reconnection physics near the X-line. The dipolarization front (DF) is formed at the leading edge of the exhaust by its interaction with the pre-existing plasma sheet. The results of a new study using THEMIS are presented showing that the exhaust does not simply push pre-existing material out of the way, but that via kinetic effects, some pre-existing plasma sheet plasma is entrained and accelerated into the exhaust. This interaction in fact occurs over a macroscopic region, rather than simply being limited to the thin DF interface. A more general consequence is the conclusion that reconnection exhausts are not simply fed by plasma inflow across the separatrices, but are also fed by plasma from the region into which the jet is propagating; the implications of this finding are discussed.

Speaker: Dr Jonathan Eastwood (Imperial College London)

Slides Nordita2015_Eastwood.pdf

14:25 Observations of cold ion heating in the exhaust region at the Earth's subsolar magnetopause

Recent studies show that cold ions (energies up to tens of eV) of ionospheric origin are present in the magnetosphere and often reach the magnetopause, participating in magnetic reconnection. At low latitudes they are abundant and even dominate over the hot magnetospheric ions. Owing to their smaller gyroradius, they remain magnetized down to smaller scales than the hot ions and therefore introduce a new length-scale into the reconnection process. We use Cluster observations of the subsolar magnetopause during 2007 and 2008 to investigate how these cold ions are heated when entering the exhaust at different locations of the separatrix region. In some crossings we observe cold ions well inside the reconnection jet, while in other crossings the cold ions are effectively heated in the separatrices while entering into the jet. We suggest that near the diffusion region the E fields are strong and energization of cold ions is efficient, while far away from the diffusion region the E fields are weaker and energization is less efficient.

Speaker: Dr Sergio Toledo Redondo (Swedish Institute of Space Physics)

Slides Nordita2015_ToledoRedondo.pdf

14:50 Electron betatron acceleration in the dipolarization front driven by magnetic reconnection

A large scale two-dimensional (2-D) particle-in-cell (PIC) simulation is performed in this paper to investigate electron acceleration in the dipolarization front (DF) region during magnetic reconnection. It is found that the DF is mainly driven by an ion outflow which also generates a positive potential region behind the DF. The DF propagates with an almost constant speed and gets growing, while the electrons in the DF region can be highly energized in the perpendicular direction due to betatron acceleration. For the first time, we reveal that there exists a velocity threshold, only the electrons below the threshold can be trapped by the parallel electric potential in the DF region and then energetized by betatron acceleration.

Speaker: Prof. Quanming Lu (University of Science and Technology of China)

Slides Nordita2015_Lu.pdf

15:15 → 15:45 Coffee break

15:45 → 17:00 Afternoon II

Convener: Dr Slavik Olshevsky (KU Leuven)

15:45 Magnetic Reconnection in the Magnetotail: Onset Mechanisms and Structure of Exhaust Jets in 3D

Magnetic reconnection is widely accepted as the driver of dynamics in the Earth’s magnetotail despite the difficulty in understanding how reconnection can be initiated in a current sheet with curved magnetic field lines associated with a small normal B_z component. In particular, reconnection is the favored mechanism for explaining the generation of bursty bulk flows and dipolarization fronts despite the lack of any obvious mechanism to produce the characteristic cross-tail width of 1-3 R_E for such fronts observed in the tail plasma sheet. The results of recent particle-in-cell simulations in 2D and 3D that bear on these issues will be discussed. 2D simulations show that an isolated B_z “hump” configuration does not produce tearing instabilities. At most, it can generate an ideal-like instability with a growth rate an order of magnitude smaller than previous estimates in an open system that leads to an earthward shift of the hump and an erosion of the tailward side. Such an unstable hump configuration is unlikely to be produced by external driving of a current sheet with no B_z accumulation. In 3D simulations the imposition of an effective anomalous resistivity localized in the cross-tail direction is used to study the structure of the exhaust jets produced by reconnection. Relatively narrow fronts (<10 d_i) expand in the ion-drift direction to reach widths of 15-20 d_i . Broader initial fronts (25-50 d_i) tend to form a 10-15 d_i width higher speed structure on the dawn side of the front. All of these fronts exhibit a tendency to filament into structures of order 1- 2 d_i in width, apparently due to the action of the ballooning/interchange instability. At longer times, these finger structures tend to aggregate into structures of order 5 d_i in width. The implications of these results will be discussed.

Speaker: Dr Philip Pritchett (University of California, Los Angeles)

Slides Nordita2015_Pritchett.pdf

16:10 Conversion of electromagnetic energy at plasma jet fronts

We use multi-spacecraft observations by Cluster and MMS in the magnetotail and 3D PIC simulations to investigate conversion of electromagnetic energy at the front of a fast plasma jet. Such plasma jet can be produced as a result of magnetic reconnection. Jet fronts are known to have a sharp increase of magnetic field (referred to as dipolarization fronts in the magnetospheric physics) as well as sharp gradients in plasma density and temperature. These sharp gradients at the front generate broadband turbulence in the lower-hybrid frequency range, which have amplitudes several times larger than the convective field, wave potential comparable to electron thermal energy, and perpendicular wavelength of the order of several electron gyro-scales. Despite the large wave amplitudes, we find only moderate dissipation due to these waves in the front reference frame, which goes into heating of electrons. We find that the major dissipation is happening in the Earth (laboratory) frame and it is related to reflection and acceleration of ions from the jet front. This dissipation operates at scales of the order several ion inertial lengths, and the primary contribution to E*J is coming from the convective electric field of the front (E=Vfront_x B) and the current flowing at the front.

Speaker: Dr Yuri Khotyaintsev (Swedish Institute of Space Physics)

Slides Nordita2015_Khotyaintsev.pdf

16:25 Three-dimensional kinetic picture of magnetic reconnection, buoyancy and flapping in the magnetotail

Magnetic reconnection in the Earth’s magnetotail has important features that distinguish it from similar processes in other space plasma regions, laboratory plasmas and in the simplest theoretical models. First, the very possibility of spontaneous reconnection has been questioned because of the stabilizing effect of electrons magnetized by the north-south (Bz) magnetic field component. Second, magnetotail reconnection often competes with non-reconnection ballooning/interchange and flapping motions, and relative roles of these processes remain unclear. Third, it is distinguished by the so-called dipolarization fronts, kinetic- scale shock- like plasma structures, which dominate the energy conversion. We discuss the kinetic and MHD theory of the magnetotail reconnection onset to identify the equilibria for which the spontaneous reconnection is possible. We show that such equilibria must have a tailward gradient of the Bz field, which is indeed observed in the magnetotail prior to substorm onset. Then we employ 3D full-particle simulations to demonstrate that when the tail becomes unstable with respect to spontaneous growth of reconnection (ion tearing) mode the instability results in the formation of dipolarization fronts. It is accompanied by changes in magnetic topology, which extend in the dawn-dusk direction over the size of the simulation box, suggesting that reconnection onset causes a macroscale reconfiguration of the real magnetotail. The front formation and acceleration is also accompanied by interchange and flapping motions. They significantly disturb the primary dipolarization front but do not destroy it. We find that dipolarization fronts are indeed the main regions of energy conversion in the magnetotail and that the temperature increase near fronts is consistent with recent THEMIS observations. Finally, we discuss virtual satellite observations, which reveal the potential of the present MMS observations in resolving the primary plasma modes associated with the magnetotail reconnection.

Speaker: Dr Mikhail Sitnov (JHU/APL)

Slides Nordita2015_Sitnov.pdf

Wednesday, 12 August

09:00 → 10:40 Morning III

Convener: Prof. Åke Nordlund (Niels Bohr Institute)

09:00 Reconnection Effects in the Heliosheath and Heliopause

Speaker: Prof. Merav Opher (Boston University)

Slides Nordita2015_Opher.pdf

09:30 Radiation from accelerated particles in relativistic jets with shocks, shear-flow, and possible reconnection

We investigated particle acceleration and shock structure associated with an unmagnetized relativistic jet propagating into an unmagnetized plasma. Strong magnetic fields generated in the trailing shock contribute to the electron’s transverse deflection and acceleration. Kinetic Kelvin- Helmholtz instability (kKHI) is also responsible to create strong DC and AC magnetic fields. The velocity shears in core-sheath jets create strong magnetic field perpendicular to the jet. We examine how the Lorentz factors of jets affect the growth rates of kKHI. We have calculated, self-consistently, the radiation from electrons accelerated in these turbulent magnetic fields in the shocks. We found that the synthetic spectra depend on the bulk Lorentz factor of the jet, its temperature and strength of the generated magnetic fields. We will investigate synthetic spectra from accelerated electrons in strong magnetic fields generated by the combined effects of shock and kKHI injecting relativistic jets in ambient plasmas. The calculated properties of the emerging radiation provide our understanding of the complex time evolution and/or spectral structure in gamma-ray bursts, relativistic jets in general, and supernova remnants. We will implement a helical magnetic field in relativistic jets and investigate its effects on instabilities and possible reconnection.

Speaker: Dr Kenichi Nishikawa (Univerisyt of Alabama in Huntsville)

Slides Nordita2015_Nishikawa.pdf

09:55 Instabilities and Magnetic Reconnection Near the Heliopause

Recent simulations of the solar wind (SW) interaction with the local interstellar medium (LISM) performed by our team showed that the heliopause is the subject of various hydrodynamic instabilities which allow for an efficient, but spotty, mixing of the SW and LISM plasmas. The analysis of the coupling between the heliospheric and interstellar magnetic fields (HMF and ISMF) shows the likelihood of magnetic reconnection. Besides, our simulations show spontaneous transition to a stochastic behavior of magnetic field in the region near the heliopause where the heliospheric current sheet is compressed to distances of the order of the observed small-scale fluctuations in the HMF. We discuss these issues in the context of our new MHD and kinetic simulations in the vicinity of the heliopause.

Speaker: Prof. Nikolai Pogorelov (Department of Space Science, University of Alabama in Huntsville)

Slides Nordita2015_Pogorelov.pdf

10:40 → 11:10 Coffee break

11:10 → 12:25 Pre-noon III

Convener: Dr Ivo Furno (EPFL)

11:10 Recent progresses on laser driven magnetic reconnection

Laser driven magnetic reconnection (LDMR) is constructed with self- generated B fields has been experimentally and theoretically studied extensively, where more than Mega-Gauss strong B fields are spontaneously generated in high-power laser-plasma interactions, which located on the target surface and produced by non- parallel temperature and density gradients of expanding plasmas. For the properties of short lived and strong B fields in laser plasmas, LDMR opened up a new territory in a parameter regime not covered before. Here we will report the recent LDMR experimental results performed on Shenguang II lasers, which is aimed to understand the basic physical processes, such as particle accelerations, scale of diffusion region, and guide fields effects et al.

Speaker: Dr Jiayong Zhong (Department of Astronomy, Beijing Normal University)

Slides Nordita2015_Zhong.pdf

11:35 Dependence of magnetic dissipation on magnetic Prandtl number

Using direct numerical simulations of three-dimensional hydromagnetic turbulence, either with helical or non-helical forcing, we show that the ratio of kinetic-to-magnetic energy dissipation always increases with the magnetic Prandtl number, i.e., the ratio of kinematic viscosity to magnetic diffusivity. This dependence can be approximated by a power law, but the exponent is not the same in all cases. For non-helical turbulence, the exponent is around 1/3, while for helical turbulence it is between 0.6 and 2/3. In the statistically steady state, the rate of the energy conversion from kinetic into magnetic by the dynamo must be equal to the Joule dissipation rate. We emphasize that for both small-scale and large-scale dynamos, the efficiency of energy conversion depends sensitively on the magnetic Prandtl number, and thus on the microphysical dissipation process. To understand this behavior, we also study shell models of turbulence and one- dimensional passive and active scalar models. We conclude that the magnetic Prandtl number dependence is qualitatively best reproduced in the one-dimensional model as a result of dissipation via localized Alfven kinks. For many astrophysical systems, the microscopic energy dissipation mechanism is not of Spitzer type, as assumed here. It is not obvious how this would affect our results. Unfortunately, the question of energy dissipation is not routinely examined in astrophysical fluid dynamics, nor is it always easy to determine energy dissipation rates, because many astrophysical fluid codes ignore explicit dissipation and rely entirely on numerical prescriptions needed to dissipate energy when and where needed. Our present work highlights once again that this can be a questionable procedure, because it means that even non-dissipative aspects, such as the strength of the dynamo which is characterized by the work done against the Lorentz force, are then ill-determined.

Speaker: Prof. Axel Brandenburg (Nordita)

Slides Nordita2015_Brandenburg.pdf

12:00 Magnetic reconnection during formation of bipolar structures from stratified helical dynamos

We study different regimes of magnetic reconnection during formation of bipolar structures using direct numerical simulations of the equations of magnetohydrodynamics with external random forcing and in the presence of gravity. The domain is divided into two parts: a lower layer with the helical forcing and an upper layer with the non-helical forcing with a smooth transition in between. At early times, a large-scale helical dynamo develops in the bottom layer, and at later times the dynamo saturates, but the vertical magnetic field continues to develop and rises to form dynamic bipolar structures at the top. Later, the magnetic reconnection of the bipolar structures causes their evolution and destruction, and new bipolar structures form in other place, etc. This is the first example of magnetic flux concentrations, owing to strong density stratification, from self-consistent dynamo simulations that generate bipolar, super-equipartition strength, magnetic structures whose energy density can exceeds the turbulent kinetic energy by even a factor of ten.

Speaker: Prof. Igor Rogachevskii (Ben-Gurion University of the Negev and Nordita)

Slides Nordita2015_Rogachevskii.pdf

12:25 → 14:00 Lunch

14:00 → 15:15 Post-noon III

Convener: Dr Daniel Graham (IRF - Uppsala)

14:00 The electron diffusion region in asymmetric and symmetric magnetic reconnection

The electron diffusion region (EDR) holds the ultimate mystery of how magnetic reconnection can occur in a collisionless plasma, and is the target region of the first science priority for the newly launched Magnetospheric Multi-scale (MMS) mission. Processes of elctron acceleration, mixing and heating in the EDR can be understood by analyzing the electron distribution functions with knowledge of the DC field structures. For reconnection with symmetric upstream conditions and negligible guide field, the EDR electron energization depends primarily on the reconnection electric field, parallel pre-acceleration before entering the EDR, and gyro- turning by the reconnected magnetic field. For asymmetric reconnection with a negligible guide field, the in-plane electric field dominates over the reconnection electric field in energizing electrons in the EDR. Structures of the in-plane electric field, plasma flow patterns, and electron distribution functions for symmetric and asymmetric reconnection in PIC simulations will be contrasted to provide predictions for the EDR encounters of MMS.

Speaker: Dr Li-Jen Chen (NASA Goddard Space Flight Center)

14:25 Electron acceleration in the separatrix region during magnetic reconnection

One long outstanding issue in the study of magnetic reconnection is exactly where and how electrons are energized, although the observations have demonstrated that a substantial part of magnetic energy is converted into energetic electrons in reconnection. Here, we present the first evidence of electron acceleration by parallel electric field (up to -20 mV/m) carried by the Double Layer (DL) in the separatrix region during reconnection. Based on theory and previous auroral observations, the DL is inferred to propagate away from X-line at a velocity about ion acoustic speed and thereby accelerates electrons effectively on its way. The identification of multiple similar DLs within mere 16s interval indicates that they are persistently produced and play an important role in energy conversion during reconnection. The observations suggest electrons are energized during reconnection not only in certain fixed regions as expected previously but in any region where as long as the DL reaches.

Speaker: Dr Rongsheng Wang (Institute of Geology and Geophysics, Chinese Academy of Sciences)

Slides Nordita2015_Wang.pdf

14:50 Evidence of magnetic field switch-off in PIC simulations of collisionless magnetic reconnection with guide field

We describe here the first observation of switch-off of the in plane tangential component of the magnetic field in Particle In Cell (PIC) simulations of magnetic reconnection, a situation reminding of Petschek’s switch-off. Switch-off is obtained through a slow shock / rotational discontinuity compound structure. Two external slow shocks located in correspondence of the separatrices reduce the tangential component of the magnetic field, but not to zero. Two anomalous rotational discontinuities in the internal part of the exhausts then perform the final switch-off. Both the slow shocks and the rotational discontinuities are characterized as such through the analysis of their Rankine-Hugoniot jump conditions. Large domain sizes and long simulated times are needed to observe the development of the structures, which are disrupted at earlier times by the presence of plasmoids. A moderate guide field is used to suppress the development of the firehose instability in the exhaust.

Speaker: Dr Maria Elena Innocenti (University of Leuven, Belgium)

Slides Nordita2015_Innocenti.pdf

15:15 → 15:45 Coffee break

15:45 → 16:35 Afternoon III

Convener: Dr Yuri Khotyaintsev (IRF - Uppsala)

15:45 Theory and Simulations of Magnetic Reconnection at the Dayside Magnetopause

Magnetic reconnection at the dayside magnetopause is a crucial facet of solar wind-magnetospheric coupling, as it drives magnetospheric convection and is necessary for magnetic energy storage in the magnetotail. Many fundamental questions about dayside reconnection remain insufficiently answered quantitatively and even qualitatively, such as the location and local efficiency of reconnection for arbitrary solar wind conditions. The situation is complicated compared to simple two-dimensional models because the dayside naturally is usually asymmetric, has significant flow shear due to the solar wind (especially when the interplanetary magnetic field has a northward component), and manifestly has a three-dimensional structure. This talk will summarize recent work on a number of aspects of dayside reconnection, including the properties of asymmetric reconnection with a flow shear, the efficiency of dayside reconnection, and the local properties of reconnection at the dayside. Theoretical predictions for each of these will be compared to two-dimensional local numerical simulations of reconnection and naturally occurring reconnection in self-consistent three-dimensional global magnetospheric magnetohydrodynamic simulations.

Speaker: Paul Cassak (West Virginia University)

Slides Nordita2015_Cassak.pdf

16:10 Measures of diffusion regions applied to 2D PIC reconnection simulations

The primary goal of the current NASA-MMS mission is to "identify and study diffusion regions during magnetic reconnection in Earth's magnetopause and magnetotail. Yet the term diffusion region is often misunderstood and can be ambiguous. Different conditions for a region to be a "diffusion region" are interpreted theoretically, related to each other and applied to 2D PIC simulations. None of the conditions is both necessary and sufficient for topological reconnection to occur. During magnetic reconnection in a kinetic plasma key differences are found between the locations of diffusion regions in the electron fluid, the ion fluid and a single (MHD) fluid.

Speaker: Prof. Martin Goldman (University of Colorado at Boulder)

Slides Nordita2015_Goldman.pdf

19:00 → 20:00 The City of Stockholm invites you to the Stockholm City Hall

The reception is hosted by the City of Stockholm (http://www.stockholm.se/stadshuset)

Thursday, 13 August

09:00 → 10:40 Morning IV

Convener: Prof. Brandenburg Axel (Nordita)

09:00 Numerical simulation of collsionless magnetic reconnection and its coupling to turbulent and kinetic processes

Magnetic reconnection is a multi-scale phenomenon which in collisionless plasmas involves electron and ion kinetic processes as well as macro-scale plasma flows. Only numerical simulation approaches allow to bridge the huge scale- gaps between the relevant particle and flow dynamics. We review the different approaches taken so far to cope with the arising difficulties and draw conclusions about most appropriate ways to solve the reconnection problem for hot and dilute magnetized plasmas.

Speaker: Prof. Joerg Buechner (Max-Planck-Institut fuer Sonnensystemforschung)

Slides Nordita2015_Buechner.pdf

09:30 Particle acceleration in turbulent magnetic reconnection

Nonthermal particles are ubiquitous in space and astrophysical plasmas, and explosive phenomena such as supernova remnant shocks, solar flares, and terrestrial substroms have demonstrated evidence for the production of high-energy particles. Yet the particle acceleration mechanism remains an unresolved issue. After the innovative idea of the stochastic acceleration by Enrico Fermi in 1949, many researchers have investigated the diffusive shock acceleration model to explain nonthermal particle, but the diffusive shock acceleration alone cannot explain the observed efficient nonthermal particles. Instead of the shock acceleration, magnetic reconnection is now thought to be another important agent. We argue that the combination of the original Fermi acceleration model and the magnetic reconnection process is one of the possible paths to accelerate the thermal plasmas to the high energy nonthermal particles. We also discuss that the turbulent magnetic reconnection with many magnetic islands/plasmoids can naturally happen in many space and astrophysical settings.

Speaker: Prof. Masahiro Hoshino (The University of Tokyo)

Slides Nordita2015_Hoshino.pdf

10:15 Characterization of Magnetic discontinuities from MHD to sub-proton scales

The intermittent properties of the turbulent field at different scales down towards electron scales have been studied using CLUSTER magnetic field data in solar wind. We found that turbulence, in some cases, orginizes itself in unidimensional current sheets well descibed by an Harris equilibrium on electron scales. These structures could be those structures that numerical simulations indicate as important sites of energy dissipation and particle heating occurring at kinetic scales. Indeed, most of the time they are consistent with ongoing magnetic reconnection.

Speaker: Dr Antonella Greco (Physics Department, University of Calabria, Italy)

Slides Nordita2015_Greco.pdf

10:40 → 11:10 Coffee break

11:10 → 12:25 Pre-noon IV

Convener: Dr Andris Vaivads (IRF - Uppsala)

11:10 Turbulence modeling approach to fast reconnection

Turbulence modeling provides a powerful tool for investigating realistic turbulence with large-scale inhomogeneities. An attempt to study fast reconnection with the aid of self-consistent turbulence model is introduced. Turbulence related to magnetic reconnection is not homogeneous-isotropic at all. Its statistical properties depend on the large-scale configurations of the magnetic and flow fields. Because of the field configuration intrinsic to the reconnection and its asymmetry, statistical properties of turbulence should be described not only by the intensity information of turbulence (energy) but also by the structural information of it (some pseudoscalars). This suggests that the turbulence effect is not only for enhancing (as usually considered) but also for suppressing the transport. Combination of the transport enhancement and suppression gives the possibility of a very localized strong transport. The fast reconnection phenomena are interpreted from this viewpoint. Another interesting topic is shock--turbulence interaction in magnetic reconnection. The density variance, which is dominant near the strong density variation, and its effect on turbulence in the vicinity of the shock are investigated. A turbulence model for such strong compressible case is proposed. Finally, on the basis of the realistic turbulence model, some implications for the measurement of turbulence quantities using satellite observation near the reconnection region are presented.

Speaker: Dr Nobumitsu Yokoi (Institute of Industrial Science, University of Tokyo)

Slides Nordita2015_Yokoi.pdf

11:35 Turbulent reconnection

Turbulence is a natural state of high Reynolds flows and is ubiquitous in astrophysical systems. In most cases turbulence is externally driven, but one also expects the outflows induced by reconnection to get turbulent. High resolution numerical resolution reconnection simulations in 3D also show the transition to the turbulent state. Therefore it is essential to understand how turbulence affects magnetic reconnection. I shall be discussing the model of fast turbulent reconnection suggested in Lazarian & Vishniac 1999, its numerical and observational testing and its more recent extensions to the processes of reconnection in relativistic fluids and partially ionized gas. I shall show simulations obtained with different numerical schemes that support the theoretical expectations and discuss simulations of self- sustained turbulent reconnection. Finally, I shall touch upon the vast astrophysical applications of the turbulent reconnection theory.

Speaker: Prof. Alexander Lazarian (University of Wisconsin, Madison, USA)

Slides Nordita2015_Lazarian.pdf

12:00 In situ study of reconnection and associated electron heating in thin current sheets

We present an in situ study of thin current sheets and associated electron heating in turbulent space plasma. We use Cluster observations made in the turbulent plasma downstream of the Earth's quasi-parallel shock and we analyze the properties of ion-scale current sheets. Intermittent structures were identified using the Partial Variance of Increments method. We studied the distribution of the identified structures as a function of their magnetic shear angle, the PVI index and the electron heating. We observed a distinct population of high PVI (<3) structures that accounted for ~20% of the total. Enhancement of the estimated electron temperature within these current sheets suggest that they are important for local electron heating and energy dissipation. Evidence of ongoing reconnection was found in a subset of this population. In those cases, multi-spacecraft observations allow to study electron distributions and wave activity at different locations around the x-line. We discuss the possible mechanisms of electron heating and acceleration and the role of thin current sheets for dissipation in turbulent reconnection.

Speaker: Alexandros Chasapis (Laboratoire des Physiques des Plasmas)

12:25 → 14:00 Lunch

14:00 → 15:15 Post-noon IV

Convener: Dr Sergio Toledo redondo (IRF - Uppsala)

14:00 Oxygen motion in the Magnetotail of Earth

How plasma stored in the Magnetotail is returned back to Earth has great implication of the Earth magnetosphere responds to changes in the solar wind. In the magnetotail both oxygen and protons are impacted by fast tail changes such as magnetic reconnection and cross tail oscillations. The Cluster mission observations of the magnetotail flows shows that at low speeds protons and oxygen is moving together but at large speeds is not. This has great implications for return flow of plasma to the inner magnetosphere. Using simulations with an implicit kinetic code investigation of how fast cross tail oscillations and reconnection can decouple oxygen from the plasma sheet is presented here. This Cluster-simulation study will greatly help us interpret the observation from MMS mission, a mission where the satellites are close and large scale structures such as cross tail oscillations is more difficult to resolve.

Speaker: Dr Laila Andersson (LASP)

Slides Nordita2015_Andersson.pdf

14:25 3D MHD Stellar Atmosphere Models

Theoretical atmosphere models provide the basis for a variety of applications for astronomy. In one-dimensional (1D) atmosphere models, convection is usually treated with the mixing-length theory. However, this theory is by far not flawless and the superadiabatic regime is poorly rendered. Due to the increasing computational power, we are now capable to compute large grids of realistic three- dimensional (3D) hydrodynamical model atmospheres with the realistic treatment of the radiative transfer. We have computed the Stagger-grid, a comprehensive grid of 3D atmosphere models of late-type stars. In the course of my talk, I'll present in detail the properties of 3D stellar atmosphere models and the its interaction with the magnetic field.

Speaker: Mr Magic Zazralt (Niels Bohr Institute)

Slides Nordita2015_Magic.pdf

14:50 Modeling reconnection with particle-assisted magnetohydrodynamics

Due to its multi-scale nature, magnetic reconnection is difficult to model numerically using a full electromagnetic description of plasma in a system whose size is orders of magnitude larger than the smallest relevant spatial scales. In the context of Earth's magnetosphere reconnection has been modeled using a magnetohydrodynamic description of plasma for decades while kinetic effects have mostly been limited to predetermined spatial regions via coupling to various specialized kinetic models. Recently progress has been made towards modeling the entire magnetosphere using a kinetic description of plasma but the required computational resources are still prohibitive for most scientists. We present a method which allows the kinetic effects of ions to be included in a magnetohydrodynamic model by representing plasma both as ions and as a fluid. A novel aspect of the method is that plasma mass can be distributed arbitrarily between fluid and particles, thereby allowing kinetic effects to be included only in regions of interest. It is also possible to have a smooth transition between fluid and kinetic physics in contrast to existing computational models in which a discontinuity in physics exists when transitioning between regions of different physical descriptions of plasma. By varying the fraction of plasma mass represented by particles it is also possible to gain insight into the physical effects required for and produced by various phenomena such as magnetic reconnection.

Speaker: Ilja Honkonen (NASA/GSFC)

Slides Nordita2015_Honkonen.pdf

15:15 → 15:45 Coffee break

15:45 → 16:35 Afternoon IV

Convener: Prof. Paolo Ricci (EPFL)

15:45 Reconnection in Vlasiator: 1 The global system

Vlasiator (http://vlasiator.fmi.fi) is a newly developed, global hybrid- Vlasov simulation, which solves the six-dimensional phase space utilising the Vlasov equation for protons, while electrons are treated as a charge-neutralising fluid. The outcome of the simulation is a global reproduction of ion-scale physics where the generation of physical features can be followed in time and their consequences can be quantitatively characterised. Vlasiator produces the ion distribution functions and the related kinetic physics in unprecedented detail, in the global scale magnetospheric scale with a resolution of a couple of hundred kilometres in the ordinary space and about 30 km/s in the velocity space. We currently run Vlasiator under a southward IMF in five dimensions consisting of a three-dimensional velocity space embedded in the polar (XZ) plane. The simulation box extends 40 Earth radii (RE) in the solar wind upstream direction and a hundred RE in the nightside, thus including the dayside and the nightside reconnection sites in a single simulation volume. This introduces an opportunity to investigate kinetic reconnection physics in the global system, from the solar wind, through the magnetosheath and magnetopause, and eventually the nightside reconnection as a consequence of other processes with realistic boundary conditions. We observe the formation of 2- dimensional representations of flux transfer events at the magnetopause, and a substorm process in the nightside. We quantify reconnection rates and other relevant reconnection parameters and compare to earlier literature, with the aim of discussing the substorm process as a consequence of the global system evolution.

Speaker: Prof. Minna Palmroth (Finnish Meteorological Institute, Helsinki, Finland)

16:10 First global 3D two-way coupled MHD-EPIC simulation of a magnetosphere: Ganymede

We present the first 3D global simulation of Ganymede’s magnetosphere in a unified framework coupling the fluid and kinetic models. An MHD model describes the global interaction of solar wind with Ganymede’s magnetosphere over the whole simulation domain. A kinetic model is used only in the selected space regions, where kinetic effects dominate and MHD description fails. In particular, kinetic regions are used in Ganymede magnetotail and dayside magnetopause to correctly describe magnetic reconnection in these regions. This is the first simulation capable of combining fluid and kinetic approaches in a realistic large-scale simulation of a planet’s magnetosphere. In this work, the BATS-R-US MHD code is coupled with the iPIC3D Particle-in-Cell kinetic code. The coupling is two-way as both the MHD and kinetic codes provide reciprocal feedback. The electromagnetic fields of the fluid region are used as the boundary conditions for the electromagnetic fields in the kinetic regions; the fluid pressure tensor is used to sample distribution functions at the boundary of the kinetic regions. On the other hand, the kinetic areas provide the fluid regions the correct electromagnetic field values calculated with the kinetic approach. The coupling of the two codes has been realized within the SWMF framework, a highly scalable parallel environment for space weather modeling. The software coupling is achieved by using message passing among different BATS-R-US and iPIC3D instances that can run on different computational resources. In this talk, we present the coupling strategy that enabled the first global fluid-kinetic simulation of Ganymede’s magnetosphere and the simulation results of the solar wind interaction with Ganymede’s magnetosphere focusing on the kinetic magnetic reconnection in magnetotail and in dayside magnetopause. Finally, we discuss the impact and application of this work to other planetary magnetosphere.

Speaker: Dr Stefano Markidis (KTH)

18:00 → 21:00 Conference dinner on a boat trip

Friday, 14 August

09:00 → 10:40 Morning V

Convener: Prof. Tomas Karlsson (KTH)

09:00 Polar observation of a tripolar guide-field perturbation during a dayside magnetopause reconnection exhaust

Hall currents generate a characteristic quadrupole out-of-plane magnetic field at a single X-line for a weak background guide-field and relatively symmetric conditions of plasma density and magnetic field strength across a reconnecting current sheet. This is observed as a bipolar perturbation of the out-of-plane magnetic field (BM) across the exhaust region as, e.g., reported by Mozer et al. [2002] for a symmetric event at the magnetopause on 1 April 2001. Here we present the first observation of a tripolar BM perturbation that the Polar satellite encountered within 5 min of the bipolar event reported by Mozer et al. and in the same subsolar region of the magnetopause. The tripolar signature consists of two guide field depressions adjacent to an enhanced guide field within the exhaust. Tripolar signatures have been reported across solar wind exhausts and explained in terms of multiple X-lines at a given current sheet. A dedicated particle-in-cell simulation is explored to understand this new magnetopause observation, which occurred for a mean ratio BM/BL=0.4 between the background BM and the reconnecting field BL.

Speaker: Dr Stefan Eriksson (University of Colorado)

09:30 How far are we from complete understanding of the reconnection process?

Recent simulations have revealed complex details of magnetic reconnection than on the first glance bring into question many established concepts in magnetic reconnection. As an example, it is not clear whether the concept of the diffusion region, a theoretical construct deeply ingrained into all things reconnection, can be usefully extended to 3D reconnection. At the same time, the vast majority of the existing work and comparison with experimental/observational data are conducted within the confines of simplified models. In this talk, we review the current state of the art, and pose some of the key outstanding problems in reconnection. In addition, we outline a roadmap to achieve true closure, and assess the degree to which the traditional concepts can be usefully deployed/extended and the areas in which they need to be abandoned.

Speaker: Dr Vadim Roytershteyn

10:15 `Realistic' 3D simulations of a small flare resulting from flux emergence

We have performed three-dimensional (3d) magnetohydrodynamic simulations of magnetic flux emergence in a model that spans the convection zone and into the outer solar atmosphere with the Bifrost code. This is a ``realistic'' model, in the sense that the parameters and physical effects that control the atmosphere can be used to produce diagnostics that can be directly compared with observations. The emerging flux leads to the formation of several current sheets as it rises into the modeled corona. Multiple plasmoids are ejected from the current sheets. Reconnection occurs impulsively, producing heating and fast outflows near or in the current sheet, arranged in a manner reminiscent of the CSHKP flare model. This includes a cusp like arcade and a flux rope in the lower atmospere underneath the current sheet. We discuss the evolution of the model and several synthetic observables. We will also show observational data of similar flux emergence in the solar atmosphere.

Speaker: Prof. Viggo Hansteen (Institute of Theoretical Astrophysics, University of Oslo)

Slides Nordita2015_Hansteen.pdf

10:40 → 11:10 Coffee break

11:10 → 12:25 Pre-noon V

Convener: Yuri Khotyaintsev (IRF - Uppsala)

11:10 Beaming of particles and synchrotron radiation in relativistic magnetic reconnection at high magnetizations

We investigate the beaming of particles and the resulting synchrotron radiation in relativistic reconnection with background magnetizations of 4, 40, and 400 via 2D particle-in-cell simulations. First, we verify that the overall rate of energy conversion and the characteristics of radiation are similar at all magnetizations even for configurations including low-density background plasmas, despite the differences in reconnection physics for such plasmas found by Bessho and Bhattacharjee (2012). We then find that particles are accelerated to high energies in all simulations, but bulk outflows are only mildly relativistic with Lorentz factors ~ 3 that are insensitive to magnetization, and that the overall particle momentum and synchrotron radiation is not strongly beamed. High-energy particles in reconnection regions are beamed within an opening angle of 1/gamma, where gamma is their Lorentz factor, but the direction of that beaming changes with location from the direction of the electric field to the direction of outflow as one moves from the center of the X-point towards the magnetic island. Because the magnetic field in the center of the reconnection region is low, the high-energy radiation from reconnection is therefore oriented in two wide fans in the plane of the current sheet that are centered approximately in the directions of reconnection outflows.

Speaker: Dr Daniel Kagan (Tel Aviv University)

Slides Nordita2015_Kagan.pdf

11:35 Sub-grid Models of Turbulence for fast MHD Magnetic Reconnection

Magnetic reconnection is a very efficient process to convert magnetic energy into kinetic plasma energy. Unfortunately, models of reconnection such as Sweet Parker, do not agree with the time scale observed during the impulsive phase of a Solar flares, providing only small reconnection rates. One possible approach for reaching fast reconnection observed is by considering small scales effects of magnetic field and plasma turbulence. To catch small scale effects, sub-grid scales models of turbulence are considered in MHD numerical simulation of global scale eruption. Different sub-grid models, a nonlinear model of turbulence (Grete & al. 2015) and a Reynolds-Average Navier-Stokes (RANS) (Yokoi 2013) are tested on a double current sheet, where their associated reconnection rates are computed and compared with those of the usual resistive MHD. It shows that turbulence can increase the reconnection rate in a significant way. Finally, high resolution simulation of cascading reconnection is used to directly compute small scales quantities in order to test the validity of the RANS turbulent model

Speaker: Mr fabien widmer (Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany)

Slides Nordtia2015_Widmer.pdf

12:00 Reconnection in Vlasiator: 2. Influence of the magnetosheath waves

The global hybrid-Vlasov simulation Vlasiator (http://vlasiator.fmi.fi), developed at the Finnish Meteorological Institute, describes ions as velocity distributions functions propagated by solving the Vlasov equation and treats electrons as charge-neutralizing fluid. We present results from a new 5- dimensional simulation describing the Earth's magnetosphere in two dimensions in the polar plane and three dimensions in the velocity space under purely southward IMF. The simulation box extends 40 Earth radii (RE) in the solar wind direction up to a hundred RE in the nightside, thus covering both the dayside and the nightside reconnection sites. As a result from the reconnection at the dayside magnetopause we observe the formation of two-dimensional equivalents of flux transfer events. The formation of the flux transfer events varies spatially and temporally producing events with different velocities. Earlier, the mirror modes in the magnetosheath have been found to affect temporal variations of the velocity of reconnection jets (Laitinen et al. 2010). Waves with the characteristics of mirror modes emerge in the magnetosheath and advect toward the magnetopause in the simulation. We investigate their role on temporal and spatial variations of the dayside reconnection under steady IMF condition.

Speaker: Sanni Hoilijoki (Finnish Meteorological Institute)

12:25 → 14:00 Lunch

14:00 → 15:15 Post-noon V

Convener: Dr Pierre Henri (CNRS-Orleans)

14:00 Spectral method for the solution of the Vlasov-Maxwell equations

The Vlasov-Maxwell equations are a fundamental model for the microscopic evolution of magnetized, collisionless plasmas. Because of the wide disparity of spatial and temporal scales typical of plasmas, their numerical solution is extremely challenging and is a very active area of research. There are three main numerical approaches to the solution of the Vlasov-Maxwell equations, differing by how phase space is handled. The most common is the Particle-In-Cell (PIC) method, where phase space is discretized by using macroparticles. In the second, called Eulerian-Vlasov, a computational grid in phase space is used. The third class of methods is the Transform methods, where the plasma distribution function is decomposed in a number of moments via a basis expansion (typically using the Fourier or Hermite basis). PIC is the most popular method owing to its simplicity and robustness. It has had remarkable success for many basic plasma physics problems. It is however plagued by statistical noise associated with the macroparticles and is therefore mainly suited for problems where a high signal-to-noise ratio is acceptable. Eulerian-Vlasov and Transform methods do not suffer from statistical noise, but can be memory and resource intensive and, therefore, so far have been mainly limited to problems with a smaller number of spatial/velocity dimensions. In this work, we discuss a spectral method to solve the Vlasov-Maxwell equations by means of an expansion of the distribution function into Hermite polynomials which reduces the Vlasov equation to an infinite system for the moments of the expansion. The spatial discretization is cast either in terms of a Fourier (global) basis or through the Discontinuous-Galerkin (DG) method (local basis), while the moment equations are discretized implicitly in time with a Crank-Nicolson scheme. The resulting set of nonlinear discrete equations is solved with the Newton- Krylov technique, where GMRES is used for the inner iterations. For periodic boundary conditions, this discretization delivers a scheme that conserves the total mass, momentum and energy of the system exactly. For comparison, this is something that has not yet been accomplished for PIC. A comparison between PIC and the spectral method on standard test problems such as Landau damping, two-stream instability and ion acoustic wave shows that the spectral method can be orders of magnitude faster/more accurate than PIC. Multi-dimensional fully electromagnetic tests involving high frequency plasma waves and the whistler instability will also be presented. In multi-dimensions preconditioning strategies become crucial to maintain the scalability of the algorithm as the dimension of the Krylov space increases. We present two preconditioning strategies showing that an order of magnitude decrease in the Krylov iterations and a sizeable gain in code speed-up can be achieved. Some attempts to optimize the spectral decomposition in velocity space will also be presented, including a method where the number of Hermite modes is changed dynamically during the simulation.

Speaker: Dr Gian Luca Delzanno (Los Alamos National Laboratory)

14:25 Reconnection in Vlasiator: 3. Magnetopause-foreshock interactions

The Finnish Meteorological Institute's hybrid-Vlasov model Vlasiator (http://vlasiator.fmi.fi), which couples kinetic ion physics through Vlasov's equation with charge-neutralising fluid electrons, is used to model self-consistently the solar wind-magnetosphere interaction in two spatial and three velocity dimensions. Recent simulations in the polar plane include southward IMF in a box extending from about 40 Earth radii (RE) upstream in the solar wind to about one hundred RE downstream, thus covering the dayside and nightside reconnection sites in a single simulation volume. Dayside reconnection at the magnetopause results in the formation of the two-dimensional equivalents of flux transfer events. These magnetic islands are accelerated and move from the subsolar region towards the cusps and beyond. In doing so, they generate fast-mode waves which propagate throughout the magnetosheath and can lead to significant perturbations in the bow shock shape and position. We investigate such simulated events and their possible signatures in the magnetosheath, at the bow shock and in the foreshock.

Speaker: Yann Pfau-Kempf (Finnish Meteorological Institute, Helsinki, Finland)

14:50 Multi-point studies of reconnection exhausts in the solar wind

Magnetic reconnection at the magnetopause and magnetotail affect many magnetospheric dynamics including the onset of magnetospheric sub-storms and space weather effects, and the balance of energy in the magnetosphere. In these environments, however, asymmetric boundary conditions and imprecise knowledge of the motion of reconnection exhausts relative to spacecraft affect the extent to which we can observationally study reconnection in these environments. Solar wind reconnection exhausts, however, frequently form large- scale structures which propagate past spacecraft in a regular manner. This makes the solar wind an excellent environment in which to study basic reconnection physics. In this study we have identified multiple solar wind reconnection exhausts in the Cluster data set and further analysed an existing data set from Wind. With Cluster and Wind we analyse these exhausts with high cadence measurements. These high cadence measurements reveal further magnetic structure across the exhaust than can be seen with lower cadence measurements, and show several types of qualitatively distinct magnetic field profiles within the data set. Furthermore, we investigate asymmetries of the magnetic field profiles, and the structure of the boundaries of bifurcated reconnection exhausts.

Speaker: Mr Rishi Mistry (Imperial College London)

15:15 → 15:45 Coffee break

15:45 → 18:00 Afternoon V

Convener: Dr Stefano Markidis (KTH)

15:45 Current evolution within magnetic islands that are forced to shrink with electron cyclotron current drive

When a fusion plasma is above a critical value of beta, neoclassical tearing modes are destabilized. The resulting magnetic islands can grow to large size, allowing fast escape of the plasma from the fusion machine. The primary tactic for preventing tearing modes and reducing the size of magnetic islands in fusion machines is to apply current inside the magnetic islands. In ITER, this will be done using electron cyclotron current drive. To study the stabilization processes inside an island in detail, we have extended the JOREK code[1], a three- dimensional compressible non-linear MHD code that simulates a realistic tokamak geometry, to include two coupled equations for the applied current. We describe the evolution of the applied current, and analyze how it spreads along surfaces of constant magnetic flux inside the magnetic islands. [1]Huysmans, G. T. A., and O. Czarny. ``MHD stability in X- point geometry: simulation of ELMs.'' Nuclear fusion 47.7 (2007): 659.

Speaker: Dr Jane Pratt (University of Exeter Astrophysics)

16:10 electron physics and signatures in asymmetric magnetic reconnection

Collisionless magnetic reconnection is enabled by electron scale mechanisms and its dynamic is mainly controlled by ion scale processes. Whenever the to plasmas on both sides of the current sheet have different properties, a lot of well-known processes and their associated signatures are vastly changed. In this presentation, I will explain how one can highlight the regions of interest regarding electron scale processes in asymmetric reconnection, and discuss about the role of the initial condition on reconnection models, showing new results obtained from simulations initialized with an asymmetric kinetic equilibrium which properties are macroscopically and microscopically different from usual models.

Speaker: Dr nicolas aunai (CNRS/LPP)

16:35 Further discussion and conclusions

18:00 → 19:00 Poster Presentations

Monday, 17 August

10:30 → 12:00 Discussion Session

Tuesday, 18 August

10:30 → 12:00 Discussion Session

Wednesday, 19 August

10:30 → 12:00 Discussion Session

Thursday, 20 August

10:30 → 12:00 Discussion Session

Friday, 21 August

10:30 → 12:00 Discussion Session