IRIS-6: The Chromosphere

20 Jun 2016, 09:00 → 23 Jun 2016, 17:05 Europe/Stockholm

FR4 (AlbaNova University Centre)

The aim of this workshop is to bring together researchers interested in the physics of the solar chromosphere and its connections to the photosphere and corona from both an observational (whether with IRIS or with other observatories) and a theoretical point of view.

Venue: AlbaNova University Centre, Stockholm, Sweden.

IRIS-6 is organised by the Institute for Solar Physics which is managed by Stockholm University through its Department of Astronomy.

Important dates
15 January: Registration opens.
30 March: Last day for abstract submissions and applications for travel grants.
Abstracts for posters may still be considered until the poster slots are full.
15 May: Last day for registrations. Registration has now closed
20 May: Final day for reservations/cancellations of block-booked hotel rooms.

Sponsors: European Space Agency, Swedish Research Council

Registration: Registration has now closed. The registration fee is 2500 SEK. The registration portal is handled by BCD Travel and is situated outside this website. Payment via credit card. A RECEIPT is sent after a few days when the payment has been handled. The fee includes lunches and coffee breaks during the conference days, the welcome reception, and the Wednesday evening boat trip and dinner. You can bring a guest to the conference dinner for a fee of 1000 SEK.

Travel grants: A limited number of travel grants are available for scientists from ESA member countries. The call has now closed and applicants have been informed of the decisions.

Contributions: Invited talks are 25+5 min and contributed talks are 15+5 min long. Posters should be of the paper variety with A0 as the maximum size. Submit abstracts at the BCD registration portal.
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Invited Speakers:

Patrick Antolin

Mats Carlsson

Manolo Collados

Jaime de la Cruz Rodríguez

Ryohko Ishikawa

Lucia Kleint

David Orozco Suárez

Hardi Peter

Bart De Pontieu

Gary Verth

Alfred de Wijn

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Scientific organizing committee:

Gianna Cauzzi	INAF
Bernhard Fleck	ESA, co-chair
David Jess	QUB
Elena Khomenko	IAC
Andreas Lagg	MPS
Jorrit Leenaarts	Stockholm, chair
Joten Okamoto	ISAS
Luc Rouppe van der Voort	Oslo
Paola Testa	CfA

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Local organizing committee:

Johan Bjørgen
David Bühler
Sara Esteban Pozuelo
Jayant Joshi
Dan Kiselman	chair
Jorrit Leenaarts
Tine Libbrecht	co-chair
Mats Löfdahl
Hiva Pazira
Carolina Robustini
Andrii Sukhorukov

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Contact: iris-6.workshop@astro.su.se

pictures boat.pdf iris-6-map.jpg

Monday 20 June

08:30 Arrival and Check-In with Coffee, located at the main entrance of Albanova, 5th floor

Session 1

09:00 Welcome

1 Recent IRIS results on chromospheric heating (Invited)

The chromosphere processes all magneto-convective energy that drives coronal heating and requires a heating rate that is at least as large as that required for the corona. Yet many questions remain about what drives the chromospheric dynamics and energetics and how these are connected to the transition region and corona. I will review some recent results from IRIS in which observations and models are compared to study chromospheric heating, constraints on Alfven wave energy/dissipation, and connections between chromospheric and coronal heating.

Speaker: Bart De Pontieu (Lockheed Martin Solar & Astrophysics Laboratory)

Slides depontieu_iris6.pdf

2 Signatures Of Magnetic Reconnection Between Small-scaled Loops In The Solar Transition Region

Using high-resolution spectral and imaging data taken by the Interface Region Imaging Spectrograph (IRIS), we investigate the transition region explosive events, which are believed to be signatures of magnetic reconnection, observed in the solar transition region. We found that the explosive events are closely associated with dynamics of small-scaled loops in the solar transition region. In some cases, explosive events can be found in conjunction region of footpoints of multiple loops, suggesting that magnetic reconnection occurs due to interactions between small-scale loops. In some other cases, explosive events are associated with brightenings along the loops, however, brightenings in the same loops may not produce explosive events. We suggest that the loop brightenings associated with explosive events are results of magnetic reconnection in braidings of small-scaled loops, while normal loop brightenings cannot produce explosive events. Our study concludes that magnetic reconnection producing explosive events can result from braidings of or interaction between small-scaled loops. Such kind of magnetic reconnection should play an important role in heating the transition region plasma.

Speaker: Lidong Xia (Shandong University)

Slides Lidong_Xia_iris6.pdf

3 Flocculent flows in the chromosphere observed with SST and IRIS

Since the early days of the CRisp Imaging SpectroPolarimeter (CRISP) at the Swedish 1-m Solar Telescope (SST) the Halpha wing data of more active regions display intermittent flows at apparent chromospheric canopy heights. These flows appear to be aligned with canopy fibrils visible at the Halpha line core, with the blobs that constitute these flocculent flows" seemingly moving from one footpoint to the other. Their driving mechanism has been speculated to be a siphon flow, while their morphology may be due to thermal instabilities (with single condensations formed much the same way as in coronal rain). Here we present results from a study combining SST observations in Halpha and Ca II 8542Å with Interface Region Imaging Spectrograph (IRIS) rastering and slit-jaw imaging in C II, Si IV and Mg II. We investigate the co-spatiality of signatures in the various diagnostics, quantify their statistical properties and discuss possible formation mechanisms."

Speaker: Gregal Vissers (Institute of Theoretical Astrophysics, University of Oslo)

Slides vissers_iris6.pdf

4 Transition Region and Chromospheric Signatures of Impulsive Heating Events

We exploit the high spatial resolution and high cadence of IRIS to investigate the response of the transition region and chromosphere to energy deposition during several small flares. We find that during the impulsive phase of these events the intensities of the C II 1334 and Si IV 1403 emission lines are characterized by numerous, small-scale impulsive bursts typically lasting 60 s or less, followed by a slower decay over several minutes. These variations in intensity are usually accompanied by redshifts of 20–40 km/s, although some blueshifted profiles are also observed. For one particularly well observed event we combine the IRIS observations with co-temporal measurements of hard X-ray emission from RHESSSI, transition region density from EIS, and high-temperature coronal loops with XRT and AIA to constrain 1D hydrodynamic models of loop evolution. Many aspects of the observations can be explained with simple heating scenarios, but some cannot. The simulated Doppler shifts, for example, show very short-duration redshifts during the initial phase of the heating while the observed redshifts persist over several minutes.

Speaker: Harry Warren (NRL)

Slides warren_iris6.pdf

10:35 Coffee break

5 Hinode-IRIS Analysis of Various Chromospheric Heatings in Emerging Flux Regions (Invited)

Emerging flux regions (EFRs) are known to exhibit various local heating events in the chromosphere. In this talk, we will introduce our two topics on this issue utilizing both Hinode and IRIS observations. First, we will focus on the repeated brightenings and jet ejections seen in the light bridge structures in EFRs (Toriumi et al. 2015a,b). Combining not only the Hinode-IRIS observations but also the EFR simulations, we found that the above energy-releasing events are the recurrent magnetic reconnection between the light bridge fields and the sunspot fields, driven ultimately by continuous magneto-convective evolution within the light bridge. The latter half will be dedicated to our recent analysis on the sporadic chromospheric bursts seen in the earliest phase of the EFRs. By comparing the chromospheric spectra obtained by IRIS and the high-resolution vector fields by Hinode/SOT/SP, we categorized these events into two groups, local reconnection between colliding magnetic elements of opposite polarities (like Ellerman bombs) and continuous heating caused by fast downflows along the arch filament systems, both of which support the physical picture of EFRs. In the presentation, we would like to emphasize that such cooperative observations may open a door to further understand the thermal, magnetic, and dynamic evolution of EFRs.

Speaker: Shin Toriumi (National Astronomical Observatory of Japan)

Slides toriumi.pdf

6 IRIS observations of a light wall rooted in a light bridge

With the high tempo-spatial Interface Region Imaging Spectrograph 1330 Å images, we find that many bright structures are rooted in the light bridge of NOAA 12192, forming a light wall. The light wall is brighter than the surrounding areas, and the wall top is much brighter than the wall body. The New Vacuum Solar Telescope Hα and the Solar Dynamics Observatory 171 and 131 Å images are also used to study the light-wall properties. In 1330, 171, and 131 Å, the top of the wall has a higher emission, while in the Hα line, the wall-top emission is very low. The wall body corresponds to bright areas in 1330 Å and dark areas in the other lines. The top of the light wall moves upward and downward successively, performing oscillations in height. The deprojected mean height, amplitude, oscillation velocity, and the dominant period are determined to be 3.6 Mm, 0.9 Mm, 15.4 km s−1, and 3.9 minutes, respectively. We interpret the oscillations of the light wall as the leakage of p-modes from below the photosphere. The constant brightness enhancement of the wall top implies the existence of some kind of atmospheric heating, e.g., via the persistent small-scale reconnection or the magneto-acoustic waves. In another series of 1330 Å images, we find that the wall top in the upward motion phase is significantly brighter than in the downward phase. This kind of oscillation may be powered by the energy released due to intermittent impulsive magnetic reconnection.

Speaker: Shuhong Yang (NAOC)

Slides yang_iris6.pdf

7 Peacock jets: dynamics and magnetic topoplogy

We report on four fan-shaped sets of high-speed jets observed at the Swedish 1-m Solar Telescope. These jets appear to be rooted close to sunspots and at light-bridge edges and to be associated with the change of sign in the field polarity. We will present the results of a study of their dynamics, obtained using high-resolution imaging spectroscopy in Hα, and the photospheric and chromospheric magnetic field topology featuring these jets and obtained from azimuth de-ambiguated inversion with FALC atmosphere done with NICOLE.

Speaker: Carolina Robustini (Institutet för solfysik - Stockholm university)

Slides robustini.pdf

8 A contrail fibril

The solar chromosphere observed in H-alpha consists mostly of narrow fibrils. The longest typically originate in network or plage and arch far over adjacent internetwork. We use data from SST, IRIS and SDO to analyze one well-observed example in a quiet area. It resulted from the earlier passage of an accelerating disturbance in which the gas was heated to transition-region and even coronal temperatures, as in the spicule-II phenomenon. After this passage a dark fibril appeared much as contrails appear behind aircraft. We use Saha-Boltzmann extinction estimation to gauge the onset visibility in various diagnostics and conclude that such long H-alpha fibrils can indeed be contrail phenomena, not indicative of the thermodynamic and magnetic environment when they are observed but of more dynamic preceding happenings. Such H-alpha fibrils do not connect network across internetwork cells but represent tracks of disturbances launched from either side. They chart the magnetic field topography at launch, not at present.

Speaker: Rob Rutten (Lingezicht Astrophysics)

Slides rutten.pdf

12:50 Lunch

Session 2

9 First results from ChroMag (Invited)

The Chromosphere and Prominence Magnetometer (ChroMag) is an instrument with the goal of quantifying the intertwined dynamics and magnetism of the solar chromosphere and in prominences through imaging spectro-polarimetry of the full solar disk in a synoptic fashion. The picture of chromospheric magnetism and dynamics is rapidly developing, and a pressing need exists for breakthrough observations of chromospheric vector magnetic field measurements that can be considered to be a lower boundary of the heliospheric system. ChroMag will provide measurements that will enable scientists to study and better understand the energetics of the solar atmosphere, how prominences are formed, how energy is stored in the magnetic field structure of the atmosphere and how it is released during space weather events like flares and coronal mass ejections. ChroMag relies heavily on new tools to interpret the measurements and derive the magneto-hydrodynamic parameters of the plasma. Recent interest in the solar chromosphere and increases of computing power have led to advances in modeling that are beginning to make such tools feasible. Measurements of an instrument like ChroMag provide critical physical context for the Solar Dynamics Observatory (SDO) and Interface Region Imaging Spectrograph (IRIS) as well as ground-based observatories such as the future Daniel K. Inouye Solar Telescope (DKIST). A prototype ChroMag instrument is currently deployed in Boulder, CO, USA. We will present an overview of instrument capabilities, a progress update on the ChroMag development, and show first results.

Speaker: Alfred de Wijn (NCAR/HAO)

Slides dewijn.pdf

10 Upper chromospheric magnetic field of a sunspot penumbra: observation of fine structure

We studied magnetic field structure of a sunspot penumbra in the upper chromosphere and compared it to that in the photosphere. High spatial resolution spectropolarimetric observations were recorded with the 1.5-meter GREGOR telescope using the GREGOR Infrared Spectrograph (GRIS). The observed spectral domain includes the upper chromospheric He I triplet at 10830 Å and the photospheric Si I 10827.1 Å and Ca I 10833.4 Å lines. The upper chromospheric magnetic field is obtained by inversions applied to the He i triplet assuming a Milne-Eddington type atmosphere. The height dependent photospheric magnetic field is retrieved by inversions applied to the Si I 10827.1 Å and Ca I 10833.4 Å lines. We find that the magnetic field strength of the observed penumbra does not show variations on small spatial scales in the upper chromosphere, whereas the inclination of the magnetic field shows variations in the azimuthal direction, resembling the well known spine-interspine structure in the photospheric layers of penumbrae. The variations in the magnetic field inclination in the upper chromosphere remarkably coincide with the variations in the inclination of the photospheric field. The typical peak to peak variation in the inclination of the magnetic field in the upper chromosphere is found to be 10°-15°, compared to 20°-25° in the photosphere.

Speaker: Jayant Joshi (ISP Stockholm)

Slides Joshi_IRIS6.pdf

11 Jet-Like Features In Sunspot Atmospheres

Modern high-resolution observations have highlighted the ubiquity of small-scale structures in sunspot umbrae. Umbral micro-jets, extended bright features which were originally identified within Ca II H filtergrams, are one interesting example of such structuring. These bright elongated events have been shown to occur on sub-arcsecond scales, have lifetimes of the order minutes, and to sometimes occur co-spatial to umbral dots. In this talk, we present an analysis of these jet-like features using a range of data collected using the Swedish Solar Telescope. Initially, the basic properties of these events (including lifetimes and sizes) are measured in Ca II H time-series data before an investigation into their signatures in co-spatial CRisp Imaging SpectroPolarimeter Ca II 8542 A line scans is conducted. This research displays links between these micro-jets and a sub-set of dark extensions in the umbra, potentially similar to dynamic fibrils or umbral spikes. Finally, spectro-polarimetric measurements are studied allowing some inferences to be made regarding the local atmospheres surrounding these structures. Overall, this research provides further evidence for the jet-like nature of umbral micro-jets as well as for links between these events and at least a sub-set of dark extended structures observed in Ca II 8542 A line profiles.

Speaker: Christopher Nelson (University of Sheffield & Queen's University Belfast)

Slides Nelson.pdf

15:30 Coffee break + poster viewing

16:30 Welcome reception

Tuesday 21 June

Session 3

12 Solar flares and their connections to the lower solar atmosphere (Invited)

A large part of the energy of solar flares goes into heating, radiation, and mass motion in the lower solar atmosphere. By combining IRIS data with different instruments, such as RHESSI, SDO, and ground-based telescopes, we can infer the properties from the photosphere to the corona during flares. In this talk, I will give an introduction to flare observations with IRIS. IRIS data can for example be used to constrain velocities of filament eruptions, continuum emission during flares, chromospheric evaporation, and state-of-the-art models of solar flares.

Speaker: Lucia Kleint (FHNW Switzerland)

Slides kleint.pdf

13 Spectroscopic analysis of the pre-flare period leading up to the X-class flare on the 29th March 2014

At 17:48 UT on the 29th of March 2014, NOAA active region (AR) 12017 produced an X1 flare observed simultaneously by an unprecedented number of observatories. Among these were the Interface Region Imaging Spectrometer (IRIS) and Hinode’s Extreme Ultraviolet Imaging Spectrometer (EIS), joint observations from which provide an excellent opportunity to investigate the dynamics of the solar atmosphere for the period leading up to the X-flare. This work presents the results of the study of two areas of the AR: Region A, the site of a C-class flare observed over an hour prior to the X-flare, and Region B, an area of the filament which shows intriguing non-thermal velocities and Doppler blue shifts roughly 40 minutes prior to the X-flare. The strongly blue shifted plasma in Region B is observed approximately simultaneously in the Chromosphere, Transition region and the Corona. Once triggered, region B continued to show dynamics in the time leading up to the X-flare. This is interpreted as the rising of the filament in this highly localised region. This rise of the filament combined with the preceding C-class flare is suggested to cause the destabilisation of the magnetic fields in the AR leading to the X-flare and subsequent filament eruption. We shall also discuss these results in relation to models of the evolution of the non-potential magnetic field present in this active region.

Speaker: Magnus Woods (UCL-Mullard Space Science Laboratory)

Slides woods.pdf

14 Numerical simulations of MgII lines in solar flares

Using numerical RHD code Flarix we simulate the behaviour of MgII lines in the flaring chromosphere heated by electron beams. This contribution will focus on the role of partial redistribution, non-thermal collisional rates, and radiative losses. The role of different types of the temporal heating profiles, e.g. a long duration heating versus pulse heating will be assessed as well. The results will be discussed in terms of recent IRIS observations.

Speaker: Jana Kasparova (Astronomical Institute of the CAS)

Slides kasparova.pdf

15 Diagnostics of continuum enhancement during two X1.0 flares observed by IRIS

The study of continuum enhancement during a flare is of immense interest owing to the fact that it represents significant fraction of the impulsive energy release and thus provides useful diagnostics of the energy deposition and emission processes in the photospheric and chromospheric heights. We study the spatial and temporal evolution of continuum enhancement (CE) and co-temporal multi-wavelength emission during two X1.0 flares of March 29, 2014 and October 25, 2014, respectively. In-depth analysis of the spectrographic mode observations obtained by IRIS in several wavelengths is carried out to derive observable parameters of CE viz. intensity contrast, life-time etc. Morphological evolution of CE is also studied by analyzing the SDO/HMI continuum filtergrams. Further, we estimate the spatial and temporal evolution of co-temporal photospheric magnetic field parameters viz. flux, gradient and tilt angle of the active region during the flares. The spatial and spectral evolution of X-ray sources during the flares are derived from the RHESSI observations. We present a comparative overview of the aforesaid parameters for the both the flares. Next, we synthesize theoretical emission employing 1-D radiative transfer code. A grid of flare atmospheres are generated based on the pre-existing semi-empirical flare atmospheres viz. F2 and F3 which are then employed to synthesize the continuum emission in the wavelength same as that studied from IRIS observations. The parameters derived from the synthesized profile are then compared with the observed CE parameters as well as magnetic-field parameters.

Speaker: Arun Kumar Awasthi (Institute of Astronomy, University of Wroclaw)

Slides awasthi_IRIS_VI_workshop.pdf

10:30 Coffee break

16 Chromospheric magnetic fields and inversions (Invited)

I will review the current state of non-LTE inversions and magnetic field inference in the solar chromosphere.

Speaker: Jaime de la Cruz Rodriguez (Stockholm University)

Slides inversion_delacruz.pdf

17 Observation of Ellerman Bomb emission features in He I D3 and He I λ10830

Ellerman Bombs (EBs) are short-lived emission features observed in the wings of Balmer lines of hydrogen. So far, no distinct signature of EBs has been found in the He I λ10830 line, and observations of EBs in He I D3 have never been reported. We aim to study the signature of EBs in neutral helium triplet lines using SST/TRIPPEL raster scans, featuring the Hβ, He I D3 and He I λ10830 spectral regions. We also obtained raster scans with IRIS and make use of the SDO/AIA 1700 Å channel. Three of the EBs in our data show distinct emission signatures in He I D3. In some cases, there are some weaker emission effects in He I λ10830 as well. The helium lines seem to have two components: a broadened and blueshifted emission component, probably associated with the EB, and an absorption component, probably due to absorption in the overlying chromosphere. With HeLiX+, we disentangle the two components in the spectra. Recently, there has been a discussion on the nature of IRIS bombs and Ellerman Bombs observed in the Si IV 1400 Å spectral lines with IRIS. It has been suggested that Ellerman Bombs are possibly much hotter than previously thought. We indeed believe that very high temperatures are needed to create EB emission signatures in neutral helium triplet lines.

Speaker: Tine Libbrecht (Institute for Solar Physics, Stockholm University)

Slides IRIS6.pdf

18 Simulated EB-like events

Recent SST observations have given a new definition of Ellerman bombs (EBs). They are short-lived flame-like features visible in the wings of Halpha, away from the disc center. EBs seem to be associated with a peculiar combination of signatures in different observables. Also, they could be placed on, energetically, a low end of the whole spectrum of events that might have the same physical process triggering them. Can we simulate them? How are they formed? What field configuration produces them? We address these issues by using MURaM simulations.

Speaker: Sanja Danilovic (Max Planck Institute for Solar System Research)

Slides t.pdf

19 Searching for the alignment between chromospheric fibrils and magnetic fields

Solar images obtained in the core of strong chromospheric lines show fibrils that appear to be tracing the magnetic field lines. They have been historically used as proxies of magnetic fields for many purposes. In this work we use a Bayesian hierarchical model to analyze several tens of thousands of pixels in spectro-polarimetric chromospheric images and compare the alignment between the field azimuth inferred from the linear polarization induced by the Zeeman effect and the direction of the fibrils in the image. Fibrils appear to be statistically well aligned with the magnetic field azimuth.

Speaker: Andres Asensio Ramos (Instituto de Astrofisica de Canarias)

Slides aasensio.key

12:45 Lunch

Session 4

20 The chromosphere and transition region as seen with CLASP (Invited)

The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is a NASA sounding-rocket experiment, which was launched from White Sands in the US on September 3, 2015, and successfully made the observations during its 5 minutes ballistic flight. The CLASP observations showed that the scattering polarization in the hydrogen Lya line (121.57 nm), which originates in the upper chromosphere and transition region, is about a few percent in the wings and of the order of 0.1% in the core, as had been theoretically predicted, with the conspicuous spatial variations of ~10 arcsec. The behavior of the wing polarization (i.e., the amplitude and clear center-to-limb variation (CLV)) is consistent with the theoretical prediction. However, the line center polarization, which via the Hanle effect is sensitive to magnetic field strengths of 10-100 G, did not show the CLV in the Q/I amplitude that is found when solving the Lya scattering polarization problem in the available 1D and 3D atmospheric models. The additional theoretical investigations that are being performed indicate that this curious feature of the CLASP observations can be understood in terms of the magnetization and/or geometrical complexity of the chromosphere-corona transition region. It is fortunate that one of channels covered the Si III line (120.65 nm), which showed measurable scattering polarization signals of a few %. This polarization could facilitate the interpretation of the scattering polarization observed in the Lya line, because it also originates in the upper chromosphere and its critical field strength for the Hanle effect is 295 G. By comparing the signals observed in Lya and the Si III line, we find some hints of the possible operation of the Hanle effect in an enhanced network region. CLASP also succeeded in obtaining images of the Lya intensity with a very high temporal cadence of 0.6 sec, finding evidence of ubiquitous high-frequency (

Speaker: Ryohko Ishikawa (National Astronomical Observatory of Japan)

21 Flux emergence and reconnection in the solar chromosphere

While the flux emergence of photospheric small-scale structures already are well observed and understood, our understanding of the impact of the emerging flux on the energetics and magnetic structure of the above-lying atmosphere is insufficient, as is the fate of the rising magnetic field and how it couples the different regions of the atmosphere. In this talk we will present realistic" 3D models of flux emergence and discuss the reconnection that ensues as the newly emerged field expands into the chromosphere."

Speaker: Viggo Hansteen (Institutt for teoretisk astrofysikk, Universitetet i Oslo)

15:05 Coffee break + poster viewing

22 Spectral analysis and modelling of solar flare chromospheric condensation

We follow up on our recent analysis of the X-Class flare SOL2014-09-10T17:45, where we studied the impulsive phase dynamics of tens of individual flaring kernels", in both coronal (Fe XXI) and chromospheric (MgII) lines observed at high cadence with IRIS. We concentrate here on the chromospheric aspect of the phenomenon, extending the analysis to multiple spectral lines of Mg II, Fe II, Si I, C II. We show that many flaring kernels display high velocity downflows in the spectra of all these chromospheric lines, exhibiting distinct, transient and strongly redshifted spectral components. From preliminary modelling using RADYN with the thick-target interpretation, the presence of two spectral components appears to be consistent with a high flux beam of accelerated electrons. In particular the highest energy electrons can heat the denser, lower layers of the atmosphere, while the bulk of the beam energy, deposited higher in the atmosphere, is sufficient to produce chromospheric evaporation with a corresponding condensation."

Speaker: David Graham (INAF Osservatorio Astrofisico di Arcetri)

Slides dgraham_IRIS6.key

23 Self-Consistently Modeling the Chromospheric Response to Flare-Accelerated Electrons and Return Currents

During solar flares, copious electrons are accelerated to high energies. These travel along magnetic field lines in the Sun's atmosphere, colliding with and heating the ambient plasma. In addition to this direct heating, these particles induce a return current that further heats the atmosphere via resistive dissipation. The return current produces an electric field that balances the electric field produced by the flare-accelerated electrons. These electrons will be decelerated by the return current electric field, but that alters the flare-accelerated electron spectrum, which in turn alters the return current. Thus, the propagation of these high energy electrons and their response in the presence of the return current is a coupled non-linear problem. Additionally, return current heating is dependent upon the plasma density and temperature and, therefore, is coupled with the hydrodynamic response to the flare. To self-consistently account for these processes requires a model that can simulate how the flare-accelerated electrons, return current, and hydrodynamic state of the solar atmosphere affect each other. Here we report on such a model. We have produced simulations of the radiative hydrodynamic response of the solar atmosphere to flare heating using the RADYN code. RADYN has been coupled with an additional code that solves the Fokker-Planck kinetic theory, which describes the interaction of flare-accelerated particles with the return current as well as with the ambient plasma. We specifically study the response of the solar chromosphere where a majority of flare heating occurs.

Speaker: Joel Allred (NASA/GSFC)

Slides iri6_allred.pptx

24 IRIS and RHESSI observations of the chromospheric response to energy input during the 29th March 2014 flare

In the common flare scenario beams of accelerated electrons deposit large amounts of energy into the chromosphere, heating it to MK temperatures and causing chromospheric evaporation, i.e. the expansion of the heated plasma into the magnetic loop. What is the importance of energy deposition by electron beams in driving evaporation relative to other types of energy deposition such as by thermal conduction? We present simultaneous EUV and X-ray observations of chromospheric evaporation in the flare SOL2014-03-29T17:48. IRIS observations of the FeXXI line indicate evaporating plasma at a temperature of 10 MK along the flare ribbon during the flare peak and several minutes into the decay phase. Hard X-ray footpoints are observed for two minutes during the peak of the flare. Their locations coincide with the locations of the upflows in parts of the southern flare ribbon consistent with a scenario of beam driven chromospheric evaporation. However, in other parts of the southern ribbon and in the northern ribbon the observed upflows are not coincident with a HXR source in time nor space, most prominently during the decay phase. In this case evaporation is likely triggered due to energy input by a conductive flux that is established between the hot (25 MK) coronal source, which is present during the whole observed time-interval, and the chromosphere. These observations suggest that conduction driven evaporation dominates not only during the decay phase but also during the flare peak.

Speaker: Marina Battaglia (University of Applied Sciences Northwestern Switzerland)

Slides battaglia_iris_6.pdf

Wednesday 22 June

Session 5

25 Recent developments in modelling of the chromosphere (Invited)

The chromosphere is arguably the most difficult and least understood domain of solar physics. All at once it represents the transition from optically thick to thin radiation escape, from gas-pressure domination to magnetic-pressure domination, from neutral to ionised state, from MHD to plasma physics, and from near-equilibrium (LTE") to non-equilibrium conditions. The heating requirements of the solar chromosphere are not easily determined since the radiative cooling is dominated by optically thick spectral lines that form far from equilibrium. Energy estimates are therefore very model dependent. 1D semi-empirical model atmospheres indicate that to maintain the quiet, average solar chromosphere, the required energy input is in the range 2-12 kW/m2 but these models neglect many important aspects like the dynamics of the chromosphere, non-equilibrium ionization effects and spatial structuring. In this talk, we will present 3D "realistic" radiation-MHD simulations spanning the solar atmosphere from the convection zone to the corona, and synthetic observations calculated from the simulations. We will present a variety of simulations showing the strong dependency of the heating and dynamics of the chromosphere on the magnetic field configuration. "

Speaker: Mats Carlsson (Institute of Theoretical Astrophysics, University of Oslo)

Slides Carlsson.pdf

26 Density diagnostics from the OIV-SIV intercombination lines observed by IRIS

The intensity of the OIV and SIV intercombination lines around 1400 Å observed with the Interface Region Imaging Spectrograph (IRIS) provide a useful tool to diagnose the electron number density in the solar transition region plasma. We measure the electron density in a variety of solar features, including a loop and a bright point in the AR NOAA 12356 and at the ribbons of the 22-June-2015 X-class flare observed by IRIS. In particular, we obtain high densities (≈1013 cm-3) during the impulsive phase of the flare. Finally, we investigate the effects of the high density and non-Maxwellian electron distribution on the atomic data used for the density diagnostics.

Speaker: Vanessa Polito (University of Cambridge)

Slides polito_v.pdf

27 Modeling formation of chromospheric lines in PRD using 3D model atmospheres.

We studied the formation of the most practically important resonance lines such as H I Ly-a, Mg II k&h, and Ca II K&H in the chromosphere using 3D MHD snapshots produced by the Bifrost numerical code. For this purpose, we implemented effects of partial redistribution (PRD) using the hybrid approximation of Leenaarts et al. (2012) into our radiative transfer code. We discuss some technical and computational difficulties we face when simulating spectral line formation in PRD. We demonstrate how the most realistic 3D PRD case is different from traditional but simpler 3D CRD and 1D PRD approximations by showing separately the roles of PRD and full 3D radiative transfer. We explain what physical properties of the chromosphere can be inferred from observations of all these lines. This investigation provides a more realistic interpretation of observations made by the IRIS satellite, the CLASP rocket experiment, and the CHROMIS instrument on the SST.

Speaker: Andrii Sukhorukov (Institutet för solfysik, Stockholms universitet)

28 Three-dimensional simulation of chromospheric jets with twisted chromospheric magnetic field lines

A three-dimensional simulation by the Tokyo code (tentative name) is presented to investigate the effect of three-dimensionality on the scale of chromospheric jets and the dependence on the photospheric magnetic field structure. We newly develop a numerical code for the radiation magnetohydrodynamic modeling of solar atmosphere. The code includes the effect of non-local radiative transfer in the photosphere and optically thin radiative cooling in the upper layer. The Spitzer-type thermal conduction and latent heat of partial ionization is also taken into account. The tall chromospheric jets with the maximum height of 10–11 Mm and lifetime of 8—10 min are formed above the strong magnetic field concentration. The strongly entangled magnetic field lines are formed in the chromosphere, which helps to drive these tall chromospheric jets through the Lorentz force. We also find that the produced chromospheric jets form a cluster with the diameter of several Mm with finer strands.

Speaker: Haruhisa Iijima (Institute for Space-Earth Environmental Research (ISEE), Nagoya University)

10:30 Coffee break

29 The chromosphere and its connections to the transition region and corona (Invited)

There is an everlasting mass cycle between the chromosphere and the upper atmosphere, the transition region and the corona. Many suggestions have been made on the nature of this mass cycle, from handwaving arguments to 3D MHD models. However, each of these suggestions has its problems and it is unclear what really governs the mass exchange. For example the transition region emission in the quiet Sun seems to be dominated by spicule-type emission, and spicules are not (yet) well represented in 3D models. In active regions many of the cooler transition region structures seem to be present in the form of small dynamic cool loops and it remains to be seen how loops reaching coronal temperatures are actually connected to such cooler loops. Since long components in the profiles of transition region and coronal lines have been identified in particular in the blue wings, indicating upflows that might indicate the injection of plasma into the corona. But then the question remains unanswered if such upflows are sufficient to feed the corona with mass. Finally, when discussing the connection from the chromosphere to the transition region and corona one also has to consider the relation to the magnetic structure. While tiny loops in the network exist, these unresolved fine structures might not be the dominant source of emission in the transition region, which also is part of solving the puzzle that seemingly loop-like structures are found in unipolar magnetic regions. Here the connection of the transition region structures to the chromosphere might be closer than we think so far, and many of the elongated transition region structures might be closely related to the fibrils in the chromosphere. For these and other observations the data from IRIS have been contributing key information that hopefully will further help unraveling the complex interaction from the chromosphere to the transition region and corona.

Speaker: Hardi Peter (MPI for Solar System Research)

30 Response of chromospheric jets in the corona

The solar chromosphere and transition region plays an important role in understanding the interaction between the relatively cool photospheric plasma and the hot multi-million degree corona. It is only beginning to be understood where and how the required energy is generated, transported and dissipated. It appears that the chromosphere and transition region plays a key role in the process. Dynamic jets as observed with high resolution Interface Region Imaging Spectrograph (IRIS) are currently being studied. We found that these jets are associated with coronal dynamics. This association between jets seen in the chromosphere and material heated to millions degree temperatures seen in the corona, suggests that they play an important role in supplying and also heating the plasma to maintain the corona. Our observations indicates that a common process which generates small-scale jets also generates propagating intensity disturbances (PDs) in the corona. It is most likely that the observed PDs are magneto-acoustic waves which propagate to higher heights while the cool jet material falls back.

Speaker: Tanmoy Samanta (Indian Institute of Astrophysics)

31 Low-lying loops observed with IRIS and the SST: connecting the dots

Low-lying loops have been observed by IRIS at the solar limb in transition region temperatures. It has been suggested that these loops constitute a connection between the corona and transition region, previously known as unresolved fine structure (UFS). In this work we make use of coordinated observations between IRIS and the Swedish 1-m Solar Telescope to study these low-lying loops. We discuss how the loops appear in different chromospheric and transition region lines, and how this can be used to constrain their formation. We also discuss the similarities and differences between these loops and spicules, which are also structures rooted in the magnetic network.

Speaker: Tiago M. D. Pereira (University of Oslo)

Slides Pereira.pdf

32 New insights into the chromosphere-corona mass cycle

Observations of solar transition region emission lines reveal the presence of redshifts up to temperatures of about 300,000 K and blueshifts for higher temperatures. The apparent large downward flows in the lower transition region would lead to an emptying of the corona, in contrast to what is being observed, thus some mechanism must be responsible for maintaining the mass balance between the photosphere and corona. We use the Bifrost stellar atmosphere code to perform 3D radiation MHD simulations of the solar atmosphere and to study the temporal evolution of mass flows into and out of the solar corona. By adding tracer particles to the simulations and analyzing their characteristics over time, we provide new insights on the physical processes driving these mass flows and on their role in the chromosphere-corona mass cycle.

Speaker: Pia Zacharias (University of Oslo)

Slides IRIS6_PiaZacharias.pdf

12:45 Lunch

Session 6

33 Multi-fluid effects in the solar atmosphere (Invited)

The deepest layers of the solar atmosphere are only partially ionised. As a result, the presence of neutrals cannot be neglected and they can influence magnetic structures in various aspects. Neutrals do not feel directly the effects of the Lorentz force and are only sensitive to it through collisions with charged particles. The net effect is that these collisions have an impact on the electric currents and, as a consequence, they can influence the magnetic equilibrium, they can give rise to magnetic instabilities, they can have an impact on wave propagation and can give rise to a number of energy dissipation mechanisms. Under a single-fluid approach, all these effects appear through the generalised Ohm's law, in which a number of non-ideal terms appear. These terms have different importance, and at different spatial and temporal scales, depending on the magnetic configuration and at different heights on the solar atmosphere. Two-fluid or multi-fluid approaches have also been attempted. Even if there are no definite evidences yet of the effects induced by the presence of neutrals on some solar phenomena, there is an increasing opinion that observations and numerical simulations with higher and higher spatial resolution are required to identify them. In this contribution, all these aspects will be reviewed under three points of view: advances under purely theoretical analyses, progress in numerical simulations and attempts to observationally determine the different behaviour of the neutral and charged components of the solar plasma.

Speaker: Manuel Collados (Instituto de Astrofisica de Canarias)

Slides Collados.pdf

34 Observational consequences of ion-neutral effects in the low solar atmosphere

The complexity of the chromosphere is due to various regime changes that take place across it. Consequently, the interpretation of chromospheric observations is a challenging task. It is thus crucial to combine these observations with advanced radiative-MHD numerical modeling. Because the photosphere, chromosphere and transition region are partially ionized, the interaction between ionized and neutral particles has important consequences on the magneto-thermodynamics of these regions. We implemented the effects of partial ionization using the generalized Ohm’s law in the Bifrost code (Gudiksen et al. 2011) which solves the full MHD equations with non-grey and non-LTE radiative transfer and thermal conduction along magnetic field lines. The implementation of partial ionization effects impact our modeled radiative-MHD atmosphere, such as producing chromospheric heating and diffusion of magnetic field into the upper-chromosphere among others. We will describe, in detail, the differences in the physical processes and chromospheric synthetic observations from simulations with and without ion-neutral interaction effects. This detailed description is important for the interpretation of recent and future chromospheric observations.

Speaker: Juan Martinez-Sykora (BAERI)

35 Emergence of granular-sized magnetic bubbles through the solar atmosphere: the path to the transition region

We study the ascent of granular-sized magnetic bubbles from the solar photosphere through the chromosphere into the transition region and above, for the first time. Such events occurred in a flux emerging region in NOAA 11850 on September 25, 2013. During that time, the first co-observing campaign between the Swedish 1-m Solar Telescope and the IRIS spacecraft was carried out. Simultaneous observations of the chromospheric H$\alpha$ 656.28 nm and \ion{Ca}{2} 854.2 nm lines, plus the photospheric \ion{Fe}{1} 630.25 nm line, were made with the CRISP spectropolarimeter at the SST reaching a spatial resolution of 0.14. At the same time, IRIS was performing a four-step dense raster of the said emerging flux region, taking slit-jaw images at 133 (C~{\sc ii}, transition region), 140 (\ion{Si}{4}, transition region), 279.6 (\ion{Mg}{2} k, core, upper chromosphere), and 283.2 nm (\ion{Mg}{2} k, wing, photosphere). Spectroscopy of several lines was performed by the IRIS spectrograph in the far and near ultraviolet, of which we have used the \ion{Si}{4} 140.3 and the \ion{Mg}{2} k 279.6 nm lines. Coronal images from the Atmospheric Imaging Assembly of the Solar Dynamics Observatory were used to investigate the possible coronal signatures of the flux emergence events. The photospheric and chromospheric properties of small-scale emerging magnetic bubbles have been described in detail in Ortiz et al. (2014). Here we are able to follow such structures up to the transition region. We describe the properties, including temporal delays, of the observed flux emergence in all layers. We believe this may be an important mechanism of transporting energy and magnetic flux from subsurface layers to the transition region and corona. "

Speaker: V. Hansteen

36 Spectral signatures of spicules in the IRIS channels: observations and preliminary modeling.

IRIS observations of chromospheric and CCTR lines, with their unique spatial and temporal capabilities, allow for a complete diagnostic of the highly dynamical spicules from their cool roots to their expansion and disappearance into the corona. We analyzed both spectral and slit-jaw observations near the south pole of the sun, with the slit perpendicular to the limb. Here we will present the main characteristics of the average Mg II (not only h and k but also the 3p-3d) line profiles along with those of the CI, OI and Si IV lines and we compare with 1D non-LTE modelling. We find that most profiles parameters are reproduced by the model, except for the k/h ratio; we discuss this last issue in terms of velocity fields in very fine (non resolved) structures. We will also present results on the spatial and spectral time evolution during the ~ 1 hour long interval of observation.

Speaker: Costas Alissandrakis (University of Ioannina)

37 Small scale chromospheric fibrils observed by SUNRISE 2

The balloon-borne SUNRISE observatory allows obtaining observations in the UV with unprecedented temporal stability and spatial resolution. During its second scientific flight the Sunrise Filter Imager (SUFI) was used to record a time series of narrow-band intensity images in the Ca II H line lasting for approximately one hour at a cadence of 7 seconds. This unique data set enabled us to identify and track small scale fibrils observed in this line and analyse their morphological properties and their temporal evolution. In a next step, we combined the information extracted from the identification and tracking of the fibrils in the intensity images with the spectro-polarimetic data recorded simultaneously with the Imaging Magnetograph eXperiment (IMaX), to identify the foot-points of these structures and correlate them with the brightness variation and temporal evolution observed in the Ca II H images. This correlation allows us to characterise the magnetic configuration of the fibrils, for example whether they are small-scale loops, or part of a longer magnetic structure.

Speaker: Ricardo Gafeira (Max Planck Institute for Solar System Research)

Slides Gafeira.pdf

16:05 LOC announcement

18:00 Conference dinner

Thursday 23 June

Session 7

38 First results from the He I 10830 spectrolarimeter at GREGOR (Invited)

Spectropolarimetry in the He I 1083nm multiplet offers a unique opportunity to observe magnetic fields in the chromosphere. There, the magnetic fields are expected to be much lower in intensity than in the photosphere. In this regard, the polarization signals of the He I 1083nm multiplet are sensitive to atomic level polarization and to the joint action of the Hanle and Zeeman effects, which makes this spectral line ideal for the determination of a wide range of magnetic field strengths in chromospheric and coronal structures. The information about the magnetic field vector, encoded onto the Stokes I, Q, U, and V profiles, can be successfully extracted with present techniques. Moreover, it is located in the near-IR where atmospheric seeing effects are less severe, so much that telescopes can achieve their diffraction limit, although at less spatial resolution than in the visible. Here, a review of the first, superb spectropolarimetric observations taken in the He I 1083 nm multiplet with the GREGOR Infrared Spectrograph (GRIS), a near-infrared scanning spectropolarimeter installed at the German GREGOR 1.5-meter solar telescope located in the Observatorio del Teide, Tenerife, is given. Some of the scientific challenges faced by the new facility are discussed.

Speaker: David Orozco Suárez (Instituto de Astrofísica de Andalucía)

Slides IRIS6-Orozco.pdf

39 Relationship between magnetic field and Mg II line profiles in a tornado-like prominence

Coordinated observations of solar prominences can be a challenge to perform, and even more so to analyse. During international campaigns in 2014 and 2015 we focused on studying tornado-like prominences with a combination of space-based satellites (IRIS, Hinode and SDO) and ground based telescopes (THEMIS, Meudon MSDP). We measured the magnetic field strength and orientation in these tornado-like structures using the He I D3 line from THEMIS, and studied the prominence plasma behaviour at a range of temperatures using IRIS, Hinode and SDO. Through a robust 2D cross correlation method to co-align data from the different instruments, we are able to probe plasma and magnetic properties in the prominence on a pixel-by-pixel basis, allowing for analysis of conditions in the tornado. We present data from 15 July 2014, wherein we show spectral analysis of the Mg II h and k lines from IRIS, line profile analysis of Hinode/EIS lines, and inversion results from THEMIS. We also study the relationship between line profile shape and magnetic field for both the Mg II lines and a number of lines observed by the Hinode/EIS instrument. In the optically thick Mg II and He II lines we mostly see cool loops and threads of material in this prominence. In coronal lines from EIS and SDO/AIA we see the tornado-like columns as dark features above the limb, absorbing the background emission. Mg II profiles show a mixture of reversed and non-reversed profiles in the prominence, with the reversed profiles mostly occurring where the magnetic field is strongest, reaching between 20 and 50 G in some parts. There is a horizontal magnetic field in these structures, parallel to the limb, which is unambiguous in our observations.

Speaker: Peter Levens (University of Glasgow)

Slides Levens.pdf

40 Chromospheric response to prolonged small-scale reconnections

Magnetic reconnection is thought to be responsible for eruptive events on the Sun, from large-scale CMEs to small-scale events that are barely resolved. The height in the solar atmosphere where the reconnection takes place may significantly influence the morphology and appearance of the events. This might be the main distinction between events such as e.g. Ellerman bombs, IRIS bombs, or explosive events which are observed in spectral lines forming at different temperatures showing different line profile characteristics. Here we report a prolonged reconnection event caused by a magnetic feature moving from a sunspot to an adjacent pore. Data-driven nonlinear force-free field extrapolations and magneto-frictional models suggest that the strong dominant sunspot and the pore of the same polarity lead to a canopy-type field structure at low heights. The moving magnetic feature of the opposite polarity reconnects with the overlying canopy. The models suggest a height of the reconnection only about 500 km above tau=1. The IRIS observations of this event show complex Mg II, C II, and Si IV line profiles, in part similar to those reported earlier for IRIS bombs. We find evidence for strong spatial, velocity discontinuities from red to blueshifts in the Si IV Dopplergrams at the reconnection site. Additionally, multiple blueshifted components along the same line of sight are also observed. Further, there is evidence that the downflowing plasma from the reconnection site interacts with chromospheric plasma, increasing the apparent brightness of the structure. The magnetic modelling of these events supports the earlier suggestions that the location of the reconnection of these active region UV bursts is very low in the atmosphere, in the low chromosphere or even in the photosphere. Small-scale events like this may give insights into the dynamics and complex evolution of large-scale eruptive events on the Sun.

Speaker: Lakshmi Pradeep Chitta (Max Planck Institute for Solar System Research)

Slides chitta_iris6_2016_online.pdf

41 Probing the solar chromosphere with ALMA

Atacama Large Millimeter/Submillimeter Array (ALMA) is opening a new chapter in the study of the Sun by providing a leap in spatial resolution and sensitivity compared to currently available millimeter wavelength observations. Using state of the art radiation MHD simulations of the solar atmosphere we demonstrate the huge potential of ALMA observations for uncovering the nature of the solar chromosphere. We show that ALMA will provide a reliable probe of the chromospheric thermal structure as well as a powerful new diagnostic of magnetic field at chromospheric heights.

Speaker: Maria Loukitcheva (Max-Planck-Institut fuer Sonnensystemforschung)

10:30 Coffee break

42 Resonant Absorption as Feeding Mechanism for Alfvenic Turbulence - Observable Signatures in a Solar Prominence (Invited)

The solar atmosphere is permeated with MHD waves of Alfvenic character. Such waves are considered important energy carriers through which the heating and morphology of the corona may be explained. The highly inhomogeneous coronal environment, particularly in solar prominences, ensures the co-existence of different wave modes that are expected to be coupled through the robust and efficient mechanism of resonant absorption. While energy transfer between wave modes is ensured, significant dissipation and heating is far more difficult to achieve. Alfvenic turbulence has been suggested as a possible dissipation mechanism for Alfvenic MHD waves in the solar atmosphere. In this talk I will show how resonant absorption is key in achieving a sustained generation of Alfvenic turbulence and braiding, through which significant heating can be achieved. The observational signatures of this mechanism have been detected in a solar prominence by combining high resolution imaging and spectroscopic instruments such as Hinode and IRIS.

Speaker: Patrick Antolin (School of Mathematics and Statistics, University of St Andrews)

Slides antolin_iris6_june2016.pdf

43 Large-Amplitude Oscillations in Prominences

Large-amplitude oscillations in prominences are among the most spectacular phenomena of the solar atmosphere. Such an oscillations involve motions with velocities above 20 km/s, and large portions of the filament that move in phase. These are triggered by energetic disturbances as flares and jets. These oscillations are an excellent tool to probe the not directly measurable filament morphology. In addition, the damping of these motions can be related with the process of evaporation of chromospheric plasma associated to coronal heating. In these talk I will review recent observational and theoretical progress on large-amplitude seismology on prominences.

Speaker: Manuel Luna (Instituto de Astrofísica de Canarias (IAC))

Slides luna.pdf

44 Tether-cutting Reconnection between Two Solar Filaments Triggering Outflows and a Coronal Mass Ejection

Triggering mechanisms of solar eruptions have long been a challenge. A few previous case studies have indicated that preceding gentle filament merging via magnetic reconnection may launch following intense eruption, according to the tether-cutting (TC) model. However, the detailed process of TC reconnection between filaments has not been exhibited yet. In this work, we report the high-resolution observations from the Interface Region Imaging Spectrometer (IRIS) of TC reconnection between two sheared filaments in NOAA active region 12146. The TC reconnection commenced on ∼15:35 UT on 2014 August 29 and triggered an eruptive GOES C4.3-class flare ∼8 minutes later. An associated coronal mass ejection appeared in the field of view of the Solar and Heliospheric Observatory/LASCO C2 about 40 minutes later. Thanks to the high spatial resolution of IRIS data, bright plasma outflows generated by the TC reconnection are clearly observed, which moved along the subarcsecond fine-scale flux tube structures in the erupting filament. Based on the imaging and spectral observations, the mean plane-of-sky and line-of-sight velocities of the TC reconnection outflows are separately measured to be ∼79 and 86 km s−1, which derives an average real speed of ∼120 km s−1. In addition, it is found that spectral features, such as peak intensities, Doppler shifts, and line widths in the TC reconnection region are evidently enhanced compared to those in the nearby region just before the flare.

Speaker: Huadong Chen (National Astronomical Observatories, Chinese Academy of Sciences)

45 Three-dimensional MHD Simulation of Prominence Formation by Radiative Condensation

We show that topological change of coronal magnetic field can trigger radiative condensation for in-situ prominence formation by 3D MHD simulation including thermal conduction and optically thin radiative cooling. The multi-wavelength observation by SDO/AIA (Bergeret al., 2012) has found the process of in-situ prominence formation, in which the cool dense plasma of prominence came from radiative condensation in the corona without plasma injection from the chromosphere. In our previous study, we have proposed a model for in-situ prominence formation and demonstrated it by 2D simulation (Kaneko & Yokoyama, 2015). In our model, converging and shearing motions are imposed at the footpoint of a coronal arcade, and a flux rope is formed through the reconnection at the polarity inversion line (PIL). Inside the flux rope, radiative cooling overwhelms background heating due to the dense coronal plasmas levitated by magnetic field. The thermal imbalance can not be suppressed by thermal conduction along the closed magnetic loops of the flux rope, leading to radiative condensation. In our previous 2D simulation, the flux rope was approximated to be of the infinite length, hence, thermal conduction was effective only along poloidal magnetic field. In this case, radiative condensation is more likely to be triggered due to absence of thermal conduction along toroidal magnetic field. In the present study, we demonstrate that our prominence formation model does work in the case of flux rope with a finite length. By imposing converging and shearing motion localized along the PIL, the flux rope with a finite length is formed. When the length of flux rope is sufficiently long, radiative condensation is triggered and prominence (filament) is formed along PIL.

Speaker: Takafumi Kaneko (The University of Tokyo)

Slides kaneko_iris6_20160623.pdf

12:45 Lunch

Session 8

46 MHD waves in the solar chromosphere (Invited)

Within the last decade, due to significant improvements in the spatial and temporal resolution of chromospheric data, MHD wave studies in this fascinating region of the Sun's atmosphere have risen to the forefront of solar physics research. In this talk we review the most interesting MHD wave-related discoveries by space and ground-based instruments such as Hinode, IRIS, CRISP/SST and ROSA/DST. We will also discuss related developments in chromospheric MHD wave modelling, both from an analytical and numerical point of view.

Speaker: Gary Verth (Sheffield University)

Slides Verth_IRIS_talk_2016.pdf

47 Chromospheric heating due to transverse waves in small magnetic elements

We characterise magnetohydrodynamics (MHD) waves in small magnetic elements and investigate their propagation in the solar atmosphere by exploiting coordinated observations with the Swedish 1 m Solar Telescope (SST) and the Interface Region Imaging Spectrograph (IRIS). We use wavelet transform to analyse (horizontal) displacement oscillations in magnetic bright points in several passbands sampling various heights within the solar photosphere and chromosphere. Cross correlations between wavelet power spectra of the transverse oscillations in the same magnetic elements observed at different atmospheric heights, and of oscillations in emission peaks of the IRIS Mg II H & K lines result in positive phase lags at high frequencies (up to 30 mHz). These reveal a direct evidence for upward propagation of transverse waves in the magnetic elements under study, and more importantly, an evident heating signature at the heights sampled by the Mg II H & K lines, i.e., the upper chromosphere. Together with estimates of height differences between the atmospheric layers sampled by the employed spectral bands (and hence phase speeds of the propagating waves) as well as the determined velocity amplitudes, we estimate an energy flux on the order of 36 kW/m^2, that is sufficient to heat the quiet chromosphere. The heating signatures from the IRIS spectra suggest that this energy, or at least a fraction of that, is likely released in the upper chromosphere.

Speaker: Shahin Jafarzadeh (Institute of Theoretical Astrophysics, University of Oslo)

Slides Shahin_Jafarzadeh.pdf

15:05 Coffee break + poster viewing

48 Generation of upward Alfvénic waves by the chromospheric shock waves

Classically, the solar chromosphere was thought to be a dissipative medium for Alfvén waves, in that those generated by the convective motions were thought to dissipate most of their energy there. However, recent high-resolution observations near the coronal base strongly suggest that the upward Alfvén waves are generated or amplified in the chromosphere by the large amplitude acoustic (shock) waves. This means that the classical wave-turbulence modelings of the solar corona and the solar wind need some modification, because most of such modelings neglect the interaction between the acoustic waves and the Alfvén waves. Understanding the Alfvén wave generation in the solar chromosphere is essential for more realistic modeling. We have performed both numerical simulations of one-dimensional MHD systems and the observations of the transverse Doppler velocities using off-limb spicule observation by IRIS. We first have investigated numerically the nonlinear interaction between large-amplitude acoustic (shock) waves and small-amplitude Alfvén waves in the chromosphere. We have found that, if the plasma beta is around unity, Alfvén waves are trapped and amplified around the shock front, leading to the strong amplification of the Alfvén waves. From this process, only upward Alfvén waves are generated. Since this process is likely to occur for strong acoustic shock waves propagating in a beta-unity plasma, the solar chromosphere is a favorable place for this process. We have compared the power-spectrum of the transverse velocity between the simulations and observations. Mg II k line is used for deriving the observational transverse velocities in the chromosphere. The similarity of the power spectrum strongly suggests that the generation of upward Alfvénic waves occur in the middle chromosphere under the formation height of Mg II k line.

Speaker: Munehito Shoda (The University of Tokyo)

Slides 20160623_shoda.pdf

49 Simulations of Alfven wave driving of the solar chromosphere - efficient heating and spicule launching.

By driving Alfven and kink waves into an expanding flux tube we demonstrated that with a Poynting flux on 2x10^7 erg/s/cm^2 we can reproduce a heating profile broadly consistent with the mid to upper chromosphere. The heating mechanism is through ponderomotive coupling of Alfven waves to shock and it is the shocks which dissipate and heat. These same shocks also produce jets similar to spicules (Type-I).

Speaker: Tony Arber (University of Warwick)

Slides Arber.pdf

50 Wave heating of the partially-ionised atmosphere

The most energetic part of the Sun, interior, due to its plasma parameters is hidden below the solar surface and invisible to the observer. Nevertheless, the solar interior generates the energy and provokes atmospheric magnetic activity. Despite great progress in both observational and simulation methods, the mechanism of energy transport from the solar convection zone into the upper atmosphere, and the upper-atmospheric heating mechanism remain the main unresolved problems in solar physics. Waves in magnetic field concentrations were shown to carry sufficient energy to provide the chromospheric heating. They create currents, which can be effectively dissipated due to ion-neutral interaction in partially ionised chromospheric plasma. In this presentation, we analyse the role of non-ideal plasma effects in the solar atmospheric energy transport. Using numerical magneto-hydrodynamic modelling we create detailed models of magnetic flux tubes, which take into account the effects of partial ionisation and ion-neutral interaction in the solar atmospheric plasma. We show that compressible and incompressible oscillations in solar magnetic fields are, indeed, able to dissipate efficiently and provide sufficient energy to compensate the chromospheric radiative losses.

Speaker: Sergiy Shelyag (Northumbria University)

Slides IRIS_220616.pptx

17:00 Concluding remarks