Sunspot Formation: Theory, Simulations and Observations

9 Mar 2015, 09:00 → 13 Mar 2015, 18:30 Europe/Stockholm

Oskar Klein Auditorium (Albanova, Stockholm)

Axel Brandenburg (Nordita), Göran Scharmer (Stockholm University), Illa R. Losada (Nordita), Nishant Singh (Nordita)

Video recordings

Please let us know if any of the recordings should not be made public.

Abstracts

Venue

Oskar Klein Auditorium, Roslagstullsbacken 21, level 4.

Nordita, Stockholm, Sweden

Background

Scientific recording of sunspots started with Galileo in 1609. Since Hale (1908) we know that sunspots are strong concentrations of magnetic field of up to 4000 gauss. They could be formed by subsurface magnetic flux tubes piercing the surface. Meanwhile, numerical simulations by many different groups suggest that strong magnetic fields could be generated in the bulk of the convection zone. This would mean that sufficiently strong magnetic fields may be generated not far from the surface. However, at the surface the magnetic field appears to be strongly concentrated into only a few isolated spots - in stark contrast to the more diffuse magnetic field beneath the surface. This is still a mystery.

Scientific Organizing Committee

The detailed plan of the meeting has been established by the Scientific Organizing Committee including Axel Brandenburg (PI, Nordita), Mark Cheung (Lockheed, Palo Alto), Laurent Gizon (MPS, Göttingen), Petri Käpylä (Helsinki University), Alexander Kosovichev (Big Bear), Åke Nordlund (NBI), Matthias Rempel (HAO/NCAR), Göran Scharmer (Co-PI, Stockholm University), Alexandra Tritschler (NSO, New Mexico), and Jörn Warnecke (MPS, Göttingen).

Local Organizing Committee

Illa R. Losada (Chair, Nordita and Stockholm University), Sarah Jabbari (Nordita and Stockholm University), Bidya Binay Karak (Nordita), Hiva Pazira (Stockholm University), Nishant Singh (Nordita), Tine Libbrecht (Stockholm University), Carolina Robustini (Stockholm University), Anne Jifält (Nordita, conference secretary), and also Elizabeth Yang (Nordita, conference secretary).

Scope of the meeting

To set the scene, the meeting will start with a session on global properties of the Sun's magnetic field. Next, we need to discuss observational constraints and results from global helioseismology. This will lead to the consideration of what causes flux emergence from an observational and theoretical point of view. There are conflicting ideas about deep seated versus distributed magnetic field generation in the Sun that need to be highlighted. Next, we focus on local magnetic flux concentrations. There are new theoretical results that can potentially be checked using local helioseismology. One of the particular characteristics of sunspots are surface effects relevant to spot formation. There are new ideas about enhanced cooling mechanisms relevant to spot formation. The region around sunspots, i.e., the so-called penumbra, is another important example that demonstrates the interaction of magnetic fields with convection. The importance of magnetic effects is also evidenced by the occurrence of umbral dots and light bridges. Finally, we open the forum to discuss opposing views on sunspot formation and try a reach a coherent picture of solar activity.

Invited speakers (confirmed)

Karel Schrijver, Lockheed, Palo Alto (session 1)
Tom Duvall, Goddard & Göttingen (session 2)
Bob Stein, Michigan State Univ. (session 3)
Irina Kitiashvili, CTR, Stanford (session 3)
Aaron Birch, MPS, Göttingen (session 5)
Yuhong Fan, HAO, Boulder (session 4)
Hotta Hideyuki, HAO, Boulder (session 4)
Junwei Zhao, Stanford (session 5)
Hannah Schunker, MPS, Göttingen (session 5)
Robert Cameron, MPS, Göttingen (session 6)
Rolf Schlichenmaier, KIS, Freiburg (session 7)
Matthias Rempel, HAO, Boulder (session 7)
Mark Cheung, Lockheed, Palo Alto (session 9)
+ others (to be confirmed)

Preliminary program

We plan to organize 8 sessions with talks and discussions and one last summarizing session. In each of the session we will try to have observational, theoretical and simulation points of view. Most of the talks will be 30 minutes long. Additional contributions will be accommodated as shorter talks or posters. There will be a discussion session at the end of each day. The topics for each day include:

Monday am: Session 1. Global properties of the Sun's magnetic field

Monday pm: Session 2. Observational constraints and results from global helioseismology

Tuesday am: Session 3. Flux emergence: observational and theoretical results

Tuesday pm: Session 4. Deep seated vs. distributed magnetic field generation in the Sun

Wednesday am: Session 5. Local flux concentration: theoretical results and local helioseismology

Wednesday pm: Session 6. Surface effects relevant to spot formation. Cooling mechanisms

Thursday am: Session 7. Penumbra formation. Magnetic effects, umbral dots, light bridges, etc.

Thursday pm: Session 8. Opposing views on sunspot formation.

Friday: Session 9. Coherent picture of solar activity.

Registration

Registration form.

Registration deadline (extended): 23 January 2015

Registration fee: 2000 SEK/214.4 EUR/240.42 US Dollar

The registration fee includes lunches, conference dinner and excursion.

Payment method : bank tranfer.

Please, it is very important to write the reference number, your name (or surname) and the code of the conference "SUNSPOT2015”. E.g.:

Ref: “4940165 Foo Bar SUNSPOT2015”

Swedish participants:

Plusgiro 15657-0

Bankgiro 5050-0206 (Danske bank)

Non-Swedish/Overseas participants:

Bank: Swedbank, 10534 Stockholm

Bankkonto: 890119243778462 Iban: SE7480000890119243778462 Swift/bic: SWEDSESS

Travel stipends

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.

Accommodation

Reduced rates are available at Arcadia Hotel and BizApartments If you need help, please contact the conference secretary Anne Jifält (alternatively Elizabeth Yang).

Sponsored by:

conference photo IMGP9042.jpg

more information dinner.pdf

Monday, 9 March

08:00 Registration

Global properties of the Sun's magnetic field

Chair 1

Convener: Axel Brandenburg (Nordita)

1 Global properties of the Sun's magnetic field

The large-scale magnetic field of the Sun is shaped by the interplay of active-region emergence, random-walk dispersal in supergranulation, and advective transport in the meridional flow and differential rotation. In that process, active region nests, the plage state, and flow-field interactions are important to the formation and evolution of active regions, and likely to the global dynamo process. In this talk, I plan to review observations and numerical experiments, looking for an emerging picture, while identifying some of the open questions.

Speaker: Karel Schrijver (Lockheed Martin Advanced Technology Center)

Slides Stockholm2015_Schrijver.pdf

2 Study of sunspot group evolution on a large statistical sample

There are different mechanisms affecting the formation of sunspot groups e.g. buoyancy on the emerging flux ropes, drag force, tension, etc. To check the roles of these theoretical mechanisms one needs to study the evolutionary phases of sunspots and sunspot groups on a large statistical sample. Our study is based on the SOHO/MDI – Debrecen Data (SDD). Its high-temporal resolution allows to study the emergence and decay of the sunspot groups in detail. We investigate the distance and the growth/decay rates as well as the asymmetry between the compactness of the different polarity parts of active regions in time.

Speaker: Judit Muraközy (Heliophysical Observatory of Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences)

Slides Murakozy_@Nordita_oralpresentation.pdf

3 Solar cycle properties from Schwabe data

The long term properties of sunspots aid us in better understanding of the relation between solar dynamo and surface magnetic fields, polar fields and open flux. The continuous observation of sunspots are not available before 1825 but sunspot numbers are available back to 1610. Reconstruction methods are available to derive the sunspot butterfly diagram from sunspot numbers. Studying long term solar cycle properties and updating the reconstruction method would lead to an efficient way of reproducing sunspot emergence. Analysing historical observations of sunspots would contribute more to the properties of sunspots. Samuel Heinrich Schwabe has recorded his observations of the Sun during 1825 - 1867, which have been digitized and the positions and sizes of sunspots were obtained from that. With that, the physical areas of sunspots and tilt angles of the sunspot groups were calculated for that period. The reconstruction method was then tested for Schwabe data and found to reproduce the butterfly diagram. The evolution of magnetic field on the surface of the Sun were then derived using a Surface Flux Transport Model.

Speaker: Senthamizh Pavai Valliappan (Leibniz Institute for Astrophysics)

Slides ppt_Pavai.pdf

4 Understanding stellar cycle and noise to search for Earth-like exoplanets

The radial velocity method is a powerful way to search for exoplanetary systems and it led to many discoveries of exoplanets in the last 20 years. Nowadays, understanding stellar activity and noise is a key factor for achieving a substantial improvement in such technique. Radial-velocity data are time-series containing the effect of both planets and stellar disturbances: the detection of Earth-like planets requires to improve the signal-to-noise ratio, i.e. it is central to understand the noise present in the data. Noise is caused by physical processes which operate on different time-scales, oftentimes acting in a non-periodic fashion. We present here an approach to such problem: to look for multifractal structures in the time-series coming from radial velocity measurements, identifying the underlying long-range correlations and fractal scaling properties, connecting them to the underlying physical processes (stellar oscillations, stellar wind, granulation, rotation, magnetic activity). This method has been previously applied to satellite data related to Arctic sea albedo, relevant for identify trends and noise in the Arctic sea ice (Agarwal, Moon, Wettlaufer, 2012). Here we suggest to use such analysis for exoplanetary data related to possible Earth-like planets.

Speaker: Fabio Del Sordo (NoRDITA/Yale University)

10:40 Coffee break

Observational constraints and results from global helioseismology

5 Flows in the Convection Zone with Potential Relevance to the Sunspot Problem.

Recent results on convection zone flows will be examined, including the depth structure of supergranulation, the effect of the Coriolis force on supergranular flows, and upper limits on the amplitude of convection at different depths.

Speaker: Thomas Duvall (MPS Gottingen)

Slides duvall_talk.pdf

6 The radial gradient of the near-surface shear layer of the Sun

Helioseismology has provided unprecedented information about the internal rotation of the Sun. One of the important achievements was the discovery of two radial shear layers: one near the bottom of the convection zone (the tachocline) and one near the surface. These shear layers may be important ingredients for explaining the magnetic cycle of the Sun. We measure the logarithmic radial gradient of the rotation rate ($\dd\ln\Omega/\dd\ln r$) near the surface of the Sun using 15 years of f~mode rotational frequency splittings from the Michelson Doppler Imager (MDI) and four years of data from the Helioseismic and Magnetic Imager (HMI). We model the angular velocity of the Sun in the upper $\sim 10$~Mm as changing linearly with depth and use a multiplicative optimally localized averaging inversion to infer the gradient of the rotation rate as a function of latitude. Both the MDI and HMI data show that $\dd\ln\Omega/\dd\ln r$ is close to $-1$ from the equator to 60$^{\circ}$ latitude and stays negative up to 75$^{\circ}$ latitude. However, the value of the gradient is different for MDI and HMI for latitudes above $60^{\circ}$. Additionally, there is a significant difference between the value of $\dd\ln\Omega/\dd\ln r$ using an older and recently reprocessed MDI data for latitudes above $30^\circ$. We could reliably infer the value of $\dd\ln\Omega/\dd\ln r$ up to 60$^{\circ}$, but not above this latitude, which will hopefully constrain theories of the near-surface shear layer and dynamo. Furthermore, the recently reprocessed MDI splitting data are more reliable than the older versions which contained clear systematic errors in the high degree f modes.

Speaker: Atefeh Barekat (Max-Planck-Institut für Sonnensystemforschung (MPS))

7 Prediction of solar activity (Wolf numbers) based on nonlinear dynamo

A one-dimensional nonlinear dynamo model (no R-model) is used for prediction of solar activity (Wolf numbers). The nonlinearity includes the dynamical equation for the magnetic helicity (the magnetic part of the alpha effect) and algebraic nonlinearity of the total alpha effect. The diffusion flux of magnetic helicity is taken into account. The Gnevyshev-Ohl rule works when only the dynamic nonlinearity is taking into account. This model reproduces enough well the Maunder Minimum of the solar activity. Authors: N. Kleeorin, Ya. Kleeorin, S. Porshnev, I. Rogachevskii, D. Sokoloff

Speaker: Nathan Kleeorin (Ben-Gurion University of the Negev)

8 Helicity observations as a constraint of dynamo models

We present our observations on current and kinetic helicities that provide constrains on solar dynamo models. We find that: (1) Current helicity in active regions present a clear hemispheric helicity sign rule but this rule also shows a solar-cycle variation. (2) Strong and weak magnetic fields show opposite helicity signs in active regions and this property may also has a solar-cycle variation. (3) The hemispheric helicity sign rule is also evident in global magnetic field outside the active regions and it is preserved for the whole solar cycle. (4) No solar-cycle variation on subsurface kinetic helicity presents in local helioseismology observation, which poses a question on how could a solar-cycle-independent kinetic helicity produce a solar-cycle-dependent current helicity. Finally, we will also present some preliminary results on using above observational constrains to check two dynamo simulation models, namely, in Miesch & Brown (2012) and in Fan & Fang (2014).

Speaker: Mei Zhang (National Astronomical Observatory of China)

Slides ZhangMei.pdf

13:10 Lunch

Flux emergence: observational and theoretical results

9 One Model Doesn't Fit All: Recent Results of a Detailed Analysis of Sunspot Demographics

Besides the detailed observation and simulations of individual sunspots, the statistical analysis of sunspot populations can provide crucial hints on the mechanisms that shape magnetism in the photosphere. In this presentation we will discuss the results of an ongoing statistical analysis of 11 different sunspot, sunspot group, and bipolar magnetic region databases. The focus of our work has been a detailed characterization of the area and flux distributions of photospheric magnetic structures, and how it is shaped by the solar cycle. Our results suggest the existence of two distinct populations of sunspots: a population of groups and active regions, and second of pores and ephemeral regions. We find that only the properties of the former population, the active regions, is found to vary with the solar cycle, while the statistical properties of pores and ephemeral regions remain constant in time. In the context of this workshop, this suggests that perhaps there is not a unique mechanism that can explain the formation of all sunspots.

Speaker: Andres Munoz-Jaramillo (Montana State University)

Slides AMJ_Stockholm_2015.pdf

10 Emergence and Decay of Sunspots Groups Observed with HMI

We report on the formation and decay of active regions as observed with HMI vector B and Ic data. Observed quantities such as flux emergence rates and maximum footpoint separations can be used to test the accuracy of numerical simulations of sunspot formation. We calculated the emergence and decay rates with and without intensity contours, i.e., we differentiate between flux emergence and sunspot formation. We compare observed values to recent numerical simulations of Rempel & Cheung (2014) and others. We observe emergence to occur at a rate that is an order of magnitude slower than what is produced in simulations. Decay rates are half that of emergence. We also find that sunspots form simultaneously with flux emergence without a lag time.

Speaker: Aimee Norton (Stanford University)

Slides Norton-Nordita-Upload.pdf

15:40 Coffee break

Flux emergence: observational and theoretical results

Convener: Rainer Arlt (Potsdam)

11 Small-scale emerging flux and local dynamo action

The surface of the Sun is covered by continuously emerging and evolving ``magnetic carpet''. I will present results of realistic numerical simulations showing that the magnetic carpet can result from the turbulent dynamo operating in a shallow subsurface layer. The simulation results reveal details of the dynamo mechanism and formation of small-scale magnetic structures.

Speaker: Irina Kitiashvili (NASA Ames Research Center)

12 Joy's law revisited by using the new Debrecen tilt angle database

A refined form of the well known Joy's law has been formulated by using the new Debrecen tilt angle data. It has been found that the latitudinal distribution of the tilt angles is not merely a monotonously increasing function, but it has a plateau between the latitudes of about 15-25 degrees, where the toroidal field is the strongest. This may imply an impact of the azimuthal magnetic field on the Coriolis force acting on the emerging flux ropes. The new tilt angle databases are derived from two Debrecen sunspot databases, the DPD (Debrecen Photoheliographic Data, 1974 - ) and SDD (SOHO/MDI-Debrecen Data, 1996-2010) along with online tools developed for data selection. The new data have been compared with the traditional sunspot group tilt angle data bases of Mount Wilson Observatory (1917-85) and Kodaikanal Solar Observatory (1906-87) and the deviations have been analysed. Various filters are tested to improve the reliability of the data to establish a combined dataset. This work has received funding from the European Community's Seventh Framework Programme (FP7/2012-2015) under grant agreement No. 284461 (eHEROES).

Speaker: András Ludmány (MTA Debrecen Heliophysical Observatory)

Slides Joy_law_Ludmany.pdf

13 Discussion on the topics of the first day

Speaker: Rainer Arlt

18:00 Wine & Cheese

Tuesday, 10 March

Flux emergence: observational and theoretical results

Convener: Karel Schrijver (Lockheed)

14 Towards understanding the subsurface structure of sunspots

Traditionally, the subsurface structure of sunspots has been inferred using local helioseismic techniques treating the sunspot as a weak perturbation to the waves, however, we show that the surface of the sunspot has a strong effect on the waves. Using 3D MHD simulations of the response of seismic waves to sunspot models with various perturbations, we find that subsurface sound-speed perturbations to a sunspot model introduce smaller travel-time-shifts than perturbations to the quiet-Sun. One way to avoid the strong surface effect is to study sunspots before they emerge at the surface.
In an effort to helioseismically detect and study sunspot regions before, during and after emergence, we have identified 105 emerging sunspot regions (and corresponding quiet-Sun control regions) observed by SDO/HMI between May 2010 to December 2012. The Doppler velocities were then analysed using helioseismic holography to obtain travel-time maps at the surface and at various depths below. This is the largest catalogue of emerging sunspot regions observed by SDO-HMI to-date, containing over 8TB of data, processed and stored in the German Data Centre for SDO housed at the MPS. Here we present the surface magnetic field properties and flow maps of the dataset.

Speaker: Hannah Schunker (Max Planck Institute for Solar System Research)

Slides schunker_public.pdf

15 Properties of sunspot formation as seen from high-resolution observations

The evolution of NOAA11024 from a protospot into a fully-developed sunspot was followed in detail from high-resolution observations. The affluence of continuously emerging magnetic flux assembling the developing spot, the action of twist and shear motions, the presence of a broad magnetic canopy beyond the spot intensity boundaries, the role of light bridges as natural path to gather the new inflowing flux and the presence of abnormal (counter-)Evershed plasma flows seem to be all some of the necessary ingredients to ignite the formation of penumbrae within time scales of 2 hours (and of about 5 hours to shape a fully-fledge sunspot). These and other observational properties will be put together in order to draw a comprehensive picture of the formation of NOAA11024 in particular and of sunspot formation in general.

Speaker: Nazaret Bello Gonzalez (Kiepenheuer-Institut fuer Sonnenphysik)

Slides nbello_obs_sunspot_formation.pdf

16 Examining the relationship between buoyant velocity and magnetic field of an emerging active region

Early theoretical studies of flux emergence showed that solar magnetic fields are brought up to the surface by magnetic buoyancy. In recent years, sophisticated numerical models have been developed to include other effects, such as solar rotation, convection, twisting and geometry of the magnetic flux tube. The objective of this work is to examine how significant these effects are on the emerging process of an active region. The strategy is to compare the observed relationship between the rising velocity and magnetic field strength of an emerging active region (EAR) with the relationship derived from the magnetic buoyancy theory. The level of discrepancy/consistency can be used to infer whether magnetic buoyancy is the main mechanism The vector magnetograms and Dopplergrams from HMI/SDO are used for this study. The results will be discussed in this presentation.

Speaker: Chia-Hsien Lin (National Central University)

Slides ChiaLin.pdf

10:20 Coffee break

Flux emergence: observational and theoretical results

Convener: Alexander Kosovichev (Big Beer)

17 Flux emergence: simulations and observations

Simulation results will be shown for two cases: weak, non-buoyant and strong, buoyant uniform, untwisted, horizontal magnetic fields advected into the computational domain by inflows at 20 Mm depth. Convection both shreds the magnetic field into many thin fibrils as well as keeps the larger-scale structure confined within the inflow supergranule cell. Convective upflows and downflows bend the field into serpentine loops. Magnetic field first emerges as horizontal field over granules with vertical legs in the intergranular lanes and elongates the granules in the field direction. Because of the fibril substructure, the field first emerges in a mixed polarity, "pepper and salt" pattern. As the underlying, large-scale loop emerges, the opposite polarity components at the surface stream into uni-polar flux concentrations at its nearly vertical legs. The three most obvious differences between the weak and strong field cases are: first, the buoyant loop emerges much more quickly (1 day vs 2 days from 20 Mm); second, the weak field does not distort the supergranule-scale convective cells while the strong field enlarges the cells in the field direction which results in the opposity polarit legs spreading apart; third, the field is more horizontal surrounding the leg remaining behind in the strong field case.

Speaker: Bob Stein (Michigan State University)

Slides Stein15.pdf

Deep seated vs. distributed magnetic field generation in the Sun

Convener: Alexander Kosovichev (New Jersey Institute of Technology)

18 Emerging flux in a Solar-like Convective Dynamo

We report the results of a magneto-hydrodynamic (MHD) simulation of a convective dynamo in a model solar convective envelope driven by the solar radiative diffusive heat flux. The convective dynamo produces a large-scale mean magnetic field that exhibits irregular cyclic behavior with oscillation time scales ranging from about 5 to 15 yr and undergoes irregular polarity reversals. The mean axisymmetric toroidal magnetic field is of opposite signs in the two hemispheres and is concentrated at the bottom of the convection zone. The presence of the magnetic fields is found to play an important role in the self-consistent maintenance of a solar-like differential rotation in the convective dynamo model. Without the magnetic fields, the convective flows drive a differential rotation with a faster rotating polar region. In the midst of magneto-convection, we found the emergence of strong super-equipartition flux bundles at the surface, exhibiting properties that are similar to emerging solar active regions.

Speaker: Yuhong Fan (NCAR)

Slides sunspotworkshop_talk_yfan.pdf

19 Scrutinizing the transport of magnetic flux from the tachocline to the surface of solar-like stars.

We would like to introduce a robust but still under work method to probe the existence of flux tubes in low mass stars. This method is based on the ability of numerical simulations to predict a magnetic-field--rotation relation for solar-like stars. Thanks to the observations compiled up to the present day, this theoretical relation can be compared with the corresponding observed relation. Investigating and developing such a method could help disregarding or confirming the the rise of magnetic flux-tubes from a tachocline.

Speaker: Yori Fournier (Leibniz Institute for Astrophysics)

20 Global solar dynamo simulation with and without tachocline: a quest for the dynamo location

The solar dynamo, responsible for sunspot formation, is thought to be operating somewhere within the solar interior. Unfortunately, the available observation techniques are not able to observe magnetic field in inner convection zone but only at the solar surface. There are, however, several observational constrains like the cycle period, magnetic field migration or the phase lag between toroidal and poloidal fields, that give us some hints about the solar dynamo location, In this work we contrast the results of global implicit large eddy simulations, with and without tachocline, with these observational links with the aim to identify the most probable mechanism involved in the formation of sunspots. The dynamo models operating in the convection zone only result in a ~2yr cycle period. Their evolution is consistent with dynamo waves migrating upwards in agreement with the Parker-Yoshimura sign rule. Although the upper part or the solar convection zone presents a naturally developed near-surface shear layer (negative at all latitudes), the migration of the magnetic field is poleward. Models that include in the domain a fraction of the radiative zone evolve in a different way due to the development of a tachocline. The intense toroidal magnetic field generated there evolves in a time-scale of the order of decades. It is confined to a sharp region at the interface between radiative and convective zones, however it governs the evolution of the magnetic field of the rest of the domain. The latitudinal shear in the convection zone seems to be only marginally important for the global field generation. These models also develop a near-surface shear layer that makes the magnetic field to migrate towards the equator at the upper part of the domain. The dynamo solution depends on the Rossby number, with results going from steady magnetic fields (with strong poloidal component) for the rapid rotating cases to oscillatory dynamos with long cycle-period for the slow rotating simulations.

Speaker: Gustavo Guerrero (Universidade Federal de Minas Gerais)

Slides nordita-guerrero.pdf

13:00 Lunch

Deep seated vs. distributed magnetic field generation in the Sun

Convener: Aimee Norton (Stanford University)

21 High resolution calculations of solar global convection and dynamo

Recent results of high-resolution MHD calculation of solar convection zone will be shown. Higher-resolution calculations enable us to investigate the efficient small-scale dynamo in the solar global convection zone. In our highest resolution, efficient small-scale dynamo is excited throughout the convection zone and magnetic field can reach almost equipartition strength. The Lorentz feedback to the thermal convection is significant. In the talk, possible influence from small-scale dynamo to energy transport, differential rotation and large-scale dynamo is discussed.

Speaker: Hideyuki Hotta (HAO/NCAR)

Slides hotta_sunspot.pdf

22 Understanding the equatorward migration of the Sun’s magnetic field

Sunspots occurring on the solar surface following a typical pattern during the activity cycle. At the beginning of the cycle they appear at high latitude, whereas at the end they appear close to the equator. This is associated with an underlying strong toroidal field which migrates equatorward. Since a few years this behavior has been reproduced in global convective dynamo simulations. I will present results from our simulations of global convective dynamos. All of these simulations produce cyclic and migrating mean magnetic fields. Through detailed comparisons, we show that the migration direction can be clearly explained by an alpha-Omega dynamo wave following the Parker-Yoshimura rule. This lead to the conclusion, that the equatorward migration in this and other work is due to a positive (negative) alpha-effect in the northern (southern) hemisphere and a negative radial gradient of rotation outside the inner tangent cylinder of these models. This idea is supported by a strong correlation between negative radial shear and toroidal field strength in the region of equatorward propagation. In the Sun the only region, where the rotation rate possesses a negative radial gradient, is in the near-surface shear layer. A positive alpha-effect there would lead to an equatorward propagating dynamo wave. Furthermore, I will present results of combined simulation of solar dynamo and a coronal envelope. These simulation give us indication, that the latitudinal temperature variation play an important role in generating the differential rotation profile of the Sun through turbulent Reynold stresses. I will show, that the meridional component of the Reynolds stress can explain the formation of a near-surface shear layer in the Sun.

Speaker: Jörn Warnecke (Max-Planck-Institut für Sonnesystemforschung)

Slides joern2015_03_sunspotsI.pdf

23 Grand minima of sunspots and dynamo models

One of the most striking aspects of 11-year sunspot cycles is that there have been times in the past when sunspots did not appear for several years. The most well-known example of this is the Maunder minimum during 1645-1715. Analyses of cosmogenic isotopes (14C and 10Be) indicate that there have been about 27 grand minima in the last 11,000 yr. In my talk I shall address the question how grand minima are produced in dynamo models. First, we shall summarize the results of modeling grand minima using mean-field models, particularly the flux transport dynamo model. Then we shall explore whether the grand minima are self-consistently reproduced in three-dimensional direct numerical simulations of large-scale dynamos.

Speaker: Bidya Binay Karak (Nordita)

Slides sunspot_meet_nordita_karak.pdf

24 Magnetic coupling of the solar atmosphere and the role of sunspots

Sunspots are ideal candidates for probing and understanding the coupling between various layers of the solar atmosphere. In this talk I shall describe how this coupling spans a broad range of spatial and temporal scales in the context of existing theoretical interpretations and the ever increasing need for coordinated observations between ground and space-based facilities.

Speaker: Rohan Eugene Louis (Leibniz Institute for astrophysics (AIP))

16:10 Coffee break

Surface effects relevant to spot formation. Cooling mechanisms

Convener: Neal Hurlburt (Lockheed Martin Solar and Astrophysics Lab)

25 Near surface effects relevant to sunspot formation.

We will discuss the role of near surface flows, based on observations.

Speaker: Robert Cameron (Max Planck Institute for Solar System Research)

Slides Nordita_Cameron.pdf

26 MHD simulations of sunspots and starspots with MURaM

The existence of starspots on cool main-sequence stars other than the Sun is known from observations. However, the properties of these spots (their sizes, temperature, magnetic field strength, etc.) are not well constrained. We aim at determining some of these properties in two- and three-dimensional radiative MHD simulations of the near-surface layers in a local-box setup. In a first parameter study, we try to reproduce the observed properties of sunspots starting from various initial conditions. We find that a homogeneous and vertical magnetic field of up to 4kG is not sufficient to suppress the onset of vigorous convection. Only simulations with a substantial vertical gradient in the magnetic field and with field strengths of > 6 kG at a depth of a few Mm below the surface reproduce the observed surface properties of sunspots. This is evidence against sunspots being shallow structures. We will now apply the same numerical setup to other cool main-sequence stars.

Speaker: Benjamin Beeck (Max Planck Institute for Solar System Research)

27 Non-LTE Calculation of Sunspot Spectra Based on Optically-Thick Semiempirical Modeling.

Models of sunspot spectra based on Sumer observations are used to calculate line profiles observed by IRIS at various disk positions. The models are used to calculate net radiative cooling rates, which indicate the required heating as a function of height.

Speaker: Eugene Avrett (Center for Astrophysics)

Slides Sunspot_Avrett.pdf

Wednesday, 11 March

Local flux concentration: theoretical results and local helioseismology

Convener: Serge Koutchmy (Institut d'Astrophysique de Paris CNRS & UPMC)

28 A Fast-Moving Wave Implying Acoustic Wave Sources Beneath Sunspots Surface

Through cross-correlating oscillation signals at different locations, we are able to reconstruct how waves propagate away from a virtual wave source inside a sunspot in the photospheric level. Before helioseismic waves are visible from the source inside the sunspot, a surprisingly fast-moving wave is detected propagating along the sunspot's radial direction from the inside to the outside, extending about 15~Mm beyond the sunspot boundary. The wave has an apparent phase velocity of 45 km/s. The observation is consistent with a magnetoacoustic wave, excited at approximately 5~Mm beneath the sunspot surface, sweeping across the photosphere, although it is not clear how this wave is excited at that depth. This wave may help open a new window to study the internal structure and dynamics of sunspots.

Speaker: Junwei Zhao (Stanford University)

Slides Zhao-fastwave.pdf

29 MHD Waves and Directional Effects in Sunspots

Magnetic field dominates wave behaviour in sunspots. The direction of the magnetic field has profound effects on mode conversion, "travel times" (as inferred from local helioseismology), and on wave polarization. These in turn affect what we see when observing spots from different orientations. I will discuss recent work at Monash on both travel time dependence on field direction, and on synthesising observables from wave simulations. (Work with Hamed Moradi, Damien Przybylski, Sergiy Shelyag)

Speaker: Paul Cally (Monash University)

Slides Cally_spot.pdf

10:10 Coffee break

Local flux concentration: theoretical results and local helioseismology

Convener: Gustavo Guerrero (Universidade Federal de Minas Gerais)

30 Flows associated with emerging active regions

We have carried out a study of about one hundred emerging sunspot regions observed by the HMI instrument. For each active region, we measure flows by applying helioseismic holography to time series of Doppler images and also by applying local correlation tracking to intensity images. We show that these two methods give very similar results in non-magnetic regions. We find that the active regions in our survey tend to emerge in regions where the horizontal flows are converging. Comparison of the observed flows with results from MURaM simulations of flux emergence show that the initial rise speed of magnetic flux concentrations at depths of about 20 Mm cannot be substantially larger than the convective flow speeds.

Speaker: Aaron Birch (Max Planck Institute for Solar System Research)

Slides Birch_Nordita.pdf

31 A HEK survey of flows around sunspots

Flows in the vicinity of sunspots are investigated, focusing on the statistical behavior of radial and rotational motions. We identify datasets for study using the sunspot records captured in the Heliophysics Events Knowledgebase (HEK) using the sunspot detection method developed under the EGSO and SDO/FFT projects.

Speaker: Neal Hurlburt (Lockheed Martin Solar & Astrophysics Laboratory)

Slides HEKsunspots2015marHurlburt.pdf

32 Effects of rotation and stratification on magnetic flux concentrations

In a strongly stratified turbulent layer, a uniform horizontal magnetic field can become unstable to spontaneously form local flux concentrations due to a negative contribution of turbulence to the large-scale (mean-field) magnetic pressure. This mechanism, called the negative effective magnetic pressure instability (NEMPI), is of interest in connection with dynamo scenarios where most of the magnetic field resides in the bulk of the convection zone, and not at the bottom. Recent work using the mean-field hydromagnetic equations has shown that NEMPI becomes suppressed at rather low rotation rates with Coriolis numbers as low as 0.1. Here we extend these earlier investigations by studying the effects of rotation both on the development of NEMPI and on the effective magnetic pressure. We also quantify the kinetic helicity from direct numerical simulations (DNS) and compare with earlier work. To calculate the rotational effect on the effective magnetic pressure we consider both DNS and analytical studies using the $\tau$ approach. To study the effects of rotation on the development of NEMPI we use both DNS and mean-field calculations of the 3D hydromagnetic equations in a Cartesian domain. We find that the growth rates of NEMPI from earlier mean-field calculations are well reproduced with DNS, provided the Coriolis number is below about 0.06. In that case, kinetic and magnetic helicities are found to be weak. For faster rotation, dynamo action becomes possible. However, there is an intermediate range of rotation rates where dynamo action on its own is not yet possible, but the rotational suppression of NEMPI is being alleviated. Production of magnetic flux concentrations through the suppression of turbulent pressure appears to be possible only in the upper-most layers of the Sun, where the convective turnover time is less than 2 hours.

Speaker: Illa Rivero Losada

Slides illa_sunspot.pdf

33 Bipolar region formation in stratified two-layer turbulence

Abstract: This work presents an extensive study of the previously discovered formation of bipolar flux concentrations in a two-layer model. We relate the formation process to the negative effective magnetic pressure instability (NEMPI), which is a possible mechanism to explain the origin of sunspots. In our simulations we use a Cartesian domain of isothermal stratified gas which is divided into two layers. In the lower layer, turbulence is forced with transverse non-helical random waves, whereas in the upper layer no flow is induced. An initially weak uniform horizontal magnetic field is imposed in the entire domain. In this study we vary the stratification by changing the gravitational acceleration, magnetic Reynolds number, the strength of the imposed magnetic field and the size of the domain to investigate their influence on the formation process. Bipolar magnetic structure formation takes place over a large range of parameters. The magnetic structures become more intensive for higher stratification. The large fluid Reynolds numbers allow for the generation of flux concentrations when the magnetic Prandtln umber is between 0.1 and 1. The magnetic field in bipolar regions increases with higher imposed field strength until the field becomes comparable to the equipartition field strength of the turbulence. A larger horizontal extent enables the flux concentrations to become stronger and more coherent. The size of the bipolar structures turns out to be independent of the domain size. Bipolar flux concentrations are correlated with strong large-scale downward and converging flows and can therefore be explained by NEMPI.

Speaker: Jörn Warnecke (Max-Planck-Institut für Sonnesystemforschung)

Slides 2015_03_sunspotJoern.pdf

34 Magnetic field concentrations from convection simulations

We present preliminary results from high resolution magneto-convection simulations where we find the formation of flux concentrations from an initially uniform magnetic field. We compute the effective magnetic pressure but find that the concentrations appear also in places where it is positive. The structures appear in roughly ten convective turnover times and live close to a turbulent diffusion time. The time scales are compatible with the negative effective magnetic pressure instability (NEMPI), although structure formation is not restricted to regions where the effective magnetic pressure is negative.

Speaker: Petri Käpylä (University of Helsinki)

13:00 Lunch

14:30 Free time

16:30 Conference dinner with boat trip

Boat trip through the Stockholm Archipielago with dinner on board.

Thursday, 12 March

Local flux concentration: theoretical results and local helioseismology

Convener: Dhrubaditya Mitra (NORDITA)

35 Time-distance helioseismology of emerging active regions

I will present results of helioseismology analysis of subsurface flows and sound-speed structures of the largest emerging active regions observed by Solar Dynamics Observatory.

Speaker: Alexander Kosovichev (New Jersey Institute of Technology)

36 Fanning out of the f-mode in presence of nonuniform magnetic fields

We show that in the presence of a magnetic field that is varying harmonically in space, the fundamental or f-mode in a stratified layer is altered in such a way that it fans out in the diagnostic k-omega diagram, with mode power also within the fan. In our simulations, the surface is defined by a temperature and density jump in a piecewise isothermal layer. Unlike our previous work (Singh et al. 2014) where a uniform magnetic field was considered, here we employ a nonuniform magnetic field together with hydromagnetic turbulence at length scales much smaller than those of the magnetic fields. The expansion of the f-mode is stronger for fields confined to the layer below the surface. In some of those cases, the k-omega diagram also reveals a new class of low frequency vertical stripes at multiples of twice the horizontal wavenumber of the background magnetic field. We argue that the study of the f-mode expansion might be a new and sensitive tool to determining subsurface magnetic fields with azimuthal or other horizontal periodicity. Some preliminary results from the analysis of HMI data will also be presented.

Speaker: Nishant Singh (Nordita)

37 Atmospheric seismology of solar active regions

We present our preliminary results from multi-height observations taken for active regions with the HELLRIDE (HELioseismology Large Regions Interferometric Device) Instrument at the Vacuum Tower Telescope (VTT). The goal of our work is to study the differences between propagation of solar acoustic waves in the case of the quiet Sun atmosphere and the atmosphere above the active regions.

Speaker: Aneta Wisniewska (Kiepenheuer-Institut fuer Sonnenphysik)

Slides Stockholm_2015_Sunspot_A.Wisniewska.pdf

10:10 Coffee break

Local flux concentration: theoretical results and local helioseismology

Convener: Robert Cameron (MPS)

38 Intense bipolar structures from dynamo simulations

We show, from direct numerical simulations of stratified MHD, that bipolar structures can emerge from from dynamo simulations. The emerged structures show intriguing dynamical behaviour. The emergence may be attributed to the Negative Effective Magnetic Pressure Instability (NEMPI).

Speaker: Dhrubaditya Mitra

Slides dhrubaSolSurf_short.pdf

39 Formation of intense bipolar regions in spherical dynamo simulations with stratification

We study a system of a highly stratified turbulent plasma. In such a system, when the magnetic Reynolds number is large enough and there is a background field of suitable strength, a new effect will play role in concentrating magnetic fields such that it leads to the formation of magnetic spots and bipolar regions. This effect is due to the fact that the turbulent pressure is suppressed by the large-scale magnetic field, which adds a negative term to the total mean-field (effective) pressure. This leads to an instability, which is known as the negative effective magnetic pressure instability (NEMPI). Direct numerical simulations (DNS) of isothermally forced turbulence have shown that NEMPI leads to the formation of spots in the presence of an imposed field. Our main aim now is to use NEMPI to explain the formation of active regions and sunspots. To achieve this goal, we need to move progressively to more realistic models. Here we extend our model by allowing the magnetic field to be generated by a dynamo. A dynamo plays an important role in solar activity. Therefore, it is of interest to investigate NEMPI in the presence of dynamo-generated magnetic fields. Mean-field simulations (MFS) of such systems in spherical geometry have shown how these two instabilities work in concert. In fact NEMPI will be activated as long as the strength of the magnetic field generated by the dynamo is in a proper range (for more detail see Jabbari et al. 2013). In our new study, we use DNS to investigate a similar system. The turbulence is forced in the entire spherical shell, but the forcing is made helical in the lower 30% of the shell, similar to the model of Mitra et al. (2014). We perform simulations using the Pencil Code for different density contrasts and other input parameters. We applied vertical field boundary conditions in the r direction. The results show that, when the stratification is high enough, intense bipolar regions form and as time passes, they expand, merge and create giant structures. At the same time, the new structures appear at different latitudes. By extending in φ direction, the size of the bipolar regions decreases. When the helical zone is thinner, the structures appear at a later time.

Speaker: Sarah Jabbari (PhD student)

40 Theories of sunspots formation: state of the art

A review of different theories of sunspots formation is presented. Comparisons with numerical simulations is made.

Speaker: Igor Rogachevskii (Ben-Gurion University of the Negev)

Slides Sunspots-Igor.pdf

41 Discussion session: Local flux concentration

Speaker: Robert Cameron

13:00 Lunch

Penumbra formation. Magnetic effects, umbral dots, light bridges, etc.

Convener: Aaron Birch (Max Planck Institute for Solar System Research)

42 Clustering of Flares and Clustering of Sunspots

In this study we tested for groups of flares (flare clusters) in which successive flares occur within a fixed time - the linking window. The data set used is the flare waiting times provided by the X-ray flare detectors on the Geostationary Operational Environmental Satellites (GOES). The study was limited to flares of magnitude C5 and greater obtained during cycle 23. While many flares in a cluster may come from the same active region, the larger clusters often have origins in multiple regions. The longest cluster of the last cycle lasted more than 42 days with an average time separation between successive flares of 5 hours, where no two flares were separated by more than 36 hours. The flare rate in clusters is 4 to 6 time greater than the rate in solar maximum outside of flares. The are indications that flare clustering is associated with periods of multiple sunspot nests.

Speaker: Alan Title (Stanford Lockheed Institute)

43 Albanova colloquium: Zooming in on star and protoplanetary disk formation

daptive mesh refinement (AMR) simulations with the RAMSES code allow us to zoom in to sub-AU scales from Giant Molecular Cloud (GMC) outer scales (~ 40pc). With grid sizes down to ~100 AU (16 levels of factor 2 refinement) we can follow supernova driving of star formation over Myr time scales. With 22 level of refinement we can follow the accretion history of individual stars over ~100 kyr with grid sizes down to ~2 AU, and with 29 levels of refinement we can study the dynamics of accretion disks with jets and disk winds over time scales ~100 yr, with grid sizes down to ~0.015 AU. We are currently implementing AMR radiative energy transfer and non-equilibrium chemistry with dust in the RAMSES code, and also developing a new moving-mesh AMR code (DISPATCH), intended for the near future era of exa-scale computing.

Speaker: Åke Nordlund

16:15 Coffee break

Penumbra formation. Magnetic effects, umbral dots, light bridges, etc.

Convener: Aaron Birch (Max Planck Institute for Solar System Research)

44 Penumbra formation

The various ways of energy transport in radiatively driven magnetoconvection determine the structure of sunspots. At first glance, sunspots are composed of a dark umbra and a brighter penumbra. The darkness of the umbra is readily explained by the tension of the magnetic field lines that supresses convection. Yet, it is clear that even in the darkest part of the umbra, radiative and conductive heat transport are not sufficient to explain the observed photospheric temperatures. Efficient convection must be present everywhere in sunspots. Indeed, with increasing spatial resolution in the last, say, 30 years the existence of umbral fine structure became apparent. I will present some images obtained with GREGOR that show this fine structure at a spatial resolution of 0.08 arcsec. These images demonstrate that magneto-convection in the umbra operates at different intensity levels. In contrast, the magneto-convective mode in the penumbra seems to be more uniform. It always produces filaments that look alike and that lead to the same spatially-averaged brightness. The penumbra is characterized and defined by its brightness relative to the umbra. It also always exhibits the Evershed flow. Yet, the crucial question is: Is there a distinct magnetic property that makes the difference between umbral und penumbral mode of magneto-convection? In this talk we will argue that, yes, there is a canonical value for the vertical component of the magnetic field that makes the difference. The formation of the penumbra is triggered by large inclination, but the penumbra mode of convection can only prevail, if B(vertical) is smaller than a well defined canonical value.

Speaker: Rolf Schlichenmaier (Kiepenheuer-Institut fuer Sonnenphysik)

45 Peeking at small-scale convection in sunspot penumbrae with high-resolution spectropolarimetry

We study the evolution of small-scale convective motions in a sunspot located close to disk center using a temporal sequence of high resolution observations acquired at the SST. We calculate Dopplergrams in photospheric layers and filter them for subsonic oscillations. Lateral downflows appear everywhere in the center side penumbra as small, weak, redshifted patches next to blueshifted flow channels. These patches merge and fragment frequently and have an intermittent life. They show median LOS velocities of 210 m/s, widths of 0.16”, lifetimes of 3.2 minutes and move together with the hosting filaments reacting to their shape variations. The evolution of the penumbral velocity field is surprisingly similar to that of the quiet Sun, except for the shape of the structures. We conclude that penumbral filaments can be understood as elongated convective cells with upflows along their length, weak downflows at the edges and a strong downflow at the end. These results support the existence of overturning convection in the penumbra (Scharmer et al. 2008), which seems to be backed up by the latest 3D numerical simulations (Rempel et al. 2009, Rempel 2011, 2012). However, from a kinematical point of view, the velocity field is also compatible with other theoretical scenarios, such as convective rolls (Danielson 1961) or twisted horizontal magnetic flux tubes (Borrero 2007).

Speaker: Sara Esteban Pozuelo (Instituto de Astrofísica de Andalucía (IAA-CSIC))

Friday, 13 March

Penumbra formation. Magnetic effects, umbral dots, light bridges, etc.

Convener: Illa Losada (Nordita & Stockholm University)

46 On the penumbra - Evershed flow - moat connection: A modeling perspective

We present a series of high-resolution sunspot simulations that cover a time span of up to 100 hours. The simulation domain covers the upper 18 Mm of the convection zone and we use open boundaries that do not maintain the initial field structure against decay driven by convective motions. We consider two setups: A sunspot simulation with penumbra, a "naked-spot" simulation in which we removed the penumbra after 20 hours through a change in the magnetic top boundary condition. Both spots are surrounded by a moat flow of a few 100 m/s flow speed in the photosphere extending to about twice the spot radius and several Mm in depth. The flow system is more extended for the spot with penumbra. In addition we find in proximity of both spots a region with significantly reduced convective flow velocities and downflow filling factor. Both effects stabilize the spots against decay, in the case of the spot with penumbra this leads to an almost stationary solution over the time span simulated.

Speaker: Matthias Rempel (HAO/NCAR)

47 On sunspots and penumbrae

Discussion of factors that influence the structure of (model an real) sunspots and their penumbrae

Speaker: Åke Nordlund

48 Physical properties of a sunspot chromosphere with umbral flashes

We present new high-resolution spectro-polarimetric Ca II 8542 observations of umbral flashes in sunspots. At nearly 0.18", and spanning about one hour of continuous observation, this is the most detailed dataset published thus far. Our study involves non-LTE inversions to quantify temperatures, mass flows and the full magnetic field vector geometry.

Speaker: Jaime de la Cruz Rodriguez (Institute for Solar Physics, Stockholm University)

10:20 Coffee break

Penumbra formation. Magnetic effects, umbral dots, light bridges, etc.

Convener: Hannah Schunker (Max Planck Institute for Solar System Research)

49 SST observations and interpretations of penumbral fine structure

During the last ten years, our understanding of sunspot fine structure and heating through convection has improved dramatically thanks to a combination of numerical MHD simulations and high-resolution imaging and spectropolarimetric data. I will summarise some of the recent observations, with emphasis on data from the Swedish 1-m Solar Telescope (SST), and how these and simple models led to a fundamental shift in our understanding of the sunspot penumbra and its Evershed flow.

Speaker: Göran Scharmer (Kungliga Vetenskapsakademien)

50 Observational Diagnostics of Magnetoconvection in a Sunspot Umbra

The three-dimensional simulation of magnetoconvection in a sunspot umbra by V\"ogler and Sch\"ussler (2006) was an important step towards more realistic sunspot and active-region simulations. In spite of the idealized boundary conditions in this simulation (it describes a periodic umbra-like atmosphere isolated from the surrounding quiet sun) it predicted the typical coffee-bean shape of umbral-dots that was later on confirmed by high-resolution observations. Here we use snapshots from a similar simulation run to synthetize the monochromatic continuum intensity (0.4 to 1.6 \mu m) and the Fe I 6302 \AA lines. The continuum and its center-to-limb variation are compared to observations available in the literature. The Fe lines are spatially and spectrally degraded to replicate realistic observations with Hinode SP/SOT. Several inversion techniques are applied to this quasi-Hinode data to retrieve magnetic field, temperature and velocity as if it were a real observation. These results are then compared to the original values in the snapshot and the sensitivity of different methods is estimated.

Speaker: Nikola Vitas (Instituto de Astrofísica de Canarias)

Slides vitas_stockholm_2015.pdf

51 Single sunspot structures and dynamics from the SOT (Hinode) and AIA (SDO) observations

The free of seeing effects high resolution images of a single sunspot observed near the centre of the Sun with the SOT (Hinode) are accuratly deconvolved. The new PSF deduced from the transit of Venus and from the extreme limb data takes into account both the diffraction of origin smearing and the stray light from the optics. It considerably improved the details near the diffraction limit, including the structure on penumbral and umbral features. A 45 min movie was assembled showing a wealth of phenomena reminiscent of the dynamical emerging magnetic field ropes in the penumbra. It includes the proper motions of inwardly moving and splitting peripheral and more central bright umbral dots. The ubiquitous bright inner penumbra “ring” is discussed with a possible interpretation of non radiative origin. Outwardly moving dark features at the outer boundaries of the penumbra seem reminiscent of the Evershed effect and it is suggested that they correspond to the moving magnetic features of the super- penumbra seen much further out with magnetograms. A discussion of the possible origin of outwardly propagating coherent penumbral waves is also attempted. The magnetic structure at high coronal level is finally discussed using extrapolations from the HMI(SDO) magnetograms by making a comparison with EM structures observed with the AIA (SDO) imager suggesting a non potential force free approximation can be assumed in the corona, in addition to a sub- photospheric ring current dynamo model of a sunspot as proposed by Larmor and Alfven.

Speaker: Serge Koutchmy (Institut d'Astrophysique de Paris CNRS & UPMC)

Poster Poster_Good_Kou_Ajab_PSF_Deconv_SOT_Hinode_Stockholm_8_March_2015.pdf

Slides Présentation_Serge_Kou_Single_Sunspots_5.pdf

12:00 Lunch

Different views and coherent picture of sunspot formation.

Convener: Jörn Warnecke (Max Planck Institute for Solar System Research)

52 What causes downflows in magnetic flux concentrations?

To produce strong flux concentrations, there has to be a downflow that pulls gas down. Since the flow is constrained to be along field lines, there must be inflow from above and the sides, which concentrates the field further. Thermal convection associated with the hierarchical nature of the supergranulation may be one such mechanism. Another one is the negative effective magnetic pressure instability. Given that the field becomes dynamically important, the interplay between flow and magnetic field must be important. In the context of magneto-convection, a segregation between magnetized and unmagnetized regions has been seen for some time (Tao et al. 1998, ApJL 496, L39), and is often associated with flux expulsion, although no predictive model has yet been constructed. Flux segregation leads to larger scale structures, as is also predicted by the negative effective magnetic pressure instability. However, there is a striking analogy with the inverse cascade behavior of the underlying dynamo process: in both cases, pseudo-scalars are involved (g.Omega and A.B in the dynamo process and g.B and u.B in the flux segregation process) and in both cases one sees the growth of large-scale magnetic structures. It is therefore tempting to argue that both are associated with the presence of quadratic invariants of the underlying equations (magnetic helicity A.B and cross helicity u.B).

Speaker: Axel Brandenburg (Nordita)

Slides StockholmSpots15Axel.pdf

53 Coherent Picture of Solar Activity

This is a moderated discussion about unresolved questions regarding sunspot formation and solar activity.

Speaker: Mark Cheung (Lockheed Martin Solar & Astrophysics Laboratory)

13:50 Round table

15:00 Coffee break

15:30 BBQ!