Author name code: schuessler ADS astronomy entries on 2022-09-14 author:"Schuessler, Manfred" ------------------------------------------------------------------------ Title: Loss of toroidal magnetic flux by emergence of bipolar magnetic regions Authors: Cameron, R. H.; Schüssler, M. Bibcode: 2020A&A...636A...7C Altcode: 2020arXiv200205436C The polarity of the toroidal magnetic field in the solar convection zone periodically reverses in the course of the 11/22-year solar cycle. Among the various processes that contribute to the removal of "old-polarity" toroidal magnetic flux is the emergence of flux loops forming bipolar regions at the solar surface. We quantify the loss of subsurface net toroidal flux by this process. To this end, we determine the contribution of an individual emerging bipolar loop and show that it is unaffected by surface flux transport after emergence. Together with the linearity of the diffusion process this means that the total flux loss can be obtained by adding the contributions of all emerging bipolar magnetic regions. The resulting total loss rate of net toroidal flux amounts to 1.3 × 1015 Mx s-1 during activity maxima and 6.1 × 1014 Mx s-1 during activity minima, to which ephemeral regions contribute about 90 and 97%, respectively. This rate is consistent with the observationally inferred loss rate of toroidal flux into interplanetary space and corresponds to a decay time of the subsurface toroidal flux of about 12 years, also consistent with a simple estimate based on turbulent diffusivity. Consequently, toroidal flux loss by flux emergence is a relevant contribution to the budget of net toroidal flux in the solar convection zone. The consistency between the toroidal flux loss rate due to flux emergence and what is expected from turbulent diffusion, and the similarity between the corresponding decay time and the length of the solar cycle are important constraints for understanding the solar cycle and the Sun's internal dynamics. Title: Solar activity: periodicities beyond 11 years are consistent with random forcing Authors: Cameron, R. H.; Schüssler, M. Bibcode: 2019A&A...625A..28C Altcode: Power spectra of solar activity based on historical records of sunspot numbers and on cosmogenic isotopes show peaks with enhanced power apart from the dominant 11-year solar cycle, such as the 90-year Gleissberg cycle or the 210-year de Vries cycle. In a previous paper we have shown that the overall shape of the power spectrum is well represented by the results of the generic normal form model for a noisy and weakly nonlinear limit cycle, with parameters all determined by observations. Using this model as a null case, we show here that all local peaks with enhanced power, apart from the 11-year band, are consistent with realization noise. Even a 3σ peak is expected to occur with a probability of about 0.25 at least once among the 216 period bins resolved by the cosmogenic isotope data. This casts doubt upon interpretations of such peaks in terms of intrinsic periodicities of the solar dynamo process. Title: Solar activity: intrinsic periodicities beyond 11 years Authors: Cameron, Robert; Schuessler, Manfred Bibcode: 2019arXiv190305398C Altcode: Power spectra of solar activity based on historical records of sunspot numbers and on cosmogenic isotopes show peaks with enhanced power apart from the dominant 11-year solar cycle, such as the 90-year Gleissberg cycle or the 210-year de Vries cycle. In a previous paper we have shown that the overall shape of the power spectrum is well represented by the results of the generic normal form model for a noisy and weakly nonlinear limit cycle, with parameters all determined by observations. Using this model as a null case, we show here that all local peaks with enhanced power, apart from the 11-year band, are consistent with realisation noise. Even a $3\sigma$ peak is expected to occur with a probability of about 0.25 at least once among the 216 period bins resolved by the cosmogenic isotope data. This casts doubt upon interpretations of such peaks in terms of intrinsic periodicities of the solar dynamo process. Title: Origin of the hemispheric asymmetry of solar activity Authors: Schüssler, M.; Cameron, R. H. Bibcode: 2018A&A...618A..89S Altcode: 2018arXiv180710061S The frequency spectrum of the hemispheric asymmetry of solar activity shows enhanced power for the period ranges around 8.5 years and between 30 and 50 years. This can be understood as the sum and beat periods of the superposition of two dynamo modes: a dipolar mode with a (magnetic) period of about 22 years and a quadrupolar mode with a period between 13 and 15 years. An updated Babcock-Leighton-type dynamo model with weak driving as indicated by stellar observations shows an excited dipole mode and a damped quadrupole mode in the correct range of periods. Random excitation of the quadrupole by stochastic fluctuations of the source term for the poloidal field leads to a time evolution of activity and asymmetry that is consistent with the observational results. Title: Solar Magnetoconvection and Small-Scale Dynamo Authors: Borrero, J. M.; Jafarzadeh, S.; Schüssler, M.; Solanki, S. K. Bibcode: 2018smf..book..275B Altcode: No abstract at ADS Title: Observing and modeling the poloidal and toroidal fields of the solar dynamo Authors: Cameron, R. H.; Duvall, T. L.; Schüssler, M.; Schunker, H. Bibcode: 2018A&A...609A..56C Altcode: 2017arXiv171007126C Context. The solar dynamo consists of a process that converts poloidal magnetic field to toroidal magnetic field followed by a process that creates new poloidal field from the toroidal field.
Aims: Our aim is to observe the poloidal and toroidal fields relevant to the global solar dynamo and to see if their evolution is captured by a Babcock-Leighton dynamo.
Methods: We used synoptic maps of the surface radial field from the KPNSO/VT and SOLIS observatories, to construct the poloidal field as a function of time and latitude; we also used full disk images from Wilcox Solar Observatory and SOHO/MDI to infer the longitudinally averaged surface azimuthal field. We show that the latter is consistent with an estimate of the longitudinally averaged surface azimuthal field due to flux emergence and therefore is closely related to the subsurface toroidal field.
Results: We present maps of the poloidal and toroidal magnetic fields of the global solar dynamo. The longitude-averaged azimuthal field observed at the surface results from flux emergence. At high latitudes this component follows the radial component of the polar fields with a short time lag of between 1-3 years. The lag increases at lower latitudes. The observed evolution of the poloidal and toroidal magnetic fields is described by the (updated) Babcock-Leighton dynamo model. Title: Solar Magnetoconvection and Small-Scale Dynamo. Recent Developments in Observation and Simulation Authors: Borrero, J. M.; Jafarzadeh, S.; Schüssler, M.; Solanki, S. K. Bibcode: 2017SSRv..210..275B Altcode: 2015SSRv..tmp..113B; 2015arXiv151104214B A number of observational and theoretical aspects of solar magnetoconvection are considered in this review. We discuss recent developments in our understanding of the small-scale structure of the magnetic field on the solar surface and its interaction with convective flows, which is at the centre of current research. Topics range from plage areas in active regions over the magnetic network shaped by supergranulation to the ubiquituous `turbulent' internetwork fields. On the theoretical side, we focus upon magnetic field generation by small-scale dynamo action. Title: Observing and modelling the poloidal and toroidal magnetic fields of the global dynamo Authors: Cameron, Robert; Duvall, Thomas; Schüssler, Manfred; Schunker, Hannah Bibcode: 2017SPD....4830601C Altcode: The large scale solar dynamo is a cycle where poloidal flux is generated from toroidal flux, and toroidal flux is generated from poloidal flux. The toroidal and poloidal fields can be inferred from observations, and the Babcock-Leighton model shows how differential rotation and flux emergence explain the observed evolution of the fields. Title: The solar magnetic field: from complexity to simplicity (and back) Authors: Schüssler, Manfred Bibcode: 2017SPD....4820201S Altcode: Observations reveal a stunning complexity of the magnetic field due to its interaction with turbulent convection. Numerical simulations and observations strongly suggest that most of the small-scale field is generated by small-scale dynamo action. The fundamental nature of this process makes it potentially relevant in a broad variety of astrophysical settings.On the other hand, the global nature of the 11-year cycle reveals a surprising simplicity. This suggests a description of the global dynamo process in terms of relatively simple concepts. During the last decades, studies of magnetic flux transport at the solar surface provided crucial information about the workings of the dynamo process. They confirm the visionary approach proposed Babcock and Leighton. A recent update of their model permits a full study of the space spanned by the few remaining parameters in order to identify the regions with solar-like solutions.Observations of other cool stars suggest that the relatively slow rotation of the Sun puts it near to the threshold for which global dynamo action ceases. This suggests a further simplification of the dynamo model in terms of a generic normal form for a weakly nonlinear system. Including the inherent randomness brought about by the flux emergence process leads to a stochastic model whose parameters are fixed by observations. The model results explain the variability of the solar cycle amplitudes from decadal to millennial time scales.However, the true complexity of the processes cannot be ignored. Simulations indicate that the connection between the toroidal field in the convection zone and the magnetic flux emerging at the surface is highly complex and non-trivial. This is an important "loose end" of Babcock-Leighton-type dynamo models. Furthermore, internal differential rotation, convective flows, meridional flows, and tilt angles are largely unknown in stars other the Sun and presently cannot be reliably inferred from theoretical models or simulations. Consequently, models for the solar dynamo cannot be easily extended to stars with different rotation rate, structure, or evolutionary state. Title: Understanding Solar Cycle Variability Authors: Cameron, R. H.; Schüssler, M. Bibcode: 2017ApJ...843..111C Altcode: 2017arXiv170510746C The level of solar magnetic activity, as exemplified by the number of sunspots and by energetic events in the corona, varies on a wide range of timescales. Most prominent is the 11-year solar cycle, which is significantly modulated on longer timescales. Drawing from dynamo theory, together with the empirical results of past solar activity and similar phenomena for solar-like stars, we show that the variability of the solar cycle can be essentially understood in terms of a weakly nonlinear limit cycle affected by random noise. In contrast to ad hoc “toy models” for the solar cycle, this leads to a generic normal-form model, whose parameters are all constrained by observations. The model reproduces the characteristics of the variable solar activity on timescales between decades and millennia, including the occurrence and statistics of extended periods of very low activity (grand minima). Comparison with results obtained with a Babcock-Leighton-type dynamo model confirm the validity of the normal-mode approach. Title: The solar magnetic field: from complexity to simplicity (and back) Authors: Schüssler, Manfred Bibcode: 2017AAS...23030001S Altcode: The Sun is the only astrophysical object that permits a detailed study of the basic processes governing its magnetic field. Observations reveal stunning complexity due to the interaction with turbulent convection. Numerical simulations and observations strongly suggest that most of the small-scale field is generated by a process called small-scale dynamo action. The fundamental nature of this process makes it a candidate for magnetic field generation in a broad variety of astrophysical settings.On the other hand, the global nature of the 11-year cycle (as exhibited, for instance, by the polarity laws of sunspot groups and the regularly reversing axial dipole field) reveals a surprising simplicity. This suggests a description of the global dynamo process underlying the solar cycle in terms of relatively simple concepts. Insufficient knowledge about the structure of magnetic field and flows in the convection zone requires the introduction of a variety of free parameters (or even free functions), which severely impairs the explanatory power of most such models. However, during the last decades, surface observations of plasma flows and magnetic flux emergence, together with studies of magnetic flux transport, provided crucial information aboutthe workings of the dynamo process. They confirm the visionary approach proposed already in the 1960s by Babcock and Leighton. A recent update of their model permits a full study of the space spanned by the few remaining parameters in order to identify the regions with solar-like solutions.Observations of other cool stars show that the magnetic activity level decreases strongly with stellar rotation rate. The relatively slow rotation of the Sun puts it near to the threshold at which global dynamo action ceases. This suggests a further simplification of the dynamo model in terms of a generic normal form for a weakly nonlinear system. Including the inherent randomness brought about by the flux emergence process leads to a stochastic model whose parameters are fixed by observations. The model results explain the variability of the solar cycle amplitudes from decadal to millennial time scales. Title: An update of Leighton's solar dynamo model Authors: Cameron, R. H.; Schüssler, M. Bibcode: 2017A&A...599A..52C Altcode: 2016arXiv161109111C In 1969, Leighton developed a quasi-1D mathematical model of the solar dynamo, building upon the phenomenological scenario of Babcock published in 1961. Here we present a modification and extension of Leighton's model. Using the axisymmetric component (longitudinal average) of the magnetic field, we consider the radial field component at the solar surface and the radially integrated toroidal magnetic flux in the convection zone, both as functions of latitude. No assumptions are made with regard to the radial location of the toroidal flux. The model includes the effects of (I) turbulent diffusion at the surface and in the convection zone; (II) poleward meridional flow at the surface and an equatorward return flow affecting the toroidal flux; (III) latitudinal differential rotation and the near-surface layer of radial rotational shear; (iv) downward convective pumping of magnetic flux in the shear layer; and (v) flux emergence in the form of tilted bipolar magnetic regions treated as a source term for the radial surface field. While the parameters relevant for the transport of the surface field are taken from observations, the model condenses the unknown properties of magnetic field and flow in the convection zone into a few free parameters (turbulent diffusivity, effective return flow, amplitude of the source term, and a parameter describing the effective radial shear). Comparison with the results of 2D flux transport dynamo codes shows that the model captures the essential features of these simulations. We make use of the computational efficiency of the model to carry out an extended parameter study. We cover an extended domain of the 4D parameter space and identify the parameter ranges that provide solar-like solutions. Dipole parity is always preferred and solutions with periods around 22 yr and a correct phase difference between flux emergence in low latitudes and the strength of the polar fields are found for a return flow speed around 2 m s-1, turbulent diffusivity below about 80 km2s-1, and dynamo excitation not too far above the threshold (linear growth rate less than 0.1 yr-1). Title: Solar Dynamics, Rotation, Convection and Overshoot Authors: Hanasoge, S.; Miesch, M. S.; Roth, M.; Schou, J.; Schüssler, M.; Thompson, M. J. Bibcode: 2017hdsi.book...85H Altcode: No abstract at ADS Title: Solar Cycle 25: Another Moderate Cycle? Authors: Cameron, R. H.; Jiang, J.; Schüssler, M. Bibcode: 2016ApJ...823L..22C Altcode: 2016arXiv160405405C Surface flux transport simulations for the descending phase of Cycle 24 using random sources (emerging bipolar magnetic regions) with empirically determined scatter of their properties provide a prediction of the axial dipole moment during the upcoming activity minimum together with a realistic uncertainty range. The expectation value for the dipole moment around 2020 (2.5 ± 1.1 G) is comparable to that observed at the end of Cycle 23 (about 2 G). The empirical correlation between the dipole moment during solar minimum and the strength of the subsequent cycle thus suggests that Cycle 25 will be of moderate amplitude, not much higher than that of the current cycle. However, the intrinsic uncertainty of such predictions resulting from the random scatter of the source properties is considerable and fundamentally limits the reliability with which such predictions can be made before activity minimum is reached. Title: The turbulent diffusion of toroidal magnetic flux as inferred from properties of the sunspot butterfly diagram Authors: Cameron, R. H.; Schüssler, M. Bibcode: 2016A&A...591A..46C Altcode: 2016arXiv160407340C Context. In order to match observed properties of the solar cycle, flux-transport dynamo models require the toroidal magnetic flux to be stored in a region of low magnetic diffusivity, typically located at or below the bottom of the convection zone.
Aims: We infer the turbulent magnetic diffusivity affecting the toroidal field on the basis of empirical data.
Methods: We considered the time evolution of mean latitude and width of the activity belts of solar cycles 12-23 and their dependence on cycle strength. We interpreted the decline phase of the cycles as a diffusion process.
Results: The activity level of a given cycle begins to decline when the centers of its equatorward propagating activity belts come within their (full) width (at half maximum) from the equator. This happens earlier for stronger cycles because their activity belts are wider. From that moment on, the activity and the belt width decrease in the same manner for all cycles, independent of their maximum activity level. In terms of diffusive cancellation of opposite-polarity toroidal flux across the equator, we infer the turbulent diffusivity experienced by the toroidal field, wherever it is located, to be in the range 150-450 km2 s-1. Strong diffusive latitudinal spreading of the toroidal flux underneath the activity belts can be inhibited by an inflow toward the toroidal field bands in the convection zone with a magnitude of several meters per second.
Conclusions: The inferred value of the turbulent magnetic diffusivity affecting the toroidal field agrees, to order of magnitude, with estimates based on mixing-length models for the solar convection zone. This is at variance with the requirement of flux-transport dynamo models. The inflows required to keep the toroidal field bands together before they approach the equator are similar to the inflows toward the activity belts observed with local helioseismology. Title: Solar Dynamics, Rotation, Convection and Overshoot Authors: Hanasoge, S.; Miesch, M. S.; Roth, M.; Schou, J.; Schüssler, M.; Thompson, M. J. Bibcode: 2015SSRv..196...79H Altcode: 2015SSRv..tmp...24H; 2015arXiv150308539H We discuss recent observational, theoretical and modeling progress made in understanding the Sun's internal dynamics, including its rotation, meridional flow, convection and overshoot. Over the past few decades, substantial theoretical and observational effort has gone into appreciating these aspects of solar dynamics. A review of these observations, related helioseismic methodology and inference and computational results in relation to these problems is undertaken here. Title: Three-dimensional simulations of near-surface convection in main-sequence stars. III. The structure of small-scale magnetic flux concentrations Authors: Beeck, B.; Schüssler, M.; Cameron, R. H.; Reiners, A. Bibcode: 2015A&A...581A..42B Altcode: 2015arXiv150504739B Context. The convective envelopes of cool main-sequence stars harbour magnetic fields with a complex global and local structure. These fields affect the near-surface convection and the outer stellar atmospheres in many ways and are responsible for the observable magnetic activity of stars.
Aims: Our aim is to understand the local structure in unipolar regions with moderate average magnetic flux density. These correspond to plage regions covering a substantial fraction of the surface of the Sun (and likely also the surface of other Sun-like stars) during periods of high magnetic activity.
Methods: We analyse the results of 18 local-box magnetohydrodynamics simulations covering the upper layers of the convection zones and the photospheres of cool main-sequence stars of spectral types F to early M. The average vertical field in these simulations ranges from 20 to 500 G.
Results: We find a substantial variation of the properties of the surface magnetoconvection between main-sequence stars of different spectral types. As a consequence of a reduced efficiency of the convective collapse of flux tubes, M dwarfs lack bright magnetic structures in unipolar regions of moderate field strength. The spatial correlation between velocity and the magnetic field as well as the lifetime of magnetic structures and their sizes relative to the granules vary significantly along the model sequence of stellar types.

Movies associated to Fig. A.1 are available in electronic form at http://www.aanda.org Title: Three-dimensional simulations of near-surface convection in main-sequence stars. IV. Effect of small-scale magnetic flux concentrations on centre-to-limb variation and spectral lines Authors: Beeck, B.; Schüssler, M.; Cameron, R. H.; Reiners, A. Bibcode: 2015A&A...581A..43B Altcode: 2015arXiv150504744B Context. Magnetic fields affect the local structure of the photosphere of stars. They can considerably influence the radiative properties near the optical surface, flow velocities, and the temperature and pressure profiles. This has an impact on observables such as limb darkening and spectral line profiles.
Aims: We aim at understanding qualitatively the influence of small magnetic flux concentrations in unipolar plage regions on the centre-to-limb variation of the intensity and its contrast and on the shape of spectral line profiles in cool main-sequence stars.
Methods: We analyse the bolometric and continuum intensity and its angular dependence of 24 radiative magnetohydrodynamic simulations of the near-surface layers of main-sequence stars with six different sets of stellar parameters (spectral types F to early M) and four different average magnetic field strengths (including the non-magnetic case). We also calculated disc-integrated profiles of three spectral lines.
Results: The small magnetic flux concentrations formed in the magnetic runs of simulations have a considerable impact on the intensity and its centre-to-limb variation. In some cases, the difference in limb darkening between magnetic and non-magnetic runs is larger than the difference between the spectral types. Spectral lines are not only broadened owing to the Zeeman effect, but are also strongly affected by the modified thermodynamical structure and flow patterns. This indirect magnetic impact on the line profiles is often bigger than that of the Zeeman effect.
Conclusions: The effects of the magnetic field on the radiation leaving the star can be considerable and is not restricted to spectral line broadening and polarisation by the Zeeman effect. The inhomogeneous structure of the magnetic field on small length scales and its impact on (and spatial correlation with) the local thermodynamical structure and the flow field near the surface influence the measurement of the global field properties and stellar parameters. These effects need to be taken into account in the interpretation of observations.

Appendix A is available in electronic form at http://www.aanda.org Title: The Cause of the Weak Solar Cycle 24 Authors: Jiang, J.; Cameron, R. H.; Schüssler, M. Bibcode: 2015ApJ...808L..28J Altcode: 2015arXiv150701764J The ongoing 11 year cycle of solar activity is considerably less vigorous than the three cycles before. It was preceded by a very deep activity minimum with a low polar magnetic flux, the source of the toroidal field responsible for solar magnetic activity in the subsequent cycle. Simulation of the evolution of the solar surface field shows that the weak polar fields and thus the weakness of the present cycle 24 are mainly caused by a number of bigger bipolar regions emerging at low latitudes with a “wrong” (i.e., opposite to the majority for this cycle) orientation of their magnetic polarities in the north-south direction, which impaired the growth of the polar field. These regions had a particularly strong effect since they emerged within +/- 10^\circ latitude from the solar equator. Title: A Method for Measuring Active Region Filling Factors on Solar-Type Stars Authors: Giampapa, Mark Steven; Andretta, Vincenzo; Beeck, Benjamin; Reiners, Ansgar; Schussler, Manfred Bibcode: 2015TESS....120101G Altcode: Radiative diagnostics of “activity” in the Sun and solar-type stars are spatially associated with sites of emergent magnetic flux. The magnetic fields themselves are widely regarded as the surface manifestations of a dynamo mechanism. The further development of both dynamo theory and models of the non-radiative heating of outer stellar atmospheres requires a knowledge of stellar magnetic field properties. In this context, it becomes important to determine the surface distribution, or at least the fractional coverage of, magnetic active regions as one critical constraint for dynamo models. But, while information on the spatial distribution of activity on stellar surfaces can be gathered in some special cases (mostly rapid rotators), such measurements have always been elusive in more solar-like stars. We discuss the challenges and results obtained from a method that relies on the non-linear response of the two principal He I triplet lines (at 1083 nm and 587.6 nm) to infer useful constraints on the fractional area coverage of magnetic active regions on solar-type stars. Title: The crucial role of surface magnetic fields for the solar dynamo Authors: Cameron, Robert; Schüssler, Manfred Bibcode: 2015Sci...347.1333C Altcode: 2015arXiv150308469C Sunspots and the plethora of other phenomena occurring in the course of the 11-year cycle of solar activity are a consequence of the emergence of magnetic flux at the solar surface. The observed orientations of bipolar sunspot groups imply that they originate from toroidal (azimuthally orientated) magnetic flux in the convective envelope of the Sun. We show that the net toroidal magnetic flux generated by differential rotation within a hemisphere of the convection zone is determined by the emerged magnetic flux at the solar surface and thus can be calculated from the observed magnetic field distribution. The main source of the toroidal flux is the roughly dipolar surface magnetic field at the polar caps, which peaks around the minima of the activity cycle. Title: MHD Simulations of Near-Surface Convection in Cool Main-Sequence Stars Authors: Beeck, Benjamin; Schussler, Manfred; Reiners, Ansgar Bibcode: 2015csss...18..467B Altcode: 2014arXiv1408.1802B The solar photospheric magnetic field is highly structured owing to its interaction with the convective flows. Its local structure has a strong influence on the profiles of spectral lines not only by virtue of the Zeeman effect, but also through the modification of the thermodynamical structure (e.g. line weakening in hot small-scale magnetic structures). Many stars harbor surface magnetic fields comparable to or larger than the Sun at solar maximum. Therefore, a strong influence of the field on the surface convection and on spectral line profiles can be expected.

We carried out 3D local-box MHD simulations of unipolar magnetized regions (average fields of 20, 100, and 500G) with parameters corresponding to six main-sequence stars (spectral types F3V to M2V). The influence of the magnetic field on the convection and the local thermodynamical structure were analyzed in detail. For three spectral lines, we determined the impact of the magnetic field on the disc-integrated Stokes-I profiles. Line weakening has in many cases a stronger impact on the spectral line profiles than the Zeeman effect. Moreover, for some stars, the correlation between the magnetic field and the vertical velocity strongly influences the line shapes. These effects can impair determinations of stellar magnetic fields since currently used methods neglect the local structure of the magnetic field and its interaction with the convective flows. The MHD simulations presented can be used to quantify these effects and thus help to improve magnetic field measurements of cool main-sequence stars. Title: Generalized Investigation of the Rotation-Activity Relation: Favoring Rotation Period instead of Rossby Number Authors: Reiners, A.; Schüssler, M.; Passegger, V. M. Bibcode: 2014ApJ...794..144R Altcode: 2014arXiv1408.6175R Magnetic activity in Sun-like and low-mass stars causes X-ray coronal emission which is stronger for more rapidly rotating stars. This relation is often interpreted in terms of the Rossby number, i.e., the ratio of rotation period to convective overturn time. We reconsider this interpretation on the basis of the observed X-ray emission and rotation periods of 821 stars with masses below 1.4 M ⊙. A generalized analysis of the relation between X-ray luminosity normalized by bolometric luminosity, L X/L bol, and combinations of rotational period, P, and stellar radius, R, shows that the Rossby formulation does not provide the solution with minimal scatter. Instead, we find that the relation L X/L bolvpropP -2 R -4 optimally describes the non-saturated fraction of the stars. This relation is equivalent to L XvpropP -2, indicating that the rotation period alone determines the total X-ray emission. Since L X is directly related to the magnetic flux at the stellar surface, this means that the surface flux is determined solely by the star's rotation and is independent of other stellar parameters. While a formulation in terms of a Rossby number would be consistent with these results if the convective overturn time scales exactly as L_bol-1/2, our generalized approach emphasizes the need to test a broader range of mechanisms for dynamo action in cool stars. Title: Effects of the Scatter in Sunspot Group Tilt Angles on the Large-scale Magnetic Field at the Solar Surface Authors: Jiang, J.; Cameron, R. H.; Schüssler, M. Bibcode: 2014ApJ...791....5J Altcode: 2014arXiv1406.5564J The tilt angles of sunspot groups represent the poloidal field source in Babcock-Leighton-type models of the solar dynamo and are crucial for the build-up and reversals of the polar fields in surface flux transport (SFT) simulations. The evolution of the polar field is a consequence of Hale's polarity rules, together with the tilt angle distribution which has a systematic component (Joy's law) and a random component (tilt-angle scatter). We determine the scatter using the observed tilt angle data and study the effects of this scatter on the evolution of the solar surface field using SFT simulations with flux input based upon the recorded sunspot groups. The tilt angle scatter is described in our simulations by a random component according to the observed distributions for different ranges of sunspot group size (total umbral area). By performing simulations with a number of different realizations of the scatter we study the effect of the tilt angle scatter on the global magnetic field, especially on the evolution of the axial dipole moment. The average axial dipole moment at the end of cycle 17 (a medium-amplitude cycle) from our simulations was 2.73 G. The tilt angle scatter leads to an uncertainty of 0.78 G (standard deviation). We also considered cycle 14 (a weak cycle) and cycle 19 (a strong cycle) and show that the standard deviation of the axial dipole moment is similar for all three cycles. The uncertainty mainly results from the big sunspot groups which emerge near the equator. In the framework of Babcock-Leighton dynamo models, the tilt angle scatter therefore constitutes a significant random factor in the cycle-to-cycle amplitude variability, which strongly limits the predictability of solar activity. Title: Comparison of solar photospheric bright points between Sunrise observations and MHD simulations Authors: Riethmüller, T. L.; Solanki, S. K.; Berdyugina, S. V.; Schüssler, M.; Martínez Pillet, V.; Feller, A.; Gandorfer, A.; Hirzberger, J. Bibcode: 2014A&A...568A..13R Altcode: 2014arXiv1406.1387R Bright points (BPs) in the solar photosphere are thought to be the radiative signatures (small-scale brightness enhancements) of magnetic elements described by slender flux tubes or sheets located in the darker intergranular lanes in the solar photosphere. They contribute to the ultraviolet (UV) flux variations over the solar cycle and hence may play a role in influencing the Earth's climate. Here we aim to obtain a better insight into their properties by combining high-resolution UV and spectro-polarimetric observations of BPs by the Sunrise Observatory with 3D compressible radiation magnetohydrodynamical (MHD) simulations. To this end, full spectral line syntheses are performed with the MHD data and a careful degradation is applied to take into account all relevant instrumental effects of the observations. In a first step it is demonstrated that the selected MHD simulations reproduce the measured distributions of intensity at multiple wavelengths, line-of-sight velocity, spectral line width, and polarization degree rather well. The simulated line width also displays the correct mean, but a scatter that is too small. In the second step, the properties of observed BPs are compared with synthetic ones. Again, these are found to match relatively well, except that the observations display a tail of large BPs with strong polarization signals (most likely network elements) not found in the simulations, possibly due to the small size of the simulation box. The higher spatial resolution of the simulations has a significant effect, leading to smaller and more numerous BPs. The observation that most BPs are weakly polarized is explained mainly by the spatial degradation, the stray light contamination, and the temperature sensitivity of the Fe i line at 5250.2 Å. Finally, given that the MHD simulations are highly consistent with the observations, we used the simulations to explore the properties of BPs further. The Stokes V asymmetries increase with the distance to the center of the mean BP in both observations and simulations, consistent with the classical picture of a production of the asymmetry in the canopy. This is the first time that this has been found also in the internetwork. More or less vertical kilogauss magnetic fields are found for 98% of the synthetic BPs underlining that basically every BP is associated with kilogauss fields. At the continuum formation height, the simulated BPs are on average 190 K hotter than the mean quiet Sun, the mean BP field strength is found to be 1750 G, and the mean inclination is 17°, supporting the physical flux-tube paradigm to describe BPs. On average, the synthetic BPs harbor downflows increasing with depth. The origin of these downflows is not yet understood very well and needs further investigation. Title: Physical causes of solar cycle amplitude variability Authors: Cameron, R. H.; Jiang, J.; Schüssler, M.; Gizon, L. Bibcode: 2014JGRA..119..680C Altcode: The level of solar activity varies from cycle to cycle. This variability is probably caused by a combination of nonlinear and random effects. Based on surface flux transport simulations, we show that the observed inflows into active regions and toward the activity belts provide an important nonlinearity in the framework of Babcock-Leighton model for the solar dynamo. Inclusion of these inflows also leads to a reproduction of the observed proportionality between the open heliospheric flux during activity minima and the maximum sunspot number of the following cycle. A substantial component of the random variability of the cycle strength is associated with the cross-equatorial flux plumes that occur when large, highly tilted sunspot groups emerge close to the equator. We show that the flux transported by these events is important for the amplitude of the polar fields and open flux during activity minima. The combined action of inflows and cross-equatorial flux plumes provides an explanation for the weakness of the polar fields at the end of solar cycle 23 (and hence for the relative weakness of solar cycle 24). Title: Can Surface Flux Transport Account for the Weak Polar Field in Cycle 23? Authors: Jiang, Jie; Cameron, Robert H.; Schmitt, Dieter; Schüssler, Manfred Bibcode: 2014crh..book..289J Altcode: No abstract at ADS Title: Three-dimensional simulations of near-surface convection in main-sequence stars. II. Properties of granulation and spectral lines Authors: Beeck, B.; Cameron, R. H.; Reiners, A.; Schüssler, M. Bibcode: 2013A&A...558A..49B Altcode: 2013arXiv1308.4873B Context. The atmospheres of cool main-sequence stars are structured by convective flows from the convective envelope that penetrate the optically thin layers and lead to structuring of the stellar atmospheres analogous to solar granulation. The flows have considerable influence on the 3D structure of temperature and pressure and affect the profiles of spectral lines formed in the photosphere.
Aims: For the set of six 3D radiative (M)HD simulations of cool main-sequence stars described in the first paper of this series, we analyse the near-surface layers. We aim at describing the properties of granulation of different stars and at quantifying the effects on spectral lines of the thermodynamic structure and flows of 3D convective atmospheres.
Methods: We detected and tracked granules in brightness images from the simulations to analyse their statistical properties, as well as their evolution and lifetime. We calculated spatially resolved spectral line profiles using the line synthesis code SPINOR. To enable a comparison to stellar observations, we implemented a numerical disc-integration, which includes (differential) rotation.
Results: Although the stellar parameters change considerably along the model sequence, the properties of the granules are very similar. The impact of the 3D structure of the atmospheres on line profiles is measurable in disc-integrated spectra. Line asymmetries caused by convection are modulated by stellar rotation.
Conclusions: The 3D structure of cool stellar atmospheres as shaped by convective flows has to be taken into account when using photospheric lines to determine stellar parameters. Title: Three-dimensional simulations of near-surface convection in main-sequence stars. I. Overall structure Authors: Beeck, B.; Cameron, R. H.; Reiners, A.; Schüssler, M. Bibcode: 2013A&A...558A..48B Altcode: 2013arXiv1308.4874B Context. The near-surface layers of cool main-sequence stars are structured by convective flows, which are overshooting into the atmosphere. The flows and the associated spatio-temporal variations of density and temperature affect spectral line profiles and thus have an impact on estimates of stellar properties such as effective temperature, gravitational acceleration, and abundances.
Aims: We aim at identifying distinctive properties of the thermodynamic structure of the atmospheres of different stars and understand their causes.
Methods: We ran comprehensive 3D radiation hydrodynamics simulations of the near-surface layers of six simulated stars of spectral type F3V to M2V with the MURaM code. We carry out a systematic parameter study of the mean stratifications, flow structures, and the energy flux in these stars.
Results: We find monotonic trends along the lower main sequence in granule size, flow velocity, and intensity contrast. The convection in the M-star models differs substantially from that of the hotter stars, mainly owing to the more gradual transition from convective to radiative energy transport.
Conclusions: While the basic mechanisms driving surface convection in cool stars are the same, the properties of the convection vary along the main sequence. Apart from monotonic trends in rms velocity, intensity contrast, granule size, etc., there is a transition between "naked" and "hidden" granulation around spectral type K5V caused by the (highly non-linear) temperature dependence of the opacity. These variations have to be taken into account when stellar parameters are derived from spectra.

Appendix A is available in electronic form at http://www.aanda.org Title: Limits to solar cycle predictability: Cross-equatorial flux plumes Authors: Cameron, R. H.; Dasi-Espuig, M.; Jiang, J.; Işık, E.; Schmitt, D.; Schüssler, M. Bibcode: 2013A&A...557A.141C Altcode: 2013arXiv1308.2827C Context. Within the Babcock-Leighton framework for the solar dynamo, the strength of a cycle is expected to depend on the strength of the dipole moment or net hemispheric flux during the preceding minimum, which depends on how much flux was present in each hemisphere at the start of the previous cycle and how much net magnetic flux was transported across the equator during the cycle. Some of this transport is associated with the random walk of magnetic flux tubes subject to granular and supergranular buffeting, some of it is due to the advection caused by systematic cross-equatorial flows such as those associated with the inflows into active regions, and some crosses the equator during the emergence process.
Aims: We aim to determine how much of the cross-equatorial transport is due to small-scale disorganized motions (treated as diffusion) compared with other processes such as emergence flux across the equator.
Methods: We measure the cross-equatorial flux transport using Kitt Peak synoptic magnetograms, estimating both the total and diffusive fluxes.
Results: Occasionally a large sunspot group, with a large tilt angle emerges crossing the equator, with flux from the two polarities in opposite hemispheres. The largest of these events carry a substantial amount of flux across the equator (compared to the magnetic flux near the poles). We call such events cross-equatorial flux plumes. There are very few such large events during a cycle, which introduces an uncertainty into the determination of the amount of magnetic flux transported across the equator in any particular cycle. As the amount of flux which crosses the equator determines the amount of net flux in each hemisphere, it follows that the cross-equatorial plumes introduce an uncertainty in the prediction of the net flux in each hemisphere. This leads to an uncertainty in predictions of the strength of the following cycle. Title: No evidence for planetary influence on solar activity Authors: Cameron, R. H.; Schüssler, M. Bibcode: 2013A&A...557A..83C Altcode: 2013arXiv1307.5988C Context. Recently, Abreu et al. (2012, A&A. 548, A88) proposed a long-term modulation of solar activity through tidal effects exerted by the planets. This claim is based upon a comparison of (pseudo-)periodicities derived from records of cosmogenic isotopes with those arising from planetary torques on an ellipsoidally deformed Sun.
Aims: We examined the statistical significance of the reported similarity of the periods.
Methods: The tests carried out by Abreu et al. were repeated with artificial records of solar activity in the form of white or red noise. The tests were corrected for errors in the noise definition as well as in the apodisation and filtering of the random series.
Results: The corrected tests provide probabilities for chance coincidence that are higher than those claimed by Abreu et al. by about 3 and 8 orders of magnitude for white and red noise, respectively. For an unbiased choice of the width of the frequency bins used for the test (a constant multiple of the frequency resolution) the probabilities increase by another two orders of magnitude to 7.5% for red noise and 22% for white noise.
Conclusions: The apparent agreement between the periodicities in records of cosmogenic isotopes as proxies for solar activity and planetary torques is statistically insignificant. There is no evidence for a planetary influence on solar activity. Title: Modelling stellar activity cycles using deep-seated dynamos and surface flux transport Authors: Işık, Emre; Schmitt, Dieter; Schüssler, Manfred Bibcode: 2013IAUS..294..431I Altcode: 2012arXiv1211.1414I We investigate the relations between tachocline-based dynamos and the surface flux transport mechanisms in stars with outer convection zones. Using our combined models of flux generation and transport, we demonstrate the importance of the buoyant rise of magnetic flux, which physically determines the emergence latitudes and tilt angles of bipolar magnetic regions. The combined effects of the dynamo strength, flux rise, and surface transport lead to various cyclic and non-cyclic time series of total unsigned surface magnetic flux. Title: Solar magneto-convection Authors: Schüssler, Manfred Bibcode: 2013IAUS..294...95S Altcode: 2012arXiv1211.6253S An overview is given about recent developments and results of comprehensive simulations of magneto-convective processes in the near-surface layers and photosphere of the Sun. Simulations now cover a wide range of phenomena, from whole active regions, over individual sunspots and pores, magnetic flux concentrations and vortices in intergranular lanes, down to the intricate mixed-polarity structure of the magnetic field generated by small-scale dynamo action. The simulations in concert with high-resolution observations have provided breakthroughs in our understanding of the structure and dynamics of the magnetic fields in the solar photosphere. Title: Can Surface Flux Transport Account for the Weak Polar Field in Cycle 23? Authors: Jiang, Jie; Cameron, Robert H.; Schmitt, Dieter; Schüssler, Manfred Bibcode: 2013SSRv..176..289J Altcode: 2011SSRv..tmp..212J; 2011SSRv..tmp...69J; 2011arXiv1104.4183J; 2011SSRv..tmp..136J; 2011SSRv..tmp..368J To reproduce the weak magnetic field on the polar caps of the Sun observed during the declining phase of cycle 23 poses a challenge to surface flux transport models since this cycle has not been particularly weak. We use a well-calibrated model to evaluate the parameter changes required to obtain simulated polar fields and open flux that are consistent with the observations. We find that the low polar field of cycle 23 could be reproduced by an increase of the meridional flow by 55% in the last cycle. Alternatively, a decrease of the mean tilt angle of sunspot groups by 28% would also lead to a similarly low polar field, but cause a delay of the polar field reversals by 1.5 years in comparison to the observations. Title: Coupled model for the formation of an active region corona Authors: Chen, Feng; Bingert, Sven; Peter, Hardi; Cameron, Robert; Schüssler; , Manfred; Cheung, Mark C. M. Bibcode: 2013enss.confE..21C Altcode: We will present the first model that couples the formation of an active region corona to a model of the emergence. This allows us to study when, where, and why active region loops form, and how they evolve. For this we use an existing 3D radiation MHD model of the emergence of an active region through the upper convection zone and the photosphere as a lower boundary for a coronal model. Our 3D MHD coronal model accounts for the braiding of the magnetic field lines that induces currents in the corona that is getting filled with the emerging magnetic field. Starting with a basically field-free atmosphere we follow the flux emergence until numerous individually identifiable hot coronal loops have been formed. The temperatures in the coronal loops of well above 1 MK are reached at densities corresponding to actually observed active region loops. The loops develop over a very short time period of the order of several minutes through the evaporation of material from the chromosphere. Because we have full access to the heating rate as a function of time and space in our computational domain we can determine the conditions under which these loops form. Title: On the relation between continuum brightness and magnetic field in solar active regions Authors: Danilovic, S.; Röhrbein, D.; Cameron, R. H.; Schüssler, M. Bibcode: 2013A&A...550A.118D Altcode: Context. Variations of solar irradiance are mainly determined by the changing coverage of the visible solar disk with magnetic flux concentrations. The relationship between brightness and field strength is an important ingredient for models and reconstructions of irradiance variations.
Aims: We assess the effect of limited observational resolution on the relationship between brightness and magnetic field by comparing comprehensive MHD simulations with observational results.
Methods: Simulations of magnetoconvection representing the near-surface layers of a plage region were used to determine maps of the continuum brightness and Stokes profiles for the Fe i line at 630.22 nm. After convolving with instrumental profiles, synthetic observations of the magnetic field were generated by applying a Stokes inversion code. We compare the resulting relation between brightness and apparent vertical magnetic field to the corresponding outcome derived from real observations of a plage region with the Hinode satellite.
Results: Consideration of the image smearing effects due to the limited resolution of the observations transform the largely monotonic relation between brightness and field strength at the original resolution of the simulations into a profile with a maximum at intermediate field strength, which is in good agreement with the observations.
Conclusions: Considering the effect of limited observational resolution renders the relation between brightness and magnetic field from comprehensive MHD simulations consistent with observational results. This is a necessary prerequisite for the utilization of simulations for models and reconstruction of solar irradiance variations. Title: Multiwavelength Study of the Region Around the ANTARES Neutrino Excess Authors: Schüssler; , Fabian; Brun, P.; Chaves, R. C. G.; Glicenstein, J. -F.; Kosack, K.; Moulin, E.; Peyaud, B.; Wouters, D.; H. E. S. S. Collaboration; Stolarczyk, T.; Vallage, B. Bibcode: 2013ICRC...33.2686S Altcode: 2013arXiv1307.6074S The ANTARES collaboration reported the results of a search for point-like neutrino sources using data taken in the period 2007-2010. An unbinned maximum likelihood based all-sky search yielded a cluster of 9 (5) events within a cone of 3 (1) degrees around (R.A., Dec) = (-46.5deg, -65.0deg). The trial factor corrected p-value of 2.6% (2.2 sigma) is not significant enough to claim the observation of an astrophysical point source. However, it currently constitutes the most significant localized neutrino excess observed by ANTARES. Here we present a multi-wavelength analysis including optical to X-ray archival data and a dedicated analysis of gamma-ray data from Fermi-LAT. In order to cover the TeV domain, dedicated observations with the H.E.S.S. telescope array were carried out. We present these data and discuss implications of the results in terms of signatures for a cosmic-ray acceleration site. Title: Are the strengths of solar cycles determined by converging flows towards the activity belts? Authors: Cameron, R. H.; Schüssler, M. Bibcode: 2012A&A...548A..57C Altcode: 2012arXiv1210.7644C It is proposed that the observed near-surface inflows towards the active regions and sunspot zones provide a nonlinear feedback mechanism that limits the amplitude of a Babcock-Leighton-type solar dynamo and determines the variation of the cycle strength. This hypothesis is tested with surface flux transport simulations including converging latitudinal flows that depend on the surface distribution of magnetic flux. The inflows modulate the build-up of polar fields (represented by the axial dipole) by reducing the tilt angles of bipolar magnetic regions and by affecting the cross-equator transport of leading-polarity magnetic flux. With flux input derived from the observed record of sunspot groups, the simulations cover the period between 1874 and 1980 (corresponding to solar cycles 11 to 20). The inclusion of the inflows leads to a strong correlation of the simulated axial dipole strength during activity minimum with the observed amplitude of the subsequent cycle. This in agreement with empirical correlations and in line with what is expected from a Babcock-Leighton-type dynamo. The results provide evidence that the latitudinal inflows are a key ingredient in determining the amplitude of solar cycles. Title: First Results from the SUNRISE Mission Authors: Solanki, S. K.; Barthol, P.; Danilovic, S.; Feller, A.; Gandorfer, A.; Hirzberger, J.; Jafarzadeh, S.; Lagg, A.; Riethmüller, T. L.; Schüssler, M.; Wiegelmann, T.; Bonet, J. A.; González, M. J. M.; Pillet, V. M.; Khomenko, E.; Yelles Chaouche, L.; Iniesta, J. C. d. T.; Domingo, V.; Palacios, J.; Knölker, M.; González, N. B.; Borrero, J. M.; Berkefeld, T.; Franz, M.; Roth, M.; Schmidt, W.; Steiner, O.; Title, A. M. Bibcode: 2012ASPC..455..143S Altcode: The SUNRISE balloon-borne solar observatory consists of a 1m aperture Gregory telescope, a UV filter imager, an imaging vector polarimeter, an image stabilization system, and further infrastructure. The first science flight of SUNRISE yielded high-quality data that reveal the structure, dynamics, and evolution of solar convection, oscillations, and magnetic fields at a resolution of around 100 km in the quiet Sun. Here we describe very briefly the mission and the first results obtained from the SUNRISE data, which include a number of discoveries. Title: Vortices, shocks, and heating in the solar photosphere: effect of a magnetic field Authors: Moll, R.; Cameron, R. H.; Schüssler, M. Bibcode: 2012A&A...541A..68M Altcode: 2012arXiv1201.5981M
Aims: We study the differences between non-magnetic and magnetic regions in the flow and thermal structure of the upper solar photosphere.
Methods: Radiative MHD simulations representing a quiet region and a plage region, respectively, which extend into the layers around the temperature minimum, are analyzed.
Results: The flow structure in the upper photospheric layers of the two simulations is considerably different: the non-magnetic simulation is dominated by a pattern of moving shock fronts while the magnetic simulation shows vertically extended vortices associated with magnetic flux concentrations. Both kinds of structures induce substantial local heating. The resulting average temperature profiles are characterized by a steep rise above the temperature minimum due to shock heating in the non-magnetic case and by a flat photospheric temperature gradient mainly caused by Ohmic dissipation in the magnetic run.
Conclusions: Shocks in the quiet Sun and vortices in the strongly magnetized regions represent the dominant flow structures in the layers around the temperature minimum. They are closely connected with dissipation processes providing localized heating. Title: Joint Response of the Helium Lines to Chromospheric Heating in Solar-type Stars Authors: Giampapa, Mark S.; Andretta, V.; Beeck, B.; Reiners, A.; Schussler, M. Bibcode: 2012AAS...22020314G Altcode: We present a preliminary report on simultaneous observations of the He I 5876 and 10830 triplet lines, respectively, in a sample of solar-type stars. The near-IR spectra were obtained with the VLT and CRIRES instrument while the visible spectra were acquired with the MPG/ESO 2.2-m telescope in conjunction with the FEROS spectrograph. The correlation of the observed strengths of these lines will be examined and their potential as diagnostics of active region area coverage in solar-type stars will be discussed.

The NSO is operated by AURA under a cooperative agreement with the National Science Foundation. Title: Simulations of the solar near-surface layers with the CO5BOLD, MURaM, and Stagger codes Authors: Beeck, B.; Collet, R.; Steffen, M.; Asplund, M.; Cameron, R. H.; Freytag, B.; Hayek, W.; Ludwig, H. -G.; Schüssler, M. Bibcode: 2012A&A...539A.121B Altcode: 2012arXiv1201.1103B Context. Radiative hydrodynamic simulations of solar and stellar surface convection have become an important tool for exploring the structure and gas dynamics in the envelopes and atmospheres of late-type stars and for improving our understanding of the formation of stellar spectra.
Aims: We quantitatively compare results from three-dimensional, radiative hydrodynamic simulations of convection near the solar surface generated with three numerical codes (CO5BOLD, MURaM, and Stagger) and different simulation setups in order to investigate the level of similarity and to cross-validate the simulations.
Methods: For all three simulations, we considered the average stratifications of various quantities (temperature, pressure, flow velocity, etc.) on surfaces of constant geometrical or optical depth, as well as their temporal and spatial fluctuations. We also compared observables, such as the spatially resolved patterns of the emerging intensity and of the vertical velocity at the solar optical surface as well as the center-to-limb variation of the continuum intensity at various wavelengths.
Results: The depth profiles of the thermodynamical quantities and of the convective velocities as well as their spatial fluctuations agree quite well. Slight deviations can be understood in terms of differences in box size, spatial resolution and in the treatment of non-gray radiative transfer between the simulations.
Conclusions: The results give confidence in the reliability of the results from comprehensive radiative hydrodynamic simulations. Title: MHD Simulations Reveal Crucial Differences Between Solar and Very Cool Star Magnetic Structures Authors: Beeck, B.; Schüssler, M.; Reiners, A. Bibcode: 2011ASPC..448.1071B Altcode: 2011arXiv1101.3848B; 2011csss...16.1071B We carried out 3D radiative magnetohydrodynamic simulations of the convective and magnetic structure in the surface layers (uppermost part of the convection zone and photosphere) of main-sequence stars of spectral types F3 to M2. The simulation results were analyzed in terms of sizes and properties of the convection cells (granules) and magnetic flux concentrations as well as velocity, pressure, density, and temperature profiles. Our numerical simulations show for the first time a qualitative difference in the magneto-convection between solar-like stars and M dwarfs. Owing to higher surface gravity, lower opacity (resulting in higher density at optical depth unity), and more stable downflows, small-scale magnetic structures concentrate into pore-like configurations of reduced intensity. This implies that in very cool stars magnetic surface structures like plage regions and starspots significantly differ from the solar example. Such a difference would have major impact on the interpretation of Doppler imaging data and the analysis of M dwarf spectra. Title: Can Overturning Motions in Penumbral Filaments BE Detected? Authors: Bharti, Lokesh; Schuessler, Manfred; Rempel, Matthias Bibcode: 2011sdmi.confE..79B Altcode: Numerical simulations indicate that the filamentation of sunspot penumbrae and the associated systematic outflow (the Evershed effect) are due to convectively driven fluid motions constrained by the inclined magnetic field. We investigate whether these motions, in particular the upflows in the bright filaments and the downflows at their edges, can be reliably observed with existing instrumentation. We use a snapshot from a sunspot simulation to calculate two-dimensional maps of synthetic line profiles for the spectral lines Fe I 7090.4 Å and C I 5380.34 Å. The maps are spatially and spectrally degraded according to typical instrument properties. Line-of-sight velocities are determined from line bisector shifts. We find that the detectability of the convective flows is strongly affected by spatial smearing, particularly so for the downflows. Furthermore, the line-of-sight velocities are dominated by the Evershed flow unless the observation is made very near the disk center. These problems may have compromised recent attempts to detect overturning penumbral convection. Lines with a low formation height are best suited for detecting the convective flows. Title: Can Overturning Motions in Penumbral Filaments Be Detected? Authors: Bharti, Lokesh; Schüssler, Manfred; Rempel, Matthias Bibcode: 2011ApJ...739...35B Altcode: 2011arXiv1107.0398B Numerical simulations indicate that the filamentation of sunspot penumbrae and the associated systematic outflow (the Evershed effect) are due to convectively driven fluid motions constrained by the inclined magnetic field. We investigate whether these motions, in particular the upflows in the bright filaments and the downflows at their edges, can be reliably observed with existing instrumentation. We use a snapshot from a sunspot simulation to calculate two-dimensional maps of synthetic line profiles for the spectral lines Fe I 7090.4 Å and C I 5380.34 Å. The maps are spatially and spectrally degraded according to typical instrument properties. Line-of-sight velocities are determined from line bisector shifts. We find that the detectability of the convective flows is strongly affected by spatial smearing, particularly so for the downflows. Furthermore, the line-of-sight velocities are dominated by the Evershed flow unless the observation is made very near the disk center. These problems may have compromised recent attempts to detect overturning penumbral convection. Lines with a low formation height are best suited for detecting the convective flows. Title: Vortices in simulations of solar surface convection Authors: Moll, R.; Cameron, R. H.; Schüssler, M. Bibcode: 2011A&A...533A.126M Altcode: 2011arXiv1108.0800M We report on the occurrence of small-scale vortices in simulations of the convective solar surface. Using an eigenanalysis of the velocity gradient tensor, we find the subset of high-vorticity regions in which the plasma is swirling. The swirling regions form an unsteady, tangled network of filaments in the turbulent downflow lanes. Near-surface vertical vortices are underdense and cause a local depression of the optical surface. They are potentially observable as bright points in the dark intergranular lanes. Vortex features typically exist for a few minutes, during which they are moved and twisted by the motion of the ambient plasma. The bigger vortices found in the simulations are possibly, but not necessarily, related to observations of granular-scale spiraling pathlines in "cork animations" or feature tracking.

Three movies are available in electronic form at http://www.aanda.org Title: Decay of a simulated mixed-polarity magnetic field in the solar surface layers Authors: Cameron, R.; Vögler, A.; Schüssler, M. Bibcode: 2011A&A...533A..86C Altcode: 2011arXiv1108.1155C Magnetic flux is continuously being removed and replenished on the solar surface. To understand the removal process we carried out 3D radiative MHD simulations of the evolution of patches of photospheric magnetic field with equal amounts of positive and negative flux. We find that the flux is removed at a rate corresponding to an effective turbulent diffusivity, ηeff, of 100-340 km2 s-1, depending on the boundary conditions. For average unsigned flux densities above about 70 Gauss, the percentage of surface magnetic energy coming from different field strengths is almost invariant. The overall process is then one where magnetic elements are advected by the horizontal granular motions and occasionally come into contact with opposite-polarity elements. These reconnect above the photosphere on a comparatively short time scale after which the U loops produced rapidly escape through the upper surface while the downward retraction of inverse-U loops is significantly slower, because of the higher inertia and lower plasma beta in the deeper layers. Title: The Sun at high resolution: first results from the Sunrise mission Authors: Solanki, S. K.; Barthol, P.; Danilovic, S.; Feller, A.; Gandorfer, A.; Hirzberger, J.; Lagg, A.; Riethmüller, T. L.; Schüssler, M.; Wiegelmann, T.; Bonet, J. A.; Pillet, V. Martínez; Khomenko, E.; del Toro Iniesta, J. C.; Domingo, V.; Palacios, J.; Knölker, M.; González, N. Bello; Borrero, J. M.; Berkefeld, T.; Franz, M.; Roth, M.; Schmidt, W.; Steiner, O.; Title, A. M. Bibcode: 2011IAUS..273..226S Altcode: The Sunrise balloon-borne solar observatory consists of a 1m aperture Gregory telescope, a UV filter imager, an imaging vector polarimeter, an image stabilization system and further infrastructure. The first science flight of Sunrise yielded high-quality data that reveal the structure, dynamics and evolution of solar convection, oscillations and magnetic fields at a resolution of around 100 km in the quiet Sun. Here we describe very briefly the mission and the first results obtained from the Sunrise data, which include a number of discoveries. Title: Is there a non-monotonic relation between photospheric brightness and magnetic field strength in solar plage regions? Authors: Röhrbein, D.; Cameron, R.; Schüssler, M. Bibcode: 2011A&A...532A.140R Altcode: Context. The relationship between the brightness and field strength of small-scale solar magnetic features is an important factor for solar irradiance variations and a constraint for simulations of solar magneto-convection.
Aims: We wish to clarify the origin of the apparent discrepancy between observational results and radiative MHD simulations.
Methods: Maps of (bolometric) brightness and magnetic field strength from the simulation of a plage region were convolved and rebinned to mimic observations obtained with telescopes with finite aperture.
Results: Image smearing changes the monotonic relation between brightness and field strength obtained at the original resolution of the simulation into a profile with a maximum at intermediate field strength, which is in qualitative agreement with the observations. This result is mainly due to the smearing of strong magnetic fields at the bright edges of magnetic structures into the weakly magnetized adjacent areas.
Conclusions: Observational and simulation results are qualitatively consistent with each other if the finite spatial resolution of the observations is taken into account. Title: Universality of the Small-scale Dynamo Mechanism Authors: Moll, R.; Pietarila Graham, J.; Pratt, J.; Cameron, R. H.; Müller, W. -C.; Schüssler, M. Bibcode: 2011ApJ...736...36M Altcode: 2011arXiv1105.0546M We quantify possible differences between turbulent dynamo action in the Sun and the dynamo action studied in idealized simulations. For this purpose, we compare Fourier-space shell-to-shell energy transfer rates of three incrementally more complex dynamo simulations: an incompressible, periodic simulation driven by random flow, a simulation of Boussinesq convection, and a simulation of fully compressible convection that includes physics relevant to the near-surface layers of the Sun. For each of the simulations studied, we find that the dynamo mechanism is universal in the kinematic regime because energy is transferred from the turbulent flow to the magnetic field from wavenumbers in the inertial range of the energy spectrum. The addition of physical effects relevant to the solar near-surface layers, including stratification, compressibility, partial ionization, and radiative energy transport, does not appear to affect the nature of the dynamo mechanism. The role of inertial-range shear stresses in magnetic field amplification is independent from outer-scale circumstances, including forcing and stratification. Although the shell-to-shell energy transfer functions have similar properties to those seen in mean-flow driven dynamos in each simulation studied, the saturated states of these simulations are not universal because the flow at the driving wavenumbers is a significant source of energy for the magnetic field. Title: Universality of the Small-Scale Dynamo Mechanism Authors: Pietarila Graham, Jonathan; Moll, R.; Pratt, J.; Cameron, R.; Mueller, W.; Schuessler, M. Bibcode: 2011SPD....42.1621P Altcode: 2011BAAS..43S.1621P We quantify possible differences between turbulent dynamo action in the Sun and the dynamo action studied in idealized simulation. For this purpose we compare Fourier-space shell-to-shell energy transfer rates of three incrementally more complex dynamo simulations: an incompressible, periodic simulation driven by random flow, a simulation of Boussinesq convection, and a simulation of fully compressible convection that includes physics relevant to the near-surface layers of the Sun. For each of the simulations studied, we find that energy is transferred from the turbulent flow to the magnetic field from length-scales in the inertial range of the energy spectrum. The addition of physical effects relevant to the solar near-surface layers, including stratification, compressibility, partial ionization, and radiative energy transport, does not appear to affect the nature of the dynamo mechanism. The role of inertial-range shear stresses in magnetic field amplification is independent from outer-scale circumstances, including forcing and stratification. Although shell-to-shell energy transfer functions have similar properties in each simulation studied, the saturated states of these simulations are not universal; the flow at the driving scales is a significant source of energy for the magnetic field. The mechanism of energy-transfer in kinematic small-scale dynamo simulations exhibits universal properties.

This work has been supported by the Max-Planck Society in the framework of the Interinstitutional Research Initiative Turbulent transport and ion heating, reconnection and electron acceleration in solar and fusion plasmas</u> of the MPI for Solar System Research, Katlenburg-Lindau, and the Institute for Plasma Physics, Garching (project MIF-IF-A-AERO8047). Title: The solar magnetic field since 1700. II. Physical reconstruction of total, polar and open flux Authors: Jiang, J.; Cameron, R. H.; Schmitt, D.; Schüssler, M. Bibcode: 2011A&A...528A..83J Altcode: 2011arXiv1102.1270J We have used semi-synthetic records of emerging sunspot groups based on sunspot number data as input for a surface flux transport model to reconstruct the evolution of the large-scale solar magnetic field and the open heliospheric flux from the year 1700 onward. The statistical properties of the semi-synthetic sunspot group records reflect those of the observed Royal Greenwich Observatory photoheliographic results. These include correlations between the sunspot numbers and sunspot group latitudes, longitudes, areas and tilt angles. The reconstruction results for the total surface flux, the polar field, and the heliospheric open flux (determined by a current sheet source surface extrapolation) agree well with the available observational or empirically derived data and reconstructions. We confirm a significant positive correlation between the polar field during activity minimum periods and the strength of the subsequent sunspot cycle, which has implications for flux transport dynamo models for the solar cycle. Just prior to the Dalton minimum, at the end of the 18th century, a long cycle was followed by a weak cycle. We find that introducing a possibly "lost" cycle between 1793 and 1800 leads to a shift of the minimum of the open flux by 15 years which is inconsistent with the cosmogenic isotope record. Title: The solar magnetic field since 1700. I. Characteristics of sunspot group emergence and reconstruction of the butterfly diagram Authors: Jiang, J.; Cameron, R. H.; Schmitt, D.; Schüssler, M. Bibcode: 2011A&A...528A..82J Altcode: 2011arXiv1102.1266J We use the historic record of sunspot groups compiled by the Royal Greenwich Observatory together with the sunspot number to derive the dependence of the statistical properties of sunspot emergence on the cycle phase and strength. In particular we discuss the latitude, longitude, area and tilt angle of sunspot groups as functions of the cycle strength and of time during the solar cycle. Using these empirical characteristics the time-latitude diagram of sunspot group emergence (butterfly diagram) is reconstructed from 1700 onward on the basis of the Wolf and group sunspot numbers. This reconstruction will be useful in studies of the long-term evolution of the Sun's magnetic field. Title: Magnetic flux generation and transport in cool stars Authors: Işık, E.; Schmitt, D.; Schüssler, M. Bibcode: 2011A&A...528A.135I Altcode: 2011arXiv1102.0569I Context. The Sun and other cool stars harbouring outer convection zones manifest magnetic activity in their atmospheres. The connection between this activity and the properties of a deep-seated dynamo generating the magnetic flux is not well understood.
Aims: By employing physical models, we study the spatial and temporal characteristics of the observable surface field for various stellar parameters.
Methods: We combine models for magnetic flux generation, buoyancy instability, and transport, which encompass the entire convection zone. The model components are: (i) a thin-layer αΩ dynamo at the base of the convection zone; (ii) buoyancy instabilities and the rise of flux tubes through the convection zone in 3D, which provides a physically consistent determination of emergence latitudes and tilt angles; and (iii) horizontal flux transport at the surface.
Results: For solar-type stars and rotation periods longer than about 10 days, the latitudinal dynamo waves generated by the deep-seated αΩ dynamo are faithfully reflected by the surface distribution of magnetic flux. For rotation periods of the order of two days, however, Coriolis acceleration of rising flux loops leads to surface flux emergence at much higher latitudes than the dynamo waves at the bottom of the convection zone reach. A similar result is found for a K0V star with a rotation period of two days. In the case of a rapidly rotating K1 subgiant, overlapping dynamo waves lead to noisy activity cycles and mixed-polarity fields at high latitudes.
Conclusions: The combined model reproduces the basic observed features of the solar cycle. The differences between the latitude distributions of the magnetic field at the bottom of the convection zone and the emerging surface flux grow with increasing rotation rate and convection zone depth, becoming quite substantial for rapidly rotating dwarfs and subgiants. The dynamical evolution of buoyantly rising magnetic flux should be considered as an essential ingredient in stellar dynamo models.

3 movies are only available in electronic form at http://www.aanda.org Title: Intensity contrast from MHD simulations and HINODE observations Authors: Afram, N.; Unruh, Y. C.; Solanki, S. K.; Schüssler, M.; Lagg, A.; Vögler, A. Bibcode: 2011A&A...526A.120A Altcode: 2010arXiv1011.6102A Context. Changes in the solar surface area, which is covered by small-scale magnetic elements, are thought to cause long-term changes in the solar spectral irradiance, which are important for determining the impact on Earth's climate.
Aims: To study the effect of small-scale magnetic elements on the total and spectral irradiance, we derive their contrasts from 3-D MHD simulations of the solar atmosphere. These calculations are necessary because measurements of small-scale flux tube contrasts are confined to a few wavelengths and affected by scattered light and instrument defocus, even for space observations.
Methods: To test the contrast calculations, we compare rms contrasts from simulations with those obtained with the broad-band filter imager mounted on the Solar Optical Telescope (SOT) onboard the Hinode satellite and also analyse centre-to-limb variations (CLV). The 3-D MHD simulations include the interaction between convection and magnetic flux tubes. They are performed by assuming non-grey radiative transfer and using the MURaM code. The simulations have an average vertical magnetic field of 0 G, 50 G, and 200 G. Emergent intensities are calculated with the spectral synthesis code ATLAS9 and are convolved with a theoretical point-spread function to account for the properties of the observations' optical system.
Results: We find reasonable agreement between simulated and observed intensity distributions in the visible continuum bands. Agreement is poorer for the CN and G-bands. The analysis of the simulations uncovers a potentially more realistic centre-to-limb behaviour than calculations based on 1-D model atmospheres.
Conclusions: We conclude that starting from 3-D MHD simulations represents a powerful approach to obtaining intensity contrasts for a wide wavelength coverage and different positions across on the solar disk. This also paves the way for future calculations of facular and network contrast as a function of magnetic fluxes. Title: The Sunrise Mission Authors: Barthol, P.; Gandorfer, A.; Solanki, S. K.; Schüssler, M.; Chares, B.; Curdt, W.; Deutsch, W.; Feller, A.; Germerott, D.; Grauf, B.; Heerlein, K.; Hirzberger, J.; Kolleck, M.; Meller, R.; Müller, R.; Riethmüller, T. L.; Tomasch, G.; Knölker, M.; Lites, B. W.; Card, G.; Elmore, D.; Fox, J.; Lecinski, A.; Nelson, P.; Summers, R.; Watt, A.; Martínez Pillet, V.; Bonet, J. A.; Schmidt, W.; Berkefeld, T.; Title, A. M.; Domingo, V.; Gasent Blesa, J. L.; del Toro Iniesta, J. C.; López Jiménez, A.; Álvarez-Herrero, A.; Sabau-Graziati, L.; Widani, C.; Haberler, P.; Härtel, K.; Kampf, D.; Levin, T.; Pérez Grande, I.; Sanz-Andrés, A.; Schmidt, E. Bibcode: 2011SoPh..268....1B Altcode: 2010arXiv1009.2689B; 2010SoPh..tmp..224B The first science flight of the balloon-borne Sunrise telescope took place in June 2009 from ESRANGE (near Kiruna/Sweden) to Somerset Island in northern Canada. We describe the scientific aims and mission concept of the project and give an overview and a description of the various hardware components: the 1-m main telescope with its postfocus science instruments (the UV filter imager SuFI and the imaging vector magnetograph IMaX) and support instruments (image stabilizing and light distribution system ISLiD and correlating wavefront sensor CWS), the optomechanical support structure and the instrument mounting concept, the gondola structure and the power, pointing, and telemetry systems, and the general electronics architecture. We also explain the optimization of the structural and thermal design of the complete payload. The preparations for the science flight are described, including AIV and ground calibration of the instruments. The course of events during the science flight is outlined, up to the recovery activities. Finally, the in-flight performance of the instrumentation is discussed. Title: Small-scale dynamo in solar surface simulations Authors: Graham, J. P.; Moll, R.; Cameron, R.; Schüssler, M. Bibcode: 2010AGUFMNG51C..01G Altcode: A magneto-convection simulation incorporating essential physical processes governing solar surface convection exhibits turbulent small-scale dynamo action. By presenting a derivation of the energy balance equation and transfer functions for compressible magnetohydrodynamics (MHD), we quantify the source of magnetic energy on a scale-by-scale basis. We rule out the two alternative mechanisms for the generation of small-scale magnetic field in the simulations: tangling of magnetic field lines associated with the turbulent cascade and Alfvenization of small-scale velocity fluctuations ("turbulent induction"). Instead, we find the dominant source of small-scale magnetic energy is stretching by inertial-range fluid motions of small-scale magnetic field lines against the magnetic tension force to produce (against Ohmic dissipation) more small-scale magnetic field. The scales involved become smaller with increasing Reynolds number, which identifies the dynamo as a small-scale turbulent dynamo. Comparisons are made between the details of the dynamo mechanism in compressible magneto-convection, Boussinesq convection, and randomly-forced incompressible turbulence. Net energy transfers (kinematic phase): work against magnetic tension (stretching) is 95% of magnetic energy generated; work against magnetic pressure (compression) is 5%. The latter is involved in the breaking down larger-scale field (25%) into smaller-scale field (30%) as part of the cascade. The dominant producer of magnetic energy is the stretching of magnetic field lines against the magnetic tension force (turbulent dynamo action).

Fluid motions at a scale of ~140km create magnetic energy predominately at a scale of ~65km. As the three wave-vectors must form a triad, the scale of the magnetic field being stretched must is 80+/-40km. All 3 scales are in the inertial range: this is turbulent small-scale dynamo. Title: SUNRISE: Instrument, Mission, Data, and First Results Authors: Solanki, S. K.; Barthol, P.; Danilovic, S.; Feller, A.; Gandorfer, A.; Hirzberger, J.; Riethmüller, T. L.; Schüssler, M.; Bonet, J. A.; Martínez Pillet, V.; del Toro Iniesta, J. C.; Domingo, V.; Palacios, J.; Knölker, M.; Bello González, N.; Berkefeld, T.; Franz, M.; Schmidt, W.; Title, A. M. Bibcode: 2010ApJ...723L.127S Altcode: 2010arXiv1008.3460S The SUNRISE balloon-borne solar observatory consists of a 1 m aperture Gregory telescope, a UV filter imager, an imaging vector polarimeter, an image stabilization system, and further infrastructure. The first science flight of SUNRISE yielded high-quality data that revealed the structure, dynamics, and evolution of solar convection, oscillations, and magnetic fields at a resolution of around 100 km in the quiet Sun. After a brief description of instruments and data, the first qualitative results are presented. In contrast to earlier observations, we clearly see granulation at 214 nm. Images in Ca II H display narrow, short-lived dark intergranular lanes between the bright edges of granules. The very small-scale, mixed-polarity internetwork fields are found to be highly dynamic. A significant increase in detectable magnetic flux is found after phase-diversity-related reconstruction of polarization maps, indicating that the polarities are mixed right down to the spatial resolution limit and probably beyond. Title: Bright Points in the Quiet Sun as Observed in the Visible and Near-UV by the Balloon-borne Observatory SUNRISE Authors: Riethmüller, T. L.; Solanki, S. K.; Martínez Pillet, V.; Hirzberger, J.; Feller, A.; Bonet, J. A.; Bello González, N.; Franz, M.; Schüssler, M.; Barthol, P.; Berkefeld, T.; del Toro Iniesta, J. C.; Domingo, V.; Gandorfer, A.; Knölker, M.; Schmidt, W. Bibcode: 2010ApJ...723L.169R Altcode: 2010arXiv1009.1693R Bright points (BPs) are manifestations of small magnetic elements in the solar photosphere. Their brightness contrast not only gives insight into the thermal state of the photosphere (and chromosphere) in magnetic elements, but also plays an important role in modulating the solar total and spectral irradiance. Here, we report on simultaneous high-resolution imaging and spectropolarimetric observations of BPs using SUNRISE balloon-borne observatory data of the quiet Sun at the disk center. BP contrasts have been measured between 214 nm and 525 nm, including the first measurements at wavelengths below 388 nm. The histograms of the BP peak brightness show a clear trend toward broader contrast distributions and higher mean contrasts at shorter wavelengths. At 214 nm, we observe a peak brightness of up to five times the mean quiet-Sun value, the highest BP contrast so far observed. All BPs are associated with a magnetic signal, although in a number of cases it is surprisingly weak. Most of the BPs show only weak downflows, the mean value being 240 m s-1, but some display strong down- or upflows reaching a few km s-1. Title: Transverse Component of the Magnetic Field in the Solar Photosphere Observed by SUNRISE Authors: Danilovic, S.; Beeck, B.; Pietarila, A.; Schüssler, M.; Solanki, S. K.; Martínez Pillet, V.; Bonet, J. A.; del Toro Iniesta, J. C.; Domingo, V.; Barthol, P.; Berkefeld, T.; Gandorfer, A.; Knölker, M.; Schmidt, W.; Title, A. M. Bibcode: 2010ApJ...723L.149D Altcode: 2010arXiv1008.1535D We present the first observations of the transverse component of a photospheric magnetic field acquired by the imaging magnetograph SUNRISE/IMaX. Using an automated detection method, we obtain statistical properties of 4536 features with significant linear polarization signal. We obtain a rate of occurrence of 7 × 10-4 s-1 arcsec-2, which is 1-2 orders of magnitude larger than the values reported by previous studies. We show that these features have no characteristic size or lifetime. They appear preferentially at granule boundaries with most of them being caught in downflow lanes at some point. Only a small percentage are entirely and constantly embedded in upflows (16%) or downflows (8%). Title: Fully Resolved Quiet-Sun Magnetic flux Tube Observed with the SUNRISE/IMAX Instrument Authors: Lagg, A.; Solanki, S. K.; Riethmüller, T. L.; Martínez Pillet, V.; Schüssler, M.; Hirzberger, J.; Feller, A.; Borrero, J. M.; Schmidt, W.; del Toro Iniesta, J. C.; Bonet, J. A.; Barthol, P.; Berkefeld, T.; Domingo, V.; Gandorfer, A.; Knölker, M.; Title, A. M. Bibcode: 2010ApJ...723L.164L Altcode: 2010arXiv1009.0996L Until today, the small size of magnetic elements in quiet-Sun areas has required the application of indirect methods, such as the line-ratio technique or multi-component inversions, to infer their physical properties. A consistent match to the observed Stokes profiles could only be obtained by introducing a magnetic filling factor that specifies the fraction of the observed pixel filled with magnetic field. Here, we investigate the properties of a small magnetic patch in the quiet Sun observed with the IMaX magnetograph on board the balloon-borne telescope SUNRISE with unprecedented spatial resolution and low instrumental stray light. We apply an inversion technique based on the numerical solution of the radiative transfer equation to retrieve the temperature stratification and the field strength in the magnetic patch. The observations can be well reproduced with a one-component, fully magnetized atmosphere with a field strength exceeding 1 kG and a significantly enhanced temperature in the mid to upper photosphere with respect to its surroundings, consistent with semi-empirical flux tube models for plage regions. We therefore conclude that, within the framework of a simple atmospheric model, the IMaX measurements resolve the observed quiet-Sun flux tube. Title: Quiet-sun Intensity Contrasts in the Near-ultraviolet as Measured from SUNRISE Authors: Hirzberger, J.; Feller, A.; Riethmüller, T. L.; Schüssler, M.; Borrero, J. M.; Afram, N.; Unruh, Y. C.; Berdyugina, S. V.; Gandorfer, A.; Solanki, S. K.; Barthol, P.; Bonet, J. A.; Martínez Pillet, V.; Berkefeld, T.; Knölker, M.; Schmidt, W.; Title, A. M. Bibcode: 2010ApJ...723L.154H Altcode: We present high-resolution images of the Sun in the near-ultraviolet spectral range between 214 nm and 397 nm as obtained from the first science flight of the 1 m SUNRISE balloon-borne solar telescope. The quiet-Sun rms intensity contrasts found in this wavelength range are among the highest values ever obtained for quiet-Sun solar surface structures—up to 32.8% at a wavelength of 214 nm. We compare the rms contrasts obtained from the observational data with theoretical intensity contrasts obtained from numerical magnetohydrodynamic simulations. For 388 nm and 312 nm the observations agree well with the numerical simulations whereas at shorter wavelengths discrepancies between observed and simulated contrasts remain. Title: Mesogranular structure in a hydrodynamical simulation Authors: Matloch, Ł.; Cameron, R.; Shelyag, S.; Schmitt, D.; Schüssler, M. Bibcode: 2010A&A...519A..52M Altcode: 2010arXiv1007.0387M
Aims: We analyse mesogranular flow patterns in a three-dimensional hydrodynamical simulation of solar surface convection in order to determine its characteristics.
Methods: We calculate divergence maps from horizontal velocities obtained with the local correlation tracking (LCT) method. Mesogranules are identified as patches of positive velocity divergence. We track the mesogranules to obtain their size and lifetime distributions. We vary the analysis parameters to verify if the pattern has characteristic scales.
Results: The characteristics of the resulting flow patterns depend on the averaging time and length used in the analysis.
Conclusions: We conclude that the mesogranular patterns do not exhibit intrinsic length and time scales. Title: Simulation of the Formation of a Solar Active Region Authors: Cheung, M. C. M.; Rempel, M.; Title, A. M.; Schüssler, M. Bibcode: 2010ApJ...720..233C Altcode: 2010arXiv1006.4117C We present a radiative magnetohydrodynamics simulation of the formation of an active region (AR) on the solar surface. The simulation models the rise of a buoyant magnetic flux bundle from a depth of 7.5 Mm in the convection zone up into the solar photosphere. The rise of the magnetic plasma in the convection zone is accompanied by predominantly horizontal expansion. Such an expansion leads to a scaling relation between the plasma density and the magnetic field strength such that B vprop rhov1/2. The emergence of magnetic flux into the photosphere appears as a complex magnetic pattern, which results from the interaction of the rising magnetic field with the turbulent convective flows. Small-scale magnetic elements at the surface first appear, followed by their gradual coalescence into larger magnetic concentrations, which eventually results in the formation of a pair of opposite polarity spots. Although the mean flow pattern in the vicinity of the developing spots is directed radially outward, correlations between the magnetic field and velocity field fluctuations allow the spots to accumulate flux. Such correlations result from the Lorentz-force-driven, counterstreaming motion of opposite polarity fragments. The formation of the simulated AR is accompanied by transient light bridges between umbrae and umbral dots. Together with recent sunspot modeling, this work highlights the common magnetoconvective origin of umbral dots, light bridges, and penumbral filaments. Title: Changes of the Solar Meridional Velocity Profile During Cycle 23 Explained by Flows Toward the Activity Belts Authors: Cameron, R. H.; Schüssler, M. Bibcode: 2010ApJ...720.1030C Altcode: 2010arXiv1007.2548C The solar meridional flow is an important ingredient in Babcock-Leighton type models of the solar dynamo. Global variations of this flow have been suggested to explain the variations in the amplitudes and lengths of the activity cycles. Recently, cycle-related variations in the amplitude of the P 1 2 term in the Legendre decomposition of the observed meridional flow have been reported. The result is often interpreted in terms of an overall variation in the flow amplitude during the activity cycle. Using a semi-empirical model based upon the observed distribution of magnetic flux on the solar surface, we show that the reported variations of the P 1 2 term can be explained by the observed localized inflows into the active region belts. No variation of the overall meridional flow amplitude is required. Title: Quiet-Sun intensity contrasts in the near ultraviolet Authors: Hirzberger, Johann; Feller, Alex; Riethmüller, Tino L.; Schüssler, Manfred; Borrero, Juan M.; Afram, Nadine; Unruh, Yvonne C.; Berdyugina, Svetlana V.; Gandorfer, Achim; Solanki, Sami K.; Barthol, Peter; Bonet, Jose A.; Martínez Pillet, Valentin; Berkefeld, Thomas; Knölker, Michael; Schmidt, Wolfgang; Title, Alan M. Bibcode: 2010arXiv1009.1050H Altcode: We present high-resolution images of the Sun in the near ultraviolet spectral range between 214 nm and 397 nm as obtained from the first science flight of the 1-m Sunrise balloon-borne solar telescope. The quiet-Sun rms intensity contrasts found in this wavelength range are among the highest values ever obtained for quiet-Sun solar surface structures - up to 32.8% at a wavelength of 214 nm. We compare with theoretical intensity contrasts obtained from numerical magneto-hydrodynamic simulations. For 388 nm and 312 nm the observations agree well with the numerical simulations whereas at shorter wavelengths discrepancies between observed and simulated contrasts remain. Title: Surface Flux Transport Modeling for Solar Cycles 15-21: Effects of Cycle-Dependent Tilt Angles of Sunspot Groups Authors: Cameron, R. H.; Jiang, J.; Schmitt, D.; Schüssler, M. Bibcode: 2010ApJ...719..264C Altcode: 2010arXiv1006.3061C We model the surface magnetic field and open flux of the Sun from 1913 to 1986 using a surface flux transport model, which includes the observed cycle-to-cycle variation of sunspot group tilts. The model reproduces the empirically derived time evolution of the solar open magnetic flux and the reversal times of the polar fields. We find that both the polar field and the axial dipole moment resulting from this model around cycle minimum correlate with the strength of the following cycle. Title: The Effect of Activity-related Meridional Flow Modulation on the Strength of the Solar Polar Magnetic Field Authors: Jiang, J.; Işik, E.; Cameron, R. H.; Schmitt, D.; Schüssler, M. Bibcode: 2010ApJ...717..597J Altcode: 2010arXiv1005.5317J We studied the effect of the perturbation of the meridional flow in the activity belts detected by local helioseismology on the development and strength of the surface magnetic field at the polar caps. We carried out simulations of synthetic solar cycles with a flux transport model, which follows the cyclic evolution of the surface field determined by flux emergence and advective transport by near-surface flows. In each hemisphere, an axisymmetric band of latitudinal flows converging toward the central latitude of the activity belt was superposed onto the background poleward meridional flow. The overall effect of the flow perturbation is to reduce the latitudinal separation of the magnetic polarities of a bipolar magnetic region and thus diminish its contribution to the polar field. As a result, the polar field maximum reached around cycle activity minimum is weakened by the presence of the meridional flow perturbation. For a flow perturbation consistent with helioseismic observations, the polar field is reduced by about 18% compared to the case without inflows. If the amplitude of the flow perturbation depends on the cycle strength, its effect on the polar field provides a nonlinearity that could contribute to limiting the amplitude of a Babcock-Leighton type dynamo. Title: SUNRISE Impressions from a successful science flight Authors: Schmidt, W.; Solanki, S. K.; Barthol, P.; Berkefeld, T.; Gandorfer, A.; Knölker, M.; Martínez Pillet, V.; Schüssler, M.; Title, A. Bibcode: 2010AN....331..601S Altcode: SUNRISE is a balloon-borne telescope with an aperture of one meter. It is equipped with a filter imager for the UV wavelength range between 214 nm and 400 nm (SUFI), and with a spectro-polarimeter that measures the magnetic field of the photosphere using the Fe I line at 525.02 nm that has a Landé factor of 3. SUNRISE performed its first science flight from 8 to 14 June 2009. It was launched at the Swedish ESRANGE Space Center and cruised at an altitude of about 36 km and geographic latitudes between 70 and 74 degrees to Somerset Island in northern Canada. There, all data, the telescope and the gondola were successfully recovered. During its flight, Sunrise achieved high pointing stability during 33 hours, and recorded about 1.8 TB of science data. Already at this early stage of data processing it is clear that SUNRISE recorded UV images of the solar photosphere, and spectropolarimetric measurements of the quiet Sun's magnetic field of unprecedented quality. Title: The solar cycle and the current solar minimum Authors: Cameron, R.; Jiang, J.; Schmitt, D.; Schuessler, M. Bibcode: 2010EGUGA..1215494C Altcode: In this talk we discuss the evolution of the Sun's large-scale magnetic field, on timescales relevant to the solar cycle. This evolution can be modeled using the surface flux transport equations, and we will outline the ingredients which go into the model. Special attention will be paid to the term describing the emergence of new flux onto the solar surface. The results of the model will be compared against observations covering most of the twentieth century, and in particular we will discuss what determines the surface field during solar minima. Title: Turbulent Small-Scale Dynamo Action in Solar Surface Simulations Authors: Pietarila Graham, Jonathan; Cameron, Robert; Schüssler, Manfred Bibcode: 2010ApJ...714.1606P Altcode: 2010ApJ...714.1606G; 2010arXiv1002.2750P We demonstrate that a magneto-convection simulation incorporating essential physical processes governing solar surface convection exhibits turbulent small-scale dynamo action. By presenting a derivation of the energy balance equation and transfer functions for compressible magnetohydrodynamics, we quantify the source of magnetic energy on a scale-by-scale basis. We rule out the two alternative mechanisms for the generation of the small-scale magnetic field in the simulations: the tangling of magnetic field lines associated with the turbulent cascade and Alfvénization of small-scale velocity fluctuations ("turbulent induction"). Instead, we find that the dominant source of small-scale magnetic energy is stretching by inertial-range fluid motions of small-scale magnetic field lines against the magnetic tension force to produce (against Ohmic dissipation) more small-scale magnetic field. The scales involved become smaller with increasing Reynolds number, which identifies the dynamo as a small-scale turbulent dynamo. Title: Probing quiet Sun magnetism using MURaM simulations and Hinode/SP results: support for a local dynamo Authors: Danilovic, S.; Schüssler, M.; Solanki, S. K. Bibcode: 2010A&A...513A...1D Altcode: 2010arXiv1001.2183D Context. Owing to the limited spatial resolution and the weak polarization signal coming from the quietest regions on the Sun, the organization of the magnetic field on the smallest scales is largely unknown.
Aims: We obtain information about the magnetic flux present in the quiet Sun by comparing radiative MHD simulations with observations, with particular emphasis on the role of surface dynamo action.
Methods: We synthesized Stokes profiles on the basis of the MHD simulation results. The profiles are degraded by taking the properties of the spectropolarimeter (SP) into account onboard the Hinode satellite. We used simulation runs with different magnetic Reynolds numbers (Rm) and observations at different heliocentric angles with different levels of noise.
Results: Simulations with an imposed mixed-polarity field and Rm below the threshold for dynamo action reproduce the observed vertical flux density, but do not display a high enough horizontal flux density. Surface dynamo simulations at the highest Rm feasible at the moment yield a ratio of the horizontal and vertical flux density consistent with observational results, but the overall amplitudes are too low. Based on the properties of the local dynamo simulations, a tentative scaling of the magnetic field strength by a factor 2-3 reproduces the signal observed in the internetwork regions.
Conclusions: We find agreement with observations at different heliocentric angles. The mean field strength in internetwork implied by our analysis is roughly 170 G at the optical depth unity. Our study shows that surface dynamo could be responsible for most of the magnetic flux in the quiet Sun outside the network, given that the extrapolation to higher Rm is valid. Title: The small-scale solar surface dynamo Authors: Pietarila Graham, Jonathan; Danilovic, Sanja; Schuessler, Manfred Bibcode: 2010arXiv1003.0347P Altcode: The existence of a turbulent small-scale solar surface dynamo is likely, considering existing numerical and laboratory experiments, as well as comparisons of a small-scale dynamo in MURaM simulations with Hinode observations. We find the observed peaked probability distribution function (PDF) from Stokes-V magnetograms is consistent with a monotonic PDF of the actual vertical field strength. The cancellation function of the vertical flux density from a Hinode SP observation is found to follow a self-similar power law over two decades in length scales down to the ~200 km resolution limit. This provides observational evidence that the scales of magnetic structuring in the photosphere extend at least down to 20 km. From the power law, we determine a lower bound for the true quiet-Sun mean vertical unsigned flux density of ~43 G, consistent with our numerically-based estimates that 80% or more of the vertical unsigned flux should be invisible to Stokes-V observations at a resolution of 200 km owing to cancellation. Our estimates significantly reduce the order-of-magnitude discrepancy between Zeeman- and Hanle-based estimates. Title: A comparison of measured and simulated solar network contrast Authors: Afram, N.; Unruh, Y. C.; Solanki, S. K.; Schüssler, M.; Mathew, S. K. Bibcode: 2010IAUS..264...63A Altcode: 2009arXiv0910.0976A Long-term trends in the solar spectral irradiance are important to determine the impact on Earth's climate. These long-term changes are thought to be caused mainly by changes in the surface area covered by small-scale magnetic elements. The direct measurement of the contrast to determine the impact of these small-scale magnetic elements is, however, limited to a few wavelengths, and is, even for space instruments, affected by scattered light and instrument defocus. In this work we calculate emergent intensities from 3-D simulations of solar magneto-convection and validate the outcome by comparing with observations from Hinode/SOT. In this manner we aim to construct the contrast at wavelengths ranging from the NUV to the FIR. Title: Properties of simulated sunspot umbral dots Authors: Bharti, L.; Beeck, B.; Schüssler, M. Bibcode: 2010A&A...510A..12B Altcode: 2009arXiv0911.5068B Realistic 3D radiative MHD simulations reveal the magneto-convective processes underlying the formation of the photospheric fine structure of sunspots, including penumbral filaments and umbral dots. Here we provide results from a statistical analysis of simulated umbral dots and compare them with reports from high-resolution observations. A multi-level segmentation and tracking algorithm has been used to isolate the bright structures in synthetic bolometric and continuum brightness images. Areas, brightness, and lifetimes of the resulting set of umbral dots are found to be correlated: larger umbral dots tend to be brighter and live longer. The magnetic field strength and velocity structure of umbral dots on surfaces of constant optical depth in the continuum at 630 nm indicate that the strong field reduction and high velocities in the upper parts of the upflow plumes underlying umbral dots are largely hidden from spectro-polarimetric observations. The properties of the simulated umbral dots are generally consistent with the results of recent high-resolution observations. However, the observed population of small, short-lived umbral dots is not reproduced by the simulations, possibly owing to insufficient spatial resolution. Title: High resolution imaging and polarimetry with SUNRISE, a balloon-borne stratospheric solar observatory Authors: Barthol, Peter; Chares, Bernd; Deutsch, Werner; Feller, Alex; Gandorfer, Achim; Grauf, Bianca; Hirzberger, Johann; Meller, Reinhard; Riethmueller, Tino; Schuessler, Manfred; Solanki, Sami K.; Knoelker, Michael; Martinez Pillet, Valentin; Schmidt, Wolfgang; Title, Alan Bibcode: 2010cosp...38.4063B Altcode: 2010cosp.meet.4063B SUNRISE is an international collaboration for the development and operation of a meter-class balloon-borne stratospheric solar observatory. Prime science goal is the study of structure and dynamics of the magnetic field in the solar atmosphere and the interaction of the magnetic field with convective plasma flows. These processes are studied by high resolution imaging in the UV and polarimetry at visible wavelengths. The instrument has been successfully launched on June 8, 2009 from ESRANGE, Kiruna, Northern Sweden. During the more than 5 days flight about 1.5 TByte of scientific data were collected. The paper gives an overview of the instrument and mission, examples of the scientific output will also be presented. SUNRISE is a joint project of the Max-Planck-Institut fuer Sonnensystemforschung (MPS), Katlenburg-Lindau, with the Kiepenheuer-Institut fuer Sonnenphysik (KIS), Freiburg, the High-Altitude Observatory (HAO), Boulder, the Lockheed-Martin Solar and Astrophysics Lab. (LMSAL), Palo Alto, and the Spanish IMaX consortium. Title: UV intensity distributions of the quiet Sun observed with Sunrise Authors: Hirzberger, Johann; Feller, A.; Riethmueller, T.; Borrero, J. M.; Schüssler, M.; Barthol, P.; Berkefeld, T.; Gandorfer, A.; Knoelker, M.; Martínez Pillet, V.; Schmidt, W.; Solanki, S.; Title, A. Bibcode: 2010cosp...38.1735H Altcode: 2010cosp.meet.1735H High resolution solar images in the near UV have been obtained with the Solar UV Filtergraph (SUFI) onboard the Sunrise balloon borne observatory, amongst others in wavelength regions not accessible from the ground. We present intensity distributions of the quiet Sun at different heliocentric angles, from disk center to the solar limb. These results, obtained in spectral windows at 214 nm, 313 nm (OH band), 388 nm (CN band) and 396.7 nm (CaIIH), represent an important validation of numerical models of the solar photosphere and are, thus, fundamental ingredients for our understanding of the thermal processes in the solar surface region. Title: Magnetic field intensification: comparison of 3D MHD simulations with Hinode/SP results Authors: Danilovic, S.; Schüssler, M.; Solanki, S. K. Bibcode: 2010A&A...509A..76D Altcode: 2009arXiv0910.1211D Context. Recent spectro-polarimetric observations have provided detailed measurements of magnetic field, velocity and intensity during events of magnetic field intensification in the solar photosphere.
Aims: By comparing with synthetic observations derived from MHD simulations, we investigate the physical processes underlying the observations, as well as verify the simulations and the interpretation of the observations.
Methods: We consider the temporal evolution of the relevant physical quantities for three cases of magnetic field intensification in a numerical simulation. In order to compare with observations, we calculate Stokes profiles and take into account the spectral and spatial resolution of the spectropolarimeter (SP) on board Hinode. We determine the evolution of the intensity, magnetic flux density and zero-crossing velocity derived from the synthetic Stokes parameters, using the same methods as applied to the Hinode/SP observations to derive magnetic field and velocity information from the spectro-polarimetric data.
Results: The three events considered show a similar evolution: advection of magnetic flux to a granular vertex, development of a strong downflow, evacuation of the magnetic feature, increase of the field strength and the appearance of the bright point. The magnetic features formed have diameters of 0.1-0.2´´. The downflow velocities reach maximum values of 5-10 km s-1 at τ = 1. In the largest feature, the downflow reaches supersonic speed in the lower photosphere. In the same case, a supersonic upflow develops approximately 200 s after the formation of the flux concentration. We find that synthetic and real observations are qualitatively consistent and, for one of the cases considered, also agree very well quantitatively. The effect of finite resolution (spatial smearing) is most pronounced in the case of small features, for which the synthetic Hinode/SP observations miss the bright point formation and also the high-velocity downflows during the formation of the smaller magnetic features.
Conclusions: The observed events are consistent with the process of field intensification by flux advection, radiative cooling, and evacuation by strong downflow found in MHD simulations. The quantitative agreement of synthetic and real observations indicates the validity of both the simulations and the interpretations of the spectro-polarimetric observations. Title: Relation between the Sunrise photospheric magnetic field and the Ca II H bright features Authors: Jafarzadeh, Shahin; Hirzberger, J.; Feller, A.; Lagg, A.; Solanki, S. K.; Pietarila, A.; Danilovic, S.; Riethmueller, T.; Barthol, P.; Berkefeld, T.; Gandorfer, A.; Knülker, M.; Martínez Pillet, V.; Schmidt, W.; Schüssler, M.; Title, A. Bibcode: 2010cosp...38.2856J Altcode: 2010cosp.meet.2856J Recent observations from the Sunrise balloon-borne solar telescope have enabled us to reach an unprecedented high spatial resolution on the solar surface with the near-ultraviolet photo-spheric and chromospheric images as well as the magnetograms. We use these high resolution observations to investigate the structure of the solar upper photosphere and lower chromosphere as well as their temporal evolutions. We study the relation between the inter-granular Ca II 397 nm bright structures in images obtained by the Sunrise Filter Imager (SuFI) and their corresponding photospheric vector magnetic field computed from the Imaging Magnetogram eXperiment (IMaX) observations. The targets under study are in a quiet Sun region and close to disc-centre. Title: Modeling the Sun's Open Magnetic Flux and the Heliospheric Current Sheet Authors: Jiang, J.; Cameron, R.; Schmitt, D.; Schüssler, M. Bibcode: 2010ApJ...709..301J Altcode: 2009arXiv0912.0108J By coupling a solar surface flux transport model with an extrapolation of the heliospheric field, we simulate the evolution of the Sun's open magnetic flux and the heliospheric current sheet (HCS) based on observational data of sunspot groups since 1976. The results are consistent with measurements of the interplanetary magnetic field near Earth and with the tilt angle of the HCS as derived from extrapolation of the observed solar surface field. This opens the possibility for an improved reconstruction of the Sun's open flux and the HCS into the past on the basis of empirical sunspot data. Title: Simulation of a flux emergence event and comparison with observations by Hinode Authors: Yelles Chaouche, L.; Cheung, M. C. M.; Solanki, S. K.; Schüssler, M.; Lagg, A. Bibcode: 2009A&A...507L..53Y Altcode: 2009arXiv0910.5737Y Aims: We study the observational signature of flux emergence in the photosphere using synthetic data from a 3D MHD simulation of the emergence of a twisted flux tube.
Methods: Several stages in the emergence process are considered. At every stage we compute synthetic Stokes spectra of the two iron lines Fe I 6301.5 Å and Fe I 6302.5 Å and degrade the data to the spatial and spectral resolution of Hinode's SOT/SP. Then, following observational practice, we apply Milne-Eddington-type inversions to the synthetic spectra in order to retrieve various atmospheric parameters and compare the results with recent Hinode observations.
Results: During the emergence sequence, the spectral lines sample different parts of the rising flux tube, revealing its twisted structure. The horizontal component of the magnetic field retrieved from the simulations is close to the observed values. The flattening of the flux tube in the photosphere is caused by radiative cooling, which slows down the ascent of the tube to the upper solar atmosphere. Consistent with the observations, the rising magnetized plasma produces a blue shift of the spectral lines during a large part of the emergence sequence.

Figure 3 is only available in electronic form at http://www.aanda.org Title: The Small-Scale Solar Surface Dynamo (Keynote) Authors: Pietarila Graham, J.; Danilovic, S.; Schüssler, M. Bibcode: 2009ASPC..415...43P Altcode: The existence of a turbulent small-scale solar surface dynamo is likely, considering existing numerical and laboratory experiments, as well as comparisons of a small-scale dynamo in MURaM simulations with Hinode observations. We find the observed peaked probability distribution function (PDF) from Stokes-V magnetograms is consistent with a monotonic PDF of the actual vertical field strength. The cancellation function of the vertical flux density from a Hinode SP observation is found to follow a self-similar power law over two decades in length scales down to the ≈200 km resolution limit. This provides observational evidence that the scales of magnetic structuring in the photosphere extend at least down to 20 km. From the power law, we determine a lower bound for the true quiet-Sun mean vertical unsigned flux density of ≈43 G, consistent with our numerically-based estimates that 80% or more of the vertical unsigned flux should be invisible to Stokes-V observations at a resolution of 200 km owing to cancellation. Our estimates significantly reduce the order-of-magnitude discrepancy between Zeeman- and Hanle-based estimates. Title: Radiative MHD simulations of sunspot structure Authors: Rempel, M.; Schuessler, M.; Cameron, R.; Knoelker, M. Bibcode: 2009AGUFMSH53B..07R Altcode: For a long time radiative MHD simulations of entire sunspots from first principles were out of reach due to insufficient computing resources. Over the past 4 years simulations have evolved from 6x6x2 Mm size domains focusing on the details of umbral dots to simulations covering a pair of opposite polarity sunspots in a 100x50x6 Mm domain. Numerical simulations point toward a common magneto convective origin of umbral dots and filaments in the inner and outer penumbra. Most recent simulations also capture the processes involved in the formation of an extended outer penumbra with strong horizontal outflows averaging around 5 km/s in the photosphere. In this talk I will briefly review the progress made in this field over the past 4 years and discuss in detail the magneto convective origin of penumbral fine structure as well as the Evershed flow. Title: Radiative MHD simulation of an Emerging Flux Region Authors: Cheung, C.; Rempel, M.; Title, A. M.; Schuessler, M. Bibcode: 2009AGUFMSH51A1267C Altcode: We present a radiation magnetohydrodynamics (MHD) simulation of the birth of an active region. The simulation models the rise of a magnetic flux bundle from the convection zone into the solar photosphere. Observational properties of the simulation are consistent with recent, high-cadence and high spatial resolution observations of emerging flux regions taken by Hinode/SOT. Observational properties common to both simulation and observation include the hierarchical formation of progressively larger photospheric magnetic structures, the formation and disappearance of light bridges, umbral dots as well as penumbral filaments. Title: Solar Surface Emerging Flux Regions: A Comparative Study of Radiative MHD Modeling and Hinode SOT Observations Authors: Cheung, M.; Schüssler, M.; Tarbell, T. D.; Title, A. M. Bibcode: 2009ASPC..415...79C Altcode: We present results from three-dimensional radiative MHD simulations of the rise of buoyant magnetic flux tubes through the convection zone and into the photosphere. Due to the strong stratification of the convection zone, the rise results in a lateral expansion of the tube into a magnetic sheet, which acts as a reservoir for small-scale flux emergence events at the scale of granulation. The interaction of the convective downflows and the rising magnetic flux tube undulates it to form serpentine field lines that emerge into the photosphere. Observational characteristics of the simulated emerging flux regions are discussed in the context of new observations from Hinode SOT. Title: Radiative MHD simulations of sunspot structure Authors: Rempel, M.; Schüssler, M.; Cameron, R.; Knölker, M. Bibcode: 2009iac..talk..192R Altcode: 2009iac..talk..106R No abstract at ADS Title: Modelling of solar mesogranulation Authors: Matloch, L.; Cameron, R.; Schmitt, D.; Schüssler, M. Bibcode: 2009A&A...504.1041M Altcode: We study whether mesogranulation flow patterns at the solar surface can arise solely from the statistical properties of granules and intergranular lanes. We have developed one- and two-dimensional models with local interaction rules between the artificial “granules” mimicking the actual physical processes on the solar surface. Defining mesogranulation according to the age of intergranular (downflow) lanes corresponding to the often applied “cork method”, as well as the areas of divergence of the horizontal velocity (two-dimensional model), we find that mesogranular patterns are present in our models. Our study of the dependence of the properties of the mesogranular patterns on the model parameter and interaction rules reveals that the patterns do not possess intrinsic length and time scales.

Appendix is only available in electronic from at http://www.aanda.org Title: Comparison of the thin flux tube approximation with 3D MHD simulations Authors: Yelles Chaouche, L.; Solanki, S. K.; Schüssler, M. Bibcode: 2009A&A...504..595Y Altcode: 2009arXiv0910.1796Y Context: The structure and dynamics of small vertical photospheric magnetic flux concentrations has been often treated in the framework of an approximation based upon a low-order truncation of the Taylor expansions of all quantities in the horizontal direction, together with the assumption of instantaneous total pressure balance at the boundary to the non-magnetic external medium. Formally, such an approximation is justified if the diameter of the structure (a flux tube or a flux sheet) is small compared to all other relevant length scales (scale height, radius of curvature, wavelength, etc.). The advent of realistic 3D radiative MHD simulations opens the possibility of checking the consistency of the approximation with the properties of the flux concentrations that form in the course of a simulation.
Aims: We carry out a comparative analysis between the thin flux tube/sheet models and flux concentrations formed in a 3D radiation-MHD simulation.
Methods: We compare the distribution of the vertical and horizontal components of the magnetic field in a 3D MHD simulation with the field distribution in the case of the thin flux tube/sheet approximation. We also consider the total (gas plus magnetic) pressure in the MHD simulation box.
Results: Flux concentrations with super-equipartition fields are reasonably well reproduced by the second-order thin flux tube/sheet approximation. The differences between approximation and simulation are due to the asymmetry and the dynamics of the simulated structures. Title: Penumbral Structure and Outflows in Simulated Sunspots Authors: Rempel, M.; Schüssler, M.; Cameron, R. H.; Knölker, M. Bibcode: 2009Sci...325..171R Altcode: 2009arXiv0907.2259R Sunspots are concentrations of magnetic field on the visible solar surface that strongly affect the convective energy transport in their interior and surroundings. The filamentary outer regions (penumbrae) of sunspots show systematic radial outward flows along channels of nearly horizontal magnetic field. These flows were discovered 100 years ago and are present in all fully developed sunspots. By using a comprehensive numerical simulation of a sunspot pair, we show that penumbral structures with such outflows form when the average magnetic field inclination to the vertical exceeds about 45 degrees. The systematic outflows are a component of the convective flows that provide the upward energy transport and result from anisotropy introduced by the presence of the inclined magnetic field. Title: Radiative MHD Simulations of Sunspot Structure Authors: Rempel, Matthias D.; Schuessler, M.; Cameron, R.; Knoelker, M. Bibcode: 2009SPD....40.0604R Altcode: We summarize the recent progress made in magneto convection simulations of sunspot structure. Over the past 4 years simulations have evolved from local 6x6x2 Mm size domains focusing on the details of umbral dots to simulations covering a pair of opposite polarity spots in a 100x50x6 Mm domain. The simulations point out the common magneto convective origin of umbral dots and filaments in the inner penumbra and most recently also reveal the processes involved in the formation of an extended outer penumbra with strong horizontal outflows averaging around 5 km/s in the photosphere. Title: Countercell Meridional Flow and Latitudinal Distribution of the Solar Polar Magnetic Field Authors: Jiang, J.; Cameron, R.; Schmitt, D.; Schüssler, M. Bibcode: 2009ApJ...693L..96J Altcode: Recent observations indicate that the latitudinal profile of the magnetic flux shows a pronounced decrease close to the solar north pole during the minimum phase of solar cycle 23. Using a surface flux transport model, we have performed numerical experiments to study the conditions that could lead to such a latitudinal distribution. We find that a strong decrease of the magnetic field near the poles results if a weak countercell of the meridional flow at high latitudes with an equatorward speed of a few m s-1 is present. Title: Turbulent Magnetic Fields in the Quiet Sun: Implications of Hinode Observations and Small-Scale Dynamo Simulations Authors: Pietarila Graham, Jonathan; Danilovic, Sanja; Schüssler, Manfred Bibcode: 2009ApJ...693.1728P Altcode: 2008arXiv0812.2125P Using turbulent MHD simulations (magnetic Reynolds numbers up to ≈8000) and Hinode observations, we study effects of turbulence on measuring the solar magnetic field outside active regions. First, from synthetic Stokes V profiles for the Fe I lines at 6301 and 6302 Å, we show that a peaked probability distribution function (PDF) for observationally derived field estimates is consistent with a monotonic PDF for actual vertical field strengths. Hence, the prevalence of weak fields is greater than would be naively inferred from observations. Second, we employ the fractal self-similar geometry of the turbulent solar magnetic field to derive two estimates (numerical and observational) of the true mean vertical unsigned flux density. We also find observational evidence that the scales of magnetic structuring in the photosphere extend at least down to an order of magnitude smaller than 200 km: the self-similar power-law scaling in the signed measure from a Hinode magnetogram ranges (over two decades in length scales and including the granulation scale) down to the ≈200 km resolution limit. From the self-similar scaling, we determine a lower bound for the true quiet-Sun mean vertical unsigned flux density of ~50 G. This is consistent with our numerically based estimates that 80% or more of the vertical unsigned flux should be invisible to Stokes V observations at a resolution of 200 km owing to the cancellation of signal from opposite magnetic polarities. Our estimates significantly reduce the order-of-magnitude discrepancy between Zeeman- and Hanle-based estimates. Title: Towards Long-Term Solar Irradiance Modelling: Network Contrasts from Magneto-Convection Simulations Authors: Unruh, Y. C.; Solanki, S. K.; Schüssler, M.; Vögler, A.; Garcia-Alvarez, D. Bibcode: 2009AIPC.1094..768U Altcode: 2009csss...15..768U Solar irradiance changes on a wide range of time scales and is a key driver of the Earth's climate where secular variability in particular is relevant. This is, however, not well understood and our knowledge relies on reconstructions based on sunspot numbers and similar proxies.

The prime candidate to produce secular variability is a change in the surface coverage of small-scale magnetic elements. Direct observational determination of the flux emitted by these magnetic elements is difficult, especially as information covering a large spectral range is needed. Here we present a theoretical approach to this problem using intensity calculations from 3-D simulations of solar magneto-convection and compare these with the intensity calculations used in the successful semi-empirical S ATIRE models at disk centre. Eventually, such a comparison should lead to the removal of the last free parameter from S ATIRE-based irradiance reconstruction. Title: Radiative Magnetohydrodynamic Simulation of Sunspot Structure Authors: Rempel, M.; Schüssler, M.; Knölker, M. Bibcode: 2009ApJ...691..640R Altcode: 2008arXiv0808.3294R Results of a three-dimensional MHD simulation of a sunspot with a photospheric size of about 20 Mm are presented. The simulation has been carried out with the MURaM code, which includes a realistic equation of state with partial ionization and radiative transfer along many ray directions. The largely relaxed state of the sunspot shows a division in a central dark umbral region with bright dots and a penumbra showing bright filaments of about 2-3 Mm length with central dark lanes. By a process similar to the formation of umbral dots, the penumbral filaments result from magnetoconvection in the form of upflow plumes, which become elongated by the presence of an inclined magnetic field; the upflow is deflected in the outward direction while the magnetic field is weakened and becomes almost horizontal in the upper part of the plume near the level of optical depth unity. A dark lane forms owing to the piling up of matter near the cusp-shaped top of the rising plume that leads to an upward bulging of the surfaces of constant optical depth. The simulated penumbral structure corresponds well to the observationally inferred interlocking-comb structure of the magnetic field with Evershed outflows along dark-laned filaments with nearly horizontal magnetic field and overturning perpendicular ("twisting") motion, which are embedded in a background of stronger and less inclined field. Photospheric spectral lines are formed at the very top and somewhat above the upflow plumes, so that they do not fully sense the strong flow as well as the large field inclination and significant field strength reduction in the upper part of the plume structures. Title: Solar Surface Emerging Flux Regions: A Comparative Study of Radiative MHD Modeling and Hinode SOT Observations Authors: Cheung, M. C. M.; Schüssler, M.; Tarbell, T. D.; Title, A. M. Bibcode: 2008ApJ...687.1373C Altcode: 2008arXiv0810.5723C We present results from numerical modeling of emerging flux regions on the solar surface. The modeling was carried out by means of three-dimensional (3D) radiative MHD simulations of the rise of buoyant magnetic flux tubes through the convection zone and into the photosphere. Due to the strong stratification of the convection zone, the rise results in a lateral expansion of the tube into a magnetic sheet, which acts as a reservoir for small-scale flux emergence events at the scale of granulation. The interaction of the convective downflows and the rising magnetic flux tube undulates it to form serpentine field lines that emerge into the photosphere. Observational characteristics, including the pattern of the emerging flux regions, the cancellation of surface flux and associated high-speed downflows, the convective collapse of photospheric flux tubes, the appearance of anomalous darkenings, the formation of bright points, and the possible existence of transient kilogauss horizontal fields are discussed in the context of new observations from the Hinode Solar Optical Telescope. Implications for the local helioseismology of emerging flux regions are also discussed. Title: A Robust Correlation between Growth Rate and Amplitude of Solar Cycles: Consequences for Prediction Methods Authors: Cameron, R.; Schüssler, M. Bibcode: 2008ApJ...685.1291C Altcode: We consider the statistical relationship between the growth rate of activity in the early phase of a solar cycle with its subsequent amplitude on the basis of four data sets of global activity indices (Wolf sunspot number, group sunspot number, sunspot area, and 10.7 cm radio flux). In all cases, a significant correlation is found: stronger cycles tend to rise faster. Owing to the overlapping of sunspot cycles, this correlation leads to an amplitude-dependent shift of the solar minimum epoch. We show that this effect explains the correlations underlying various so-called precursor methods for the prediction of solar cycle amplitudes and also affects the prediction tool of Dikpati et al. based on a dynamo model. Inferences as to the nature of the solar dynamo mechanism resulting from predictive schemes which (directly or indirectly) use the timing of solar minima should therefore be treated with caution. Title: 3D MHD Simulations of Sunspot Structure Authors: Rempel, M.; Schüssler, M. Bibcode: 2008ESPM...12..3.9R Altcode: We present results of a 3D MHD simulation of a sunspot with a photospheric size of about 20 Mm carried out with the MURaM MHD code. The simulation covers a time span of about 12 hours. The largely relaxed state of the sunspot shows a division in a central dark umbral region with bright dots and a penumbra showing bright filaments of about 3 to 4 Mm length with central dark lanes. By a process similar to the formation of umbral dots, the penumbral filaments result from magneto-convection in the form of upflow plumes, which become elongated by the presence of an inclined magnetic field: the upflow is deflected in the outward direction and bends down the magnetic field to become almost horizontal in the upper part of the plume near the level of optical depth unity. At the same time, roll-type motion leads to a flow perpendicular to the filament axis and to downflow near its edges. Expansion and flux expulsion leads to a strong reduction of the field strength in the upper part of the rising plume, where a dark lane forms owing to the piling up of matter near the cusp-shaped top and the upward bulging of the surfaces of constant optical depth. The simulated penumbral structure corresponds well to the observationally inferred interlocking-comb structure of the magnetic field with Evershed outflows along dark-laned filaments with nearly horizontal magnetic field and roll-type perpendicular motion, which are embedded in a background of stronger and less inclined field. Photospheric spectral lines are formed at the very top and somewhat above the upflow plumes, so that they do not fully sense the strong flow as well as the large field inclination and significant field strength reduction in the upper part of the plume structures. Title: Storage of Magnetic Flux in the Solar Convective Overshoot Region Authors: Isik, E.; Holzwarth, V.; Schüssler, M. Bibcode: 2008ESPM...12..3.3I Altcode: The stability of magnetic fields in the solar interior is of critical importance for dynamo models. Instabilities induced by external flows are relevant for the storage of magnetic field in the stably stratified lower convective overshoot region. We have investigated the effects of finite perturbations and external flows on the stability and dynamics of thin magnetic flux tubes in mechanical equilibrium in the overshoot region. We have used a model convection zone based on the non-local mixing length approximation. Numerical simulations have shown that the friction-induced flux tube instability occurs already for field strengths well below the critical value for the onset of Parker instability, though with significantly longer growth times. Above a certain amplitude of the radial perturbation, nonlinear effects introduced by the external stratification become sufficient to affect the further growth of the instability. We have carried out a systematic parameter study with flux tubes subject to external radial flows. We have obtained ranges for the maximum speed and the duration of the flow, which allow for the storage of magnetic flux tubes with near-equipartition field strengths in the overshoot region, for times comparable with the dynamo amplification time. Title: How Well Do Zeeman Measurements Reflect the Turbulent Solar Magnetic Field? Authors: Pietarila, J. Graham; Danilovic, S.; Schüssler, M. Bibcode: 2008ESPM...12.3.13P Altcode: We employ the turbulent nature of the magnetic field in the solar photosphere to constrain interpretations of Zeeman-polarimetry-based observations. Using higher Reynolds number, more turbulent, simulations of the solar convection zone than have previously been reported, we compare the distribution and cancellation statistics of the magnetic field itself with the statistics derived from simulated Stokes profiles. A favorable comparison between the cancellation statistics of the observables and the field allow for a prediction at the approximated magnetic Reynolds number of the sun, ReM = 1e6, and a comparison with Hinode observations. The difference between the probability distribution functions (PDFs) from simulations, Zeeman-based polarimetry, and interpretations of the Hanle effect are also examined and a possible explanation is suggested. Title: MHD Simulation: From the Convection Zone to the Corona and Beyond (in 30 Minutes) Authors: Schüssler, M. Bibcode: 2008ESPM...12..1.1S Altcode: The all-encompassing ab-initio simulation reaching from the tachocline out to the solar wind is not feasible within the foreseeable future. Therefore, we have to approach the problems of magnetic flux generation, transport, emergence, magneto-convective interaction, equilibrium, dynamics, and dissipation in a piecemeal fashion, focussing upon a realistic (as far as possible) description of the physics in more restricted domains. The last decade has seen significant progress along this line with large-scale simulations of the deep convection zone, of the near-surface layers and photosphere, and in the corona. It has been realized that the magnetic field is the coupling agent of all these domains and first attempts have been made to address the magnetic coupling in simulations. I will go through the various physical domains, and briefly review the respective state of the art, but mainly focus upon the open questions, problems and challenges ahead - of which there are plenty. Title: Comparison of Magnetoconvection Simulations with the Approximation of Thin Flux Tubes Authors: Yelles Chaouche, L.; Solanki, S.; Schuessler, M. Bibcode: 2008ESPM...12..3.8Y Altcode: The structure and dynamics of small vertical photospheric magnetic flux concentrations has been often treated in the framework of an approximation based upon a low-order truncation of the Taylor expansions of all quantities in the horizontal direction, together with the assumption of instantaneous total pressure balance at the boundary to the non-magnetic external medium. Formally, such an approximation is justified if the diameter of the structure (a flux tube or a flux sheet) is small compared to all other relevant length scales (scale height, radius of curvature, wavelength, etc.). The advent of realistic 3D radiative MHD simulations opens the possibility to check the consistency of the approximation with the properties of the flux concentrations that form in the course of the simulation. We make a comparative analysis between the thin flux tube/sheet model and flux concentrations existing in a 3D radiation-MHD simulation. We have found that for flux concentration well above the equipartition distribution, the MHD magnetic structures are reasonably well reproduced by the second-order thin flux tube/sheet approximation. The differences between approximation and simulation are due to the asymmetry and the dynamics of the simulated structures. Title: The intensity contrast of solar granulation: comparing Hinode SP results with MHD simulations Authors: Danilovic, S.; Gandorfer, A.; Lagg, A.; Schüssler, M.; Solanki, S. K.; Vögler, A.; Katsukawa, Y.; Tsuneta, S. Bibcode: 2008A&A...484L..17D Altcode: 2008arXiv0804.4230D Context: The contrast of granulation is an important quantity characterizing solar surface convection.
Aims: We compare the intensity contrast at 630 nm, observed using the Spectro-Polarimeter (SP) aboard the Hinode satellite, with the 3D radiative MHD simulations of Vögler & Schüssler (2007, A&A, 465, L43).
Methods: A synthetic image from the simulation is degraded using a theoretical point-spread function of the optical system, and by considering other important effects.
Results: The telescope aperture and the obscuration by the secondary mirror and its attachment spider, reduce the simulated contrast from 14.4% to 8.5%. A slight effective defocus of the instrument brings the simulated contrast down to 7.5%, close to the observed value of 7.0%.
Conclusions: A proper consideration of the effects of the optical system and a slight defocus, lead to sufficient degradation of the synthetic image from the MHD simulation, such that the contrast reaches almost the observed value. The remaining small discrepancy can be ascribed to straylight and slight imperfections of the instrument, which are difficult to model. Hence, Hinode SP data are consistent with a granulation contrast which is predicted by 3D radiation MHD simulations. Title: Strong horizontal photospheric magnetic field in a surface dynamo simulation Authors: Schüssler, M.; Vögler, A. Bibcode: 2008A&A...481L...5S Altcode: 2008arXiv0801.1250S Context: Observations with the Hinode spectro-polarimeter have revealed strong horizontal internetwork magnetic fields in the quiet solar photosphere.
Aims: We aim to interpret the observations with results from numerical simulations.
Methods: Radiative MHD simulations of dynamo action by near-surface convection are analyzed with respect to the relation between vertical and horizontal magnetic field components.
Results: The dynamo-generated fields show a clear dominance of the horizontal field in the height range where the spectral lines used for the Hinode observations are formed. The ratio between the averaged horizontal and vertical field components is consistent with the values derived from the observations. This behavior results from the intermittent nature of the dynamo field with polarity mixing on small scales in the surface layers.
Conclusions: Our results provide further evidence that local near-surface dynamo action contributes significantly to the solar internetwork fields. Title: Magnetic Flux Emergence in the Solar Photosphere Authors: Cheung, M. C. M.; Schüssler, M.; Moreno-Insertis, F. Bibcode: 2008ASPC..384..181C Altcode: 2008csss...14..181C The most prominent magnetic structures on the surface of the Sun are bipolar active regions. These magnetic complexes are comprised of a hierarchy of magnetic structures of different sizes, the largest of which are sunspots. Observations indicate that the appearance of active regions on the solar surface result from the emergence of bundles of magnetic flux from the underlying convection zone. We study the emergence process by means of 3D radiation MHD simulations. In the simulations, an initially buoyant magnetic flux tube is introduced into the near-surface layers of the convection zone. Subject to the buoyancy force, the flux tube rises towards the photosphere. Our simulations highlight the importance of magneto-convection on the evolution of the magnetic flux tube. The external convective flow field has an important influence on the emergence morphology of the emerging magnetic field. Depending on the initial properties of the magnetic flux tube (e.g. field strength, twist, entropy etc.), flux emergence may lead to a disturbance of the local granulation pattern. The observational signatures associated with emerging magnetic flux in our simulations are in qualitative and quantitative agreement with observational studies of emerging flux regions on the Sun. Title: Are solar cycles predictable? Authors: Schüssler, M. Bibcode: 2007AN....328.1087S Altcode: 2007arXiv0712.1917S Various methods (or recipes) have been proposed to predict future solar activity levels - with mixed success. Among these, some precursor methods based upon quantities determined around or a few years before solar minimum have provided rather high correlations with the strength of the following cycles. Recently, data assimilation with an advection-dominated (flux-transport) dynamo model has been proposed as a predictive tool, yielding remarkably high correlation coefficients. After discussing the potential implications of these results and the criticism that has been raised, we study the possible physical origin(s) of the predictive skill provided by precursor and other methods. It is found that the combination of the overlap of solar cycles and their amplitude-dependent rise time (Waldmeier's rule) introduces correlations in the sunspot number (or area) record, which account for the predictive skill of many precursor methods. This explanation requires no direct physical relation between the precursor quantity and the dynamo mechanism (in the sense of the Babcock-Leighton scheme or otherwise). Title: A coupled model of magnetic flux generation and transport in stars Authors: Işik, E.; Schmitt, D.; Schüssler, M. Bibcode: 2007AN....328.1111I Altcode: 2011arXiv1111.2453I We present a combined model for magnetic field generation and transport in cool stars with outer convection zones. The mean toroidal magnetic field, which is generated by a cyclic thin-layer \alpha\Omega dynamo at the bottom of the convection zone is taken to determine the emergence probability of magnetic flux tubes in the photosphere. Following the nonlinear rise of the unstable thin flux tubes, emergence latitudes and tilt angles of bipolar magnetic regions are determined. These quantities are put into a surface flux transport model, which simulates the surface evolution of magnetic flux under the effects of large-scale flows and turbulent diffusion. First results are discussed for the case of the Sun and for more rapidly rotating solar-type stars.

Movies are available via http://www.aip.de/AN/movies Title: Photospheric Magnetic Flux Emergence: A comparative study between Hinode/SOT Observations and MHD simulations Authors: Cheung, M. C.; Schüssler, M.; Moreno-Insertis, F.; Tarbell, T. D. Bibcode: 2007AGUFMSH53A1073C Altcode: With high angular resolution, high temporal cadence and a stable point spread function, the Solar Optical Telescope (SOT) onboard the Hinode satellite is the ideal instrument for the study of magnetic flux emergence and its manifestations on the solar surface. In this presentation, we focus on the development of ephemeral regions and small active regions. In many instances, SOT has been able to capture the entire emergence process from beginning to end: i.e. from the initial stages of flux appearance in granule interiors, through the intermediate stages of G-band bright point formation, and finally to the coalescence of small vertical flux elements to form pores. To investigate the physics of the flux emergence process, we performed 3D numerical MHD simulations with the MURaM code. The models are able to reproduce, and help us explain, various observational signatures of magnetic flux emergence. Title: Radiative magnetohydrodynamic simulations of solar pores Authors: Cameron, R.; Schüssler, M.; Vögler, A.; Zakharov, V. Bibcode: 2007A&A...474..261C Altcode: Context: Solar pores represent a class of magnetic structures intermediate between small-scale magnetic flux concentrations in intergranular lanes and fully developed sunspots with penumbrae.
Aims: We study the structure, energetics, and internal dynamics of pore-like magnetic structures by means of exploratory numerical simulations.
Methods: The MURaM code has been used to carry out several 3D radiative MHD simulations for pores of various sizes and with different boundary conditions.
Results: The general properties of the simulated pores (morphology, continuum intensity, magnetic field geometry, surrounding flow pattern, mean height profiles of temperature, pressure, and density) are consistent with observational results. No indications for the formation of penumbral structure are found. The simulated pores decay by gradually shedding magnetic flux into the surrounding pattern of intergranular downflows (“turbulent erosion”). When viewed under an angle (corresponding to observations outside solar disc center), granules behind the pore appear brightened.
Conclusions: Radiative MHD simulations capture many observed properties of solar pores. Title: Flow instabilities of magnetic flux tubes. II. Longitudinal flow Authors: Holzwarth, V.; Schmitt, D.; Schüssler, M. Bibcode: 2007A&A...469...11H Altcode: 2007arXiv0704.3685H Context: Flow-induced instabilities are relevant for the storage and dynamics of magnetic fields in stellar convection zones and possibly also in other astrophysical contexts.
Aims: We continue the study started in the first paper of this series by considering the stability properties of longitudinal flows along magnetic flux tubes.
Methods: A linear stability analysis was carried out to determine criteria for the onset of instability in the framework of the approximation of thin magnetic flux tubes.
Results: In the non-dissipative case, we find Kelvin-Helmholtz instability for flow velocities exceeding a critical speed that depends on the Alfvén speed and on the ratio of the internal and external densities. Inclusion of a friction term proportional to the relative transverse velocity leads to a friction-driven instability connected with backward (or negative energy) waves. We discuss the physical nature of this instability. In the case of a stratified external medium, the Kelvin-Helmholtz instability and the friction-driven instability can set in for flow speeds significantly lower than the Alfvén speed.
Conclusions: Dissipative effects can excite flow-driven instability below the thresholds for the Kelvin-Helmholtz and the undulatory (Parker-type) instabilities. This may be important for magnetic flux storage in stellar convection zones and for the stability of astrophysical jets. Title: Stokes diagnostics of simulated solar magneto-convection Authors: Shelyag, S.; Schüssler, M.; Solanki, S. K.; Vögler, A. Bibcode: 2007A&A...469..731S Altcode: 2007astro.ph..3490S We present results of synthetic spectro-polarimetric diagnostics of radiative MHD simulations of solar surface convection with magnetic fields. Stokes profiles of Zeeman-sensitive lines of neutral iron in the visible and infrared spectral ranges emerging from the simulated atmosphere have been calculated in order to study their relation to the relevant physical quantities and compare with observational results. We have analyzed the dependence of the Stokes-I line strength and width as well as of the Stokes-V signal and asymmetries on the magnetic field strength. Furthermore, we have evaluated the correspondence between the actual velocities in the simulation with values determined from the Stokes-I (Doppler shift of the centre of gravity) and Stokes-V profiles (zero-crossing shift). We confirm that the line weakening in strong magnetic fields results from a higher temperature (at equal optical depth) in the magnetic flux concentrations. We also confirm that considerable Stokes-V asymmetries originate in the peripheral parts of strong magnetic flux concentrations, where the line of sight cuts through the magnetopause of the expanding flux concentration into the surrounding convective donwflow. Title: Magnetic Flux Emergence In Granular Convection: Radiative MHD Simulations And Hinode SOT Observations Authors: Cheung, Mark; Schüssler, M.; Moreno-Insertis, F.; Tarbell, T.; SOT Team Bibcode: 2007AAS...210.9425C Altcode: 2007BAAS...39..221C We model the emergence of buoyant magnetic flux from the convection zone into the photosphere by means of 3D radiative MHD simulations using the MURaM code. In a series of simulations, we study how an initially buoyant magnetic flux tube rises in the presence of granular convection. The simulations take into account the effects of radiative energy exchange, ionization effects in the equation of state and compressibility. An emphasis of this talk is the comparison of observational diagnostics from the simulations with recent observations from Hinode SOT. Title: Magnetic flux emergence in granular convection: radiative MHD simulations and observational signatures Authors: Cheung, M. C. M.; Schüssler, M.; Moreno-Insertis, F. Bibcode: 2007A&A...467..703C Altcode: 2007astro.ph..2666C Aims:We study the emergence of magnetic flux from the near-surface layers of the solar convection zone into the photosphere.
Methods: To model magnetic flux emergence, we carried out a set of numerical radiative magnetohydrodynamics simulations. Our simulations take into account the effects of compressibility, energy exchange via radiative transfer, and partial ionization in the equation of state. All these physical ingredients are essential for a proper treatment of the problem. Furthermore, the inclusion of radiative transfer allows us to directly compare the simulation results with actual observations of emerging flux.
Results: We find that the interaction between the magnetic flux tube and the external flow field has an important influence on the emergent morphology of the magnetic field. Depending on the initial properties of the flux tube (e.g. field strength, twist, entropy etc.), the emergence process can also modify the local granulation pattern. The emergence of magnetic flux tubes with a flux of 1019 Mx disturbs the granulation and leads to the transient appearance of a dark lane, which is coincident with upflowing material. These results are consistent with observed properties of emerging magnetic flux.

Movies are only available in electronic form at http://www.aanda.org Title: Photospheric magnetoconvection Authors: Cameron, Robert; Vögler, Alexander; Schüssler, Manfred Bibcode: 2007IAUS..239..475C Altcode: No abstract at ADS Title: A solar surface dynamo Authors: Vögler, A.; Schüssler, M. Bibcode: 2007A&A...465L..43V Altcode: 2007astro.ph..2681V Context: Observations indicate that the "quiet" solar photosphere outside active regions contains considerable amounts of magnetic energy and magnetic flux, with mixed polarity on small scales. The origin of this flux is unclear.
Aims: We test whether local dynamo action of the near-surface convection (granulation) can generate a significant contribution to the observed magnetic flux.
Methods: We have carried out MHD simulations of solar surface convection, including the effects of strong stratification, compressibility, partial ionization, radiative transfer, as well as an open lower boundary.
Results: Exponential growth of a weak magnetic seed field (with vanishing net flux through the computational box) is found in a simulation run with a magnetic Reynolds number of about 2600. The magnetic energy approaches saturation at a level of a few percent of the total kinetic energy of the convective motions. Near the visible solar surface, the (unsigned) magnetic flux density reaches at least a value of about 25 G.
Conclusions: .A realistic flow topology of stratified, compressible, non-helical surface convection without enforced recirculation is capable of turbulent local dynamo action near the solar surface. Title: Solar Cycle Prediction Using Precursors and Flux Transport Models Authors: Cameron, R.; Schüssler, M. Bibcode: 2007ApJ...659..801C Altcode: 2006astro.ph.12693C We study the origin of the predictive skill of some methods to forecast the strength of solar activity cycles. A simple flux transport model for the azimuthally averaged radial magnetic field at the solar surface is used, which contains a source term describing the emergence of new flux based on observational sunspot data. We consider the magnetic flux diffusing over the equator as a predictor, since this quantity is directly related to the global dipole field from which a Babcock-Leighton dynamo generates the toroidal field for the next activity cycle. If the source is represented schematically by a narrow activity belt drifting with constant speed over a fixed range of latitudes between activity minima, our predictor shows considerable predictive skill, with correlation coefficients up to 0.95 for past cycles. However, the predictive skill is completely lost when the actually observed emergence latitudes are used. This result originates from the fact that the precursor amplitude is determined by the sunspot activity a few years before solar minimum. Since stronger cycles tend to rise faster to their maximum activity (known as the Waldmeier effect), the temporal overlapping of cycles leads to a shift of the minimum epochs that depends on the strength of the following cycle. This information is picked up by precursor methods and also by our flux transport model with a schematic source. Therefore, their predictive skill does not require a memory, i.e., a physical connection between the surface manifestations of subsequent activity cycles. Title: Magnetic flux transport on active cool stars and starspot lifetimes Authors: Işik, E.; Schüssler, M.; Solanki, S. K. Bibcode: 2007A&A...464.1049I Altcode: 2006astro.ph.12399I Context: Many rapidly rotating cool stars show signatures of large magnetic regions at all latitudes. Mid-latitude starspots and magnetic regions have characteristic lifetimes of 1 month or less, as indicated by observations using (Zeeman-) Doppler imaging techniques.
Aims: We aim to estimate the lifetimes of bipolar magnetic regions and starspots on the surfaces of cool stars. We consider different possible configurations for starspots and compare their flux variations and lifetimes based on a magnetic flux transport model.
Methods: We carry out numerical simulations of the surface evolution of bipolar magnetic regions (BMRs) and magnetic spots on stars, which have radii and surface rotational shears of AB Doradus, the Sun, and the HR 1099 primary. The surface flux transport model is based on the magnetic induction equation for radial fields under the effects of surface differential rotation, meridional flow, and turbulent diffusion due to convective flow patterns. We calculate the flux evolution and the lifetimes of BMRs and unipolar starspots, varying the emergence latitude, surface shear rate, and tilt angle.
Results: For BMRs comparable to the largest observed on the Sun, we find that varying the surface flows and the tilt angle modifies the lifetimes over a range of one month. For very large BMRs (area ~10% of the stellar surface) the assumption of a tilt angle increasing with latitude leads to a significant increase of lifetime, as compared to the case without tilt. Such regions can evolve to polar spots that live more than a year. Adopting the observed weak latitudinal shear and the radius of the active subgiant component of HR 1099, we find longer BMR lifetimes as compared to the more strongly sheared AB Dor case. Random emergence of six additional tilted bipoles in an activity belt at 60° latitude enhanced the lifetimes of polar caps up to 7 years. We have also compared the evolution and lifetime of monolithic starspots with those of conglomerates of smaller spots of similar total area. We find similar decay patterns and lifetimes for both configurations. Title: Flow instabilities of magnetic flux tubes. I. Perpendicular flow Authors: Schüssler, M.; Ferriz Mas, A. Bibcode: 2007A&A...463...23S Altcode: Context: The stability properties of filamentary magnetic structures are relevant for the storage and dynamics of magnetic fields in stellar convection zones and possibly also in other astrophysical contexts.
Aims: In a series of papers we study the effect of external and internal flows on the stability of magnetic flux tubes. In this paper we consider the effect of a flow perpendicular to a straight, horizontal flux tube embedded in a gravitationally stratified fluid. The flow acts on the flux tube by exerting an aerodynamic drag force and by modifying the pressure stratification in the background medium.
Methods: We carry out a Lagrangian linear stability analysis in the framework of the approximation of thin magnetic flux tubes.
Results: The external flow can drive monotonic and oscillatory instability (overstability). The stability condition depends on direction and magnitude of the external velocity as well as on its first and second derivatives with respect to depth. The range of the flow-driven instabilities typically extends to modes with much shorter wavelengths than for the buoyancy-driven undulatory Parker instability.
Conclusions: .Perpendicular flows with Alfvénic Mach number of order unity can drive monotonic as well as oscillatory instability of thin magnetic flux tubes. Such instability can affect the storage of magnetic flux in stellar interiors. Title: Solar mesogranulation as a cellular automaton effect Authors: Matloch, L.; Cameron, R.; Schmitt, D.; Schüssler, M. Bibcode: 2007msfa.conf..339M Altcode: We present a simple cellular automaton model of solar granulation that captures the granular cell characteristics in terms of lifetime and size distributions. We show that mesogranulation, as defined in observational data, is an intrinsic feature of such a cell system. Title: The origin of the reversed granulation in the solar photosphere Authors: Cheung, M. C. M.; Schüssler, M.; Moreno-Insertis, F. Bibcode: 2007A&A...461.1163C Altcode: 2006astro.ph.12464C Aims:We study the structure and reveal the physical nature of the reversed granulation pattern in the solar photosphere by means of 3-dimensional radiative hydrodynamics simulations.
Methods: We used the MURaM code to obtain a realistic model of the near-surface layers of the convection zone and the photosphere.
Results: The pattern of horizontal temperature fluctuations at the base of the photosphere consists of relatively hot granular cells bounded by the cooler intergranular downflow network. With increasing height in the photosphere, the amplitude of the temperature fluctuations diminishes. At a height of z=130-140 km in the photosphere, the pattern of horizontal temperature fluctuations reverses so that granular regions become relatively cool compared to the intergranular network. Detailed analysis of the trajectories of fluid elements through the photosphere reveal that the motion of the fluid is non-adiabatic, owing to strong radiative cooling when approaching the surface of optical depth unity followed by reheating by the radiation field from below. The temperature structure of the photosphere results from the competition between expansion of rising fluid elements and radiative heating. The former acts to lower the temperature of the fluid whereas the latter acts to increase it towards the radiative equilibrium temperature with a net entropy gain. After the fluid overturns and descends towards the convection zone, radiative energy loss again decreases the entropy of the fluid. Radiative heating and cooling of fluid elements that penetrate into the photosphere and overturn do not occur in equal amounts. The imbalance in the cumulative heating and cooling of these fluid elements is responsible for the reversal of temperature fluctuations with respect to height in the photosphere. Title: Magnetic flux transport and the lifetimes of spots on active cool stars Authors: Işik, E.; Schüssler, M.; Solanki, S. K. Bibcode: 2007msfa.conf..367I Altcode: We present results of numerical simulations of magnetic flux transport on the surfaces of cool stars with radii of 1 Rm and 3.3 Rm. The effects of differential rotation and the tilt angle on the lifetimes of stellar bipolar magnetic regions are discussed. The existence of long-lasting polar spots can be explained by high-latitude persistent emergence of bipolar regions. Title: Modeling the Sun's open magnetic flux Authors: Schüssler, M.; Baumann, I. Bibcode: 2006A&A...459..945S Altcode: Context: .The heliospheric magnetic field can be extrapolated from the photospheric field distribution using models based upon various approximations. Such models are required for the reconstruction of the open solar magnetic flux prior to the time of direct measurements on the basis of surface flux transport simulations.
Aims: .We evaluate the consistency of extrapolation models with direct measurements of the heliospheric magnetic field. Furthermore, we study whether extrapolations on the basis of a surface flux transport model for the photospheric magnetic field reproduce the temporal evolution of the measured near-Earth magnetic field.
Methods: .We use the potential field source surface (PFSS) model and the current sheet source surface (CSSS) model to extrapolate the heliospheric field on the basis of the Wilcox Solar Observatory (WSO) synoptic maps of the solar surface field from 1976-2005. The results are compared with the near-Earth measurements of the radial heliospheric field and its independence of latitude found with Ulysses. Furthermore, we determine extrapolations on the basis of photospheric flux distributions computed with a surface flux transport code, using as input sunspot group areas from the SOON database.
Results: .The CSSS model based upon WSO data, with a source surface located at ≥10~R⊙ and cusp surface at 1.7~R⊙, yields the best agreement with the measurements. The flux transport simulations reproduce the observed surface flux together with the open flux if the tilt angle of emerging bipolar magnetic regions is smaller than commonly assumed, but consistent with sunspot observations.
Conclusions: .The CSSS model with a source surface in the vicinity of the Alfvénic point of the solar wind leads to a good extrapolation of the heliospheric field from solar surface data. A surface flux transport model based upon sunspot data with consistently calibrated tilt angles reproduces the observed evolution of the solar total open flux. Title: Flux Emergence at the Photosphere Authors: Cheung, M. C. M.; Schüssler, M.; Moreno-Insertis, F. Bibcode: 2006ASPC..354...97C Altcode: To model the emergence of magnetic fields at the photosphere, we carried out 3D magneto-hydrodynamics (MHD) simulations using the MURaM code. Our simulations take into account the effects of compressibility, energy exchange via radiative transfer and partial ionization in the equation of state. All these physical ingredients are essential for a proper treatment of the problem. In the simulations, an initially buoyant magnetic flux tube is embedded in the upper layers of the convection zone. We find that the interaction between the flux tube and the external flow field has an important influence on the emergent morphology of the magnetic field. Depending on the initial properties of the flux tube (e.g. field strength, twist, entropy etc.), the emergence process can also modify the local granulation pattern. The inclusion of radiative transfer allows us to directly compare the simulation results with real observations of emerging flux. Title: The Dynamical Disconnection of Sunspots from their Magnetic Roots Authors: Rempel, M.; Schüssler, M. Bibcode: 2006ASPC..354..148R Altcode: After a dynamically active emergence phase, magnetic flux at the solar surface soon ceases to show strong signs of the subsurface dynamics of its parent magnetic structure. This indicates that some kind of disconnection of the emerged flux from its roots in the deep convection zone should take place. We propose a mechanism for the dynamical disconnection of the surface flux based upon the buoyant upflow of plasma along the field lines. Such flows arise in the upper part of a rising flux loop during the final phases of its buoyant ascent towards the surface. The combination of the pressure buildup by the upflow and the cooling of the upper layers of an emerged flux tube by radiative losses at the surface lead to a progressive weakening of the magnetic field in several Mm depth. When the field strength has become sufficiently low, convective motions and the fluting instability disrupt the flux tube into thin, passively advected flux fragments, thus providing a dynamical disconnection of the emerged part from its roots. We substantiate this scenario by considering the quasi-static evolution of a sunspot model under the effects of radiative cooling, convective energy transport, and pressure buildup by a prescribed inflow at the bottom of the model. For inflow speeds in the range shown by simulations of thin flux tubes, we find that the disconnection takes place in a depth between two and six Mm for disconnection times up to three days. Title: Magnetic Flux Transport on Active Cool Stars and Starspot Lifetimes Authors: Isik, E.; Schüssler, M.; Solanki, S. K. Bibcode: 2006IAUJD...8E..21I Altcode: Rapidly rotating cool stars are known to have large magnetic regions at mid- to high-latitudes. Mid-latitude starspots and magnetic regions have characteristic lifetimes on the order of one month as observed using (Zeeman-) Doppler imaging techniques. The structure and detailed morphology of starspots are not observable at present. In this study, we present numerical simulations of the surface transport of bipolar magnetic regions (BMRs) and magnetic spots on stars which have radii and surface rotational shears of AB Doradus, the Sun, and the HR 1099 primary. The surface flux transport model is based on the magnetic induction equation for radial fields under the effects of surface differential rotation, meridional flow, and turbulent diffusion due to supergranulation. We calculate flux evolution and lifetimes of BMRs with different emergence latitudes, surface shear rates, and tilt angles. For BMRs comparable to the largest ones on the Sun, we find that varying the surface flows and tilt angle modifies the lifetimes over a range of a month. For very large BMRs (area fraction ~ 0.1) the assumption of Joy's law for the tilt angle - as compared to the case with zero tilt - leads to a significant increase of lifetime. Such regions can evolve to form circumpolar spots that live more than a year. Taking the observed weak latitudinal shear and the radius of the active subgiant component of HR 1099, we find longer BMR lifetimes as compared to the more strongly sheared AB Dor case. We have also considered the effect on decay and lifetimes of starspots if they are monolithic or a conglomerate of smaller spots of similar total size. We find these different configurations differ neither in their decay patterns, nor in their lifetimes. We also give an analytical explanation for the linear decay of magnetic flux in the monolithic-spot simulation. Title: Catalysis of candidate TPS Materials for EXPERT - a Basis for TPS Design and Catalysis based in-flight Instrumentations Authors: Herdrich, G.; Auweter-Kurtz, M.; Fertig, M.; Fischer, W.; Muylaert, J. -M.; Pidan, S.; Schüssler, M.; Trabandt, U. Bibcode: 2006ESASP.631E..42H Altcode: 2006tpsh.confE..42H No abstract at ADS Title: Moving magnetic tubes: fragmentation, vortex streets and the limit of the approximation of thin flux tubes Authors: Cheung, M. C. M.; Moreno-Insertis, F.; Schüssler, M. Bibcode: 2006A&A...451..303C Altcode: Aims.We study the buoyant rise of magnetic flux tubes in a stratified layer over a range of Reynolds numbers (25 ⪉ Re ⪉ 2600) by means of numerical simulations. Special emphasis is placed on studying the fragmentation of the rising tube, its trailing wake and the formation of a vortex street in the high-Reynolds number regime. Furthermore, we evaluate the relevance of the thin flux tube approximation with regard to describing the evolution of magnetic flux tubes in the simulations.
Methods: .We used the FLASH code, which has an adaptive mesh refinement (AMR) algorithm, thus allowing the simulations to be carried out at high Reynolds numbers.
Results: .The evolution of the magnetic flux tube and its wake depends on the Reynolds number. At Re up to a few hundred, the wake consists of two counter-rotating vortex rolls. At higher Re, the vortex rolls break up and the shedding of flux into the wake occurs in a more intermittent fashion. The amount of flux retained by the central portion of the tube increases with the field line twist (in agreement with previous literature) and with Re. The time evolution of the twist is compatible with a homologous expansion of the tube. The motion of the central portion of the tube in the simulations is very well described by the thin flux tube model whenever the effects of flux loss or vortex forces can be neglected. If the flux tube has an initial net vorticity, it undergoes asymmetric vortex shedding. In this case, the lift force accelerates the tube in such a way that an oscillatory horizontal motion is super-imposed on the vertical rise of the tube, which leaves behind a vortex street. This last result is in accordance with previous simulations reported in the literature, which were carried out at lower Reynolds number.
Title: Magnetoconvection in a Sunspot Umbra Authors: Schüssler, M.; Vögler, A. Bibcode: 2006ApJ...641L..73S Altcode: 2006astro.ph..3078S Results from a realistic simulation of three-dimensional radiative magnetoconvection in a strong background magnetic field corresponding to the conditions in sunspot umbrae are shown. The convective energy transport is dominated by narrow upflow plumes with adjacent downflows, which become almost field-free near the surface layers. The strong external magnetic field forces the plumes to assume a cusplike shape in their top parts, where the upflowing plasma loses its buoyancy. The resulting bright features in intensity images correspond well (in terms of brightness, size, and lifetime) to the observed umbral dots in the central parts of sunspot umbrae. Most of the simulated umbral dots have a horizontally elongated form with a central dark lane. Above the cusp, most plumes show narrow upflow jets, which are driven by the pressure of the piled-up plasma below. The large velocities and low field strengths in the plumes are effectively screened from spectroscopic observation because the surfaces of equal optical depth are locally elevated, so that spectral lines are largely formed above the cusp. Our simulations demonstrate that nearly field-free upflow plumes and umbral dots are a natural result of convection in a strong, initially monolithic magnetic field. Title: The solar magnetic field Authors: Solanki, Sami K.; Inhester, Bernd; Schüssler, Manfred Bibcode: 2006RPPh...69..563S Altcode: 2010arXiv1008.0771S The magnetic field of the Sun is the underlying cause of the many diverse phenomena combined under the heading of solar activity. Here we describe the magnetic field as it threads its way from the bottom of the convection zone, where it is built up by the solar dynamo, to the solar surface, where it manifests itself in the form of sunspots and faculae, and beyond into the outer solar atmosphere and, finally, into the heliosphere. On the way it transports energy from the surface and the subsurface layers into the solar corona, where it heats the gas and accelerates the solar wind. Title: SUNRISE: high-resolution UV/VIS observations of the Sun from the stratosphere Authors: Solanki, S. K.; Barthol, P.; Gandorfer, A.; Schüssler, M.; Lites, B. W.; Martinez Pillet, V.; Schmidt, W.; Title, A. M. Bibcode: 2006cosp...36.2416S Altcode: 2006cosp.meet.2416S SUNRISE is a balloon-borne solar telescope with an aperture of 1m working in the UV VIS optical domain The main scientific goal of SUNRISE is to study the structure and dynamics of the magnetic field in the atmosphere of the Sun at high spatial resolution SUNRISE will provide diffraction-limited images of the photosphere and chromosphere with an unprecedented resolution down to 35km at wavelengths around 220nm Focal-plane instruments are a UV filter imager a Fabry-Perot filter magnetograph and a spectrograph polarimeter Stratospheric long-duration balloon flights of SUNRISE over the North Atlantic and or Antarctica are planned SUNRISE is a joint project of the Max-Planck-Institut fuer Sonnensystemforschung MPS Katlenburg-Lindau with the Kiepenheuer-Institut fuer Sonnenphysik KIS Freiburg the High-Altitude Observatory HAO Boulder the Lockheed-Martin Solar and Astrophysics Lab LMSAL Palo Alto and the spanish IMaX consortium The presentation will give an overview about the mission and a description of the instrumentation now at the beginning of the hardware construction phase Title: A necessary extension of the surface flux transport model Authors: Baumann, I.; Schmitt, D.; Schüssler, M. Bibcode: 2006A&A...446..307B Altcode: Customary two-dimensional flux transport models for the evolution of the magnetic field at the solar surface do not account for the radial structure and the volume diffusion of the magnetic field. When considering the long-term evolution of magnetic flux, this omission can lead to an unrealistic long-term memory of the system and to the suppression of polar field reversals. In order to avoid such effects, we propose an extension of the flux transport model by a linear decay term derived consistently on the basis of the eigenmodes of the diffusion operator in a spherical shell. A decay rate for each eigenmode of the system is determined and applied to the corresponding surface part of the mode evolved in the flux transport model. The value of the volume diffusivity associated with this decay term can be estimated to be in the range 50-100 km2 s-1 by considering the reversals of the polar fields in comparison of flux transport simulations with observations. We show that the decay term prohibits a secular drift of the polar field in the case of cycles of varying strength, like those exhibited by the historical sunspot record. Title: Simulations of Solar Pores Authors: Cameron, R.; Vögler, A.; Schüssler, M.; Zakharov, V. Bibcode: 2005ESASP.600E..11C Altcode: 2005ESPM...11...11C; 2005dysu.confE..11C No abstract at ADS Title: D Magneto-Convection and Flux Emergence in the Photosphere Authors: Cheung, M.; Schüssler, M.; Moreno-Insertis, F. Bibcode: 2005ESASP.596E..54C Altcode: 2005ccmf.confE..54C No abstract at ADS Title: Solar activity, cosmic rays, and Earth's temperature: A millennium-scale comparison Authors: Usoskin, I. G.; Schüssler, M.; Solanki, S. K.; Mursula, K. Bibcode: 2005JGRA..11010102U Altcode: Previous studies of a solar influence on climate variations have often suffered from the relatively short length of continuous direct solar observations of less than 400 years. We use two recently reconstructed series of the sunspot number and the cosmic ray flux to study this question over time intervals of up to nearly 1800 years. Comparison of the Sun-related data sets with various reconstructions of terrestrial Northern Hemisphere mean surface temperatures reveals consistently positive correlation coefficients for the sunspot numbers and consistently negative correlation coefficients for the cosmic rays. The significance levels reach up to 99% but vary strongly for the different data sets. The major part of the correlation is due to the similarity of the long-term trends in the data sets. The trend of the cosmic ray flux correlates somewhat better with the terrestrial temperature than the sunspot numbers derived from the same cosmogenic isotope data. Title: The dynamical disconnection of sunspots from their magnetic roots Authors: Schüssler, M.; Rempel, M. Bibcode: 2005A&A...441..337S Altcode: 2005astro.ph..6654S After a dynamically active emergence phase, magnetic flux at the solar surface soon ceases to show strong signs of the subsurface dynamics of its parent magnetic structure. This indicates that some kind of disconnection of the emerged flux from its roots in the deep convection zone should take place. We propose a mechanism for the dynamical disconnection of the surface flux based upon the buoyant upflow of plasma along the field lines. Such flows arise in the upper part of a rising flux loop during the final phases of its buoyant ascent towards the surface. The combination of the pressure buildup by the upflow and the cooling of the upper layers of an emerged flux tube by radiative losses at the surface lead to a progressive weakening of the magnetic field in several Mm depth. When the field strength has become sufficiently low, convective motions and the fluting instability disrupt the flux tube into thin, passively advected flux fragments, thus providing a dynamical disconnection of the emerged part from its roots. We substantiate this scenario by considering the quasi-static evolution of a sunspot model under the effects of radiative cooling, convective energy transport, and pressure buildup by a prescribed inflow at the bottom of the model. For inflow speeds in the range shown by simulations of thin flux tubes, we find that the disconnection takes place in a depth between 2 and 6 Mm for disconnection times up to 3 days. Title: Is there a phase constraint for solar dynamo models? Authors: Schüssler, M. Bibcode: 2005A&A...439..749S Altcode: 2005astro.ph..5596S The spatio-temporal relationship between the sign of the observed radial component of the magnetic field at the solar surface and the sign of the toroidal field as inferred from Hale's polarity rules for sunspots is usually interpreted as signifying the phase relation between the poloidal and the toroidal magnetic field components involved in the solar dynamo process. This has been taken as a constraint for models of the solar dynamo. This note draws attention to the fact that the observed phase relation is naturally and inevitably produced by the emergence of tilted bipolar regions and flux transport through surface flows, without any necessity of recourse to the dynamo process. Consequently, there is no constraint on dynamo models resulting from the observed phase relation. Title: Climate: How unusual is today's solar activity? (reply) Authors: Solanki, S. K.; Usoskin, I. G.; Kromer, B.; Schüssler, M.; Beer, J. Bibcode: 2005Natur.436E...4S Altcode: Muscheler et al. claim that the solar activity affecting cosmic rays was much higher in the past than we deduced from 14C measurements. However, this claim is based on a problematic normalization and is in conflict with independent results, such as the 44Ti activity in meteorites and the 10Be concentration in ice cores. Title: Comments on the structure and dynamics of magnetic fields in stellar convection zones Authors: Schuessler, Manfred Bibcode: 2005astro.ph..6050S Altcode: This paper is a ``Habilitationsschrift'', a second thesis required until recently by universities in Germany and in a few other countries to obtain the right to lecture. It was accepted by the University of Goettingen in 1990 after review by a number of german and international experts. Although the introduction and the references represent the state of research in 1990, most of the remaining content is still relevant and has never been published elsewhere. The most important part is the derivation of a linear stability formalism for thin magnetic flux tubes following an arbitrary path in a gravitationally stratified medium with a stationary velocity. It was found later (Ferriz-Mas & Schuessler, Geophys. Astrophys. Fluid Dyn. vol. 72, 209; 1995) that, for consistency, the inertial term in the equation of motion for the external medium should be included in Eq. (3.24), which leads to an additional term in the stability equations in the case of a spatially varying external velocity. This term is missing in the present text, but can be easily introduced into the formalism. The full abstract can be found at the beginning of the paper. Title: Flux tubes, surface magnetism, and the solar dynamo: constraints and open problems Authors: Schüssler, M. Bibcode: 2005AN....326..194S Altcode: The flux-tube paradigm has proven to be a remarkably useful tool to understand the connection between the dynamo process in the solar interior and its observable manifestations at the surface. After a brief review of the justification of this approach and of its successes, we discuss in some detail its loose ends and the remaining open questions - and attempt to provide some tentative answers. This includes the origin of fields much stronger than the dynamical equipartition value, the structure of the stored magnetic flux (flux tubes versus a magnetic layer) and the importance of convective pumping, as well as processes connected with flux emergence and the subsequent development of the magnetic flux. It is argued that the observations of the surface field indicate a dynamical disconnection of the emerged flux from its roots in the deep convection zone. Based on the `explosion' of magnetic flux tubes, a scenario and an illustrative model for the disconnection process are suggested. In the last part of the paper, the significance of observed properties of the surface magnetism for constraining solar dynamo models a critically discussed. It is shown that some properties of the butterfly diagram do neither confirm nor refute specific dynamo models. Furthermore, the observed phase relationship between the average toroidal and poloidal magnetic field components is shown to result from the tilt angle of bipolar magnetic regions, so that it imposes no constraint for models of the deep-seated solar dynamo. Title: Solar activity over the last 1150 years: does it correlate with climate? Authors: Usoskin, I. G.; Schüssler, M.; Solanki, S. K.; Mursula, K. Bibcode: 2005ESASP.560...19U Altcode: 2005csss...13...19U No abstract at ADS Title: Simulations of magneto-convection in the solar photosphere. Equations, methods, and results of the MURaM code Authors: Vögler, A.; Shelyag, S.; Schüssler, M.; Cattaneo, F.; Emonet, T.; Linde, T. Bibcode: 2005A&A...429..335V Altcode: We have developed a 3D magnetohydrodynamics simulation code for applications in the solar convection zone and photosphere. The code includes a non-local and non-grey radiative transfer module and takes into account the effects of partial ionization. Its parallel design is based on domain decomposition, which makes it suited for use on parallel computers with distributed memory architecture. We give a description of the equations and numerical methods and present the results of the simulation of a solar plage region. Starting with a uniform vertical field of 200 G, the processes of flux expulsion and convective field amplification lead to a dichotomy of strong, mainly vertical fields embedded in the granular downflow network and weak, randomly oriented fields filling the hot granular upflows. The strong fields form a magnetic network with thin, sheet-like structures extending along downflow lanes and micropores with diameters of up to 1000 km which form occasionally at vertices where several downflow lanes merge. At the visible surface around optical depth unity, the strong field concentrations are in pressure balance with their weakly magnetized surroundings and reach field strengths of up to 2 kG, strongly exceeding the values corresponding to equipartition with the kinetic energy density of the convective motions. As a result of the channelling of radiation, small flux concentrations stand out as bright features, while the larger micropores appear dark in brightness maps owing to the suppression of the convective energy transport. The overall shape of the magnetic network changes slowly on a timescale much larger than the convective turnover time, while the magnetic flux is constantly redistributed within the network leading to continuous formation and dissolution of flux concentrations.

Appendices A-D are only available in electronic form at http://www.edpsciences.org Title: A necessary extension of the flux transport model Authors: Baumann, I.; Schmitt, D.; Schüssler, M. Bibcode: 2005MmSAI..76..933B Altcode: 2005astro.ph.10322B Customary two-dimensional flux-transport models for the evolution of the magnetic field at the solar surface do not account for the radial structure and the volume diffusion of the magnetic field \cite{Schrijver2002}. When considering the long-term evolution of magnetic flux, this omission can lead to an unrealistic long-term memory of the system and to the suppression of polar field reversals. In order to avoid such effects, we propose an extension of the flux-transport model by a linear decay term derived consistently on the basis of the eigenmodes of the diffusion operator in a spherical shell. The value of the volume diffusivity eta associated with this term can be estimated to be in the range 50-100 km2 s-1 by considering the reversals of the polar fields in comparison of flux-transport simulations with observations. We show that the decay term prohibits a secular drift of the polar field in the case of cycles of varying strength, like those exhibited by the historical sunspot record. Title: Mechanisms of secular magnetic field variations. Authors: Solanki, S. K.; Schüssler, M. Bibcode: 2005MmSAI..76..781S Altcode: The variability of the solar magnetic field on time scales of decades and longer lies at the root of the various mechanisms by which the changing Sun could affect Earth's climate. We discuss the origin of the secular variability of both the open heliospheric flux and the total unsigned solar surface flux and review models that have been put forward to describe these variations. We propose that a combination of the effects of overlapping activity cycles and the long decay time of large-scale magnetic patterns is responsible for the secular variability of the solar magnetic field. Title: The Decay of a Simulated Pore Authors: Cameron, R.; Vögler, A.; Shelyag, S.; Schüssler, M. Bibcode: 2004ASPC..325...57C Altcode: Using MURaM -- Max-Planck Institut für Aeronomie University of Chicago Radiative Magnetohydrodynamics -- an MHD code which includes radiative transfer and partial ionization, we have studied the decay phase of a solar pore. The simulations are sufficiently realistic in their treatment of the photosphere to allow a direct comparison with observations, both current and those of upcoming missions such as SolarB. As well as discussing the structure and decay of pores, we show the formation of shallow, field aligned, convective rolls which are an important feature of our solutions. Title: Small-Scale Solar Magnetic Elements: Simulations and Observations Authors: Solanki, S. K.; Schüssler, M. Bibcode: 2004ASPC..325..105S Altcode: Both the small-scale and large-scale properties of solar features, such as sunspots and the solar corona, are influenced strongly by the small-scale structure of the underlying magnetic field. Even some global properties of the Sun, such as variations of the Sun's irradiance, depend on the local properties of small-scale magnetic features. We briefly describe these dependences, as well as recent results concerning the small-scale magnetic elements deduced from radiation MHD simulations and spectropolarimetric observations. The simulations reproduce a number of sensitive observational tests and explain, e.g. why G-band images allow only a part of the magnetic flux to be identified. Title: G-band spectral synthesis and diagnostics of simulated solar magneto-convection Authors: Shelyag, S.; Schüssler, M.; Solanki, S. K.; Berdyugina, S. V.; Vögler, A. Bibcode: 2004A&A...427..335S Altcode: Realistic simulations of radiative magneto-convection in the solar (sub)photosphere are used for a spectral synthesis of Fraunhofer's G band, which is dominated by spectral lines from the CH molecule. It is found that the spatial pattern of integrated G-band brightness closely matches the spatial structure of magnetic flux concentrations in the convective downflow regions. The brightness contrast is mainly caused by the weakening of CH lines due to the reduced CH abundance and the resulting shift of the optical depth scale in the hot and tenuous magnetic flux concentrations. Various properties of the synthetic brightness images agree well with G-band observations. These results lends credit to the observational usage of G-band bright features as proxies for magnetic flux concentrations in the solar photosphere. However, the converse is only correct in a limited sense: only a fraction of the magnetic flux concentrations turn out to be bright in the G band. Title: Evolution of the large-scale magnetic field on the solar surface: A parameter study Authors: Baumann, I.; Schmitt, D.; Schüssler, M.; Solanki, S. K. Bibcode: 2004A&A...426.1075B Altcode: Magnetic flux emerging on the Sun's surface in the form of bipolar magnetic regions is redistributed by supergranular diffusion, a poleward meridional flow and differential rotation. We perform a systematic and extensive parameter study of the influence of various parameters on the large-scale field, in particular the total unsigned surface flux and the flux in the polar caps, using a flux transport model. We investigate both, model parameters and source term properties. We identify the average tilt angle of the emerging bipolar regions, the diffusion coefficient (below a critical value), the total emergent flux and, for the polar field, the meridional flow velocity and the cycle length as parameters with a particularly large effect. Of special interest is the influence of the overlap between successive cycles. With increasing overlap, an increasing background field (minimum flux at cycle minimum) is built up, which is of potential relevance for secular trends of solar activity and total irradiance. Title: SUNRISE: high-resolution UV/VIS observations of the Sun from the stratosphere Authors: Gandorfer, Achim M.; Solanki, Sami K.; Schüssler, Manfred; Curdt, Werner; Lites, Bruce W.; Martínez Pillet, Valentin; Schmidt, Wolfgang; Title, Alan M. Bibcode: 2004SPIE.5489..732G Altcode: SUNRISE is a balloon-borne solar telescope with an aperture of 1m, working in the UV/VIS optical domain. The main scientific goal of SUNRISE is to understand the structure and dynamics of the magnetic field in the atmosphere of the Sun. SUNRISE will provide diffraction-limited images of the photosphere and chromosphere with an unpredecented resolution down to 35km at wavelengths around 220nm. Focal-plane instruments are a spectrograph/polarimeter, a Fabry-Perot filter magnetograph, and a filter imager. The first stratospheric long-duration balloon flight of SUNRISE over Antarctica is planned in winter 2006/2007. SUNRISE is a joint project of the Max-Planck-Institut fur Sonnensystemforschung (MPS), Katlenburg-Lindau, with the Kiepenheuer-Institut für Sonnenphysik (KIS), Freiburg, the High-Altitude Observatory (HAO), Boulder, the Lockheed-Martin Solar and Astrophysics Lab. (LMSAL), Palo Alto, and the Instituto de Astrofisica de Canarias, La Laguna, Tenerife. In this paper we will present an overview on the mission and give a description of the instrumentation, now, at the beginning of the hardware construction phase. Title: Unusual activity of the Sun during recent decades compared to the previous 11,000 years Authors: Solanki, S. K.; Usoskin, I. G.; Kromer, B.; Schüssler, M.; Beer, J. Bibcode: 2004Natur.431.1084S Altcode: Direct observations of sunspot numbers are available for the past four centuries, but longer time series are required, for example, for the identification of a possible solar influence on climate and for testing models of the solar dynamo. Here we report a reconstruction of the sunspot number covering the past 11,400 years, based on dendrochronologically dated radiocarbon concentrations. We combine physics-based models for each of the processes connecting the radiocarbon concentration with sunspot number. According to our reconstruction, the level of solar activity during the past 70 years is exceptional, and the previous period of equally high activity occurred more than 8,000 years ago. We find that during the past 11,400 years the Sun spent only of the order of 10% of the time at a similarly high level of magnetic activity and almost all of the earlier high-activity periods were shorter than the present episode. Although the rarity of the current episode of high average sunspot numbers may indicate that the Sun has contributed to the unusual climate change during the twentieth century, we point out that solar variability is unlikely to have been the dominant cause of the strong warming during the past three decades. Title: Does the butterfly diagram indicate a solar flux-transport dynamo? Authors: Schüssler, M.; Schmitt, D. Bibcode: 2004A&A...421..349S Altcode: 2004astro.ph..3570S We address the question whether the properties of the observed latitude-time diagram of sunspot occurrence (the butterfly diagram) provide evidence for the operation of a flux-transport dynamo, which explains the migration of the sunspot zones and the period of the solar cycle in terms of a deep equatorward meridional flow. We show that the properties of the butterfly diagram are equally well reproduced by a conventional dynamo model with migrating dynamo waves, but without transport of magnetic flux by a flow. These properties seem to be generic for an oscillatory and migratory field of dipole parity and thus do not permit an observational distinction between different dynamo approaches. Title: Approximations for non-grey radiative transfer in numerical simulations of the solar photosphere Authors: Vögler, A.; Bruls, J. H. M. J.; Schüssler, M. Bibcode: 2004A&A...421..741V Altcode: Realistic simulations of solar (magneto-)convection require an accurate treatment of the non-grey character of the radiative energy transport. Owing to the large number of spectral lines in the solar atmosphere, statistical representations of the line opacities have to be used in order to keep the problem numerically tractable. We consider two statistical approaches, the opacity distribution function (ODF) concept and the multigroup (or opacity binning) method and provide a quantitative assessment of the errors that arise from the application of these methods in the context of 2D/3D simulations. In a first step, the ODF- and multigroup methods are applied to a 1D model-atmosphere and the resulting radiative heating rates are compared. A number of 4-6 frequency bins is found to warrant a satisfactory modeling of the radiative energy exchange. Further tests in 2D model-atmospheres show the applicability of the multigroup method in realistic situations and underline the importance of a non-grey treatment. Furthermore, we address the question of an appropriate opacity average in multigroup calculations and discuss the significance of velocity gradients for the radiative heating rates. Title: Kelvin-Helmholtz and shear instability of a helical flow around a magnetic flux tube Authors: Kolesnikov, F.; Bünte, M.; Schmitt, D.; Schüssler, M. Bibcode: 2004A&A...420..737K Altcode: Magnetic flux concentrations in the solar (sub)photosphere are surrounded by strong downflows, which come into swirling motion owing to the conservation of angular momentum. While such a whirl flow can stabilize a magnetic flux tube against the MHD fluting instability, it potentially becomes subject to Kelvin-Helmholtz and shear instability near the edge of the flux tube, which may lead to twisting of the magnetic field and perhaps even to the disruption of the magnetic structure. As a first step towards studying the relevance of such instabilities, we investigate the stability of an incompressible flow with longitudinal and azimuthal (whirl) components surrounding a cylinder with a uniform longitudinal magnetic field. We find that a sharp jump of the azimuthal flow component at the cylinder boundary always leads to Kelvin-Helmholtz-type instability for sufficiently small wavelength of the perturbation. On the other hand, a smooth and wide enough transition of the azimuthal velocity towards the surface of the cylinder leads to stable configurations, even for a discontinuous profile of the longitudinal flow. Title: On the Origin of Solar Faculae Authors: Keller, C. U.; Schüssler, M.; Vögler, A.; Zakharov, V. Bibcode: 2004ApJ...607L..59K Altcode: Solar faculae appear as bright small features close to the solar limb. Recent high-resolution images show these brightenings in unprecedented detail. Our analysis of numerical MHD simulations reproduces the observed small-scale features. The simulations reveal that faculae originate from a thin layer within granules just below largely transparent magnetic flux concentrations. This is basically the ``bright wall'' model of Spruit. The dark, narrow lanes often associated with faculae occur at the opposite side of the magnetic flux concentration and are due to an extended layer with lower-than-average temperature. Title: Usoskin et al. Reply: Authors: Usoskin, Ilya G.; Solanki, Sami K.; Schüssler, Manfred; Mursula, Kalevi Bibcode: 2004PhRvL..92s9002U Altcode: A Reply to the Comment by G. M. Raisbeck and F. Yiou. Title: Distribution of magnetically confined circumstellar matter in oblique rotators Authors: Preuss, O.; Schüssler, M.; Holzwarth, V.; Solanki, S. K. Bibcode: 2004A&A...417..987P Altcode: 2004astro.ph..1599P We consider the mechanical equilibrium and stability of matter trapped in the magnetosphere of a rapidly rotating star. Assuming a dipolar magnetic field and arbitrary inclination of the magnetic axis with respect to the axis of rotation we find stable equilibrium positions a) in a (warped) disk roughly aligned with the magnetic equatorial plane and b) at two locations above and below the disk, whose distance from the star increases with decreasing inclination angle between dipole and rotation axis. The distribution of matter is not strongly affected by allowing for a spatial offset of the magnetic dipole. These results provide a possible explanation for some observations of corotating localized mass concentrations in hot magnetic stars. Title: Stokes diagnostics of magneto-convection. Profile shapes and asymmetries Authors: Khomenko, E. V.; Shelyag, S.; Solanki, S. K.; Vögler, A.; Schüssler, M. Bibcode: 2004IAUS..223..635K Altcode: 2005IAUS..223..635K We discuss the polarization signals produced in recent realistic 3D simulations of solar magnetoconvection. The Stokes profiles of the Fe I 6301.5, 6302.5, 15648 and 15652 mathrm{Å} Zeeman-sensitive spectral lines are synthesised and smeared to simulate the image degradation caused by the Earth's atmosphere and finite telescope resolution. A Principal Component Analysis approach is applied to classify the profiles. We find that the classes of Stokes V profiles as well as their amplitude and area asymmetries are very close to the observations in the network and inter-network regions. Title: Stokes Diagnostics of Magnetoconvection. Profile shapes and asymmetries. Authors: Khomenko, E. V.; Shelyag, S.; Solanki, S. K.; Vogler, A.; Schussler, M. Bibcode: 2004cosp...35.2131K Altcode: 2004cosp.meet.2131K Stokes profiles observed in the quiet Sun have a broad range of asymmetries and show a variety of shapes. These asymmetries are the result of the velocity and magnetic field gradients both in horizontal and vertical directions. We use the most recent realistic 3D simulations of magnetoconvection at the solar surface to synthesize Stokes profiles of some photospheric lines and to compare them with observations. Such comparison provides an important constrains on the MHD models allowing to conclude about their realism and, thus, to understand the nature of solar magnetoconvection. The following Zeeman-sensitive spectral lines are considered: Fe I 6301.5, 6302.5, 15648 and 15652 Å. These lines are extensively used in observations. The computed Stokes profiles of these lines were spatially smeared to simulate the effects of a telescope and atmospheric seeing. A Principal Component Analysis approach is applied to classify the profiles. The effects of spatial resolution and the amount of the magnetic flux in the MHD model on the profile shapes are discussed. The profiles of different classes are clustered together and form patches on the surface. The size of these patches decreases with increasing spatial resolution. The distributions of the amplitude and area asymmetries of Stokes V profiles are very close to the observations in network and inter-network regions. Some 15% of the profiles smeared with a 0.''5 seeing have irregular shape with 3 or more lobes. Finally, we show that simultaneous observations of the same area of the solar disc using infrared Fe I 15648, 15652 Å and the visible Fe I 6301.5, 6302.5 Å lines done under different seeing conditions (for example in the case of simultaneous observations at different telescopes) may result in different asymmetries and even different polarities of the profiles in two spectral regions observed at the same spatial point. This work was partially supported by INTAS grant 00-00084. Title: Reconstruction of solar activity for the last millennium using 10Be data Authors: Usoskin, I. G.; Mursula, K.; Solanki, S.; Schüssler, M.; Alanko, K. Bibcode: 2004A&A...413..745U Altcode: 2003astro.ph..9556U In a recent paper (Usoskin et al. 2002a), we have reconstructed the concentration of the cosmogenic 10Be isotope in ice cores from the measured sunspot numbers by using physical models for 10Be production in the Earth's atmosphere, cosmic ray transport in the heliosphere, and evolution of the Sun's open magnetic flux. Here we take the opposite route: starting from the 10Be concentration measured in ice cores from Antarctica and Greenland, we invert the models in order to reconstruct the 11-year averaged sunspot numbers since 850 AD. The inversion method is validated by comparing the reconstructed sunspot numbers with the directly observed sunspot record since 1610. The reconstructed sunspot record exhibits a prominent period of about 600 years, in agreement with earlier observations based on cosmogenic isotopes. Also, there is evidence for the century scale Gleissberg cycle and a number of shorter quasi-periodicities whose periods seem to fluctuate in the millennium time scale. This invalidates the earlier extrapolation of multi-harmonic representation of sunspot activity over extended time intervals. Title: Theoretical Models of Solar Magnetic Variability Authors: Schüssler, Manfred; Schmitt, Dieter Bibcode: 2004GMS...141...33S Altcode: Solar variability on all observationally accessible temporal and spatial scales is intimately connected with the variations of the solar magnetic field and its interaction with the non-stationary flow patterns in the convection zone. We briefly review the current status of theoretical models (of conceptual, analytical, and numerical kind) for the various manifestations of granulation to the long-term variability of the global solar cycle and its underlying dynamo mechanism. A model aiming at reconstructing the evolution of the total (unsigned) solar magnetic flux since 1610 on the basis of the group sunspot number record is discussed in the final section. Title: Solar activity and climate during the last millennium Authors: Solanki, S. K.; Usoskin, I.; Schüssler, M. Bibcode: 2004cosp...35.2535S Altcode: 2004cosp.meet.2535S The sunspot number is the longest running direct index of solar activity, with direct measurements starting in 1610. For many purposes, e.g., for comparisons with climate indices, it is still too short. We present a reconstruction of the cycle-averaged sunspot number over the last millennium based on 10Be concentrations in Greenland and Antarctic ice cores. As intermediate steps of the method, we also reconstruct the cosmic ray flux at Earth and the Sun's open magnetic flux. The reconstructions are validated by comparison with direct measurements or independent reconstructions. We also compare with records of global climate, in particular with the global temperature ("hockey stick") curve of Mann et al (1998). A reasonable agreement is found for the entire millennium, excluding only the last decades, when the two curves start diverging from each other. Title: Why Solar Magnetic Flux Concentrations Are Bright in Molecular Bands Authors: Schüssler, M.; Shelyag, S.; Berdyugina, S.; Vögler, A.; Solanki, S. K. Bibcode: 2003ApJ...597L.173S Altcode: Using realistic ab initio simulations of radiative magnetoconvection, we show that the bright structures in images taken in the ``G band,'' a spectral band dominated by lines of the CH molecule, precisely outline small-scale concentrations of strong magnetic fields on the visible solar surface. The brightening is caused by a depletion of CH molecules in the hot and tenuous magnetic structures, thus confirming the model of radiatively heated magnetic flux concentrations. These results provide a firm basis for observational studies of the evolution and dynamics of the small-scale solar magnetic field derived through ``proxy magnetometry'' with G-band images. Title: Millennium-Scale Sunspot Number Reconstruction: Evidence for an Unusually Active Sun since the 1940s Authors: Usoskin, Ilya G.; Solanki, Sami K.; Schüssler, Manfred; Mursula, Kalevi; Alanko, Katja Bibcode: 2003PhRvL..91u1101U Altcode: 2003astro.ph.10823U The extension of the sunspot number series backward in time is of considerable interest for dynamo theory, solar, stellar, and climate research. We have used records of the 10Be concentration in polar ice to reconstruct the average sunspot activity level for the period between the year 850 to the present. Our method uses physical models for processes connecting the 10Be concentration with the sunspot number. The reconstruction shows reliably that the period of high solar activity during the last 60years is unique throughout the past 1150years. This nearly triples the time interval for which such a statement could be made previously. Title: Sunrise: balloon-borne high-resolution observation of the Sun Authors: Schüssler, M.; Sunrise Team Bibcode: 2003ESASP.530..279S Altcode: 2003erbp.conf..279S Sunrise is a balloon-borne solar telescope with an aperture of 1 m, a project developed in cooperation between institutes in Germany, Spain, and the USA. The main scientific goal of Sunrise is to understand the structure and dynamics of the magnetic field in the atmosphere of the Sun. Sunrise will provide diffraction-limited images of the photosphere and chromosphere with an unpredecented resolution down to 35 km at wavelengths around 200 nm. Focal-plane instruments are a spectrograph/polarimeter, a Fabry-Perot filter magnetograph, and a filter imager. The first long-duration flight of Sunrise over Antarctica is planned in winter 2006/2007. Title: Dynamics of magnetic flux tubes in close binary stars. I. Equilibrium and stability properties Authors: Holzwarth, V.; Schüssler, M. Bibcode: 2003A&A...405..291H Altcode: 2003astro.ph..4496H Surface reconstructions of active close binary stars based on photometric and spectroscopic observations reveal non-uniform starspot distributions, which indicate the existence of preferred spot longitudes (with respect to the companion star). We consider the equilibrium and linear stability of toroidal magnetic flux tubes in close binaries to examine whether tidal effects are capable to initiate the formation of rising flux loops at preferred longitudes near the bottom of the stellar convection zone. The tidal force and the deviation of the stellar structure from spherical symmetry are treated in lowest-order perturbation theory assuming synchronised close binaries with orbital periods of a few days. The frequency, growth time, and spatial structure of linear eigenmodes are determined by a stability analysis. We find that, despite their small magnitude, tidal effects can lead to a considerable longitudinal asymmetry in the formation probability of flux loops, since the breaking of the axial symmetry due to the presence of the companion star is reinforced by the sensitive dependence of the stability properties on the stellar stratification and by resonance effects. The orientation of preferred longitudes of loop formation depends on the equilibrium configuration and the wave number of the dominating eigenmode. The change of the growth times of unstable modes with respect to the case of a single star is very small. Title: The Kelvin-Helmholz and Shear Instabilities of a Vortex Flow Around a Magnetic Flux Tube Authors: Kolesnikov, Fedor; Schüssler, Manfred Bibcode: 2003ANS...324R..64K Altcode: 2003ANS...324..I05K No abstract at ADS Title: Long-Term Cosmic Ray Intensities: Physical Reconstruction Authors: Usoskin, I. G.; Mursula, K.; Solanki, S. K.; Schuessler, M.; Kovaltsov, G. A. Bibcode: 2003ICRC....7.4041U Altcode: 2003ICRC...28.4041U Solanki et al. (2000) have recently calculated the open solar magnetic flux for the last 400 years from sunspot data. Using this reconstructed magnetic flux as an input to a simple spherically symmetric quasi-steady state model of the heliosphere, we calculate the expected differential spectra and integral intensity of galactic cosmic rays at the Earth's orbit since 1610. The calculated cosmic ray integral intensity is in good agreement with the neutron monitor measurements during the last 50 years. Moreover, using the specific yield function of cosmogenic 10 Be radionuclide production in the atmosphere, we also calculate the expected 10 Be production rate which exhibits an excellent agreement with the actual 10 Be abundance in polar ice over the last 400 years. Here we present a physical model for the long-term reconstruction of cosmic ray intensity at 1 AU. The reconstruction is based on a combination of the solar magnetic flux model and a heliospheric model. This model allows us to calculate the expected intensity of galactic cosmic rays (GCR) at the Earth's orbit for the last 400 years. Details can be found in [25]. Using the numerical recip e of Solanki et al. [21] and the group sunspot number series (Fig. 1.a) [11] we have calculated the open solar magnetic flux Fo since 1610 as shown in Fig. 1.b. In order to calculate galactic cosmic ray (GCR) spectra we use a spherically symmetric quasi-steady sto chastic simulation model described in detail elsewhere [24], which reliably describes the long-term GCR modulation during the last 50 years. In this model, the most important parameter of the heliospheric modulation of GCR is the modulation strength [10]: Φ = (D - rE )V /(3κo), where D = 100 AU is the heliospheric boundary and rE = 1 AU, V = 400 km/s is the constant solar wind velocity and κo is the rigidity indep endent part of the diffusion coefficient. Thus, all changes in the modulation strength Φ in our model are related to the changing diffusion Title: Dynamics of magnetic flux tubes in close binary stars. II. Nonlinear evolution and surface distributions Authors: Holzwarth, V.; Schüssler, M. Bibcode: 2003A&A...405..303H Altcode: 2003astro.ph..4498H Observations of magnetically active close binaries with orbital periods of a few days reveal the existence of starspots at preferred longitudes (with respect to the direction of the companion star). We numerically investigate the non-linear dynamics and evolution of magnetic flux tubes in the convection zone of a fast-rotating component of a close binary system and explore whether the tidal effects are able to generate non-uniformities in the surface distribution of erupting flux tubes. Assuming a synchronised system with a rotation period of two days and consisting of two solar-type components, both the tidal force and the deviation of the stellar structure from spherical shape are considered in lowest-order perturbation theory. The magnetic field is initially stored in the form of toroidal magnetic flux rings within the stably stratified overshoot region beneath the convection zone. Once the field has grown sufficiently strong, instabilities initiate the formation of rising flux loops, which rise through the convection zone and emerge at the stellar surface. We find that although the magnitude of tidal effects is rather small, they nevertheless lead to the formation of clusters of flux tube eruptions at preferred longitudes on opposite sides of the star, which result from the cumulative and resonant character of the action of tidal effects on rising flux tubes. The longitude distribution of the clusters depends on the initial parameters of flux tubes in the overshoot region like magnetic field strength and latitude, implying that there is no globally unique preferred longitude along a fixed direction. Title: SUNRISE: Balloon-borne High-Resolution Observation of the Sun Authors: Solanki, S. K.; Curdt, W.; Gandorfer, A.; Schüssler, M.; Lites, B. W.; Martinez Pillet, V.; Schmidt, W.; Title, A. M.; Sunrise Team Bibcode: 2003ANS...324..113S Altcode: 2003ANS...324..P20S No abstract at ADS Title: Magnetic flux tubes and the dynamo problem Authors: Schüssler, Manfred; Ferriz-Mas, Antonio Bibcode: 2003and..book..123S Altcode: 2003eclm.book..123S The observed properties of the magnetic field in the solar photosphere and theoretical studies of magneto-convection in electrically well-conducting fluids suggest that the magnetic field in stellar convection zones is quite inhomogeneous: magnetic flux is concentrated into magnetic flux tubes embedded in significantly less magnetized plasma. Such a state of the magnetic field potentially has strong implications for stellar dynamo theory since the dynamics of an ensemble of flux tubes is rather different from that of a more uniform field and new phenomena like magnetic buoyancy appear.

If the diameter of a magnetic flux tube is much smaller than any other relevant length scale, the MHD equations governing its evolution can be considerably simplified in terms of the thin-flux-tube approximation. Studies of thin flux tubes in comparison with observed properties of sunspot groups have led to far-reaching conclusions about the nature of the dynamo-generated magnetic field in the solar interior. The storage of magnetic flux for periods comparable to the amplification time of the dynamo requires the compensation of magnetic buoyancy by a stably stratified medium, a situation realized in a layer of overshooting convection at the bottom of the convection zone. Flux tubes stored in mechanical force equilibrium in this layer become unstable with respect to an undular instability once a critical field strength is exceeded, flux loops rise through the convection zone and erupt as bipolar magnetic regions at the surface. For parameter values relevant for the solar case, the critical field strength is of the order of 105 G. A field of similar strength is also required to prevent the rising unstable flux loops from being strongly deflected poleward by the action of the Coriolis force and also from `exploding' in the middle of the convection zone. The latter process is caused by the superadiabatic stratification.

The magnetic energy density of a field of 105 G is two orders of magnitude larger than the kinetic energy density of the convective motions in the lower solar convection zone. This raises serious doubts whether the conventional turbulent dynamo process based upon cyclonic convection can work on the basis of such a strong field. Moreover, it is unclear whether solar differential rotation is capable of generating a toroidal magnetic field of 105 G; it is conceivable that thermal processes like an entropy-driven outflow from exploded flux tubes leads to the large field strength required.

The instability of magnetic flux tubes stored in the overshoot region suggests an alternative dynamo mechanism based upon growing helical waves propagating along the tubes. Since this process operates only for field strengths exceeding a critical value, such a dynamo can fall into a `grand minimum' once the field strength is globally driven below this value, for instance by magnetic flux pumped at random from the convection zone into the dynamo region in the overshoot layer. The same process may act as a (re-)starter of the dynamo operation. Other non-conventional dynamo mechanisms based upon the dynamics of magnetic flux tubes are also conceivable. Title: SUNRISE: a balloon-borne telescope for high resolution solar observations in the visible and UV Authors: Solanki, Sami K.; Gandorfer, Achim M.; Schuessler, Manfred; Curdt, W.; Lites, Bruce W.; Martinez-Pillet, Valentin; Schmidt, Wolfgang; Title, Alan M. Bibcode: 2003SPIE.4853..129S Altcode: Sunrise is a light-weight solar telescope with a 1 m aperture for spectro-polarimetric observations of the solar atmosphere. The telescope is planned to be operated during a series of long-duration balloon flights in order to obtain time series of spectra and images at the diffraction-limit and to study the UV spectral region down to ~200 nm, which is not accessible from the ground. The central aim of Sunrise is to understand the structure and dynamics of the magnetic field in the solar atmosphere. Through its interaction with the convective flow field, the magnetic field in the solar photosphere develops intense field concentrations on scales below 100 km, which are crucial for the dynamics and energetics of the whole solar atmosphere. In addition, Sunrise aims to provide information on the structure and dynamics of the solar chromosphere and on the physics of solar irradiance changes. Sunrise is a joint project of the Max-Planck-Institut fuer Aeronomie (MPAe), Katlenburg-Lindau, with the Kiepenheuer-Institut fuer Sonnenphysik (KIS), Freiburg, the High-Altitude Observatory (HAO), Boulder, the Lockheed-Martin Solar and Astrophysics Lab. (LMSAL), Palo Alto, and the Instituto de Astrofi sica de Canarias, La Laguna, Tenerife. In addition, there are close contacts with associated scientists from a variety of institutes. Title: Studying magneto-convection by numerical simulation Authors: Vögler, A.; Schüssler, M. Bibcode: 2003AN....324..399V Altcode: Following a brief overview of the two main approaches to investigate the interaction between magnetic fields and convective flows near the solar surface layers by numerical simulation, namely idealized model problems and `realistic' large-eddy simulations, we present first results obtained with a newly developed MHD code. The first example concerns the realistic simulation of the magnetic field dynamics in a solar plage region while the second example demonstrates small-scale dynamo action in idealized compressible convection. Title: Simulation of Solar Magnetoconvection Authors: Vögler, A.; Shelyag, S.; Schüssler, M.; Cattaneo, F.; Emonet, T.; Linde, T. Bibcode: 2003IAUS..210..157V Altcode: No abstract at ADS Title: MHD Simulations: What's Next? Authors: Schüssler, M. Bibcode: 2003ASPC..307..601S Altcode: No abstract at ADS Title: Search for a relationship between solar cycle amplitude and length Authors: Solanki, S. K.; Krivova, N. A.; Schüssler, M.; Fligge, M. Bibcode: 2002A&A...396.1029S Altcode: The cross-correlation between time series of solar cycle length and amplitude suggests that the length precedes the amplitude. The relationship between the two is found to be more complex than a simple lag or phase shift, however. A simple empirical model is constructed which allows the amplitude of a given cycle to be predicted with relatively high accuracy from the lengths of earlier cycles. This result not only adds to the means at our disposal for predicting the amplitudes of future cycles, but also implies that the solar dynamo carries a memory of the length of one cycle over into the next. It may also have a bearing on why solar cycle length correlates better with the Earth's temperature record than cycle amplitude (Friis-Christensen & Lassen \cite{Friis-Christensen:Lassen:1991}). Thoughts on possible physical causes are presented. Title: A physical reconstruction of cosmic ray intensity since 1610 Authors: Usoskin, Ilya G.; Mursula, Kalevi; Solanki, Sami K.; Schüssler, Manfred; Kovaltsov, Gennady A. Bibcode: 2002JGRA..107.1374U Altcode: The open solar magnetic flux has been recently reconstructed by [2000, 2002] for the last 400 years from sunspot data. Using this reconstructed magnetic flux as an input to a spherically symmetric quasi-steady state model of the heliosphere, we calculate the expected intensity of galactic cosmic rays at the Earth's orbit since 1610. This new, physical reconstruction of the long-term cosmic ray intensity is in good agreement with the neutron monitor measurements during the last 50 years. Moreover, it resolves the problems related to previous reconstruction for the last 140 years based on linear correlations. We also calculate the flux of 2 GeV galactic protons and compare it to the cosmogenic 10Be level in polar ice in Greenland and Antarctica. An excellent agreement between the calculated and measured levels is found over the last 400 years. Title: Sunrise: a 1-m balloon borne solar telescope Authors: Solanki, S. K.; Schüssler, M.; Curdt, W.; Lites, B. W.; Martinez Pillet, V.; Schmidt, W.; Title, A. M.; Sunrise Team Bibcode: 2002ESASP.505...27S Altcode: 2002solm.conf...27S; 2002IAUCo.188...27S Sunrise is a light-weight solar telescope with a 1 m aperture for spectro-polarimetric observations of the solar atmosphere. The telescope is planned to be operated during a series of long-duration balloon flights in order to obtain time series of spectra and images at the diffraction-limit and to study the UV spectral region down to ≅200 nm, which is not accessible from the ground. The central aim of Sunrise is to understand the structure and dynamics of the magnetic field in the solar atmosphere. Interacting with the convective flow field, the magnetic field in the solar photosphere develops intense field concentrations on scales below 100 km, which are crucial for the dynamics and energetics of the whole solar atmosphere. In addition, Sunrise aims to provide information on the structure and dynamics of the solar chromosphere and on the physics of solar irradiance changes. Title: The formation of sunspots and starspots Authors: Schüssler, M. Bibcode: 2002AN....323..377S Altcode: The spatial scale of large solar active regions and many of their observed features indicate that they originate from the emergence of a coherent magnetic structure of rather well-ordered toroidal magnetic flux in the solar interior. Numerical simulations of the instability of magnetic flux tubes and their rise through the convection zone are consistent with the basic observed properties of sunspot groups like low emergence latitudes, tilt angles with respect to the east-west direction, and proper motions. The success of this approach in the case of the Sun motivates its application to other magnetically active stars. The effect of the Coriolis force on rising magnetic flux tubes provides an explanation for the existence of high-latitude spots on rapidly rotating active stars. Detailed models have been developed for the distribution of flux emergence latitudes depending on the rotation rate and internal structure of cool stars in various evolutionary stages. The models for very young (T-Tauri-like) stars exhibit extended latitude ranges of flux emergence, including the poles. In the case of rapidly rotating main-sequence stars, the flux tubes appear in mid to high latitudes; neither equatorial nor truly polar flux emergence is found. The trapping of flux tubes in giants with a sufficiently small (relative) core size suggests an explanation for the strong decline of X-ray emission of cool giants across the `coronal dividing line' in the Hertzsprung-Russell diagram. We give an overview of the solar and stellar results and discuss the benefits and the limitations of applying the solar paradigm to active stars. Title: Do tidal effects determine the spot distribution on active binaries? Authors: Holzwarth, V.; Schüssler, M. Bibcode: 2002AN....323..399H Altcode: Observations of spot distributions on active binaries indicate the existence of preferred longitudes, which are more pronounced if the system period is shorter. This suggests that this effect is caused by the proximity of the companion star. We explore whether preferred longitudes can be explained by tidal effects on the dynamics of magnetic flux tubes, which are thought to give rise to starspots upon emergence at the stellar surface. We find that in the case of fast-rotating binaries flux tubes erupting at mid latitudes show indeed a considerably non-uniform longitudinal distribution. Title: Thermal properties of magnetic flux tubes. I. Solution of the diffusion problem Authors: Moreno-Insertis, F.; Schüssler, M.; Glampedakis, K. Bibcode: 2002A&A...388.1022M Altcode: The heat flow and temperature structure within and surrounding a magnetic flux tube stored in mechanical equilibrium in a stellar convection zone are considered. The stationary thermal equilibrium state is determined through the analytical solution of a two-dimensional heat diffusion problem for an infinitely long cylinder with different thermal conductivities inside and outside the cylinder, both spatially variable. In the exterior of the cylinder, convective heat transport is approximated in terms of a linear diffusive process, while in its interior convection is assumed to be suppressed and only the much smaller radiative conductivity remains. The results show that, under the conditions prevailing near the bottom of the solar convection zone and in the limit of small cylinder radius, the temperature disturbance (thermal shadow) in the exterior of the insulating cylinder is almost negligible due to the large effiency of convective energy transport. The spatial dependence of the conductivities and the curvature of the external temperature profile lead to a temperature excess in the interior with respect to the undisturbed temperature profile far away from the cylinder. We show that, within the framework of the thin magnetic flux tube approximation, this temperature excess is due to a heating term equal to the negative divergence of the undisturbed radiative heat flow, as suggested earlier by Fan & Fisher (\cite{Fan:Fisher:1996}). These results are independent of the treatment of the convective transport in the exterior as long as the stratification is almost adiabatic. The consequences for the storage of magnetic flux in the solar convection zone, brought about by the enhanced buoyancy and caused by the heating effect, are discussed. Title: Structure of the magnetic field in the lower convection zone Authors: Schüssler, Manfred; Rempel, Matthias Bibcode: 2002ESASP.508..499S Altcode: 2002soho...11..499S The properties of the magnetic field and the convective flows near the base of the solar convection zone are crucial for understanding the working of the solar dynamo. We consider three aspects of this complex problem. (I) Magnetic flux needs to be stored against buoyant loss for a sufficiently long time in order to be amplified by the dynamo process. Convective pumping in strongly stratified convection is probably not sufficient for the strong fields (of order 105G) which have been inferred from the simulations of rising flux tubes. The required subadiabatically stratified storage region is likely to be generated by the asymmetric flow field (strong coherent downflows, weak upflows) characteristic for compressible convection in a stratified medium. (II) In a weakly subadiabatic region or a convective overshoot layer, the force equilibrium of a magnetic layer is very similar to that of an isolated flux tube: zero buoyancy and balance between the magnetic curvature (tension) force and the Coriolis force induced by a longitudinal flow along the field lines in a rotating system. In a strongly subadiabatic radiative region, a magnetic layer develops a different kind of force equilibrium, which involves buoyancy and a latitudinal pressure gradient. (III) A field of 105G is difficult to generate by convection or differential rotation. The outflow of plasma from an "exploded" flux tube provides an intensification mechanisms which is not limited by the Lorentz force and converts potential energy of a superadiabatic stratification into magnetic energy. Title: Magnetic variability of the Sun Authors: Schüssler, Manfred Bibcode: 2002ESASP.477....3S Altcode: 2002scsw.conf....3S The solar magnetic field is variable on all temporal and spatial scales which are accessible to observation. Convective motions and magnetic flux interact on timescales of minutes (granulation) to days (supergranulation) while the evolution of active regions and the global transport of magnetic flux on the solar surface takes place on timescales ranging from days to years. The solar activity cycle and its long-term modulation vary on scales of decades to centuries. This contribution gives a brief overview about what is known about solar magnetic variability and what is our present understanding of the underlying physics. In addition, a simple model for the secular variation of the magnetic flux on the solar surface since the Maunder minimum is presented. Title: Secular variation of the Sun's magnetic flux Authors: Solanki, S. K.; Schüssler, M.; Fligge, M. Bibcode: 2002A&A...383..706S Altcode: We present an extension of the model of \citet{Solanki:etal:2000} that allows us to reconstruct the time evolution of both the total and the open magnetic flux at the solar surface since 1700. The flux emerging in large active regions is determined using the sunspot number as a proxy, while the flux emergence in small ephemeral regions is described by an extended cycle whose amplitude and length are related to the corresponding sunspot cycle. Both types of regions contribute to the open flux, which is the source of the heliospheric field. The overlap of the activity cycles of ephemeral regions leads to a secular variation of the total cycle-related magnetic flux (active region flux + ephemeral region flux + open flux). The model results indicate that the total surface flux has doubled in the first half of the last century. The evolution of the open flux is in good agreement with the reconstruction by \citet{Lockwood:etal:1999}. Title: Reconstruction of Cosmic Ray Intensity Since 1610 Authors: Usoskin, I. G.; Mursula, K.; Solanki, S. K.; Schüssler, M.; Kovaltsov, G. A. Bibcode: 2002EGSGA..27.5173U Altcode: Open solar magnetic flux has been recently reconstructed by Solanki et al. (2000, 2002) for the last 400 years from sunspot data. Using this reconstructed magnetic flux as an input to a spherically symmetric quasi-steady state model of the heliosphere, we calculate the expected intensity of galactic cosmic rays at the Earth's orbit since 1610. This calculated cosmic ray intensity is in good agreement with the neutron monitor measurements during the last 50 years. Moreover, we calculate the flux of 2 GeV galactic protons and compare it to the cosmogenic 10Be level in polar ice in Greenland and Antarctica. An excellent agreement between the calculated and actual levels is found over the last 400 years. Title: Physical reconstruction of long-term solar activity Authors: Usoskin, I.; Solanki, S.; Schuessler, M.; Mursula, K.; Kovaltsov, G. Bibcode: 2002cosp...34E.901U Altcode: 2002cosp.meetE.901U For many applications in dynamo theory and solar-terrestrial research it is important to know the evolution of solar activity on long time scales (centuries to millennia). Previous reconstructions were based upon either multi-harmonic backward extrapolation of the known sunspot records or on the assumption of a linear relation between terrestrial proxies (e.g., cosmogenic isotope abundance) and solar activity. Here we present, for the first time, a physical reconstruction of sunspot activity on long time scales from the cosmogenic 10 Be records. We use a numerical inversion of a combined physical solar-heliospheric model (Usoskin et al., 2002), which is essentially non-linear. Using physical rather than empirical relations on all steps, we present a reconstruction of sunspot activity since the 15th century. Uncertainties of the reconstruction are discussed in details. It is important that the current high level of sunspot activity is unique on the millennium time scale. Title: Buried magnetic flux tubes in giant stars near the ``Coronal Dividing Line'' Authors: Holzwarth, V.; Schüssler, M. Bibcode: 2001A&A...377..251H Altcode: We apply the ``solar paradigm'' for stellar magnetic activity to the post-main-sequence evolution of stars in the mass range 1 Msun <= Mstar <= 3 Msun. The model starts from a strong toroidal magnetic field generated by a dynamo working in the overshoot layer below the convection envelope. Once a critical field strength is exceeded, an undulatory (Parker-type) instability leads to rising flux loops. Upon emergence at the stellar surface, they form bipolar magnetic regions and large-scale coronal loops. By considering the stability, dynamics, and rise of magnetic flux tubes along evolutionary sequences of stellar models, we find that the flux loops become trapped in the stellar interior when the depth of convective envelope exceeds about 80% of the stellar radius. Trapping is caused by an increase of field line curvature at the loop summit, so that eventually the magnetic tension force dominates over the buoyancy force. The magnetic loops find a stable equilibrium configuration within the convection zone and do not emerge at the stellar surface. The transition from emerging to trapped flux tubes falls in the range of spectral types G7 to K0 for luminosity class III giants, which is close to the observed ``coronal dividing line'' in the Hertzsprung-Russell diagram. This result is remarkably stable within large ranges of stellar parameters (mass, rotation) and flux tube parameters (field strength, magnetic flux) and depends practically exclusively on the fractional radius of the stellar radiative core. We suggest that flux tube trapping is the cause for the strong decline of stellar X-ray emission across the ``coronal dividing line''. Title: Intensification of Magnetic Fields by Conversion of Potential Energy Authors: Rempel, M.; Schüssler, M. Bibcode: 2001ApJ...552L.171R Altcode: A strong superequipartition magnetic field strength on the order of 10 T (105 G) has been inferred at the bottom of the solar convection zone. We show that the ``explosion'' of weak magnetic flux tubes, which is caused by a sudden loss of pressure equilibrium in the flux loop rising through the superadiabatically stratified convection zone, provides a mechanism that leads to a strong field: the flow of high-entropy material out of the exploded loop leads to a significant intensification of the magnetic field in the underlying flux sheet at the bottom. In contrast to the amplification by differential rotation, this process converts the potential energy of the stratification into magnetic energy and thus is not dynamically limited by the back-reaction on the flow field via the Lorentz force. Title: The Formation of One-Lobed Stokes V Profiles in an Inhomogeneous Atmosphere Authors: Ploner, S. R. O.; Schussler, M.; Solanki, S. K.; Sheminova, V. A.; Gadun, A. S.; Frutiger, C. Bibcode: 2001ASPC..236..371P Altcode: 2001aspt.conf..371P We assess the diagnostic potential of the observed pathological Stokes V profiles that differ strongly from the customary, nearly antisymmetric two-lobed shape. In particular, we consider the formation of one-lobed Stokes V profiles using the results of an MHD simulation. We find that the majority of one-lobed profiles is produced in regions of weak horizontal field with significant cancellation caused by mixed polarity along the line of sight. A minority of one-lobed profiles originates close to strong magnetic field concentrations with strong gradients of velocity and magnetic field strength. Title: Magneto-Convection Authors: Schüssler, M.; Knölker, M. Bibcode: 2001ASPC..248..115S Altcode: 2001mfah.conf..115S No abstract at ADS Title: A Model for the Decline of Coronal X-ray Emission of Cool Giant Stars Authors: Holzwarth, V.; Schüssler, M.; Solanki, S. K. Bibcode: 2001ASPC..248..259H Altcode: 2001mfah.conf..259H No abstract at ADS Title: Numerical Simulation of Solar Magneto-Convection Authors: Schüssler, M. Bibcode: 2001ASPC..236..343S Altcode: 2001aspt.conf..343S No abstract at ADS Title: An Example of Reconnection and Magnetic Flux Recycling near the Solar Surface Authors: Ploner, S. R. O.; Schüssler, M.; Solanki, S. K.; Gadun, A. S. Bibcode: 2001ASPC..236..363P Altcode: 2001aspt.conf..363P No abstract at ADS Title: Preferred Longitudes of Starspots on Magnetically Active Close Binaries Authors: Holzwarth, V.; Schüssler, M. Bibcode: 2001ASPC..248..247H Altcode: 2001mfah.conf..247H No abstract at ADS Title: Intensification of Magnetic Field in a Stellar Convection Zone by Conversion of Potential Energy Authors: Rempel, M.; Schüssler, M. Bibcode: 2001ASPC..248..165R Altcode: 2001mfah.conf..165R No abstract at ADS Title: Buried Flux Tubes in the Coronal Graveyard (CD-ROM Directory: contribs/schussle) Authors: Schüssler, M.; Holzwarth, V.; Solanki, S. K.; Charbonnel, C. Bibcode: 2001ASPC..223.1114S Altcode: 2001csss...11.1114S No abstract at ADS Title: Storage of a Strong Magnetic Field Below the Solar Convection Zone (CD-ROM Directory: contribs/rempel) Authors: Rempel, M.; Schüssler, M.; Moreno-Insertis, F.; Tóth, G. Bibcode: 2001ASPC..223..738R Altcode: 2001csss...11..738R No abstract at ADS Title: High-resolution Solar Polarimetry with Sunrise Authors: Schmidt, W.; Solanki, S. K.; Schüssler, M.; Curdt, W.; Lites, B. W.; Title, A. M.; Martinez Pillet, V. Bibcode: 2001AGM....18S1001S Altcode: Sunrise is a 1m balloon-borne solar telescope. It is equipped with a spectrograph polarimeter which combines vector-polarimetry in the visible with diagnostic spectroscopy in the visible and the UV, down to 200 nm. The instrumentation includes a filter-magnetograph and a medium-band filtergraph. The wavelength bands of the latter include the CH-band (430.6 nm) and a UV continuum at 205 nm. Diffraction limited resolution in the UV will be achieved by employing a phase diversity technique. The main telescope is based on a lightweight silicon-carbide mirror, developed within the Solar Lite program. During the long-duration flight at Antarctica, foreseen for late 2005, Sunrise will continuously observe the sun for a period of about ten days, with constant image quality across the full field of view. In-flight alignment of the telescope optics will be controlled by a wavefront sensor. The main goal of Sunrise is to understand the structure and dynamics of the magnetic field in the atmosphere of the sun. To this end, Sunrise will observe small magnetic flux concentrations with dimensions of less than 70 km with high polarimetric accuracy. At the same time, Sunrise will provide diffraction-limited filtergrams of the photosphere and chromosphere with a resolution down to 35 km at a wavelength of 200 nm. Title: Star Spot Patterns on Young Stars: Theoretical Approach (CD-ROM Directory: contribs/granzer2) Authors: Granzer, Th.; Caligari, P.; Schüssler, M.; Strassmeier, K. G. Bibcode: 2001ASPC..223.1232G Altcode: 2001csss...11.1232G No abstract at ADS Title: Evolution of the Sun's large-scale magnetic field since the Maunder minimum Authors: Solanki, S. K.; Schüssler, M.; Fligge, M. Bibcode: 2000Natur.408..445S Altcode: The most striking feature of the Sun's magnetic field is its cyclic behaviour. The number of sunspots, which are dark regions of strong magnetic field on the Sun's surface, varies with a period of about 11 years. Superposed on this cycle are secular changes that occur on timescales of centuries and events like the Maunder minimum in the second half of the seventeenth century, when there were very few sunspots. A part of the Sun's magnetic field reaches out from the surface into interplanetary space, and it was recently discovered that the average strength of this interplanetary field has doubled in the past 100 years. There has hitherto been no clear explanation for this doubling. Here we present a model describing the long-term evolution of the Sun's large-scale magnetic field, which reproduces the doubling of the interplanetary field. The model indicates that there is a direct connection between the length of the sunspot cycle and the secular variations. Title: Physical Causes of Solar Variability - Discussion Session 1b Authors: Kuhn, J. R.; Schüssler, M. Bibcode: 2000SSRv...94..177K Altcode: This report is divided into three parts: Section 1 gives a short introduction and a summary of the topics discussed. Section 2 is a position statement by J. Kuhn on the interpretation of the irradiance measurements, while Section 3 gives a position statement by M. Schüssler discussing observations of stars that could be useful for understanding solar variability. Title: Storage of magnetic flux at the bottom of the solar convection zone Authors: Rempel, M.; Schüssler, M.; Tóth, G. Bibcode: 2000A&A...363..789R Altcode: We consider the mechanical equilibrium of a layer of axisymmetric toroidal magnetic field located in a subadiabatically stratified region near the bottom of the solar convection zone, with particular emphasis on the effects of spherical geometry. We determine equilibrium configurations and simulate numerically how these are reached from a non-equilibrium initial situation. While a subadiabatic stratification is essential for suppressing the buoyancy force, the latitudinal component of the magnetic curvature force is balanced by a latitudinal pressure gradient (in the case of a large subadiabaticity, as in the radiative interior) or by the Coriolis force due to a toroidal flow along the field lines (in the case of small subadiabaticity, as in a layer of convective overshoot). The latter case is found relevant for storing the magnetic flux generated by the solar dynamo. The corresponding equilibrium properties are similar to those of isolated magnetic flux tubes. Significant variations of the differential rotation at the bottom of the convection zone in the course of the solar cycle are expected for such a kind of equilibrium. Title: Solar Magnetic Field Authors: Schüssler, M. Bibcode: 2000eaa..bookE1982S Altcode: Electrical currents flowing in the solar plasma generate a magnetic field, which is detected in the SOLAR ATMOSPHERE by spectroscopic and polarization measurements (SOLAR MAGNETIC FIELD: INFERENCE BY POLARIMETRY). The SOLAR WIND carries the magnetic field into interplanetary space where it can be measured directly by instruments on space probes.... Title: Strong Stokes V asymmetries of photospheric spectral lines: What can they tell us about the magnetic field structure? Authors: Grossmann-Doerth, U.; Schüssler, M.; Sigwarth, M.; Steiner, O. Bibcode: 2000A&A...357..351G Altcode: In an attempt to identify the mechanism responsible for the extremely asymmetric Stokes V profiles which were recently observed we analyzed several simple atmospheric configurations with separated layers of mass flow and magnetic field. We found that under appropriate conditions the models are capable of producing the observed one-lobe profiles. Title: Distribution of starspots on cool stars. II. Pre-main-sequence and ZAMS stars between 0.4 Msun and 1.7 Msun Authors: Granzer, Th.; Schüssler, M.; Caligari, P.; Strassmeier, K. G. Bibcode: 2000A&A...355.1087G Altcode: We study the dynamics of magnetic flux tubes in young stars with masses between 0.4 Msun and 1.7 Msun and for rotation rates between 0.25 Omegasun and 63 Omegasun . The resulting latitudinal emergence patterns at the stellar surface are compared with observed distributions of starspots in stellar latitude. The stellar models considered cover the range of evolutionary stages from shortly after the Hayashi phase down to the ZAMS, i.e. from the classical and weak-line T Tauri stars to the alpha -Persei stars. We use numerical simulations to follow the evolution of magnetic flux tubes from their origin at the bottom of the convection zone up to near-surface layers. We find a strong increase of emergence latitude with increasing rotation rate, a moderate decrease with increasing stellar mass, and a stronger decrease with stellar age. At very early evolutionary stages, when the central radiative zone is still quite small, we find magnetic flux emergence both in low latitudes as well as in the polar areas. High-latitude emergence is predicted in slightly more evolved pre-main-sequence and young main-sequence stars, but truly polar spots on these stars require an additional transport mechanism acting after magnetic flux emergence at the surface, probably meridional circulation or poleward slip of the `anchored' part of the erupted flux tubes. We discuss our findings in the light of the results obtained from Doppler-imaging studies. Title: Solar Variability and Climate Authors: Friis-Christensen, E.; Fröhlich, C.; Haigh, J. D.; Schüssler, M.; Von Steiger, R. Bibcode: 2000svc..book.....F Altcode: No abstract at ADS Title: Dynamics of magnetic flux tubes in close binary stars Authors: Holzwarth, Volkmar; Schüssler, Manfred Bibcode: 2000IAUS..200P.217H Altcode: Photometric and Doppler imaging observations of active binaries frequently show high latitude starspots and indicate in several cases the existence of spots at preferred longitudes (position angle with respect to the companion star). We investigate the dynamics of magnetic flux tubes in the convection zone of close, fast-rotating binary stars and explore whether the observed preferred longitudes could be caused by tidal forces and the deformation of the active star. We consider a close binary system in bound rotation with spin axes perpendicular to the orbital plane and a rotation rate of a few days. The resulting distance to the companion star is sufficiently large to consider the tidal force and the deviation from spherical structure in lowest order perturbation theory. The magnetic field is in the form of toroidal magnetic flux rings, which are stored in mechanical equilibrium within the stably stratified overshoot region beneath the convection zone until the undulatory instability initiates the rise and eruption of individual loops. Frequencies and geometry of stable as well as growth rates of unstable eigenmodes are determined by a linear stability analysis. Particular consideration is given to the question whether the effects of tidal forces and perturbations of the stellar structure can force an unstable flux tube to enter the convection zone at specific longitudes. Title: Physical Causes of Solar Variability Authors: Kuhn, J. R.; Schussler, M. Bibcode: 2000svc..book..177K Altcode: No abstract at ADS Title: Stability of magnetic flux tubes in close binary stars Authors: Holzwarth, V.; Schüssler, M. Bibcode: 2000AN....321..175H Altcode: Photometric and Doppler imaging observations of active binaries indicate the existence of starspots at preferred longitudes (position angles with respect to the companion star). We investigate the stability of magnetic flux tubes in the convection zone of close, fast-rotating binary stars and explore whether the observed preferred longitudes could be caused by tidal forces and the deformation of the active star. We assume a synchronized binary system with spin axes perpendicular to the orbital plane and a rotation period of a few days. The tidal force and the deviation from spherical structure are considered in lowest-order perturbation theory. The magnetic field is in the form of toroidal magnetic flux rings, which are stored in mechanical equilibrium within the stably stratified overshoot region beneath the convection zone until the field has grown sufficiently strong for the undulatory instability to initiate the formation of rising loops. Frequencies and geometry of stable as well as growth rates of unstable eigenmodes are determined by linear stability analysis. Particular consideration is given to the question whether the effects of tidal forces and perturbations of the stellar structure can force a rising flux loop to enter the convection zone at specific longitudes. Title: Radiative cooling of a hot flux tube in the solar photosphere Authors: Schlichenmaier, R.; Bruls, J. H. M. J.; Schüssler, M. Bibcode: 1999A&A...349..961S Altcode: Radiative energy transport is of key importance for the dynamics of slender magnetic flux tubes in the solar atmosphere, particularly so in connection with the filamentation of the sunspot penumbra. In investigations using the thin-flux-tube approximation of the MHD equations, the radiative exchange with the surrounding atmosphere has hitherto been described by the relaxation-time approach, also called `Newton's law of cooling'. The strongly nonlinear temperature-dependence of the radiative absorption coefficient and large temperature differences between the tube and its environment render this concept questionable. As a simple model of a bright penumbral filament we consider the cooling of a hot horizontal flux tube with a longitudinal flow, embedded in a non-stratified, homogeneous atmosphere at 4 800 K. We compare the results of the relaxation-time approach and of a nonlinear diffusion approximation with the numerical solution of the equation of (grey) radiative transfer. We find that the cooling times given by the relaxation-time method compare well with the results from radiative transfer as long as the initial temperature of the tube is below 7 500 K and its lateral optical depth does not exceed unity. Under these conditions, the tube cools more or less homogeneously over its cross section. For hotter and optically thick tubes, the strong temperature-dependence of the absorption coefficient leads to the formation of a cooling front, which migrates radially inward at approximately constant speed. Such inhomogeneous cooling is well represented by the nonlinear diffusion approximation. The self-similar evolution of the cooling front permits an analytical estimate of the cooling time, which provides a reasonable approximation of the result of the radiative transfer calculation. This estimate can be used to derive an improved radiative cooling term in the framework of the thin-flux-tube approximation, so that both optically thin and optically thick flux tubes can be treated adequately. The results of the radiative transfer calculations are applied to obtain an estimate of the length and brightness of penumbral bright grains. Title: Computing radiative heating on unstructured spatial grids Authors: Bruls, J. H. M. J.; Vollmöller, P.; Schüssler, M. Bibcode: 1999A&A...348..233B Altcode: We discuss the basic problems and methods involved in the design of a radiative transfer module for a 2D/3D (magneto-)hydrodynamics simulation code aimed at applications in cool-star atmospheres. Attention is focused on the difficulties arising from the unstructured triangular/tetrahedral grid and the need to minimize the communication overhead, so that the code runs efficiently on parallel computers. In a first step, we use the gray approximation and ignore scattering effects, but even then the computation of the radiative heating rate, required as a source term in the energy equation, involves several integration steps that are discussed in detail. In particular, the details of the short-characteristics solver for the radiative transfer equation, the influence of the cell size, and the accuracy of the angular integrations of the specific intensity are considered. Theoretical estimates of possible errors are in general cumbersome to obtain; instead we use simple model problems for the accuracy estimates. A plane-parallel model for the quiet Sun serves as a testground for the basics while a schematic model of a magnetic flux sheet provides an acid test for the behavior of the computational methods under typical circumstances arising during simulations. Two alternative methods to compute the radiative heating rate are compared and their weaknesses are identified. The errors are minimized by a hybrid scheme that selects a method depending on the optical path length within a grid cell. Title: Der Zyklus der Sonne. Authors: Schüssler, M.; Wöhl, H. Bibcode: 1999S&WSp...4...56S Altcode: No abstract at ADS Title: Klimaveränderung, Treibhauseffekt oder Sonnenzyklus Authors: Schmitt, D.; Schüssler, M. Bibcode: 1999S&WSp...4...64S Altcode: No abstract at ADS Title: Simulating solar MHD Authors: Schüssler, M. Bibcode: 1999AnGeo..17..578S Altcode: 1999AnGeo..17..579S; 1999AnG....17..578S Two aspects of solar MHD are discussed in relation to the work of the MHD simulation group at KIS. Photospheric magneto-convection, the nonlinear interaction of magnetic field and convection in a strongly stratified, radiating fluid, is a key process of general astrophysical relevance. Comprehensive numerical simulations including radiative transfer have significantly improved our understanding of the processes and have become an important tool for the interpretation of observational data. Examples of field intensification in the solar photosphere ('convective collapse') are shown. The second line of research is concerned with the dynamics of flux tubes in the convection zone, which has far-reaching implications for our understanding of the solar dynamo. Simulations indicate that the field strength in the region where the flux is stored before erupting to form sunspot groups is of the order of 105 G, an order of magnitude larger than previous estimates based on equipartition with the kinetic energy of convective flows. Title: Can Chromospheric Activity mimic a Polar Spot? Authors: Bruls, J. H. M. J.; Schüssler, M.; Solanki, S. K. Bibcode: 1999ASPC..158..182B Altcode: 1999ssa..conf..182B No abstract at ADS Title: Storage of toroidal magnetic field below the solar convection zone Authors: Rempel, M.; Schüssler, M.; Moreno-Insertis, F. Bibcode: 1999AGAb...15R..74R Altcode: 1999AGM....15..J15R Simulations of erupting flux tubes in the thin flux tube approximation show that essential properties of sunspots can only be explained if the initial field strength of the flux tube at the base of the convection zone is about 10 T. Such strong magnetic field can only be stored below the solar convection zone in a subadiabatic stratification. We consider mechanical equilibria in form of magnetic flux tubes and magnetic sheets and discuss the influence of radiative and convective energy transport on these configurations. In the case of magnetic flux tubes, radiative inflow of heat leads to enhanced buoyancy which causes the flux tube to move upwards and leave the storage region. In the case of magnetic sheets, the compensation of the poleward directed magnetic tension force requires a deviation of the temperature from the hydrostatic background stratification. Convective energy transport disturbs the equilibrium and leads to thermal circulations. Title: Radiative Transfer On Unstructured Triangular Grids Authors: Bruls, J.; Vollmöller, P.; Schüssler, M. Bibcode: 1999ASPC..183...44B Altcode: 1999hrsp.conf...44B No abstract at ADS Title: Radiative transfer for MHD simulations on unstructured grids Authors: Bruls, J.; Vollmöller, P.; Schüssler, M. Bibcode: 1999AGAb...15..141B Altcode: 1999AGM....15.P108B We discuss the basic problems and methods involved in the design of a radiative transfer module for a 2D/3D (magneto-)hydrodynamics simulation code aimed at applications in cool-star atmospheres. Attention is focused on the difficulties arising from the unstructured triangular/tetrahedral grid and the need to minimize the communication overhead, so that the code runs efficiently on parallel computers. In a first step, we use the gray approximation and ignore scattering effects, but even then the computation of the radiative heating rate, required as a source term in the energy equation, involves several integration steps. In particular, the details of the short-characteristics solver for the radiative transfer equation, the influence of the cell size, and the accuracy of the angular integrations of the specific intensity are considered. Theoretical estimates of possible errors are in general cumbersome to obtain; instead we use simple model problems for the accuracy estimates. A plane-parallel model for the quiet Sun serves as a testground for the basics while a schematic model of a magnetic flux provides an acid test for the behavior of the computational methods under typical circumstances arising during simulations. Two alternative methods to compute the radiative heating rate are compared and their weaknesses are identified. The errors are minimized by a hybrid scheme that selects a method depending on the optical path length within a grid cell. We have implemented the radiative transfer model in a 2D Euler code and have performed test simulations of solar surface convection. Title: Dynamics of magnetic flux tubes in evolved stars Authors: Holzwarth, V.; Schüssler, M. Bibcode: 1999AGAb...15R..71H Altcode: 1999AGM....15..J08H We explore the limits of the `solar paradigm' by applying the magnetic flux tube model, which proved to be consistent with many observed properties of sunspot groups, to subgiant and giant stars. The model starts from a strong toroidal field generated by a dynamo working at the bottom of the stellar convection zone. The field is stored within the stably stratified convective overshoot layer in the form of magnetic flux tubes. Once a critical field strength is exceeded, the undulatory (Parker-type) instability leads to flux loops rising through the convection zone. Upon emergence at the stellar surface, the flux loops form bipolar magnetic regions and starspots. We apply this model to evolved stars using evolutionary sequences for stars between 1 M_odot and 2.5 M_odot. We determine the stability limit by linear analysis und follow the development of the unstable flux loops through numerical simulation. We find that the Coriolis force leads to flux emergence at high stellar latitudes for stars with deep convection zones and large rotation rate. On the other hand, if the relative size of the stellar core falls short of a certain limit, i.e., at some stage of the rapid expansion of a star during the giant phase, flux loops formed by the instability do not surface any longer but find a new equilibrium within the star. This transition could possibly be connected with the existence of a `coronal dividing line' in the Hertzsprung-Russell diagram. Title: The formation of extremely asymmetric Stokes V profiles Authors: Steiner, O.; Grossmann-Doerth, U.; Schüssler, M.; Sigwarth, M. Bibcode: 1999AGAb...15R..10S Altcode: 1999AGM....15..A11S Recent polarimetric observations at high spatial resolution (< 1 arcsec) and with high polarimetric accuracy (noise of Stokes V/I_c < 3 cdot 10^{-4}) have revealed that about 10% of all Stokes V profiles of a quiet Sun region have an abnormal shape that strongly deviates from the more common, nearly antisymmetric profiles. 35% of the abnormal Stokes V profiles are of one-wing type, where the profile shows essentially one lobe only. We show, that a strongly asymmetric Stokes V profile can be obtained when the atmosphere, in which the profile is formed, is divided into two or more layers of different magnetic, flow, and thermal properties. A simple configuration of that kind (sometimes referred to as magnetic canopy) consists of a layer of plasma at rest with a magnetic field component parallel to the line of sight, located on top of a field-free layer with downdraft. The Stokes V asymmetry in this case sensitively depends on the position of the layers interface and may assume values up to about 80%. An asymmetry of nearly 100% (corresponding to a one-wing profile) is obtained when the plasma below the magnetic canopy is relatively cool, giving rise to the formation of a strong, redshifted spectral line in the field-free plasma beneath the canopy. If the core of this line is formed at lower temperature than the temperature of the canopy layer, the red wing of the resulting Stokes V profile is driven into emission, leading to a ``pathological'' V profile. Title: Radiative cooling of a hot flux tube in the solar photosphere Authors: Schlichenmaier, R.; Bruls, J. H. M. J.; Schüssler, M. Bibcode: 1999AGAb...15Q..75S Altcode: 1999AGM....15..J16S Radiative energy transport is of key importance for the dynamics of slender magnetic flux tubes in the solar atmosphere. In investigations using the thin-flux-tube approximation of the MHD equations, the radiative exchange with the surrounding atmosphere has hitherto been described by the relaxation-time approach, also called `Newton's law of cooling'. The strongly nonlinear temperature-dependence of the radiative absorption coefficient and large temperature differences between the tube and its environment render this concept questionable. As a simple model of a bright penumbral filament we consider the cooling of a hot horizontal flux tube with a longitudinal flow, embedded in a non-stratified, homogeneous atmosphere at 4 800 K. We compare the results of the relaxation-time approach and of a nonlinear diffusion approximation with the numerical solution of the equation of (grey) radiative transfer. We find that the cooling times given by the relaxation-time method compare well with the results from radiative transfer as long as the initial temperature of the tube is below 7 500 K and its lateral optical depth does not exceed unity. Under these conditions, the tube cools homogeneously over its cross section. For hotter and optically thick tubes, the strong temperature-dependence of the absorption coefficient leads to the formation of a cooling front, which migrates radially inward at approximately constant speed. Such inhomogeneous cooling is well represented by the nonlinear diffusion approximation. The self-similar evolution of the cooling front permits an analytical estimate of the cooling time, which provides a reasonable approximation of the result of the radiative transfer calculation. This estimate can be used to derive an improved radiative cooling term in the framework of the thin-flux-tube approximation. Title: Upwelling in a young sunspot Authors: Sigwarth, M.; Schmidt, W.; Schuessler, M. Bibcode: 1998A&A...339L..53S Altcode: An upflow of with a velocity exceeding 0.5 km*s(-1) is found in the umbra of the preceding spot in a young active region with ongoing flux emergence. A weak downflow is indicated in the corresponding spot of follower polarity. Such a flow pattern is consistent with the counter-rotation flow along a rising magnetic flux loop driven by the Coriolis force as predicted by numerical simulations. Title: Convective intensification of solar surface magnetic fields: results of numerical experiments Authors: Grossmann-Doerth, U.; Schuessler, M.; Steiner, O. Bibcode: 1998A&A...337..928G Altcode: The concentration of magnetic flux by convective flows in the solar surface layers is studied by means of two-dimensional numerical simulations with radiative transfer. We follow the evolution of an initially homogeneous, vertical magnetic field, starting from an evolved state of simulated solar granulation. The results of three simulation runs with initial field strengths, B_0, of 100 G, 200 G, and 400 G, respectively, are shown. In all cases, horizontal convective flows rapidly sweep magnetic flux into the intergranular downflow channels. The field is further amplified up to kilogauss values by partial evacuation due to a strongly accelerated downflow within the magnetic structure. The value of the field strength reached at a given depth and the size of the flux concentrations grows with the initial field strength (i.e., the amount of magnetic flux within the computional box). In the case of B_0 = 400 G, the downflow within the flux concentration becomes so strong that it `bounces' off the high density plasma in the deeper layers; the resulting upflow leads to a strong, upward moving shock and to the dispersal of the flux sheet after a lifetime of about 200 s. In the cases with less magnetic flux (B_0 100 G, 200 G), the downflow is less vigorous and the flux concentrations persist to the end of the simulation (about 5 minutes). Radiation diagnostics in the continuum and in spectral lines predicts observable signatures of the intensification process. The accelerated downflow leads to a conspicuous Doppler shift and a negative area asymmetry of Stokes V-profiles of spectral lines, while the intensification of the magnetic field may be detectable through the `magnetic line ratio' method in the visible and by direct Zeeman splitting of magnetically sensitive lines in the infrared wavelength ranges. Title: Doppler imaging: the polar SPOT controversy Authors: Bruls, J. H. M. J.; Solanki, S. K.; Schuessler, M. Bibcode: 1998A&A...336..231B Altcode: Doppler imaging studies have revealed that most rapidly rotating cool stars have high-latitude spots, which in many cases cover the stellar poles. The spectroscopic signature of polar spots is a filling in of the cores of spectral lines, which become flat-bottomed and may show bumps. Although the existence of polar spots is corroborated by spectroscopic and photometric measurements, and although theoretical models predict polar spots, they remain controversial. Most notably, it has been proposed that the line core filling in might also be caused by chromospheric activity. We present a NLTE radiative transfer analysis of 14 of the most-used Doppler-imaging lines which demonstrates that chromospheric activity can produce filling in of the observed line profiles only in a few of these lines. Moreover, such filling in is in general not of the type observed in the spectra of active stars. We are able to produce a flat-bottomed line core by concentrating the chromospheric activity near the poles, but only for two of the strongest lines, Fe i 5497 Angstroms and Fe i 6430 Angstroms. In the observations, however, also the weaker lines have flat-bottomed cores. Therefore, it is unlikely that polar spots are an artifact due to misinterpretation of the spectral signature of chromospheric activity. Nevertheless, we cannot exclude that chromospheric activity provides part of the filling in of the cores of some stronger lines; we present a diagnostic that may help to separate the contributions of chromospheric activity and spots. Title: Emerging Flux Tubes in the Solar Convection Zone. II. The Influence of Initial Conditions Authors: Caligari, P.; Schüssler, M.; Moreno-Insertis, F. Bibcode: 1998ApJ...502..481C Altcode: Numerical simulations of rising magnetic flux tubes in the solar convection zone have contributed significantly to our understanding of the basic properties of sunspot groups. They have provided an important clue to the operation of the solar dynamo by predicting strong (super-equipartition) magnetic fields near the bottom of the convection zone. We have investigated to what extent the simulation results (obtained on the basis of the thin flux tube approximation) depend on the assumptions made about the initial state of a magnetic flux tube at the start of the simulation. Two initial conditions used in the literature have been considered in detail: mechanical equilibrium (MEQ) and temperature balance (TBL). It turns out that the requirement of super-equipartition field strength is a robust feature of the simulations, largely independent of the choice of initial conditions: emergence of active regions at low latitudes and the correct dependence of their tilt angle (with respect to the east-west direction) as a function of heliographic latitude require an initial magnetic field strength on the order of 105 G. Other properties of rising flux tubes, such as the asymmetries of shape and field strength between the leading and following wings (with respect to the direction of rotation) of a rising loop, or the anchoring of part of the flux tube in the overshoot region, depend on the initial condition. Observed asymmetries in the magnetic flux distribution and of proper motions in emerging active regions favor MEQ over TBL as the proper initial condition. MEQ should also be preferred for other theoretical reasons: it allows for fewer free parameters, it requires no fine tuning for the tube geometry and background stratification in the overshoot region, and it can be easily made compatible with an encompassing model of the generation, storage, and eruption of the magnetic flux. We have also studied whether an external upflow (convective updraft) can trigger the eruption of an otherwise stably stored flux tube in the overshoot region. We find that a significant deformation and destabilization of a flux tube with equipartition field strength requires coherent upflow velocities of 20-50 m s-1 in the overshoot layer, which is an order of magnitude larger than current estimates for such velocities. Title: Dynamical Interaction of Solar Magnetic Elements and Granular Convection: Results of a Numerical Simulation Authors: Steiner, O.; Grossmann-Doerth, U.; Knölker, M.; Schüssler, M. Bibcode: 1998ApJ...495..468S Altcode: Nonstationary convection in the solar photosphere and its interaction with photospheric magnetic structures (flux sheets in intergranular lanes) have been simulated using a numerical code for two-dimensional MHD with radiative energy transfer. Dynamical phenomena are identified in the simulations, which may contribute to chromospheric and coronal heating. Among these are the bending and horizontal displacement of a flux sheet by convective flows and the excitation and propagation of shock waves both within and outside the magnetic structure. Observational signatures of these phenomena are derived from calculated Stokes profiles of Zeeman-sensitive spectral lines. We suggest that the extended red wings of the observed Stokes V profiles are due to downward coacceleration of magnetized material in a turbulent boundary layer between the flux sheet and the strong external downflow. Upward-propagating shocks in magnetic structures should be detectable if a time resolution of about 10 s is achieved, together with a spatial resolution that allows one to isolate individual magnetic structures. Determination of the complicated internal dynamics of magnetic elements requires observations with a spatial resolution better than 100 km in the solar photosphere. Title: On the Asymmetry of Bipolar Active Regions Authors: Ferriz-Mas, A.; Schüssler, M. Bibcode: 1998ASPC..155...14F Altcode: 1998sasp.conf...14F No abstract at ADS Title: A Non-LTE Analysis of Doppler Imaging Lines Authors: Bruls, J. H. M. J.; Solanki, S. K.; Schussler, M. Bibcode: 1998ASPC..154.1959B Altcode: 1998csss...10.1959B Doppler imaging studies have revealed that most stars with high activity levels have polar spots. Although their existence is corroborated by spectroscopic and photometric measurements, and although theoretical models have been produced that include polar spots, their existence remains controversial. Based on a NLTE radiative transfer analysis of the most-used Doppler-imaging lines we reject the claim that chromospheric activity might be responsible for the features in the spectral lines that are commonly interpreted as polar spots. Title: The Solar Photosphere: Open Questions Authors: Schüssler, M. Bibcode: 1998ESASP.417....3S Altcode: 1998cesh.conf....3S No abstract at ADS Title: Variability of Solar and Stellar Activity by Two Interacting Hydromagnetic Dynamos Authors: Schmitt, D.; Schussler, M.; Ferriz-Mas, A. Bibcode: 1998ASPC..154.1324S Altcode: 1998csss...10.1324S We propose a combination of two dynamos for solar and stellar magnetic activity. A strong-field dynamo operating in the overshoot layer at the base of the convection zone generating superequipartition fields concentrated in isolated flux tubes is responsible for cyclic activity (e.g., sunspots), while a turbulent weak-field dynamo in the convection zone produces a more irregular field. The combination of a threshold in field strength for dynamo action due to instability of magnetic flux tubes in the overshoot layer and random fluctuations due to magnetic fields from the turbulent convection zone leads, in the case of the Sun and solar-type stars, to activity cycles with strong amplitude variations and the occasional appearance of grand minima. Stronger fluctuations may destroy the cyclic behaviour of the overshoot layer dynamo and lead to increased but irregular activity. Such activity is observed in fast rotating cool stars. On the other hand, stars with low and non-variable magnetic activity may be in a state with only the turbulent convection zone dynamo active. Title: Predicted Starspot Distributions on Pre-MS Stars Authors: Granzer, Th.; Strassmeier, K. G.; Schussler, M.; Caligari, P. Bibcode: 1998ASPC..154.1977G Altcode: 1998csss...10.1977G We present an application of the magnetic flux-tube model of Caligari (1995, 1991) to pre-main-sequence stars. This model was originally designed as an explanation for sunspots. The primary goal is to derive the emerging latitude of flux tubes for a large parameter volume including stellar mass, age, and rotational period. Since the original model worked well in explaining certain sunspot features, the next step would be to compare our model predictions with observed starspot distributions obtained by Doppler imaging techniques. Title: Distribution of sunspot groups from asymmetric rising flux loops. Authors: Schuessler, M.; Woehl, H. Bibcode: 1997A&A...327..361S Altcode: Rising magnetic flux loops in the solar convection zone develop an asymmetric shape with a flat preceding part and a steeper following part with respect to the direction of solar rotation. By a statistical analysis of newly forming sunspot groups we test the conjecture that this geometrical asymmetry leads to an asymmetric distribution of secondary spot groups originating from the legs of the same rising flux loop as the primary group, which develops out of the loop summit. We find that ~10% of all sunspot groups actually develop secondary groups within +/-20° longitude distance from the primary group and within one day after its emergence. Those secondary groups related to small primary groups (area <100 millionths of the solar hemisphere) are predominantly located on their eastern (following) side, while for larger primary groups the secondaries are more numerous on their western (preceding) side. Both results are consistent with the geometrical asymmetry developed by rising magnetic flux loops. Title: Transmission Line Pulse Based Reliability Investigations of THz Schottky Diodes Authors: Brandt, M.; Schüssler, M.; Lin, C. I.; Simon, A.; Hartnagel, H. L. Bibcode: 1997ESASP.395...29B Altcode: 1997esae.conf...29B No abstract at ADS Title: (Erratum) The solar dynamo with meridional circulation. Authors: Choudhuri, A. R.; Schuessler, M.; Dikpati, M. Bibcode: 1997A&A...319..362C Altcode: Erratum to Astron. Astrophys. 303, L29 (1995). Title: Long-term Variation of Solar Activity by a Dynamo Based on Magnetic Flux Tubes Authors: Schussler, M.; Schmitt, D.; Ferriz-Mas, A. Bibcode: 1997ASPC..118...39S Altcode: 1997fasp.conf...39S We show that cyclic activity and Maunder-type grand minima can be explained by a dynamo driven by the instability of magnetic flux tubes in the overshoot layer at the bottom of the solar convection zone. The combination of a threshold in field strength for dynamo action and random fluctuations due to magnetic fields from the turbulent convection zone leads to activity cycles with strong amplitude variations and the occasional appearance of grand minima. Title: Numerical simulations of magnetic flux sheets. Authors: Steiner, O.; Knölker, M.; Schüssler, M. Bibcode: 1997smf..conf...31S Altcode: Non-stationary convection in the solar photosphere and its interaction with photospheric magnetic structures (flux sheets in intergranular lanes) has been simulated using a numerical code for two-dimensional MHD with radiative transfer. Dynamical phenomena, which may contribute to chromospheric and coronal heating, like bending and horizontal displacement of a flux sheet by convective flows as well as the excitation and propagation of shock waves within and outside the magnetic structure are found. Observational signature of transversal displacement and shocks are derived. It is shown that upward propagating shocks in magnetic structures should be detectable. The evolution of an initially homogeneous vertical magnetic field is followed, starting from an evolved state of a two-dimensional numerical simulation of solar granulation. Title: Origin of the Proper Motions of Emerging Bipolar Magnetic Regions Authors: Caligari, P.; Schussler, M.; Moreno-Insertis, F. Bibcode: 1997ASPC..118...76C Altcode: 1997fasp.conf...76C We have performed numerical simulations of the rise of magnetic flux tubes through the convection zone. We find that the observed proper motions of pores and sunspots in young active regions can be understood as a consequence of the Coriolis force: conservation of angular momentum leads to a retardation of the rising flux loop with respect to those parts of the flux tube that remain anchored in the overshoot layer below the convection zone proper. The result is an asymmetric shape with the following flank of the loop being more vertical than the leading part. When emerging at the solar surface, the asymmetric shape of the tube leads to proper motions which are in qualitative agreement with the observations. By studying the dependence of the asymmetry on the initial state of the flux tube we find that the observed proper motions favor a mechanical equilibrium of the magnetic field in the overshoot layer. We also find that small active regions (emerging from flux tubes with little magnetic flux) are less asymmetric and should show weaker proper motions than large bipolar regions. This prediction can be put to an observational test. Title: Observations of the quiet Sun's magnetic field. Authors: Grossmann-Doerth, U.; Keller, C. U.; Schuessler, M. Bibcode: 1996A&A...315..610G Altcode: The profiles of Stokes I and V of FeI 5247A, CrI 5247A, FeI 5250.2A and Fe I 5250.6A have been measured in the quiet photosphere with ZIMPOL, a new polarimeter, with very low noise level. The spatial resolution element of about 1Mm was essentially determined by seeing. In about 15% of the observed area, the signal-to-noise ratio was sufficiently large to determine the strength of the magnetic field and to derive various properties of the Stokes V profiles (asymmetries, zero-crossing shift and thermal line ratio). Our results show that, at least in this fraction of area, the magnetic field in the quiet Sun is dominated by strong flux concentrations which must be rather small. The data on the Stokes V properties provide observational constraints for theoretical models of flux concentrations. Title: Dynamics of magnetic flux tubes in the solar convection zone. Authors: Schüssler, M. Bibcode: 1996NAWG.1996..234S Altcode: The observed properties of sunspot groups on the surface of the Sun are consistent with the concept of magnetic flux tubes emerging from deep within the solar convection zone. Magnetic flux tubes are generated and storage in mechanical equilibrium in a subadiabatically stratified overshoot layer below the convection zone; they become unstable with respect to an undulatory (Parker-type) instability once the field strength exceeds a critical value; flux loops form, move through the convection zone and give rise to bipolar sunspot regions when they emerge at the surface. Both the stability criteria and the constraints set by the observed properties of sunspot groups (orientation, tilt angle, asymmetry, proper motions) require that the field strength at the bottom of the convection zone should be of the order of 105G, an order of magnitude larger than the equipartition field strength with respect to the convective flows. This result has profound consequences for dynamo models for the solar cycle. Title: Distribution of starspots on cool stars. I. Young and main sequence stars of 1Msun_. Authors: Schuessler, M.; Caligari, P.; Ferriz-Mas, A.; Solanki, S. K.; Stix, M. Bibcode: 1996A&A...314..503S Altcode: Sunspots are restricted to a latitude band within 30degof the solar equator. In contrast, the latitudes of spots on the surfaces of rapidly rotating cool stars can range from their polar regions, for RS CVn systems and for T Tauri stars leaving the Hayashi track, to mid latitudes for stars close to or on the main sequence. In order to find an explanation for these observed spot latitudes we have applied the criteria for the undulatory instability (Parker instability) of a toroidal magnetic flux tube embedded in the convective overshoot layer below the outer convection zone and calculated the non-linear evolution of the rising magnetic loops formed by this instability. We describe the results for a star of one solar mass in different phases of its evolution before and on the main sequence. We find that there usually is a range of latitudes at which magnetic flux can emerge on the stellar surface. The mean latitude of emergence shifts towards the poles for increasingly rapid rotation. The internal structure of the star, however, plays an almost equally important role in determining the latitude of magnetic emergence. For stars of solar mass only the youngest objects, with extremely deep convection zones, should show spots emerging at the stellar poles. Pre-main sequence stars at an age of 10^7^ y (convection zone reaching down half-way to the centre) exhibit high latitude, but not truly polar spots, while a main sequence star of one solar mass, even at high rotation rates, only shows intermediate latitude spots. These results are found to be in good agreement with Doppler images of young rapid rotators. Title: Enhanced inertia of thin magnetic flux tubes. Authors: Moreno-Insertis, F.; Schuessler, M.; Ferriz-Mas, A. Bibcode: 1996A&A...312..317M Altcode: Accelerated bodies immersed in a fluid experience enhanced inertia due to the associated co-acceleration of a certain volume of fluid in their environment. We discuss the concept of enhanced inertia in the framework of the approximation of thin flux tubes, which is widely used to describe the dynamics of concentrated magnetic structures in astrophysical objects. Previous attempts to incorporate this effect have used a local approach, in which the reaction force of the external medium on a given tube mass element solely depends on the relative acceleration of tube and environment at that element. We show that those previous formulations are inconsistent (either on physical or geometrical grounds). We present here an alternative derivation of the enhanced inertia term by geometrical means, still within a local treatment of the problem but avoiding the pitfalls of previous works. Our formulation, on the other hand, reveals a basic problem: all local approaches are bound to give incorrect answers for the reaction force in as far as they disregard the variation of the external flow in the direction parallel to the flux tube: in doing so, they generally fail to provide for global momentum conservation. An exact solution and detailed analysis for an instance of this failure is given. The discussion of this paper may be of use also in the hydrodynamical framework of vortex tube dynamics. Title: Intermittent solar activity by an on-off dynamo. Authors: Schmitt, D.; Schuessler, M.; Ferriz-Mas, A. Bibcode: 1996A&A...311L...1S Altcode: We show that the alteration between intervals of cyclic activity and grand minima like the Maunder minimum in the 17th century, which is characteristic for the long-term variation of the solar activity, can be described as on-off intermittency arising in a dymano driven by the instability of magnetic flux tubes in the overshoot layer below the convection zone. The combination of a threshold in field strength for dynamo action and random fluctuations due to magnetic fields from a turbulent convection zone dynamo leads to activity cycles with strong amplitude variations and the occasional appearance of grand minima of very low activity. The resulting pattern is in qualitative agreement with the long-term records of solar activity. Title: Polarized Radiation Diagnostics of Magnetohydrodynamic Models of the Solar Atmosphere Authors: Steiner, O.; Grossmann-Doerth, U.; Schüssler, M.; Knölker, M. Bibcode: 1996SoPh..164..223S Altcode: Solar magnetic elements and their dynamical interaction with the convective surface layers of the Sun are numerically simulated. Radiation transfer in the photosphere is taken into account. A simulation run over 18.5 minutes real time shows that the granular flow is capable of moving and bending a magnetic flux sheet (the magnetic element). At times it becomes inclined by up to 30° with respect to the vertical around the level τ5000 = 1 and it moves horizontally with a maximal velocity of 4 km/s. Shock waves form outside and within the magnetic flux sheet. The latter cause a distinctive signature in a time series of synthetic Stokes V-profiles. Such shock events occur with a mean frequency of about 2.5 minutes. A time resolution of at least 10 seconds in Stokes V recordings is needed to reveal an individual shock event by observation. Title: Convective intensification of photospheric magnetic fields. Authors: Schüssler, M.; Grossmann-Doerth, U.; Steiner, O.; Knölker, M. Bibcode: 1996AGAb...12...89S Altcode: No abstract at ADS Title: Distribution of starspots on cool stars (review) Authors: Schussler, M. Bibcode: 1996IAUS..176..269S Altcode: No abstract at ADS Title: Magnetic flux tubes and the solar dynamo - storage, instability and eruption of magnetic flux. Authors: Schüssler, M. Bibcode: 1996ASIC..481...17S Altcode: The observed properties of sunspot groups on the surface of the Sun are consistent with the concept of magnetic flux tubes emerging from deep within the solar convection zone. In order to maintain coherence and orientation during their rise in the turbulent convective flows, the magnetic field strength in these flux tubes should always exceed the equipartition value with respect to the kinetic energy density of the convective motions. Through linear stability analysis and nonlinear numerical simulations a consistent picture of the storage, instability, and rise of magnetic flux tubes has emerged in recent years. The author presents arguments which lead to this view and discusses a number of open questions in connection with the implied super-equipartition fields. Title: Flux Tube Dynamics in Active Stars Authors: Caligari, P.; Schüssler, M.; Solanki, S. K.; Schaerer, D.; Stix, M. Bibcode: 1996ApL&C..34...17C Altcode: No abstract at ADS Title: Instabilities of Magnetic Flux Tubes in a Stellar Convection Zone Authors: Ferriz-Mas, A.; Schüssler, M. Bibcode: 1996ApL&C..34....1F Altcode: No abstract at ADS Title: The solar dynamo with meridional circulation. Authors: Choudhuri, A. R.; Schussler, M.; Dikpati, M. Bibcode: 1995A&A...303L..29C Altcode: We show that meridional circulation can have a profound influence on dynamo models for the solar cycle. Motivated by the observed tilt angles of sunspot groups we assume that the generation of the poloidal field takes place near the surface, while a shear layer of radial differential rotation produces the toroidal field at the bottom of the convection zone. Both layers are coupled by a circulation with a poleward directed flow in the upper part and an equatorward flow in the deep layers of the convection zone. The circulation forces the toroidal field belts (which are responsible for the surface activity) to move equatorward. This leads to butterfly diagrams in qualitative agreement with the observations, even if the dynamo wave would propagate poleward in the absence of circulation. This result opens the possibility to construct models for the solar cycle which are based on observational data (tilt angles, differential rotation, and meridional circulation). Title: ``Explosion'' and Intensification of Magnetic Flux Tubes Authors: Moreno-Insertis, F.; Caligari, P.; Schuessler, M. Bibcode: 1995ApJ...452..894M Altcode: A magnetic flux tube anchored at the bottom of the solar convection zone and rising toward the surface as a result of an undulatory instability can be affected by a sudden catastrophic expansion and weakening of the magnetic field at its apex if the original field strength is below a few times 104 G. Such an "explosion" occurs if the flux tube evolves close enough to (adiabatic) hydrostatic equilibrium along the magnetic field lines in a super-adiabatically stratified environment. This condition is satisfied if the diameter of the tube is small enough for the drag force to dominate the dynamical evolution. For example, rising flux tubes with equipartition field (104 G) at their basis explode in the middle of the convection zone if their magnetic flux is below ≌1021 Mx.

Apart from preventing flux tubes with equipartition field from reaching the surface, the explosion process may have other consequences for the evolution of magnetic fields in the convection zone: (a) it provides a source of weak field to be acted upon by the convective flows in the course of a turbulent dynamo process; (b) upflow of matter into the inflated top of a loop intensifies the magnetic field in the submerged part of the flux tube at the bottom of the convection zone. This might constitute a mechanism to produce the strong azimuthal fields in the overshoot region suggested by recent studies of the undular instability of magnetic flux tubes. Title: Book-Review - Solar Magnetic Fields Authors: Schussler, M.; Schmidt, W.; Roberts, B. Bibcode: 1995Obs...115...97S Altcode: No abstract at ADS Title: Emerging Flux Tubes in the Solar Convection Zone. I. Asymmetry, Tilt, and Emergence Latitude Authors: Caligari, P.; Moreno-Insertis, F.; Schussler, M. Bibcode: 1995ApJ...441..886C Altcode: The process of emergence of magnetic flux from the depths of the convection zone to the surface is presented in the framework of self-consistent model for the storage of field in the lower overshoot regions and as the mechanism responsible for some of the regularities observed in active regions. We have performed numerical simulations of the emergence of flux tubes in the solar convection zone including the effects of spherical geometry and rotation. The magnetic flux tubes can be stored in mechanical equilibrium in the overshoot region, which is the natural equilibrium of the flux rings in a subadiabatic layer. An undular instability leads to the formation of loops once a critical magnetic field strength of the order of 105 G is exceeded. In the nonlinear phase of their unstable evolution, the tubes move across the convection zone on a very fast time-scale, typically about one month. The geometry and dynamics of the flux tubes studied in these simulations permit prediction of some of the observed properties of the active regions. First, the wings of the tube show a marked asymmetry of inclination and velocity, which is compatible with the observed asymmetric proper motions of sunspots and with the position of the neutral line in emerging active regions. Second, upon emergence the flux tubes show a tilt angle with respect to the equator which fits reasonably well with the observed values. Third, the flux tubes rise roughly within a cone of radial directions in the Sun so that no outbreak at high latitudes takes place. The calculations lend further support to the possibility of superequipartition field strengths in the overshoot region. The implications of the present results for the dynamo mechanism are discussed and hints for observational work are also given. Title: Solar Magnetic Fields. Authors: Schuessler, M. Bibcode: 1995RvMA....8...11S Altcode: No abstract at ADS Title: Book Review: Solar Magnetic Fields Authors: Kundu, Mukul R.; Schüssler, M.; Schmidt, W. Bibcode: 1995ComAp..18...36K Altcode: No abstract at ADS Title: Instabilities of magnetic flux tubes in a stellar convection zone II. Flux rings outside the equatorial plane Authors: Ferriz-Mas, A.; Schüssler, M. Bibcode: 1995GApFD..81..233F Altcode: Motivated by the problems of magnetic flux storage and dynamo action in stars with convection zones, we study the equilibrium and stability of magnetic flux tubes under the influence of differential rotation and stratification. The formalism developed in the first paper in this series is applied to axisymmetric, toroidal flux tubes (flux rings) lying in planes parallel to the equator at an arbitrary latitude. We assume mechanical force equilibrium, which requires neutral buoyancy of the flux tube and a longitudinal internal flow in the direction of stellar rotation. Stability against isentropic perturbations is investigated by considering both axisymmetric and non-axisymmetric, three-dimensional displacements of the equilibrium configuration. For axisymmetric modes, we find qualitative differences between the stability criteria for flux tubes within and outside the equatorial plane, where instability is generally easier to excite and overstable modes appear. In the case of non-axisymmetric perturbations, the results of a numerical study with parameter values corresponding to the bottom of the solar convection zone are discussed. The stability properties depend in a complicated way on the various parameters (e.g., latitude, magnetic field, superadiabaticity of the stratification, angular velocity and its gradient). While the magnetic field value for the onset of undulatory (Parker) instability with large growth rates is mainly determined by the stratification and the rotation rate, instabilities at somewhat lower field strengths with relatively small growth rates depend strongly on the sign and the value of the angular velocity gradient. Title: Simulation of the Interaction of Convective Flow with Magnetic Elements in the Solar Atmosphere. Authors: Steiner, O.; Grossmann-Doerth, U.; Knoelker, M.; Schuessler, M. Bibcode: 1995RvMA....8...81S Altcode: No abstract at ADS Title: Observation of Surface Activity on Cool Giants with the VLT Interferometer Authors: von der Lühe, O.; Schüssler, M.; Solanki, S. K.; Caligari, P. Bibcode: 1995svlt.conf...94V Altcode: No abstract at ADS Title: Book-Review - Solar Magnetic Fields Authors: Schussler, M.; Schmidt, W.; Priest, E. R. Bibcode: 1994Ap&SS.222..266S Altcode: No abstract at ADS Title: Waves and Instabilities of a Toroidal Magnetic Flux Tube in a Rotating Star Authors: Ferriz-Mas, A.; Schuessler, M. Bibcode: 1994ApJ...433..852F Altcode: The oscillation modes and instabilities of a toroidal flux tube lying in the equatorial plane of a differentially rotating star are investigated using the thin flux-tube approximation. The behavior of the frequencies as functions of the magnetic field strength and of the superadiabaticity is explored for both axisymmetric and nonaxisymmetric modes. In limiting cases, the modes can be identified with known normal modes of simple configurations (e.g., a plane-parallel atmosphere with constant gravity or a magnetic flux tube embedded in a homogeneous medium). The axisymmetric modes are essentially radial modes oscillating with a magnetically modified Brunt-Vaisala frequency. As to the nonaxisymmetric modes in the limit of rapid rotation (or of weak magnetic field) there is one pair of stable inertial waves, which are due to the Coriolis force, and one pair of magnetospheric modes, which are longitudinal slow modes modified by rotation. In the general case (intermediate rotation rates), the modes do not have a definite character, but their properties can be investigated analytically. The corresponding bifurcation diagrams are discussed. Title: Books-Received - Solar Magnetic Fields Authors: Schussler, M.; Schmidt, W. Bibcode: 1994Sci...266..666S Altcode: No abstract at ADS Title: A dynamo effect due to instability of magnetic flux tubes. Authors: Ferriz-Mas, A.; Schmitt, D.; Schuessler, M. Bibcode: 1994A&A...289..949F Altcode: We show that a non-axisymmetric instability of toroidal magnetic flux tubes in a rotating star provides a dynamo effect. The instability occurs in the form of propagating helical waves; their growth in amplitude causes a phase shift between the perturbations of magnetic field and velocity, which leads to an electric field (anti)parallel to the direction of the unperturbed field. Together with differential rotation, this effect is capable of driving a dynamo of the α{OMEGA}-type. In contrast to the conventional α-effect in cyclonic convection, this dynamo effect operates in strong (super-equipartition) magnetic fields which resist against distortion by convective flows. We calculate the induced electric field using results from linear stability analysis and a model of the solar convection zone which consistently includes an overshoot layer. We find that for growing magnetic field the dynamo effect occurs first in high latitudes (near the poles) in a region of weak instability which moves towards the equator as the field strength increases further. The dependence of the dynamo effect on the location of the flux tubes (in depth and latitude) and on the rotation rate is discussed. Title: Active Region Asymmetry as a Result of the Rise of Magnetic Flux Tubes Authors: Moreno-Insertis, F.; Caligari, P.; Schuessler, M. Bibcode: 1994SoPh..153..449M Altcode: The magnetic flux tubes that rise across the convection zone to produce active regions are shown to develop a difference in inclination between their preceding and follower sides. This asymmetry is such that the follower wing is more vertical (i.e., closer to the radial direction) than the preceding side. An asymmetry of this kind can be obtained as a natural consequence of the conservation of angular momentum along the rise. This process may explain a number of the observed asymmetries in morphology and behavior of the preceding and follower parts of the active regions. We present results of numerical simulations showing this effect and discuss possible observational consequences. Title: Solar Magnetic Fields Authors: Schüssler, Manfred; Schmidt, Wolfgang Bibcode: 1994smf..conf.....S Altcode: 1994QB539.M23S65... The study of solar magnetic fields includes such issues as how large-scale magnetic fields are generated in the Sun and how magnetic structures are spontaneously formed and then interact with convective flows. These are just a sample of the fundamental questions answered in this timely review of our understanding of solar magnetic fields. This volume collects together articles and research papers ranging from such topics as large-scale patterns and global dynamo action to tiny flux tubes, from the overshoot layer below the convection zone up to the corona, and from instrumental problems and theoretical methods to the latest ground-based and satellite observations. This volume provides an essential review of our knowledge to date for graduate students and researchers. Title: The deep layers of solar magnetic elements Authors: Grossmann-Doerth, U.; Knoelker, M.; Schuessler, M.; Solanki, S. K. Bibcode: 1994A&A...285..648G Altcode: We compare self-consistent theoretical models of solar magnetic flux sheets with spectropolarimetric observations of a solar plage and a network region. Our observational diagnostics mainly provide information on temperature and magnetic field of the deep photospheric layers. They are used to constrain the two free parameters of the models, viz. width and initial evacuation of the flux sheets. We find that the width of flux sheets in the network is approximately 200 km, while it is 300-350 km in an active plage. The flux sheets turn out to be less evacuated than previously thought, so they have continuum intensities close to unity. Since these are average values, however, our results do not exclude the presence of either smaller and brighter or larger and darker magnetic structures. Title: Forces on Magnetic Flux Tubes Moving in Inhomogeneous Flows Authors: Moreno-Insertis, F.; Ferriz-Mas, A.; Schussler, M. Bibcode: 1994ApJ...422..652M Altcode: The back-reaction of an incompressible two-dimensional flow of constant strain to the accelerated motion of a straight cylinder is calculated in the general case that the flow is inhomogeneous, nonstationary, and with nonvanishing (although constant) vorticity. The resulting enhanced inertia of the cylinder is basically given by its relative acceleration with respect to the background flow. Further force terms that appear because of the nonstationary and inhomogeneity of the background flow are the following: a force identical to that experienced by the mass elements of the unperturbed flow at the position of the axis of the cylinder; the customary lift force because of the circulation around the body; finally, a force term that appears only with nonvanishing relative speed between the body and the background flow and which is related to the energy which has to be imparted by the body to the surrounding fluid in order to adapt the perturbation to the new local velocity. The results have application to the dynamics of magnetic flux tubes in the convection zone and atmosphere of the Sun as well as to other astrophysical problems. Title: Dynamic interaction of convection with magnetic flux sheets: first results of a new MHD code Authors: Steiner, O.; Knölker, M.; Schüssler, M. Bibcode: 1994ASIC..433..441S Altcode: No abstract at ADS Title: Distribution of Magnetic Flux on the Surface of Rapidly Rotating Stars Authors: Caligari, P.; Schussler, M.; Stix, M.; Solanki, S. K. Bibcode: 1994ASPC...64..387C Altcode: 1994csss....8..387C No abstract at ADS Title: Alpha-effect due to instability of magnetic flux tubes and the solar dynamo Authors: Schmitt, D.; Ferriz-Mas, A.; Schüssler, M. Bibcode: 1994smf..conf..101S Altcode: No abstract at ADS Title: Dynamics of erupting magnetic flux tubes Authors: Moreno-Insertis, F.; Schüssler, M.; Caligari, P. Bibcode: 1994ASIC..433..407M Altcode: 1994ssm..work..407M The eruption of magnetic flux tubes from the overshoot layer due to instability and the dynamics of their subsequent rise through the solar convection zone are followed by numerical simulation. Special emphasis is put on the possibility of explaining observed regularities of the active regions at the surface (tilt angles, latitude of emergence, asymmetry between preceding and following parts, etc). Instability sets in with non-axisymmetric (undular) modes at azimuthal wavenumbers $m=1$ and $m=2$ if the field strength exceeds values of the order of $10^5$ G. At the same time, such strong initial fields are required to reproduce the observable properties of sunspots and active regions. Consequently, a consistent picture of storage, instability and eruption of solar magnetic fields emerges. Title: Solar magnetic fields : proceedings of the international conference held in Freiburg, Germany, June 29-July 2, 1993 Authors: Schüssler, M.; Schmidt, Wolfgang Bibcode: 1994smfp.book.....S Altcode: 1994QB539.M23S74... No abstract at ADS Title: Instability and eruption of magnetic flux tubes Authors: Caligari, P.; Ferriz-Mas, A.; Moreno-Insertis, F.; Schüssler, M. Bibcode: 1994smf..conf..139C Altcode: No abstract at ADS Title: Solar Magnetic Elements: Models Compared with Observations Authors: Grossmann-Doerth, U.; Knolker, M.; Schussler, M.; Solanki, S. K. Bibcode: 1994ASPC...68...96G Altcode: 1994sare.conf...96G No abstract at ADS Title: MHD simulations with adaptive mesh refinement Authors: Steiner, O.; Grossmann-Doerth, U.; Knölker, M.; Schüssler, M. Bibcode: 1994smf..conf..282S Altcode: No abstract at ADS Title: Flux tube dynamics in pre-main-sequence and giant stars. Authors: Caligari, P.; Schaerer, D.; Schüssler, M.; Solanki, S. Bibcode: 1994AGAb...10...92C Altcode: No abstract at ADS Title: Simulation of magneto-convection with radiative transfer Authors: Steiner, O.; Grossmann-Doerth, U.; Knölker, M.; Schüssler, M. Bibcode: 1994smf..conf..286S Altcode: No abstract at ADS Title: Instability and eruption of magnetic flux tubes in the solar convection zone. Authors: Schussler, M.; Caligari, P.; Ferriz-Mas, A.; Moreno-Insertis, F. Bibcode: 1994A&A...281L..69S Altcode: We present a consistent model of storage, instability and dynamical eruption of magnetic flux tubes in the solar convection zone and underlying overshoot region. Using a convection zone model with self-consistent overshoot layer, we calculate equilibrium configurations of magnetic flux tubes and determine their linear stability properties, taking into consideration the effects of stratification and rotation. Instability of flux tubes stored in the overshoot layer with growth times below one year requires field strengths of the order of 105 G; in many cases, the dominant mode has an azimuthal wave number of m = 2. Numerical simulations are used to follow the nonlinear evolution of such unstable flux tubes and their rise through the convection zone, from which they emerge to form active regions. The results are in accordance with the following two requirements, based on observational facts: (a) the upward motion of the tubes is not significantly deflected by the Coriolis force so that they can emerge at low latitudes, (b) their inclination with respect to the East-West direction (tilt angle) as a function of the latitude of emergence is consistent with observations of acitve regions. Title: Storage of Magnetic Flux in the Overshoot Region Authors: Moreno-Insertis, F.; Schussler, M.; Ferriz-Mas, A. Bibcode: 1993IAUS..157...41M Altcode: No abstract at ADS Title: Flux Tubes and Dynamos Authors: Schussler, M. Bibcode: 1993IAUS..157...27S Altcode: No abstract at ADS Title: Instabilities of magnetic flux tubes in a stellar convection zone I. Equatorial flux rings in differentially rotating stars Authors: Ferriz-Mas, A.; Schüssler, M. Bibcode: 1993GApFD..72..209F Altcode: The stability properties of magnetic flux tubes in stellar convection zones including overshoot regions is of considerable interest in connection with the problems of magnetic flux storage and hydromagnetic dynamo action in the Sun and other cool stars. We have developed a general formalism based on the approximation of thin flux tubes which provides a basis for a linear stability analysis of arbitrary flux tube equilibria. As a first application, the stability of axisymmetric, toroidal flux tubes (flux rings) located in the equatorial plane of a star under the influence of differential rotation and stratification has been considered. Arbitrary angular velocity differences between the interior of the flux ring and its environment are permitted. It is found that the linear evolution of radial and azimuthal perturbations (i.e., within the equatorial plane) is decoupled from that of latitudinal perturbations (perpendicular to the plane). The latitudinal instability ('poleward slip') is found to be suppressed if the matter within the flux tube rotates faster than its environment by a sufficient amount. For perturbations within the equatorial plane, both stratification (sub-order superadiabatic) of the external gas and rotation are crucial. Angular momentum conservation tends to suppress axisymmetric modes. This effect is enhanced by a faster rotation of the gas within the flux tube. Non-axisymmetric modes are more unstable since the constraint of angular momentum conservation is broken. For these modes, a slower internal rotation rate has a stabilizing effect. Within a certain range of magnetic field strengths, a second region of stability exists within the region of unstable configurations, which can extend into the superadiabatically stratified (convectively unstable) region. The character of the different modes is discussed in conjunction with the topology of the stability diagram. Title: On the Stability of Magnetic Flux Tubes in the Equator of a Star Authors: Ferriz-Mas, A.; Schussler, M. Bibcode: 1993IAUS..157...45F Altcode: No abstract at ADS Title: Modes of a flux ring lying in the equator of a star. Authors: Ferriz-Mas, A.; Schüssler, M. Bibcode: 1993spd..conf...69F Altcode: No abstract at ADS Title: Why rapid rotators have polar spots. Authors: Schuessler, M.; Solanki, S. K. Bibcode: 1992A&A...264L..13S Altcode: Starspots on magnetically active, cool stars preferentially appear near the poles. We suggest that this preference of high latitudes is due to the rapid rotation to these stars which leads to a dominance of the Coriolis force over the buoyancy force in the dynamics of magnetic flux tubes. As a consequence, flux tubes erupting from the deep parts of the stellar convection zone follow a path nearly parallel to the axis of rotation and thus necessarily surface at high latitudes, unless their initial field strength exceeds a critical value for which buoyancy becomes dominant again. It is shown that for stars with rotation periods below about 10 days flux tubes with such large field strength (of the order of 10 exp 6 G) cannot be formed and stored since they are unstable with respect to non-axisymmetric disturbances. Consequently, magnetically active stars with rapid rotation exhibit magnetic flux eruption at high latitudes and polar starspots. Title: Storage of magnetic flux tubes in a convective overshoot region Authors: Moreno-Insertis, F.; Schuessler, M.; Ferriz-Mas, A. Bibcode: 1992A&A...264..686M Altcode: Consideration is given to the suppression of the radial and polar escape of magnetic flux in the form of toroidal flux tubes (flux rings) from low latitudes in the overshoot region below the solar convection zone through the combined action of the subadiabatic ambient stratification and the rotationally induced forces. It is shown that a flux ring which is initially in thermal equilibrium with its environment and rotates with the ambient angular velocity moves radially and latitudinally towards an equilibrium configuration of lower internal temperature and larger internal rotation rate with respect to the surrounding nonmagnetic gas. Flux rings perform superposed buoyancy and inertial oscillations around their equilibrium positions. From a study of the frequencies and amplitudes of these oscillations, it is concluded that flux rings with B of less than about 100,000 G can be kept within the overshoot region if the superadiabaticity is sufficiently negative, i.e., less than about -0.00004. Title: MHD simulations with adaptive mesh refinement. Authors: Steiner, O.; Grossmann-Doerth, U.; Knölker, M.; Schüssler, M. Bibcode: 1992AGAb....7..213S Altcode: No abstract at ADS Title: Instability of magnetic flux tubes in the solar convection zone. Authors: Caligari, P.; Moreno-Insertis, F.; Schüssler, M. Bibcode: 1992AGAb....7..152C Altcode: No abstract at ADS Title: Convection Authors: Schüssler, M. Bibcode: 1992ASIC..373...81S Altcode: 1992sla..conf...81S No abstract at ADS Title: Small-Scale Photospheric Magnetic Fields Authors: Schüssler, M. Bibcode: 1992ASIC..373..191S Altcode: 1992sla..conf..191S No abstract at ADS Title: The effect of non-linear oscillations in magnetic flux tubes on Stokes V asymmetry Authors: Grossmann-Doerth, U.; Schuessler, M.; Solanki, S. K. Bibcode: 1991A&A...249..239G Altcode: The present knowledge on, and interpretation of, the asymmetry of Stokes V of spectral lines thought to be formed in and around magnetic flux tubes is reviewed. Crude models of nonlinear oscillations in the flux tube were investigated in order to explain the observed values of Stokes V amplitude and area asymmetry. It was found that flux tube models with quasi-oscillatory motions, consisting of a slow upflow and a rapid downflow in the magnetic region, as well as a downflow in the nonmagnetic surroundings, may reproduce the observations, both asymmetries and zero-crossing shift of Stokes V. Title: Structure and dynamics of magnetic fields in the solar convection zone. Authors: Schüssler, M. Bibcode: 1991NAWG.1991...25S Altcode: No abstract at ADS Title: Solar magnetic elements: results of MHD simulations. Authors: Grossmann-Doerth, U.; Knölker, M.; Schüssler, M.; Weisshaar, E. Bibcode: 1991AGAb....6...31G Altcode: No abstract at ADS Title: Filigree and flux tube physics. Authors: Spruit, H. C.; Schuessler, M.; Solanki, S. K. Bibcode: 1991sia..book..890S Altcode: This review covers the properties of the small-scale (outside sunspots) magnetic field from observational and theoretical points of view. Special emphasis is put on the physics of small isolated magnetic concentrations (flux tubes). Topics discussed include the basic observational properties, the origin and disappearance of the small-scale field, the properties of individual magnetic elements, their influence on the solar irradiance and their interaction with solar oscillations. Title: Solar magnetic elements Authors: Schüssler, Manfred Bibcode: 1991GApFD..62..271S Altcode: The interaction of magnetic fields with convective flows and radiative energy transport leads to a remarkable spatial organization of the magnetic flux permeating the solar photosphere: Most of the flux outside sunspots is concentrated in small-scale structures of large fields strength. The properties of these flux concentrations are largely uniform suggesting the concept of a prototype structure, the magnetic element. This contribution gives a brief overview of the observational results and the theoretical concepts used to describe the physics of magnetic elements. Dynamical processes like flows, oscillations and waves are considered in somewhat more detail including some recent results of numerical simulations. In addition, some consequences of the intermittent nature of photospheric magnetic fields for the structure of the solar chromosphere are briefly discussed. Title: Concentrated magnetic fields in the solar atmosphere. Authors: Schüssler, M. Bibcode: 1991ptpa.conf....1S Altcode: The solar atmosphere provides the opportunity to study magnetohydrodynamic and plasma processes on scales and under circumstances inaccessible to experimental investigation on Earth. In this contribution the author gives a summary of the present knowledge about magnetic elements, the small-scale structures into which most of the magnetic flux in the solar photosphere is concentrated. They represent an example for the formation of dissipative structures in systems far from thermal equilibrium. He discusses in particular the observationally determined properties of magnetic elements as well as theoretical concepts and results concerning their formation, stationary state, instabilities and decay. Title: Some developments in the theory of magnetic flux concentrations in the solar atmosphere Authors: Knoelker, M.; Grossmann-Doerth, U.; Schuessler, M.; Weisshaar, E. Bibcode: 1991AdSpR..11e.285K Altcode: 1991AdSpR..11..285K Most of the magnetic flux in the solar photosphere is concentrated in small-scale structures of large field strength, called magnetic elements. We discuss briefly the observationally determined properties of magnetic elements and the theoretical concepts for the origin of magnetic flux filamentation and concentration. New results of model calculations for 2D magnetic flux sheets on the basis of numerical simulation of the compressible MHD equations including a full (grey) radiative transfer are presented. Synthetic Stokes profiles of spectral lines and continuum intensity distributions serve to compare the theoretical models with observational data. Among the key results are: (1) The upper layers of the magnetic structure become hotter than the environment due to radiative illumination effects; (2) a strong convective flow evolves with horizontal velocity of 2 km/s towards the flux sheet and a narrow ``downflow jet'' with velocity up to 6 km/s adjacent to the magnetic structure; (3) both flux sheet and non-magnetic environment oscillate with a period around 5 minutes. Comparison with observed properties of solar magnetic elements reveals: (4) Calculated and semi-empirical temperature profiles as function of height in the photosphere are in reasonable agreement; (5) the calculated velocity field around flux concentrations explains the area asymmetry of the observed Stokes V-profiles including their center-limb variation; (6) the calculated continuum intensity of a flux sheet model is compatible with the values inferred from high spatial resolution observations of bright points at solar disk center; (7) the observed center-limb variation of facular contrast at low or medium spatial resolution is reproduced by arranging calculated flux sheets in arrays. We stress the importance of MHD simulation models for the analysis and interpretation of data from future facilities for high spatial resolution observations like OSL and LEST. Title: Model calculations of magnetic flux concentrations in the solar photosphere. Authors: Grossmann-Doerth, U.; Knölker, M.; Schüssler, M.; Weisshaar, E. Bibcode: 1990AGAb....5...44G Altcode: No abstract at ADS Title: Theoretical Aspects of Small-Scale Photospheric Magnetic Fields Authors: Schüssler, M. Bibcode: 1990IAUS..138..161S Altcode: No abstract at ADS Title: Non-linear dynamos. I - One-dimensional model of a thin layer dynamo Authors: Schmitt, D.; Schuessler, M. Bibcode: 1989A&A...223..343S Altcode: A simple model of a boundary layer alpha-effect dynamo at the bottom of the solar convection zone is considered in order to study nonlinear solutions. After discussing the various modes that result from the solution of the linear eigenvalue problem the results of a numerical study are presented, including two different kinds of nonlinearities: (1) quenching of the alpha-effect for increasing amplitude of the magnetic field and (2) loss of magnetic flux from the dynamo region due to magnetic buoyancy or related instabilities. It turns out that the spectrum of nonlinear solutions, especially for alpha-effect quenching, is very complicated and that the results may sensitively depend on the initial conditions. Nonlinear modes appear which have no linear counterpart. The implications for the solar dynamo are discussed; it is argued that the flux loss nonlinearity leads to results which are in better accordance with the properties of the solar cycle. Consequences for the interpretation of observations of active stars are briefly mentioned. Title: Stokes V asymmetry and shift of spectral lines Authors: Grossmann-Doerth, U.; Schuessler, M.; Solanki, S. K. Bibcode: 1989A&A...221..338G Altcode: Further evidence is given for the interpretation of the observed unshifted and asymmetric Stokes V profiles in the solar atmosphere in terms of a magnetic flux concentration expanding with height and surrounded by a downflow. A general proof is given that a V profile originating in an atmosphere in which magnetic field and flow are spatially separated along the line of sight has a zero-crossing wavelength which is unshifted with respect to the rest wavelength of the line center. Heuristic considerations for the dependence of the asymmetry of Stokes V on Zeeman shift, Doppler shift, line width, and strength are described and confirmed by exploratory calculations. Title: Observational aspects of magnetic flux sheet models Authors: Grossmann-Doerth, U.; Knölker, M.; Schüssler, M.; Weisshaar, E. Bibcode: 1989hsrs.conf..427G Altcode: No abstract at ADS Title: Dynamics of magnetic flux concentrations - The second-order thin flux tube approximation Authors: Ferriz-Mas, A.; Schuessler, M.; Anton, V. Bibcode: 1989A&A...210..425F Altcode: The thin flux tube approximation for the dynamics of magnetic flux concentrations is extended up to second order in the radial expansion to consistently include azimuthal velocities and twisted magnetic fields. The linear wave modes of a flux tube as described by the new set of equations are calculated and compared with the results of the conventional zeroth-order thin flux tube approximation. By comparison with exact solutions which are available for a uniform flux tube in a non-stratified medium, it is shown that the thin flux tube approximation (based on an expansion approach) is best suited to describe surface modes of a magnetic structure while body waves are not so well represented. Title: Models of Magnetic Flux Sheets Authors: Grossmann-Doerth, U.; Knölker, M.; Schüssler, M.; Weisshaar, E. Bibcode: 1989ASIC..263..481G Altcode: 1989ssg..conf..481G No abstract at ADS Title: Theoretical aspects and modelling of photospheric flux tubes. Authors: Knölker, M.; Schüssler, M. Bibcode: 1989ftsa.conf...17K Altcode: This paper discusses some aspects of the theoretical description of concentrated magnetic fields in the solar photosphere. The authors focus on processes leading to the formation and destruction of magnetic elements and on the properties of their quasi-equilibrium state. Results of 2D model calculations of flux slabs are discussed in some detail with emphasis on the continuum intensity and its center-to-limb variation. Title: Book-Review - Solar and Stellar Physics - 5TH European Solar Meeting - Titisee / Schwarzwald - Germany - 1987APR27-30 Authors: Schroter, E. H.; Schussler, M.; Staude, J. Bibcode: 1989AN....310...66S Altcode: No abstract at ADS Title: Book-Review - Solar and Stellar Physics / 5TH European Solar Meeting / Titisee / Schwarzwald Germany - 1987APR Authors: Schroter, E. H.; Schussler, M.; Venkatakrishnan, P. Bibcode: 1988BASI...16..248S Altcode: No abstract at ADS Title: Unshifted, asymmetric Stokes V-profiles - Possible solution of a riddle Authors: Grossmann-Doerth, U.; Schuessler, M.; Solanki, S. K. Bibcode: 1988A&A...206L..37G Altcode: We show that Stokes V-profiles originating in an atmosphere in which a magnetic field and a systematic velocity field are spatially separated along the line of sight are asymmetric with unshifted zero-crossings. Such a configuration is probably realized in the peripheral parts of magnetic flux concentrations in the solar photosphere: Since the magnetic field flares out with height, the line of sight traverses both a static magnetic region and a downdraft in the non-magnetic surroundings. V-profiles formed in these parts of the flux concentation will be strongly asymmetric but unshifted while the V-profiles from the central parts are symmetric and unshifted. The resulting mean V-profiles show the characteristics of the observed profiles, i.e. they are unshifted and asymmetric. Title: Model calculations of magnetic flux tubes. IV - Convective energy transport and the nature of intermediate size flux concentrations Authors: Knoelker, M.; Schuessler, M. Bibcode: 1988A&A...202..275K Altcode: Results of 2D-MHD simulations of solar (sub-) photospheric magnetic flux concentrations with sizes between 500 km and 1000 km are presented. A residual level of convective energy transport within the magnetic structure is assumed to be maintained by oscillatory convection or by motions driven by the Rayleigh-Taylor instability since pure radiative transport leads to density inversions. The calculated flux sheet models demonstrate the decreasing efficiency of heating by lateral influx of radiation with increasing size of the structure. Flux concentrations with sizes greater than about 500 km appear darker than the mean pahotosphere if observed at the center of the solar disk but display a significant brightness enhancement near the limb. The results are used to resolve the apparent contradiction between the data obtained with the Fourier Transform Spectrometer in network and plage regions which imply small, hot and bright structures and spectra with high spatial resolution (less than about 1 arcsec) which reveal comparatively large (1-2 arcsecs), dark magnetic structures in active regions. All observations can be reconciled if the larger structures are clusters of small magnetic elements and partly suppress the convective energy transport. Title: Book-Review - Solar and Stellar Physics Authors: Schroter, E. H.; Schussler, M. Bibcode: 1988JBAA...98..262S Altcode: No abstract at ADS Title: Book-Review - Solar and Stellar Physics Authors: Schroter, E. H.; Schussler, M. Bibcode: 1988S&T....75S.498S Altcode: No abstract at ADS Title: Model calculations of magnetic flux tubes. III - Properties of solar magnetic elements Authors: Knoelker, M.; Schuessler, M.; Weisshaar, E. Bibcode: 1988A&A...194..257K Altcode: The paper presents the results of 2D-MHD simulations of small magnetic flux concentrations in the solar photosphere. Consideration is given to the effects of extended boundary layers (i.e., a smooth transition to the nonmagnetic environment and inhibition of convective energy transport in the flux concentration surroundings). A comparison is made with observational data, and it is found that the boundary layer is likely to be thin compared to the diameter of a magnetic element while convective transport does not seem to be strongly inhibited outside the flux concentration. It is found that all models of magnetic elements are slightly unstable against fluting in the absence of a surrounding whirl flow. Title: Continuum intensity of magnetic flux concentrations - Are magnetic elements bright points? Authors: Schuessler, M.; Solanki, S. K. Bibcode: 1988A&A...192..338S Altcode: The authors use the weakening of temperature sensitive spectral lines (Fe I λ5250.2 and λ5247.1) in a strong plage region to derive a lower limit (≈1.4 of the quiet photospheric value) for the continuum intensity in magnetic flux concentrations. This suggests an identification of magnetic elements with the observed photospheric bright points. The authors discuss the implications of this result for the quantitative determination of magnetic flux and propose a procedure for obtaining information on the physical structure of the non-magnetic surroundings of flux concentrations. Title: Models of small magnetic flux concentrations in the solar photosphere. Authors: Grossmann-Doerth, U.; Knölker, M.; Schüssler, M. Bibcode: 1988AGAb....1...11G Altcode: No abstract at ADS Title: Spurious variation of photospheric magnetic flux Authors: Grossmann-Doerth, U.; Pahlke, K. -D.; Schuessler, M. Bibcode: 1987A&A...176..139G Altcode: The response of a Babcock-type magnetograph to emission from different stages in the evolution of a solar active region is simulated by computing the Stokes I and V profiles of the 525.02-nm and 630.25-nm lines of Fe I in model atmospheres representing sunspot umbrae, magnetic knots, and magnetic elements. The results are presented in tables and graphs and characterized. Breakup of a single sunspot into small hot elements with the same total magnetic flux is shown to produce a significant (as much as factor of 10) spurious loss of signal in the magnetograph. It is suggested that recent observations of flux-loss events (in unipolar regions of the solar photosphere without magnetic neutral lines) be carefully reexamined and analyzed. Title: Solar and Stellar Physics Authors: Schröter, Egon-Horst; Schüssler, Manfred Bibcode: 1987LNP...292.....S Altcode: 1987ssp..conf.....S These proceedings bring together ideas from solar and stellar physics. The sun is near enough for rather detailed observations and one chapter is devoted to the more recent experimental data from observations from space. On the other hand the multitude of stars provides a wide range of physical parameters to test hypotheses in solar and stellar astrophysics. The reader will find an illuminating overview of these fields ranging from the dynamo in the convection zone to the stellar envelopes and winds in the outer regions. In particular the importance of small-scale magnetohydrodynamic processes for the activity phenomena plays an important role in the contributions to this volume. For both students and researchers the general introduction by N. O. Weiss makes an excellent guide to this very active field of research. (See also Lecture Notes in Physics Vol. 291.) Title: Structure and Dynamics of Small Magnetic Flux Concentrations: Observation versus Theory Authors: Schüssler, M. Bibcode: 1987rfsm.conf..223S Altcode: Observational results and theoretical model calculations for small magnetic flux concentrations in the solar photosphere are compared. The formation of flux concentrations and their magnetic, thermal and velocity structure are considered. Flux tube geometry, interaction with the environment and, finally, their destruction are discussed. In the conclusion, the necessity of further development and support of MHD model calculations in connection with new observational projects is emphasized. Title: Magnetic Fields and the Rotation of the Solar Convection Zone Authors: Schuessler, M. Bibcode: 1987ASSL..137..303S Altcode: 1987isav.symp..303S Arguments are presented in favour of the conjecture that magnetic fields in the convection zone are concentrated, highly fragmented and passive to velocity fields (convection, rotation) with the only exception of the uppermost layers, where buoyancy becomes dominant. A firmer basis is given to the concept of an "anchor" or "coupling depth". It is concluded that large active regions have their origin in a region of dominating differential rotation at the lower boundary layer of the convetion zone from where they are injected into the convection zone proper. It is shown that this view is consistent with observations of the evolution of active regions and favours the cluster model of sunspots. Title: Solar Astrophysics: A Selection of Problems, Developments and Projects Authors: Schüssler, M. Bibcode: 1987MitAG..68...41S Altcode: This contribution discusses the scientific basis of two large future projects in solar physics with German participation: LEST (Large Earthbound Solar Telescope) and HRSO (High Resolution Solar Observatory). The author starts with an overview of the current problem areas in solar physics and the relation to astrophysics in general. Emphasis is laid on the key role of small-scale structures for processes of broad astrophysical relevance and the crucial part played by magnetic fields and their intermittent structure. The requirements following from this state of the scientific discourse do not only concern observation and measurement: data evaluation and interpetation, numerical simulation and theory must be supported adequately in order to make sensible use of the data gathered with expensive new observational facilities. The author discusses the interaction and cooperation of these sub-disciplines and lays down the requirements for each of them. Title: WORKSHOP I: Solar/Stellar Activity and Winds Authors: Schüssler, M. Bibcode: 1986MitAG..65..150S Altcode: No abstract at ADS Title: Berichte von den Workshops. Authors: Schüssler, M.; Trümper, J.; Offermann, D.; Porcas, R. Bibcode: 1986MitAG..65..149S Altcode: Workshop I: Solar/stellar activity and winds (M. Schüssler), Workshop II: "ROSAT", Stand des Projekts (J. Trümper), Workshop III: Variabilität der mittleren Atmosphäre - Vorbereitung eines Shuttle-Experiments (D. Offermann), Workshop IV: VLBI of galactic objects (R. Porcas). Title: MHD Models of Solar Photospheric Magnetic Flux Concentrations Authors: Schüssler, M. Bibcode: 1986ssmf.conf..103S Altcode: This contribution has two parts: the first part attempts to review the different approaches and approximations used for the theoretical description of small magnetic flux concentrations. Emphasis is laid on calculations for individual structures (flux tubes) while large-scale numerical simulations are not discussed. In the second part, a few specific topics are considered in somewhat more detail. It is pointed out that detailed model calculations are needed to provide predictions which can be tested by observations (forthcoming high-resolution measurements from space). Such models must describe very small structures like thermal and resistive boundary layers of only a few km width. Title: Heating of solar magnetic elements by downflows Authors: Hasan, S. S.; Schuessler, M. Bibcode: 1985A&A...151...69H Altcode: The idea that magnetic elements in the photosphere and lower chromosphere of the sun are heated by downflowing gas is quantitatively examined. The time-dependent hydromagnetic equations are solved numerically in the slender flux tube approximation. Viscous terms are retained, and the radiative exchange of heat between the flux tube and the ambient medium are included. Hydrogen ionization and its thermodynamic consequences are treated self-consistently. Starting from a state of hydrostatic and thermal equilibrium, the temporal response due to the onset of a downflow in the tube is studied. After a transient phase lasting a few minutes, a stationary state results that is substantially hotter than the ambient medium over a fairly large height range. Chapman's facular model can be reproduced remarkably well by adjusting the mass flux entering the tube at the upper boundary. The results are comparatively insensitive to viscosity (nu less than or equal to 10 to the 12th sq cm/s), while radiative heat exchange is significant. Some observational implications are discussed, and it is suggested that the necessary mass flux could be provided by overstable oscillations during their downflow phase. Title: Model Calculations of Solar Photospheric Flux Concentrations Authors: Knoelker, M.; Schussler, M.; Weisshaar, E. Bibcode: 1985tphr.conf..195K Altcode: No abstract at ADS Title: Model calculations of solar photospheric flux concentrations. Authors: Knölker, M.; Schüssler, M.; Weisshaar, E. Bibcode: 1985MPARp.212..195K Altcode: No abstract at ADS Title: On the structure of magnetic fields in the solar convection zone. Authors: Schuessler, M. Bibcode: 1984ESASP.220...67S Altcode: 1984ESPM....4...67S The following concepts are discussed in turn and tentatively put together in a picture of a boundary layer dynamo near the bottom of the convection zone: expulsion of magnetic flux and vorticity, fragmentation and accumulation of fields, the dominating forces that govern the evolution of a magnetic structure. Title: The interchange instability of small flux tubes Authors: Schuessler, M. Bibcode: 1984A&A...140..453S Altcode: The effect of stationary velocity fields on the interchange (fluting) instability of magnetic flux tubes is investigated theoretically. It is shown through a series of calculations that converging convective cells and internal flows along the fieldlines cannot stabilize but instead can lead to a slightly destabilizing contribution. A small flux tube (magnetic element) is found to be stable if it is surrounded by a whirl which forms in a narrow intergranular down draft. Estimates for the case of the sun (with an assumed whirl velocity of 2 km/s) yield stable tubes in between the unstable tubes with magnetic fluxes of more than 5-10 x 10 to the 17th mx when stabilized by the centrifugal force of the whirl, and more than 2-6 x 10 to the 19th mx when stabilized for buoyancy. Some of the implications of the estimates for current models of the solar surface magnetic fields are discussed. Title: Model calculations of magnetic flux tubes. I - Equations and method. II - Stationary results for solar magnetic elements Authors: Deinzer, W.; Hensler, G.; Schuessler, M.; Weisshaar, E. Bibcode: 1984A&A...139..426D Altcode: The equations, boundary conditions, geometry, approximations, and numerical methods used in model studies of the physics of magnetic flux tubes in stellar convection zones and atmospheres are presented. The methods described are then used to study the smallest flux tubes in the solar atmosphere, i.e., the magnetic elements that are thought to be responsible for facular points and filigree. The models proposed here are dynamical and include flows; the material properties (e.g., opacity and heat capacity) are calculated self-consistently. Although many limitations remain, it is shown that a considerable contribution to the understanding of the physics of photospheric flux concentrations can be made even at this level of sophistication. In addition, models of stationary magnetic flux elements are presented which are considered to constitute the basic structure of the solar photospheric magnetic field. Title: Model Calculations of Magnetic Flux Tubes - Part Two - Stationary Results for Solar Magnetic Elements Authors: Deinzer, W.; Hensler, G.; Schussler, M.; Weisshaar, E. Bibcode: 1984A&A...139..435D Altcode: Based on the methods of a preceding paper (Deinzer et al., 1984) we present models of stationary magnetic flux elements which are thought to build the basic strncture of the solar photospheric magnetic field. The main results are: a) A model with a density reduction to half of the ambient value and a reduction of the vertical convective transport coefficient by a factor 0.2 is well comparable with the semi- empirical model of Chapman (1979). b) The tube is surrounded by a dark region of gas which is significantly cooler than the undisturbed photo sphere at the same height. This reduces the "hot wall effect" and points to the necessity of an additional heating process for the upper layers of the flux tube atmosphere. c) The inclination of the isotherms with respect to the lines of constant gravitational potential in the nonmagnetic surroundings of the tube leads to a downflow around the magnetic strncture with a velocity of ∼ 1 km s-1. d) The energetics of the observable layers (τ ≍ 1) of the tube is dominated by horizontal radiative influx. Convective transport along the tube plays a secondary role. Title: Evidence for the 22-YEAR-CYCLE in the Longitudinal Distribution of Sunspots Authors: Balthasar, Horst; Schüssler, Manfred Bibcode: 1984SoPh...93..177B Altcode: It was shown in an earlier paper that preferred hemispheres of solar activity alternate with the 22-year magnetic cycle, when analyzed in the 27.0 day Bartels rotation. Using data which cover the time between 1818 and 1983 we trace back this result to 1880 (cycle 12). Before 1880 no significant correlations are found. Title: Equilibrium Models for Thin Flux Tubes in the Solar Convection Zone Authors: Anton, V.; Schüssler, M. Bibcode: 1984MitAG..62..219A Altcode: No abstract at ADS Title: Preferred longitudes of sunspot groups and high-speed solar wind streams: evidence for a "solar memory". Authors: Balthasar, H.; Schuessler, M. Bibcode: 1983SoPh...87...23B Altcode: Correlation analysis of the mean longitude distribution of sunspot groups (taken from the Greenwich Photoheliographic Results) and high-speed solar wind streams (inferred from the C9 index for geomagnetic disturbances) with the Bartels rotation period P = 27.0 days shows anti-correlation for individual cycles. Title: Self-consistent models for small photospheric flux tubes Authors: Deinzer, W.; Hensler, G.; Schmitt, D.; Schuessler, M.; Weisshaar, E. Bibcode: 1983IAUS..102...67D Altcode: The present investigation is concerned with results of a numerical study of magnetic field concentrations in the solar photosphere and upper convection zone. A two-dimensional time dependent code was developed for the full MHD equations in slab geometry for a compressible medium, taking into account the momentum equation, the equation of continuity, the induction equation for infinite conductivity, and the energy equation. A finite-element technique is used in the development procedure. Attention is given to the description of convective energy transport by the mixing-length formalism, a nearly static flux tube model, and the use of the two-dimensional radiative transfer code of Mihalas et al. (1978) for LTE diagnostics of the radiation field. Title: Simulation interplanetarer Alféenwellen Authors: Richter, A. K.; Schüssler, M.; Weisshaar, E. Bibcode: 1983MitAG..60..289R Altcode: No abstract at ADS Title: Stellar dynamo theory Authors: Schuessler, M. Bibcode: 1983IAUS..102..213S Altcode: Three topics of current interest are treated in the review: (1) The success and shortcomings of dynamo models for the solar cycle are explained, and oscillator models discussed briefly. (2) The intermittent (flux tube) nature of magnetic fields in convection zones leads to new conjectures about stellar dynamos. Arguments are given that the dynamo may operate in the overshoot region below a convective envelope. Mean-field theory for intermittent fields is illustrated. (3) The author reviews nonlinear dynamo models and some attempts to interpret observational results concerning late-type active stars. Title: On changes of the rotation velocities of stable, recurrent sunspots and their interpretation with a flux tube model Authors: Balthasar, H.; Schuessler, M.; Woehl, H. Bibcode: 1982SoPh...76...21B Altcode: The angular rotation velocities of stable, recurrent sunspots were investigated using data from the Greenwich Photoheliographic Results 1940 until 1968. We found constant rotation velocities during the passages on the solar disk with errors of about ±4 m s−1. During their lifetime these spots show a decreasing braking of their rotation velocities from 0.8 to 0.3 m s−1 per day. A plausible interpretation is found by assuming the spots to be coupled to a slowly rising subsurface flux tube and a rotation velocity which increases with depth. Title: Zweidimensionale Dynamomodelle auf der Basis magnetischer Flußröhren Authors: Schüssler, M. Bibcode: 1982MitAG..55...69S Altcode: No abstract at ADS Title: Modelle photosphärischer Magnetfeld-Konzentrationen Authors: Deinzer, W.; Hensler, G.; Schmitt, D.; Schüssler, M.; Weisshaar, E. Bibcode: 1982MitAG..55...65D Altcode: No abstract at ADS Title: The solar torsional oscillation and dynamo models of the solar cycle Authors: Schuessler, M. Bibcode: 1981A&A....94L..17S Altcode: It is shown that the solar torsional oscillation and overlapping of cycles recently discovered by Howard and LaBonte (1980) can easily be produced by the Lorentz force due to a migrating dynamo wave. Comparison with a special flux tube dynamo model (Schuessler, 1980) shows reasonable agreement with the observations. Title: Ein solarer Dynamo auf der Basis magnetischer Flußröhren. Authors: Schüssler, M. Bibcode: 1981MitAG..52..136S Altcode: No abstract at ADS Title: Ein solarer Dynamo auf der Basis Magnetischer Flußröhren Authors: Schüssler, M. Bibcode: 1981MitAG..52Q.136S Altcode: No abstract at ADS Title: Flux tube dynamo approach to the solar cycle Authors: Schuessler, M. Bibcode: 1980Natur.288..150S Altcode: In the past two decades dynamo models of solar activity and the physical foundations of the solar cycle have become acceptable. However, recent observations revealing the concentrated form of photospheric magnetic flux1, the distribution and cyclic appearance of X-ray bright points2,3 and ephemeral active regions4, as well as the discovery of sunspot brightness variations during the solar cycle5,6, have raised some questions about the dynamo theory of solar activity7. Both observation and theory8 suggest that most of the magnetic flux in the solar convection zone is in the form of concentrated isolated magnetic flux tubes. The turbulent dynamo theory9,10 has, therefore, to be modified and attempts have been made to include concentrated fields11. Nevertheless, the dynamo problem for a convective medium pervaded by concentrated flux tubes, the `flux tube dynamo', has not yet been solved. The problem is to regenerate the poloidal magnetic field out of which toroidal field is produced by differential rotation. Here the consequences and the structure of the resulting cycle are considered if a field regeneration process operating on flux tubes is assumed. A calculation similar to Leighton's magneto-kinematic model12 shows that a flux tube dynamo model can operate and reproduce the essential features of the solar cycle. It can also explain the cyclic variation of sunspot brightness and ephemeral active regions. Title: Neues zur Theorie der Sonnenaktivität. Authors: Schuessler, M. Bibcode: 1980S&W....19..331S Altcode: The current status of the theory of photospheric magnetic fields and the solar cycle theory is reviewed. Some new observations concerning the photospheric magnetic fields, the bright X-ray spots, and the ratio of the umbra radius to the penumbra radius are discussed, and their importance for these theories and their further development is examined. Title: Flows along magnetic flux tubes. I - Equilibrium and buoyancy of a slender magnetic loop in the interior of a star Authors: Schuessler, M. Bibcode: 1980A&A....89...26S Altcode: It has been speculated that loop formation may enhance the buoyant loss of magnetic flux from stellar interiors. In this paper, the effect on magnetic buoyancy of a downflow along the arms of a slender, isolated magnetic loop formed from an initially horizontal flux tube is investigated excluding the uppermost subsurface layers of the star. Loop formation, in these circumstances, is shown to be incapable of accelerating the buoyant loss of magnetic flux from regions in radiative equilibrium. For convective regions as the envelope of the sun, only very broad loops with a radius of curvature of the order of their distance to the stellar center may grow and do not enhance the buoyant loss significantly. Thus horizontal (or azimuthal) flux tubes built by differential rotation stay horizontal for most of their buoyant rise through the star. Title: Theoretical aspects of early type magnetic stars. Authors: Schüssler, M. Bibcode: 1980Nukl...25.1425S Altcode: No abstract at ADS Title: Non-linear dynamo theory: finite amplitude magnetic fields with large scale circulation in a compressible stratified medium. Authors: Schuessler, M. Bibcode: 1979A&A....72..348S Altcode: In the framework of the dynamo theory of the solar cycle self-consistent numerical solutions of the nonlinear mean-field MHD equations (including Lorentz force) within a compressible stratified medium are given for a Cartesian geometry. For both steady (alpha-squared) and oscillatory (alpha-omega) turbulent dynamos the growth of the magnetic field is limited by a mean flow driven by the Lorentz force. Magnetic buoyancy supports this mechanism but is not able to suppress dynamo action totally or to set narrow limits to the dynamo models investigated. Flow velocities of the order of 1 m/s are sufficient to limit the magnetic-field amplitude to about 10 mT (mean toroidal field of the sun). For an oscillatory dynamo of the solar type the flow pattern has a one-cell geometry with fluid rising to the surface near the spot zone (zone of maximum toroidal field in the vicinity of the equator), flowing towards the pole, and sinking down there. This may account for the observed poleward motion of the prominence zone. Title: Magnetic buoyancy revisited: analytical and numerical results for rising flux tubes. Authors: Schuessler, M. Bibcode: 1979A&A....71...79S Altcode: The buoyancy of magnetic flux tubes in the sun is reconsidered by investigating the effect of a shear flow (differential rotation) on the balance of magnetic-flux gain and loss, by examining the hydrostatic pressure stratification inside and outside a horizontal flux tube, and by numerically integrating the full set of MHD equations as an initial-value problem for the two-dimensional case. The results obtained show that: (1) flux tubes with a strong magnetic field can be produced in the solar convection zone for any realistic choice of parameters by consistent consideration of flux gain via differential rotation and flux loss due to magnetic buoyancy; (2) the buoyancy force can be determined by integration of the pressure deviation over the surface of a flux tube; and (3) rising flux tubes with a strong magnetic field experience distortion and eventually fragment. Title: Neue Ergebnisse zum Auftrieb magnetischer Flußröhren in der Konvektionszone der Sonne Authors: Schüssler, M. Bibcode: 1979MitAG..45..199S Altcode: No abstract at ADS Title: Nichtlineare Dynamomodelle im kompressiblen geschichteten Medium Authors: Schüssler, M. Bibcode: 1979MitAG..45..198S Altcode: No abstract at ADS Title: Diffusion of a strong internal magnetic field through the radiative envelope of a 2.25 M sun-star. Authors: Schuessler, M.; Paehler, A. Bibcode: 1978A&A....68...57S Altcode: Summary. Strong internal magnetic fields of stars are probably relevant for triggering supernova explosions, pulsar and white dwarf magnetic fields and the fields of some magnetic stars. We present numerical calculations for the evolution of a magnetic field produced by turbulent dynamo action in the convective core of a 2.25 M0- star. The diffusion of the field through the radiative envelope is investigated. The field reaches detectable surface values in a time less than 1 % of the familiar magnetic timescale T = R2. -1. For stars with M Mc (Mc = 2.25.. .5 M0 depending on the core field strength) the diffusion time is less than or at most of the order of the main sequence lifetime. Consequently, a core dynamo can serve as a model for magnetic stars in the mass range 1.5 M0 M Mc. Stars with 1.2 M0 M 1.5 M0 probably possess a field which is a remnant from dynamo action during the Hayashi phase. Stars with M Mc have main sequence lifetimes too short to allow for growing of a detectable surface field. Their hidden fields are required, however, for the production of the observed pulsar magnetic fields. Key words: stars - magnetic field - stellar structure and evolution - a-effect - dynamo-theory Title: On buoyant magnetic flux tube in the solar convection zone. Authors: Schuessler, M. Bibcode: 1977A&A....56..439S Altcode: Analytical calculations and numerical results are presented for slender buoyant magnetic flux tubes moving through the solar convective envelope. Conservation of mass and magnetic flux is assumed together with thermal equilibrium with the surrounding gas. Two braking mechanisms are investigated: aerodynamic drag and viscous forces. The resulting vertical velocities are always of the order of a few meters per second, causing timescales of the order of the solar cycle for ascension through the whole convection zone. Thus there is enough time for the fields to be amplified by differential rotation and the alpha effect in order to build up an alpha-omega dynamo. Title: Zur Wechselwirkung von Magnetfeldern und Strömungen im kompressiblen Medium unter dem Aspekt des Dynamo-Problems Title: Zur Wechselwirkung von Magnetfeldern und Strömungen im kompressiblen Medium unter dem Aspekt des Dynamo-Problems Authors: Schüssler, Manfred Bibcode: 1977PhDT.......317S Altcode: No abstract at ADS Title: A Dynamo Model for Magnetic Stars with Long Periods Authors: Schüssler, M. Bibcode: 1976paps.coll...39S Altcode: 1976IAUCo..32...39S No abstract at ADS Title: Axisymmetric alpha 2-dynamos in the Hayashi-phase. Authors: Schuessler, M. Bibcode: 1975A&A....38..263S Altcode: The evolution of axisymmetric magnetic fields during the Hayashi-phase is investigated on the basis of the alpha-effect dynamo mechanism. In particular, the influence of the transition from the fully convective to the fully radiative main sequence state (for A stars) is considered for rigid rotation. The fields are calculated by solving the induction equation as an initial value problem. The numerical results show that a dynamo-built magnetic field can 'survive' the transition while conserving the main part of its total flux. The topology of the field can change significantly during the evolution. For a certain phase of the transition to the main sequence there are oscillating fields, possibly giving an explanation for magnetic stars with long periods.