explanation blue bibcodes open ADS page with paths to full text
Author name code: schuessler
ADS astronomy entries on 2022-09-14
author:"Schuessler, Manfred"
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Title: Loss of toroidal magnetic flux by emergence of bipolar
magnetic regions
Authors: Cameron, R. H.; Schüssler, M.
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 × 10<SUP>15</SUP> Mx s<SUP>-1</SUP>
during activity maxima and 6.1 × 10<SUP>14</SUP> Mx s<SUP>-1</SUP>
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.
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Title: Solar activity: periodicities beyond 11 years are consistent
with random forcing
Authors: Cameron, R. H.; Schüssler, M.
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.
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Title: Solar activity: intrinsic periodicities beyond 11 years
Authors: Cameron, Robert; Schuessler, Manfred
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.
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Title: Origin of the hemispheric asymmetry of solar activity
Authors: Schüssler, M.; Cameron, R. H.
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.
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Title: Solar Magnetoconvection and Small-Scale Dynamo
Authors: Borrero, J. M.; Jafarzadeh, S.; Schüssler, M.; Solanki, S. K.
2018smf..book..275B Altcode:
No abstract at ADS
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Title: Observing and modeling the poloidal and toroidal fields of
the solar dynamo
Authors: Cameron, R. H.; Duvall, T. L.; Schüssler, M.; Schunker, H.
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. <BR /> 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. <BR /> 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. <BR /> 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.
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Title: Solar Magnetoconvection and Small-Scale Dynamo. Recent
Developments in Observation and Simulation
Authors: Borrero, J. M.; Jafarzadeh, S.; Schüssler, M.; Solanki, S. K.
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.
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Title: Observing and modelling the poloidal and toroidal magnetic
fields of the global dynamo
Authors: Cameron, Robert; Duvall, Thomas; Schüssler, Manfred;
Schunker, Hannah
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.
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Title: The solar magnetic field: from complexity to simplicity
(and back)
Authors: Schüssler, Manfred
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.
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Title: Understanding Solar Cycle Variability
Authors: Cameron, R. H.; Schüssler, M.
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.
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Title: The solar magnetic field: from complexity to simplicity
(and back)
Authors: Schüssler, Manfred
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.
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Title: An update of Leighton's solar dynamo model
Authors: Cameron, R. H.; Schüssler, M.
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<SUP>-1</SUP>,
turbulent diffusivity below about 80 km<SUP>2</SUP>s<SUP>-1</SUP>,
and dynamo excitation not too far above the threshold (linear growth
rate less than 0.1 yr<SUP>-1</SUP>).
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Title: Solar Dynamics, Rotation, Convection and Overshoot
Authors: Hanasoge, S.; Miesch, M. S.; Roth, M.; Schou, J.; Schüssler,
M.; Thompson, M. J.
2017hdsi.book...85H Altcode:
No abstract at ADS
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Title: Solar Cycle 25: Another Moderate Cycle?
Authors: Cameron, R. H.; Jiang, J.; Schüssler, M.
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.
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Title: The turbulent diffusion of toroidal magnetic flux as inferred
from properties of the sunspot butterfly diagram
Authors: Cameron, R. H.; Schüssler, M.
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. <BR /> Aims: We infer the
turbulent magnetic diffusivity affecting the toroidal field on the basis
of empirical data. <BR /> 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. <BR /> 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 km<SUP>2</SUP>
s<SUP>-1</SUP>. 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. <BR /> 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.
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Title: Solar Dynamics, Rotation, Convection and Overshoot
Authors: Hanasoge, S.; Miesch, M. S.; Roth, M.; Schou, J.; Schüssler,
M.; Thompson, M. J.
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.
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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.
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. <BR /> 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. <BR /> 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. <BR /> 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. <P
/>Movies associated to Fig. A.1 are available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201525788/olm">http://www.aanda.org</A>
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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.
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. <BR /> 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. <BR /> 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. <BR />
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. <BR /> 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. <P />Appendix A is available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201525874/olm">http://www.aanda.org</A>
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Title: The Cause of the Weak Solar Cycle 24
Authors: Jiang, J.; Cameron, R. H.; Schüssler, M.
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.
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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
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.
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Title: The crucial role of surface magnetic fields for the solar
dynamo
Authors: Cameron, Robert; Schüssler, Manfred
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.
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Title: MHD Simulations of Near-Surface Convection in Cool
Main-Sequence Stars
Authors: Beeck, Benjamin; Schussler, Manfred; Reiners, Ansgar
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. <P />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.
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Title: Generalized Investigation of the Rotation-Activity Relation:
Favoring Rotation Period instead of Rossby Number
Authors: Reiners, A.; Schüssler, M.; Passegger, V. M.
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 <SUB>⊙</SUB>. A
generalized analysis of the relation between X-ray luminosity
normalized by bolometric luminosity, L <SUB>X</SUB>/L <SUB>bol</SUB>,
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 <SUB>X</SUB>/L
<SUB>bol</SUB>vpropP <SUP>-2</SUP> R <SUP>-4</SUP> optimally describes
the non-saturated fraction of the stars. This relation is equivalent
to L <SUB>X</SUB>vpropP <SUP>-2</SUP>, indicating that the rotation
period alone determines the total X-ray emission. Since L <SUB>X</SUB>
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<SUP>-1/2</SUP>,
our generalized approach emphasizes the need to test a broader range
of mechanisms for dynamo action in cool stars.
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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.
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.
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.
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
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.
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. <BR /> 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. <BR
/> 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. <BR />
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. <BR /> 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.
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. <BR /> Aims: We aim at
identifying distinctive properties of the thermodynamic structure of
the atmospheres of different stars and understand their causes. <BR />
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. <BR /> 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. <BR />
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. <P />Appendix A is available in electronic form
at <A href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
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.
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. <BR /> 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. <BR /> Methods: We measure the cross-equatorial
flux transport using Kitt Peak synoptic magnetograms, estimating both
the total and diffusive fluxes. <BR /> 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.
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. <BR /> Aims: We examined the statistical significance of the
reported similarity of the periods. <BR /> 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. <BR /> 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. <BR
/> 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
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
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
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.
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.
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. <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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.
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.
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.
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.
2012A&A...541A..68M Altcode: 2012arXiv1201.5981M
<BR /> Aims: We study the differences between non-magnetic and
magnetic regions in the flow and thermal structure of the upper solar
photosphere. <BR /> Methods: Radiative MHD simulations representing
a quiet region and a plage region, respectively, which extend into
the layers around the temperature minimum, are analyzed. <BR />
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. <BR /> 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.
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. <P />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.
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. <BR
/> Aims: We quantitatively compare results from three-dimensional,
radiative hydrodynamic simulations of convection near the solar surface
generated with three numerical codes (CO<SUP>5</SUP>BOLD, MURaM,
and Stagger) and different simulation setups in order to investigate
the level of similarity and to cross-validate the simulations. <BR
/> 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. <BR /> 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. <BR /> 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.
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
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
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.
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. <P />Three movies are available in electronic form at <A
href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: Decay of a simulated mixed-polarity magnetic field in the
solar surface layers
Authors: Cameron, R.; Vögler, A.; Schüssler, M.
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, η<SUB>eff</SUB>, of 100-340 km<SUP>2</SUP>
s<SUP>-1</SUP>, 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.
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.
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. <BR /> Aims: We wish to clarify the origin of
the apparent discrepancy between observational results and radiative
MHD simulations. <BR /> 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. <BR /> 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. <BR /> 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.
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.
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. <P />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.
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.
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.
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. <BR /> Aims: By employing
physical models, we study the spatial and temporal characteristics of
the observable surface field for various stellar parameters. <BR />
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. <BR />
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. <BR />
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. <P />3 movies are only available in electronic form at <A
href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: Intensity contrast from MHD simulations and HINODE observations
Authors: Afram, N.; Unruh, Y. C.; Solanki, S. K.; Schüssler, M.;
Lagg, A.; Vögler, A.
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. <BR /> 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. <BR /> 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. <BR
/> 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. <BR /> 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.
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.
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). <P />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.
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.
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<SUP>-1</SUP>, but some
display strong down- or upflows reaching a few km s<SUP>-1</SUP>.
---------------------------------------------------------
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.
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<SUP>-4</SUP>
s<SUP>-1</SUP> arcsec<SUP>-2</SUP>, 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.
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.
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.
2010A&A...519A..52M Altcode: 2010arXiv1007.0387M
<BR /> Aims: We analyse mesogranular flow patterns in
a three-dimensional hydrodynamical simulation of solar surface
convection in order to determine its characteristics. <BR /> 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. <BR />
Results: The characteristics of the resulting flow patterns depend on
the averaging time and length used in the analysis. <BR /> 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.
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 rhov<SUP>1/2</SUP>. 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.
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 <SUP>1</SUP> <SUB>2</SUB> 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 <SUP>1</SUP>
<SUB>2</SUB> 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.
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.
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.
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.
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.
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
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.
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. <BR /> 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. <BR /> 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 (R<SUB>m</SUB>) and observations at different
heliocentric angles with different levels of noise. <BR /> Results:
Simulations with an imposed mixed-polarity field and R<SUB>m</SUB>
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 R<SUB>m</SUB>
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. <BR /> 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 R<SUB>m</SUB> is valid.
---------------------------------------------------------
Title: The small-scale solar surface dynamo
Authors: Pietarila Graham, Jonathan; Danilovic, Sanja; Schuessler,
Manfred
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.
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.
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
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.
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.
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. <BR /> 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. <BR /> 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. <BR /> 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<SUP>-1</SUP> 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. <BR /> 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.
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.
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.
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. <BR />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. <BR />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. <P />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.
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.
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.
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.
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.
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.
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. <P />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.
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. <BR />Aims: We carry out a comparative analysis between
the thin flux tube/sheet models and flux concentrations formed in a 3D
radiation-MHD simulation. <BR />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. <BR />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.
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.
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.
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<SUP>-1</SUP> 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
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.
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. <P
/>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.
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.
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.
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Title: A Robust Correlation between Growth Rate and Amplitude of
Solar Cycles: Consequences for Prediction Methods
Authors: Cameron, R.; Schüssler, M.
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.
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Title: 3D MHD Simulations of Sunspot Structure
Authors: Rempel, M.; Schüssler, M.
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.
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Title: Storage of Magnetic Flux in the Solar Convective Overshoot
Region
Authors: Isik, E.; Holzwarth, V.; Schüssler, M.
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.
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Title: How Well Do Zeeman Measurements Reflect the Turbulent Solar
Magnetic Field?
Authors: Pietarila, J. Graham; Danilovic, S.; Schüssler, M.
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.
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Title: MHD Simulation: From the Convection Zone to the Corona and
Beyond (in 30 Minutes)
Authors: Schüssler, M.
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.
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Title: Comparison of Magnetoconvection Simulations with the
Approximation of Thin Flux Tubes
Authors: Yelles Chaouche, L.; Solanki, S.; Schuessler, M.
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.
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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.
2008A&A...484L..17D Altcode: 2008arXiv0804.4230D
Context: The contrast of granulation is an important quantity
characterizing solar surface convection. <BR />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). <BR />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. <BR />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%. <BR />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.
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. <BR />Aims: We aim to interpret the observations
with results from numerical simulations. <BR />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. <BR />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. <BR />Conclusions:
Our results provide further evidence that local near-surface dynamo
action contributes significantly to the solar internetwork fields.
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Title: Magnetic Flux Emergence in the Solar Photosphere
Authors: Cheung, M. C. M.; Schüssler, M.; Moreno-Insertis, F.
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.
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Title: Are solar cycles predictable?
Authors: Schüssler, M.
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).
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Title: A coupled model of magnetic flux generation and transport
in stars
Authors: Işik, E.; Schmitt, D.; Schüssler, M.
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. <P />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.
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.
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Title: Radiative magnetohydrodynamic simulations of solar pores
Authors: Cameron, R.; Schüssler, M.; Vögler, A.; Zakharov, V.
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. <BR
/>Aims: We study the structure, energetics, and internal dynamics
of pore-like magnetic structures by means of exploratory numerical
simulations. <BR />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. <BR />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. <BR />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.
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. <BR />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. <BR />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. <BR />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. <BR />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.
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
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.
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. <BR />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. <BR />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 10<SUP>19</SUP> 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. <P />Movies are only available in electronic
form at http://www.aanda.org
---------------------------------------------------------
Title: Photospheric magnetoconvection
Authors: Cameron, Robert; Vögler, Alexander; Schüssler, Manfred
2007IAUS..239..475C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A solar surface dynamo
Authors: Vögler, A.; Schüssler, M.
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. <BR />Aims: We test whether local dynamo action
of the near-surface convection (granulation) can generate a significant
contribution to the observed magnetic flux. <BR />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. <BR />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. <BR
/>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.
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.
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. <BR
/>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. <BR />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 <ASTROBJ>AB Doradus</ASTROBJ>, the Sun,
and the <ASTROBJ>HR 1099</ASTROBJ> 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. <BR
/>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 <ASTROBJ>AB
Dor</ASTROBJ> 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.
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. <BR />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. <BR />Methods: We carry out a Lagrangian
linear stability analysis in the framework of the approximation of
thin magnetic flux tubes. <BR />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. <BR />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.
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.
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. <BR />Methods: We
used the MURaM code to obtain a realistic model of the near-surface
layers of the convection zone and the photosphere. <BR />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.
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.
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.<BR /> 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.<BR /> 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.<BR /> Results: .The CSSS model based
upon WSO data, with a source surface located at ≥10~R<SUB>⊙</SUB>
and cusp surface at 1.7~R<SUB>⊙</SUB>, 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.<BR /> 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.
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.
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.
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.
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.
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.<BR /> 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.<BR /> 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.<BR />
---------------------------------------------------------
Title: Magnetoconvection in a Sunspot Umbra
Authors: Schüssler, M.; Vögler, A.
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
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.
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.
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 km<SUP>2</SUP> s<SUP>-1</SUP>
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.
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.
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.
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.
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.
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.
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 <SUP>14</SUP>C
measurements. However, this claim is based on a problematic
normalization and is in conflict with independent results, such as
the <SUP>44</SUP>Ti activity in meteorites and the <SUP>10</SUP>Be
concentration in ice cores.
---------------------------------------------------------
Title: Comments on the structure and dynamics of magnetic fields in
stellar convection zones
Authors: Schuessler, Manfred
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.
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.
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.
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. <P />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.
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 km<SUP>2</SUP> s<SUP>-1</SUP> 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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 <SUP>10</SUP>Be data
Authors: Usoskin, I. G.; Mursula, K.; Solanki, S.; Schüssler, M.;
Alanko, K.
2004A&A...413..745U Altcode: 2003astro.ph..9556U
In a recent paper (Usoskin et al. 2002a), we have reconstructed
the concentration of the cosmogenic <SUP>10</SUP>Be isotope in ice
cores from the measured sunspot numbers by using physical models
for <SUP>10</SUP>Be 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 <SUP>10</SUP>Be
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
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.
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 <SUP>10</SUP>Be 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.
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
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 <SUP>10</SUP>Be 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 <SUP>10</SUP>Be 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
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.
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
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.
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.
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
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
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. <P />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
10<SUP>5</SUP> 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. <P />The magnetic energy density of
a field of 10<SUP>5</SUP> 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 10<SUP>5</SUP> G; it is conceivable that thermal processes like
an entropy-driven outflow from exploded flux tubes leads to the large
field strength required. <P />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.
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.
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.
2003IAUS..210..157V Altcode:
No abstract at ADS
---------------------------------------------------------
Title: MHD Simulations: What's Next?
Authors: Schüssler, M.
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.
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.
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 <SUP>10</SUP>Be 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
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.
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.
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.
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
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
10<SUP>5</SUP>G) 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 10<SUP>5</SUP>G 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
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.
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.
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.
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.
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
M<SUB>sun</SUB> <= M<SUB>star </SUB> <= 3 M<SUB>sun</SUB>. 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.
2001ApJ...552L.171R Altcode:
A strong superequipartition magnetic field strength on the order of
10 T (10<SUP>5</SUP> 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.
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.
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.
2001ASPC..248..259H Altcode: 2001mfah.conf..259H
No abstract at ADS
---------------------------------------------------------
Title: Numerical Simulation of Solar Magneto-Convection
Authors: Schüssler, M.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 M<SUB>sun</SUB> and 1.7 M<SUB>sun</SUB>
Authors: Granzer, Th.; Schüssler, M.; Caligari, P.; Strassmeier, K. G.
2000A&A...355.1087G Altcode:
We study the dynamics of magnetic flux tubes in young stars with masses
between 0.4 M<SUB>sun</SUB> and 1.7 M<SUB>sun</SUB> and for rotation
rates between 0.25 Omega<SUB>sun</SUB> and 63 Omega<SUB>sun</SUB>
. 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.
2000svc..book.....F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Dynamics of magnetic flux tubes in close binary stars
Authors: Holzwarth, Volkmar; Schüssler, Manfred
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.
2000svc..book..177K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Stability of magnetic flux tubes in close binary stars
Authors: Holzwarth, V.; Schüssler, M.
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.
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.
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.
1999S&WSp...4...56S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Klimaveränderung, Treibhauseffekt oder Sonnenzyklus
Authors: Schmitt, D.; Schüssler, M.
1999S&WSp...4...64S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Simulating solar MHD
Authors: Schüssler, M.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 10<SUP>5</SUP> 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<SUP>-1</SUP> 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 10<SUP>5</SUP>G, 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 1M<SUB>sun</SUB>_.
Authors: Schuessler, M.; Caligari, P.; Ferriz-Mas, A.; Solanki, S. K.;
Stix, M.
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.
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.
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.
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 τ<SUB>5000</SUB> =
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.
1996AGAb...12...89S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Distribution of starspots on cool stars (review)
Authors: Schussler, M.
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.
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.
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.
1996ApL&C..34....1F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The solar dynamo with meridional circulation.
Authors: Choudhuri, A. R.; Schussler, M.; Dikpati, M.
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.
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 10<SUP>4</SUP> 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
(10<SUP>4</SUP> G) at their basis explode in the middle of the
convection zone if their magnetic flux is below ≌10<SUP>21</SUP>
Mx. <P />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.
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.
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 10<SUP>5</SUP>
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.
1995RvMA....8...11S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Book Review: Solar Magnetic Fields
Authors: Kundu, Mukul R.; Schüssler, M.; Schmidt, W.
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.
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.
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.
1995svlt.conf...94V Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Book-Review - Solar Magnetic Fields
Authors: Schussler, M.; Schmidt, W.; Priest, E. R.
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
1994smf..conf..101S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Dynamics of erupting magnetic flux tubes
Authors: Moreno-Insertis, F.; Schüssler, M.; Caligari, P.
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
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.
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.
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.
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.
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.
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.
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 10<SUP>5</SUP> 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.
1993IAUS..157...41M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Flux Tubes and Dynamos
Authors: Schussler, M.
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.
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.
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.
1993spd..conf...69F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Why rapid rotators have polar spots.
Authors: Schuessler, M.; Solanki, S. K.
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.
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.
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.
1992AGAb....7..152C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Convection
Authors: Schüssler, M.
1992ASIC..373...81S Altcode: 1992sla..conf...81S
No abstract at ADS
---------------------------------------------------------
Title: Small-Scale Photospheric Magnetic Fields
Authors: Schüssler, M.
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.
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.
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.
1991AGAb....6...31G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Filigree and flux tube physics.
Authors: Spruit, H. C.; Schuessler, M.; Solanki, S. K.
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
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.
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.
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.
1990AGAb....5...44G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Theoretical Aspects of Small-Scale Photospheric Magnetic Fields
Authors: Schüssler, M.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
1988JBAA...98..262S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Book-Review - Solar and Stellar Physics
Authors: Schroter, E. H.; Schussler, M.
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.
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.
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.
1988AGAb....1...11G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Spurious variation of photospheric magnetic flux
Authors: Grossmann-Doerth, U.; Pahlke, K. -D.; Schuessler, M.
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
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.
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.
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.
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.
1986MitAG..65..150S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Berichte von den Workshops.
Authors: Schüssler, M.; Trümper, J.; Offermann, D.; Porcas, R.
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.
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.
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.
1985tphr.conf..195K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Model calculations of solar photospheric flux concentrations.
Authors: Knölker, M.; Schüssler, M.; Weisshaar, E.
1985MPARp.212..195K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On the structure of magnetic fields in the solar convection
zone.
Authors: Schuessler, M.
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.
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.
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.
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<SUP>-1</SUP>. 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
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.
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.
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.
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.
1983MitAG..60..289R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Stellar dynamo theory
Authors: Schuessler, M.
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.
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<SUP>−1</SUP>. During their lifetime these spots show a decreasing
braking of their rotation velocities from 0.8 to 0.3 m s<SUP>−1</SUP>
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.
1982MitAG..55...69S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Modelle photosphärischer Magnetfeld-Konzentrationen
Authors: Deinzer, W.; Hensler, G.; Schmitt, D.; Schüssler, M.;
Weisshaar, E.
1982MitAG..55...65D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The solar torsional oscillation and dynamo models of the
solar cycle
Authors: Schuessler, M.
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.
1981MitAG..52..136S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Ein solarer Dynamo auf der Basis Magnetischer Flußröhren
Authors: Schüssler, M.
1981MitAG..52Q.136S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Flux tube dynamo approach to the solar cycle
Authors: Schuessler, M.
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
flux<SUP>1</SUP>, the distribution and cyclic appearance of X-ray
bright points<SUP>2,3</SUP> and ephemeral active regions<SUP>4</SUP>,
as well as the discovery of sunspot brightness variations during
the solar cycle<SUP>5,6</SUP>, have raised some questions about the
dynamo theory of solar activity<SUP>7</SUP>. Both observation and
theory<SUP>8</SUP> 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 theory<SUP>9,10</SUP> has, therefore,
to be modified and attempts have been made to include concentrated
fields<SUP>11</SUP>. 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
model<SUP>12</SUP> 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.
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.
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.
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.
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.
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.
1979MitAG..45..199S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Nichtlineare Dynamomodelle im kompressiblen geschichteten
Medium
Authors: Schüssler, M.
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.
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.
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.
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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
1977PhDT.......317S Altcode:
No abstract at ADS
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Title: A Dynamo Model for Magnetic Stars with Long Periods
Authors: Schüssler, M.
1976paps.coll...39S Altcode: 1976IAUCo..32...39S
No abstract at ADS
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Title: Axisymmetric alpha <SUP>2</SUP>-dynamos in the Hayashi-phase.
Authors: Schuessler, M.
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.