Author name code: schlichenmaier
ADS astronomy entries on 2022-09-14
author:"Schlichenmaier, Rolf"
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Title: The European Solar Telescope
Authors: Quintero Noda, C.; Schlichenmaier, R.; Bellot Rubio, L. R.;
Löfdahl, M. G.; Khomenko, E.; Jurcak, J.; Leenaarts, J.; Kuckein,
C.; González Manrique, S. J.; Gunar, S.; Nelson, C. J.; de la Cruz
Rodríguez, J.; Tziotziou, K.; Tsiropoula, G.; Aulanier, G.; Collados,
M.; the EST team
Bibcode: 2022arXiv220710905Q
Altcode:
The European Solar Telescope (EST) is a project aimed at studying
the magnetic connectivity of the solar atmosphere, from the deep
photosphere to the upper chromosphere. Its design combines the knowledge
and expertise gathered by the European solar physics community during
the construction and operation of state-of-the-art solar telescopes
operating in visible and near-infrared wavelengths: the Swedish 1m Solar
Telescope (SST), the German Vacuum Tower Telescope (VTT) and GREGOR,
the French Télescope Héliographique pour l'Étude du Magnétisme
et des Instabilités Solaires (THÉMIS), and the Dutch Open Telescope
(DOT). With its 4.2 m primary mirror and an open configuration, EST will
become the most powerful European ground-based facility to study the Sun
in the coming decades in the visible and near-infrared bands. EST uses
the most innovative technological advances: the first adaptive secondary
mirror ever used in a solar telescope, a complex multi-conjugate
adaptive optics with deformable mirrors that form part of the optical
design in a natural way, a polarimetrically compensated telescope design
that eliminates the complex temporal variation and wavelength dependence
of the telescope Mueller matrix, and an instrument suite containing
several (etalon-based) tunable imaging spectropolarimeters and several
integral field unit spectropolarimeters. This publication summarises
some fundamental science questions that can be addressed with the
telescope, together with a complete description of its major subsystems.
Title: Characterization of magneto-convection in sunspots. The
Gough-Tayler stability criterion in MURaM sunspot simulations
Authors: Schmassmann, M.; Rempel, M.; Bello González, N.;
Schlichenmaier, R.; Jurčák, J.
Bibcode: 2021A&A...656A..92S
Altcode:
Context. Observations have shown that in stable sunspots, the umbral
boundary is outlined by a critical value of the vertical magnetic
field component. However, the nature of the distinct magnetoconvection
regimes in the umbra and penumbra is still unclear.
Aims: We
analyse a sunspot simulation in an effort to understand the origin
of the convective instabilities giving rise to the penumbral and
umbral distinct regimes.
Methods: We applied the criterion
from Gough & Tayler (1966, MNRAS, 133, 85), accounting for the
stabilising effect of the vertical magnetic field, to investigate
the convective instabilities in a MURaM sunspot simulation.
Results: We find: (1) a highly unstable shallow layer right beneath the
surface extending all over the simulation box in which convection is
triggered by radiative cooling in the photosphere; (2) a deep umbral
core (beneath −5 Mm) stabilised against overturning convection
that underlies a region with stable background values permeated
by slender instabilities coupled to umbral dots; (3) filamentary
instabilities below the penumbra nearly parallel to the surface and
undulating instabilities coupled to the penumbra which originate
in the deep layers. These deep-rooted instabilities result in the
vigorous magneto-convection regime characteristic of the penumbra; (4)
convective downdrafts in the granulation, penumbra, and umbra develop
at about 2 km s−1, 1 km s−1, and 0.1 km
s−1, respectively, indicating that the granular regime of
convection is more vigorous than the penumbra convection regime, which,
in turn, is more vigorous than the close-to-steady umbra; (5) the GT
criterion outlines both the sunspot magnetopause and peripatopause,
highlighting the tripartite nature of the sub-photospheric layers
of magnetohydrodynamic (MHD) sunspot models; and, finally, (6)
the Jurčák criterion is the photospheric counterpart of the GT
criterion in deep layers.
Conclusions: The GT criterion as a
diagnostic tool reveals the tripartite nature of sunspot structure
with distinct regimes of magneto-convection in the umbra, penumbra,
and granulation operating in realistic MHD simulations. Movies associated with Figs. 2 and 3 are available at https://www.aanda.org
Title: On the (in)stability of sunspots
Authors: Strecker, H.; Schmidt, W.; Schlichenmaier, R.; Rempel, M.
Bibcode: 2021A&A...649A.123S
Altcode: 2021arXiv210311487S
Context. The stability of sunspots is one of the long-standing unsolved
puzzles in the field of solar magnetism and the solar cycle. The thermal
and magnetic structure of the sunspot beneath the solar surface is
not accessible through observations, thus processes in these regions
that contribute to the decay of sunspots can only be studied through
theoretical and numerical studies.
Aims: We study the effects
that destabilise and stabilise the flux tube of a simulated sunspot
in the upper convection zone. The depth-varying effects of fluting
instability, buoyancy forces, and timescales on the flux tube are
analysed.
Methods: We analysed a numerical simulation of a
sunspot calculated with the MURaM code. The simulation domain has a
lateral extension of more than 98 Mm × 98 Mm and extends almost 18
Mm below the solar surface. The analysed data set of 30 hours shows a
stable sunspot at the solar surface. We studied the evolution of the
flux tube at defined horizontal layers (1) by means of the relative
change in perimeter and area, that is, its compactness; and (2) with
a linear stability analysis.
Results: The simulation shows a
corrugation along the perimeter of the flux tube (sunspot) that proceeds
fastest at a depth of about 8 Mm below the solar surface. Towards
the surface and towards deeper layers, the decrease in compactness is
damped. From the stability analysis, we find that above a depth of 2
Mm, the sunspot is stabilised by buoyancy forces. The spot is least
stable at a depth of about 3 Mm because of the fluting instability. In
deeper layers, the flux tube is marginally unstable. The stability
of the sunspot at the surface affects the behaviour of the field
lines in deeper layers by magnetic tension. Therefore the fluting
instability is damped at depths of about 3 Mm, and the decrease in
compactness is strongest at a depth of about 8 Mm. The more vertical
orientation of the magnetic field and the longer convective timescale
lead to slower evolution of the corrugation process in layers deeper
than 10 Mm.
Conclusions: The formation of large intrusions of
field-free plasma below the surface destabilises the flux tube of
the sunspot. This process is not visible at the surface, where the
sunspot is stabilised by buoyancy forces. The onset of sunspot decay
occurs in deeper layers, while the sunspot still appears stable in
the photosphere. The intrusions eventually lead to the disruption
and decay of the sunspot. The animation is available at https://www.aanda.org
This paper is mainly based on Part I of
the Ph.D. thesis "On the decay of sunspots", https://freidok.uni-freiburg.de/data/165760
Title: A distinct magnetic property of the inner penumbral
boundary. III. Analysis of simulated sunspots
Authors: Jurčák, Jan; Schmassmann, Markus; Rempel, Matthias; Bello
González, Nazaret; Schlichenmaier, Rolf
Bibcode: 2020A&A...638A..28J
Altcode: 2020arXiv200403940J
Context. Analyses of sunspot observations revealed a fundamental
magnetic property of the umbral boundary: the invariance of the
vertical component of the magnetic field.
Aims: We analyse
the magnetic properties of the umbra-penumbra boundary in simulated
sunspots and thus assess their similarity to observed sunspots. We
also aim to investigate the role of the plasma β and the ratio of
kinetic to magnetic energy in simulated sunspots in the convective
motions because these quantities cannot be reliably determined from
observations.
Methods: We used a set of non-gray simulation
runs of sunspots with the MURaM code. The setups differed in terms
of subsurface magnetic field structure and magnetic field boundary
imposed at the top of the simulation domain. These data were used to
synthesize the Stokes profiles, which were then degraded to the Hinode
spectropolarimeter-like observations. Then, the data were treated
like real Hinode observations of a sunspot, and magnetic properties
at the umbral boundaries were determined.
Results: Simulations
with potential field extrapolation produce a realistic magnetic field
configuration on the umbral boundaries of the sunspots. Two simulations
with a potential field upper boundary, but different subsurface
magnetic field structures, differ significantly in the extent of their
penumbrae. Increasing the penumbra width by forcing more horizontal
magnetic fields at the upper boundary results in magnetic properties
that are not consistent with observations. This implies that the size of
the penumbra is given by the subsurface structure of the magnetic field,
that is, by the depth and inclination of the magnetopause, which is
shaped by the expansion of the sunspot flux rope with height. None of
the sunspot simulations is consistent with the observed properties of
the magnetic field and the direction of the Evershed flow at the same
time. Strong outward-directed Evershed flows are only found in setups
with an artificially enhanced horizontal component of the magnetic
field at the top boundary that are not consistent with the observed
magnetic field properties at the umbra-penumbra boundary. We stress
that the photospheric boundary of simulated sunspots is defined by a
magnetic field strength of equipartition field value.
Title: Characterization of the umbra-penumbra boundary by the vertical
component of the magnetic field. Analysis of ground-based data from
the GREGOR Infrared Spectrograph
Authors: Lindner, P.; Schlichenmaier, R.; Bello González, N.
Bibcode: 2020A&A...638A..25L
Altcode: 2020arXiv200409956L
Context. The vertical component of the magnetic field was found to
reach a constant value at the boundary between penumbra and umbra of
stable sunspots in a recent statistical study of Hinode/SP data. This
finding has profound implications as it can serve as a criterion to
distinguish between fundamentally different magneto-convective modes
operating in the sun.
Aims: The objective of this work is to
verify the existence of a constant value for the vertical component of
the magnetic field (B⊥) at the boundary between umbra and
penumbra from ground-based data in the near-infrared wavelengths and to
determine its value for the GREGOR Infrared Spectrograph (GRIS@GREGOR)
data. This is the first statistical study on the Jurčák criterion with
ground-based data, and we compare it with the results from space-based
data (Hinode/SP and SDO/HMI).
Methods: Eleven spectropolarimetric
data sets from the GRIS@GREGOR slit-spectograph containing fully-fledged
stable sunspots were selected from the GRIS archive. SIR inversions
including a polarimetric straylight correction are used to produce
maps of the magnetic field vector using the Fe I 15648 Å and 15662 Å
lines. Averages of B⊥ along the contours between penumbra
and umbra are analyzed for the 11 data sets. In addition, contours at
the resulting B⊥const are drawn onto maps and
compared to intensity contours. The geometric difference between these
contours, ΔP, is calculated for each data set.
Results: Averaged
over the 11 sunspots, we find a value of B⊥const
= (1787 ± 100) gauss. The difference from the values previously derived
from Hinode/SP and SDO/HMI data is explained by instrumental differences
and by the formation characteristics of the respective lines that were
used. Contours at B⊥ = B⊥const and
contours calculated in intensity maps match from a visual inspection
and the geometric distance ΔP was found to be on the order of 2
pixels. Furthermore, the standard deviation between different data sets
of averages along umbra-penumbra contours is smaller for B⊥
than for B∥ by a factor of 2.4.
Conclusions: Our
results provide further support to the Jurčák criterion with the
existence of an invariable value B⊥const at the
umbra-penumbra boundary. This fundamental property of sunspots can act
as a constraining parameter in the calibration of analysis techniques
that calculate magnetic fields. It also serves as a requirement for
numerical simulations to be realistic. Furthermore, it is found
that the geometric difference, ΔP, between intensity contours
and contours at B⊥ = B⊥const
acts as an index of stability for sunspots. The data from
the GRIS instrument is publicly available in the archive at http://sdc.leibniz-kis.de.
Title: Science Requirement Document (SRD) for the European Solar
Telescope (EST) (2nd edition, December 2019)
Authors: Schlichenmaier, R.; Bellot Rubio, L. R.; Collados, M.;
Erdelyi, R.; Feller, A.; Fletcher, L.; Jurcak, J.; Khomenko, E.;
Leenaarts, J.; Matthews, S.; Belluzzi, L.; Carlsson, M.; Dalmasse,
K.; Danilovic, S.; Gömöry, P.; Kuckein, C.; Manso Sainz, R.;
Martinez Gonzalez, M.; Mathioudakis, M.; Ortiz, A.; Riethmüller,
T. L.; Rouppe van der Voort, L.; Simoes, P. J. A.; Trujillo Bueno,
J.; Utz, D.; Zuccarello, F.
Bibcode: 2019arXiv191208650S
Altcode:
The European Solar Telescope (EST) is a research infrastructure
for solar physics. It is planned to be an on-axis solar telescope
with an aperture of 4 m and equipped with an innovative suite of
spectro-polarimetric and imaging post-focus instrumentation. The EST
project was initiated and is driven by EAST, the European Association
for Solar Telescopes. EAST was founded in 2006 as an association
of 14 European countries. Today, as of December 2019, EAST consists
of 26 European research institutes from 18 European countries. The
Preliminary Design Phase of EST was accomplished between 2008 and
2011. During this phase, in 2010, the first version of the EST Science
Requirement Document (SRD) was published. After EST became a project
on the ESFRI roadmap 2016, the preparatory phase started. The goal
of the preparatory phase is to accomplish a final design for the
telescope and the legal governance structure of EST. A major milestone
on this path is to revisit and update the Science Requirement Document
(SRD). The EST Science Advisory Group (SAG) has been constituted by
EAST and the Board of the PRE-EST EU project in November 2017 and has
been charged with the task of providing with a final statement on the
science requirements for EST. Based on the conceptual design, the SRD
update takes into account recent technical and scientific developments,
to ensure that EST provides significant advancement beyond the current
state-of-the-art. The present update of the EST SRD has been developed
and discussed during a series of EST SAG meetings. The SRD develops
the top-level science objectives of EST into individual science
cases. Identifying critical science requirements is one of its main
goals. Those requirements will define the capabilities of EST and the
post-focus instrument suite. The technical requirements for the final
design of EST will be derived from the SRD.
Title: Spectropolarimetric Observations of an Arch Filament System
with GREGOR
Authors: Balthasar, H.; Gömöry, P.; González Manrique, S. J.;
Kuckein, C.; Kučera, A.; Schwartz, P.; Berkefeld, T.; Collados, M.;
Denker, C.; Feller, A.; Hofmann, A.; Schlichenmaier, R.; Schmidt,
D.; Schmidt, W.; Sigwarth, M.; Sobotka, M.; Solanki, S. K.; Soltau,
D.; Staude, J.; Strassmeier, K. G.; von der Lühe, O.
Bibcode: 2019ASPC..526..217B
Altcode: 2018arXiv180401789B
We observed an arch filament system (AFS) in a sunspot group with the
GREGOR Infrared Spectrograph attached to the GREGOR solar telescope. The
AFS was located between the leading sunspot of negative polarity and
several pores of positive polarity forming the following part of the
sunspot group. We recorded five spectro-polarimetric scans of this
region. The spectral range included the spectral lines Si I 1082.7
nm, He I 1083.0 nm, and Ca I 1083.9 nm. In this work we concentrate
on the silicon line which is formed in the upper photosphere. The
line profiles are inverted with the code 'Stokes Inversion based
on Response functions' to obtain the magnetic field vector. The
line-of-sight velocities are determined independently with a Fourier
phase method. Maximum velocities are found close to the ends of AFS
fibrils. These maximum values amount to 2.4 km s-1 next
to the pores and to 4 km s-1 at the sunspot side. Between
the following pores, we encounter an area of negative polarity that
is decreasing during the five scans. We interpret this by new emerging
positive flux in this area canceling out the negative flux. In summary,
our findings confirm the scenario that rising magnetic flux tubes
cause the AFS.
Title: Photospheric Magnetic Fields of the Trailing Sunspots in
Active Region NOAA 12396
Authors: Verma, M.; Balthasar, H.; Denker, C.; Böhm, F.; Fischer,
C. E.; Kuckein, C.; González Manrique, S. J.; Sobotka, M.; Bello
González, N.; Diercke, A.; Berkefeld, T.; Collados, M.; Feller, A.;
Hofmann, A.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Pastor Yabar,
A.; Rezaei, R.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth,
M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K.; Volkmer,
R.; von der Lühe, O.; Waldmann, T.
Bibcode: 2019ASPC..526..291V
Altcode: 2018arXiv180507752V
The solar magnetic field is responsible for all aspects of solar
activity. Sunspots are the main manifestation of the ensuing solar
activity. Combining high-resolution and synoptic observations has
the ambition to provide a comprehensive description of the sunspot
growth and decay processes. Active region NOAA 12396 emerged on 2015
August 3 and was observed three days later with the 1.5-meter GREGOR
solar telescope on 2015 August 6. High-resolution spectropolarimetric
data from the GREGOR Infrared Spectrograph (GRIS) are obtained in the
photospheric lines Si I λ1082.7 nm and Ca I λ1083.9 nm, together
with the chromospheric He I λ1083.0 nm triplet. These near-infrared
spectropolarimetric observations were complemented by synoptic
line-of-sight magnetograms and continuum images of the Helioseismic
and Magnetic Imager (HMI) and EUV images of the Atmospheric Imaging
Assembly (AIA) on board the Solar Dynamics Observatory (SDO).
Title: New Insights on Penumbra Magneto-Convection
Authors: Bello González, N.; Jurčák, J.; Schlichenmaier, R.;
Rezaei, R.
Bibcode: 2019ASPC..526..261B
Altcode:
Fully-fledged penumbrae are a well characterised phenomenon from
an observational point of view. Also, MHD simulations reproduce the
observed characteristics and provide us with insights on the physical
mechanisms possibly running behind the observed processes. Yet, how this
penumbral magneto-convection sets in is still an open question. Due to
the fact that penumbra formation is a relatively fast process (of the
order of hours), it has eluded its observation with sufficient spatial
resolution by both space- and ground-based solar observatories. Only
recently, some researchers have witnessed the onset of both orphan
and sunspot penumbrae in detail. We are one of those. In July 2009, we
observed the early stages of the NOAA 11024 AR leading sunspot while
developing its penumbra. The spectro-polarimetric dataset lead us to
new observational findings. In this contribution, we put into context
our and other authors' results to draw the overall picture of sunspot
formation. Most important, the comparison on the properties of different
types of penumbrae lead us to the conclusion that the formation of
penumbrae is not just one mechanism. While the sole cause necessary
for penumbral magneto-convection is a stably inclined magnetic field,
observations show that inclined fields can be caused by flux emergence,
to form orphan penumbrae, or by field lines transported down from
upper photospheric layers, to form sunspot penumbra. This conclusion,
together with the recent findings by Jur\čák and collaborators on
a canonical value of the vertical component of the magnetic field
blocking the action of penumbral magneto-convection in umbral areas,
is a crucial step forward towards the understanding of the coupling
of solar plasmas and magnetic fields in penumbral atmospheres.
Title: Convective blueshifts in the solar
atmosphere. III. High-accuracy observations of spectral lines in
the visible
Authors: Löhner-Böttcher, J.; Schmidt, W.; Schlichenmaier, R.;
Steinmetz, T.; Holzwarth, R.
Bibcode: 2019A&A...624A..57L
Altcode: 2019arXiv190107606L
Context. Convective motions in the solar atmosphere cause spectral lines
to become asymmetric and shifted in wavelength. For photospheric lines,
this differential Doppler shift varies from the solar disk center
to the limb.
Aims: Precise and comprehensive observations of
the convective blueshift and its center-to-limb variation improve
our understanding of the atmospheric hydrodynamics and ensuing line
formation, and provide the basis to refine 3D models of the solar
atmosphere.
Methods: We performed systematical spectroscopic
measurements of the convective blueshift of the quiet Sun with the
Laser Absolute Reference Spectrograph (LARS) at the German Vacuum
Tower Telescope. The spatial scanning of the solar disk covered
11 heliocentric positions each along four radial (meridional and
equatorial) axes. The high-resolution spectra of 26 photospheric
to chromospheric lines in the visible range were calibrated with a
laser frequency comb to absolute wavelengths at the 1 m s-1
accuracy. Applying ephemeris and reference corrections, the bisector
analysis provided line asymmetries and Doppler shifts with an
uncertainty of only few m s-1. To allow for a comparison
with other observations, we convolved the results to lower spectral
resolutions.
Results: All spectral line bisectors exhibit
a systematic center-to-limb variation. Typically, a blueshifted
"C"-shaped curve at disk center transforms into a less blueshifted
"\"-shape toward the solar limb. The comparison of all lines reveals
the systematic dependence of the convective blueshift on the line
depth. The blueshift of the line minima describe a linear decrease
with increasing line depths. The slope of the center-to-limb variation
develops a reversal point at heliocentric positions between μ = 0.7
and 0.85, seen as the effect of horizontal granular flows in the mid
photosphere. Line minima formed in the upper photosphere to chromosphere
exhibit hardly any blueshift or even a slight redshift. Synthetic
models yield considerable deviations from the observed center-to-limb
variation.
Conclusions: The obtained Doppler shifts of the
quiet Sun can serve as an absolute reference for other observations,
the relative calibration of Dopplergrams, and the necessary refinement
of atmospheric models. Based on this, the development of high-precision
models of stellar surface convection will advance the detection of
(potentially habitable) exoplanets by radial velocity measurements. The reduced spectra are only available at the CDS via anonymous ftp to
http://cdsarc.u-strasbg.fr
(ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/624/A57
Title: VizieR Online Data Catalog: Convective blueshifts in solar
atmos. (Loehner-Boettcher+, 2019)
Authors: Loehner-Boettcher, J.; Schmidt, W.; Schlichenmaier, R.;
Steinmetz, T.; Holzwarth, R.
Bibcode: 2019yCat..36240057L
Altcode:
Solar spectra were observed in eight different spectral regions (525nm,
538nm, 543nm, 557nm, 589nm, 614nm, 617nm, 630nm) with the Laser Absolute
Reference Spectrograph (LARS) at the VTT on Tenerife. The observations
were performed at different heliocentric position mu=cos(alpha)
from disk center (mu=1.0) to near the solar limb (mu=0.3,0.2). These
20min-time-averaged observations were performed along four radial
axes (north, south, east, west). The calibration of the solar spectra
with a laser frequency comb results in an absolute wavelength scale
with an Doppler accuracy of 1m/s. The spectra were recorded with the
spectrographs (resolution greater than 700000) CCD camera. Vacuum
Tower Telescope (VTT), Laser Absolute Reference Spectrograph (LARS),
May 2016 to May 2018. IF Integrated sunlight of 10arcsec field
of view AND Spectral resolution greater than 700000, Temporal average
over 20min at the respective heliocentric position mu. FITS files
contain the average solar spectrum in the same form throughout: *: 2048
pixel, dimension [*,0]: spectral intensity, dimension [*,1]: standard
deviation, dimension [*,2]: absolute wavelength in Angstroem. Data naming includes the instrument, date, time, spectral region,
heliocentric position mu, and abbreviated radial axis (except at disk
center) (2 data files).
Title: Recent advancements in the EST project
Authors: Jurčák, Jan; Collados, Manuel; Leenaarts, Jorrit; van Noort,
Michiel; Schlichenmaier, Rolf
Bibcode: 2019AdSpR..63.1389J
Altcode: 2018arXiv181100851J
The European Solar Telescope (EST) is a project of a new-generation
solar telescope. It has a large aperture of 4 m, which is necessary for
achieving high spatial and temporal resolution. The high polarimetric
sensitivity of the EST will allow to measure the magnetic field in the
solar atmosphere with unprecedented precision. Here, we summarise the
recent advancements in the realisation of the EST project regarding
the hardware development and the refinement of the science requirements.
Title: Magnetic properties of a long-lived sunspot. Vertical magnetic
field at the umbral boundary
Authors: Schmassmann, M.; Schlichenmaier, R.; Bello González, N.
Bibcode: 2018A&A...620A.104S
Altcode: 2018arXiv181009358S
Context. In a recent statistical study of sunspots in 79 active regions,
the vertical magnetic field component Bver averaged along the
umbral boundary is found to be independent of sunspot size. The authors
of that study conclude that the absolute value of Bver at the
umbral boundary is the same for all spots.
Aims: We investigate
the temporal evolution of Bver averaged along the umbral
boundary of one long-lived sunspot during its stable phase.
Methods: We analysed data from the HMI instrument on-board SDO. Contours
of continuum intensity at Ic = 0.5Iqs, whereby
Iqs refers to the average over the quiet sun areas, are used
to extract the magnetic field along the umbral boundary. Projection
effects due to different formation heights of the Fe I 617.3 nm line
and continuum are taken into account. To avoid limb artefacts, the
spot is only analysed for heliocentric angles smaller than 60°.
Results: During the first disc passage, NOAA AR 11591, Bver
remains constant at 1693 G with a root-mean-square deviation of 15 G,
whereas the magnetic field strength varies substantially (mean 2171 G,
rms of 48 G) and shows a long term variation. Compensating for formation
height has little influence on the mean value along each contour, but
reduces the variations along the contour when away from disc centre,
yielding a better match between the contours of Bver =
1693 G and Ic = 0.5Iqs.
Conclusions:
During the disc passage of a stable sunspot, its umbral boundary can
equivalently be defined by using the continuum intensity Ic
or the vertical magnetic field component Bver. Contours of
fixed magnetic field strength fail to outline the umbral boundary. Movies associated with Figs. 3 and 5 are available at https://www.aanda.org
Title: Absolute velocity measurements in sunspot umbrae
Authors: Löhner-Böttcher, J.; Schmidt, W.; Schlichenmaier, R.;
Doerr, H. -P.; Steinmetz, T.; Holzwarth, R.
Bibcode: 2018A&A...617A..19L
Altcode: 2018arXiv180408304L
Context. In sunspot umbrae, convection is largely suppressed by the
strong magnetic field. Previous measurements reported on negligible
convective flows in umbral cores. Based on this, numerous studies have
taken the umbra as zero reference to calculate Doppler velocities of
the ambient active region.
Aims: We aim to clarify the amount of
convective motion in the darkest part of umbrae, by directly measuring
Doppler velocities with an unprecedented accuracy and precision.
Methods: We performed spectroscopic observations of sunspot umbrae with
the Laser Absolute Reference Spectrograph (LARS) at the German Vacuum
Tower Telescope. A laser frequency comb enabled the calibration of
the high-resolution spectrograph and absolute wavelength positions for
13 observation sequences. A thorough spectral calibration, including
the measurement of the reference wavelength, yielded Doppler shifts
of the spectral line Ti I 5713.9 Å with an uncertainty of around 5
m s-1. A bisector analysis gave the depth-dependent line
asymmetry.
Results: The measured Doppler shifts are a composition
of umbral convection and magneto-acoustic waves. For the analysis of
convective shifts, we temporally averaged each sequence to reduce the
superimposed wave signal. Compared to convective blueshifts of up to
-350 m s-1 in the quiet Sun, sunspot umbrae yield strongly
reduced convective blueshifts around -30 m s-1. We find
that the velocity in a sunspot umbra correlates significantly with the
magnetic field strength, but also with the umbral temperature defining
the depth of the Ti I 5713.9 Å line. The vertical upward motion
decreases with increasing field strength. Extrapolating the linear
approximation to zero magnetic field reproduces the measured quiet Sun
blueshift. In the same manner, we find that the convective blueshift
decreases as a function of increasing line depth.
Conclusions:
Simply taking the sunspot umbra as a zero velocity reference for the
calculation of photospheric Dopplergrams can imply a systematic velocity
error reaching 100 m s-1, or more. Setting up a relationship
between vertical velocities and magnetic field strength provides a
remedy for solar spectropolarimetry. We propose a novel approach of
substantially increasing the accuracy of the Doppler velocities of a
sunspot region by including the magnetic field information to define
the umbral reference velocity.
Title: The magnetic nature of umbra-penumbra boundary in sunspots
Authors: Jurčák, J.; Rezaei, R.; González, N. Bello; Schlichenmaier,
R.; Vomlel, J.
Bibcode: 2018A&A...611L...4J
Altcode: 2018arXiv180108983J
Context. Sunspots are the longest-known manifestation of solar
activity, and their magnetic nature has been known for more than a
century. Despite this, the boundary between umbrae and penumbrae,
the two fundamental sunspot regions, has hitherto been solely
defined by an intensity threshold. Aim. Here, we aim at studying the
magnetic nature of umbra-penumbra boundaries in sunspots of different
sizes, morphologies, evolutionary stages, and phases of the solar
cycle.
Methods: We used a sample of 88 scans of the Hinode/SOT
spectropolarimeter to infer the magnetic field properties in at the
umbral boundaries. We defined these umbra-penumbra boundaries by
an intensity threshold and performed a statistical analysis of the
magnetic field properties on these boundaries.
Results: We
statistically prove that the umbra-penumbra boundary in stable sunspots
is characterised by an invariant value of the vertical magnetic field
component: the vertical component of the magnetic field strength
does not depend on the umbra size, its morphology, and phase of the
solar cycle. With the statistical Bayesian inference, we find that the
strength of the vertical magnetic field component is, with a likelihood
of 99%, in the range of 1849-1885 G with the most probable value
of 1867 G. In contrast, the magnetic field strength and inclination
averaged along individual boundaries are found to be dependent on the
umbral size: the larger the umbra, the stronger and more horizontal
the magnetic field at its boundary.
Conclusions: The umbra and
penumbra of sunspots are separated by a boundary that has hitherto been
defined by an intensity threshold. We now unveil the empirical law of
the magnetic nature of the umbra-penumbra boundary in stable sunspots:
it is an invariant vertical component of the magnetic field.
Title: Flows along arch filaments observed in the GRIS `very fast
spectroscopic mode'
Authors: González Manrique, S. J.; Denker, C.; Kuckein, C.; Pastor
Yabar, A.; Collados, M.; Verma, M.; Balthasar, H.; Diercke, A.;
Fischer, C. E.; Gömöry, P.; Bello González, N.; Schlichenmaier,
R.; Cubas Armas, M.; Berkefeld, T.; Feller, A.; Hoch, S.; Hofmann,
A.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Schmidt, D.; Schmidt,
W.; Sigwarth, M.; Sobotka, M.; Solanki, S. K.; Soltau, D.; Staude,
J.; Strassmeier, K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T.
Bibcode: 2017IAUS..327...28G
Altcode: 2017arXiv170102206G
A new generation of solar instruments provides improved spectral,
spatial, and temporal resolution, thus facilitating a better
understanding of dynamic processes on the Sun. High-resolution
observations often reveal multiple-component spectral line profiles,
e.g., in the near-infrared He i 10830 Å triplet, which provides
information about the chromospheric velocity and magnetic fine
structure. We observed an emerging flux region, including two small
pores and an arch filament system, on 2015 April 17 with the `very
fast spectroscopic mode' of the GREGOR Infrared Spectrograph (GRIS)
situated at the 1.5-meter GREGOR solar telescope at Observatorio del
Teide, Tenerife, Spain. We discuss this method of obtaining fast (one
per minute) spectral scans of the solar surface and its potential to
follow dynamic processes on the Sun. We demonstrate the performance
of the `very fast spectroscopic mode' by tracking chromospheric
high-velocity features in the arch filament system.
Title: Penumbral thermal structure below the visible surface
Authors: Borrero, J. M.; Franz, M.; Schlichenmaier, R.; Collados,
M.; Asensio Ramos, A.
Bibcode: 2017A&A...601L...8B
Altcode: 2017arXiv170502832B
Context. The thermal structure of the penumbra below its visible
surface (I.e., τ5 ≥ 1) has important implications for
our present understanding of sunspots and their penumbrae: their
brightness and energy transport, mode conversion of magneto-acoustic
waves, sunspot seismology, and so forth.
Aims: We aim at
determining the thermal stratification in the layers immediately
beneath the visible surface of the penumbra: τ5 ∈ [1,3]
(≈70-80 km below the visible continuum-forming layer)
Methods:
We analyzed spectropolarimetric data (I.e., Stokes profiles) in three Fe
I lines located at 1565 nm observed with the GRIS instrument attached
to the 1.5-m solar telescope GREGOR. The data are corrected for the
smearing effects of wide-angle scattered light and then subjected
to an inversion code for the radiative transfer equation in order
to retrieve, among others, the temperature as a function of optical
depth T(τ5).
Results: We find that the temperature
gradient below the visible surface of the penumbra is smaller than
in the quiet Sun. This implies that in the region τ5 ≥
1 the penumbral temperature diverges from that of the quiet Sun. The
same result is obtained when focusing only on the thermal structure
below the surface of bright penumbral filaments.
Conclusions:
We interpret these results as evidence of a thick penumbra, whereby
the magnetopause is not located near its visible surface. In addition,
we find that the temperature gradient in bright penumbral filaments is
lower than in granules. This can be explained in terms of the limited
expansion of a hot upflow inside a penumbral filament relative to
a granular upflow, as magnetic pressure and tension forces from the
surrounding penumbral magnetic field hinder an expansion like this.
Title: A distinct magnetic property of the inner penumbral
boundary. II. Formation of a penumbra at the expense of a pore
Authors: Jurčák, J.; Bello González, N.; Schlichenmaier, R.;
Rezaei, R.
Bibcode: 2017A&A...597A..60J
Altcode: 2016arXiv161201745J; 2016A&A...597A..60J
Context. We recently presented evidence that stable
umbra-penumbra boundaries are characterised by a distinct
canonical value of the vertical component of the magnetic
field, Bstablever. In order to trigger
the formation of a penumbra, large inclinations in the magnetic
field are necessary. In sunspots, the penumbra develops and
establishes by colonising both umbral areas and granulation, that
is, penumbral magneto-convection takes over in umbral regions with
Bver<Bstablever, as well as
in granular convective areas. Eventually, a stable umbra-penumbra
boundary settles at Bstablever.
Aims:
Here, we aim to study the development of a penumbra initiated at
the boundary of a pore, where the penumbra colonises the entire pore
ultimately.
Methods: We have used Hinode/SOT G-band images to
study the evolution of the penumbra. Hinode/SOT spectropolarimetric
data were used to infer the magnetic field properties in the studied
region.
Results: The penumbra forms at the boundary of a pore
located close to the polarity inversion line of NOAA 10960. As the
penumbral bright grains protrude into the pore, the magnetic flux in
the forming penumbra increases at the expense of the pore magnetic
flux. Consequently, the pore disappears completely giving rise to an
orphan penumbra. At all times, the vertical component of the magnetic
field in the pore is smaller than Bstablever
≈ 1.8 kG.
Conclusions: Our findings are in an agreement
with the need of Bstablever for establishing
a stable umbra-penumbra boundary: while Bver in
the pore is smaller than Bstablever, the
protrusion of penumbral grains into the pore area is not blocked,
a stable pore-penumbra boundary does not establish, and the pore
is fully overtaken by the penumbral magneto-convective mode. This
scenario could also be one of the mechanisms giving rise to orphan
penumbrae. The movie associated to Fig. 1 is available at http://www.aanda.org
Title: Canonical Bver value on umbra/penumbra boundaries
Authors: Jurcak, Jan; Bello González, Nazaret; Schlichenmaier, Rolf;
Rezaei, Reza
Bibcode: 2017psio.confE.112J
Altcode:
No abstract at ADS
Title: Slipping reconnection in a solar flare observed in high
resolution with the GREGOR solar telescope
Authors: Sobotka, M.; Dudík, J.; Denker, C.; Balthasar, H.; Jurčák,
J.; Liu, W.; Berkefeld, T.; Collados Vera, M.; Feller, A.; Hofmann,
A.; Kneer, F.; Kuckein, C.; Lagg, A.; Louis, R. E.; von der Lühe, O.;
Nicklas, H.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth,
M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.;
Volkmer, R.; Waldmann, T.
Bibcode: 2016A&A...596A...1S
Altcode: 2016arXiv160500464S
A small flare ribbon above a sunspot umbra in active region 12205 was
observed on November 7, 2014, at 12:00 UT in the blue imaging channel
of the 1.5 m GREGOR telescope, using a 1 Å Ca II H interference
filter. Context observations from the Atmospheric Imaging Assembly
(AIA) onboard the Solar Dynamics Observatory (SDO), the Solar Optical
Telescope (SOT) onboard Hinode, and the Interface Region Imaging
Spectrograph (IRIS) show that this ribbon is part of a larger one
that extends through the neighboring positive polarities and also
participates in several other flares within the active region. We
reconstructed a time series of 140 s of Ca II H images by means of the
multiframe blind deconvolution method, which resulted in spatial and
temporal resolutions of 0.1″ and 1 s. Light curves and horizontal
velocities of small-scale bright knots in the observed flare ribbon
were measured. Some knots are stationary, but three move along the
ribbon with speeds of 7-11 km s-1. Two of them move in the
opposite direction and exhibit highly correlated intensity changes,
which provides evidence of a slipping reconnection at small spatial
scales. Movies associated to Figs. 1 and 2 are available at http://www.aanda.org
Title: Deep probing of the photospheric sunspot penumbra: no evidence
of field-free gaps
Authors: Borrero, J. M.; Asensio Ramos, A.; Collados, M.;
Schlichenmaier, R.; Balthasar, H.; Franz, M.; Rezaei, R.; Kiess, C.;
Orozco Suárez, D.; Pastor Yabar, A.; Berkefeld, T.; von der Lühe,
O.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Soltau, D.; Volkmer, R.;
Waldmann, T.; Denker, C.; Hofmann, A.; Staude, J.; Strassmeier, K. G.;
Feller, A.; Lagg, A.; Solanki, S. K.; Sobotka, M.; Nicklas, H.
Bibcode: 2016A&A...596A...2B
Altcode: 2016arXiv160708165B
Context. Some models for the topology of the magnetic field in
sunspot penumbrae predict regions free of magnetic fields or with
only dynamically weak fields in the deep photosphere.
Aims:
We aim to confirm or refute the existence of weak-field regions in
the deepest photospheric layers of the penumbra.
Methods:
We investigated the magnetic field at log τ5 = 0 is
by inverting spectropolarimetric data of two different sunspots
located very close to disk center with a spatial resolution of
approximately 0.4-0.45''. The data have been recorded using the GRIS
instrument attached to the 1.5-m solar telescope GREGOR at the El
Teide observatory. The data include three Fe I lines around 1565 nm,
whose sensitivity to the magnetic field peaks half a pressure scale
height deeper than the sensitivity of the widely used Fe I spectral
line pair at 630 nm. Before the inversion, the data were corrected
for the effects of scattered light using a deconvolution method with
several point spread functions.
Results: At log τ5
= 0 we find no evidence of regions with dynamically weak (B<
500 Gauss) magnetic fields in sunspot penumbrae. This result is much
more reliable than previous investigations made on Fe I lines at 630
nm. Moreover, the result is independent of the number of nodes employed
in the inversion, is independent of the point spread function used to
deconvolve the data, and does not depend on the amount of stray light
(I.e., wide-angle scattered light) considered.
Title: Spectropolarimetric observations of an arch filament system
with the GREGOR solar telescope
Authors: Balthasar, H.; Gömöry, P.; González Manrique, S. J.;
Kuckein, C.; Kavka, J.; Kučera, A.; Schwartz, P.; Vašková, R.;
Berkefeld, T.; Collados Vera, M.; Denker, C.; Feller, A.; Hofmann,
A.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Pastor Yabar, A.;
Rezaei, R.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth,
M.; Sobotka, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier,
K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T.
Bibcode: 2016AN....337.1050B
Altcode: 2016arXiv160901514B
Arch filament systems occur in active sunspot groups, where a fibril
structure connects areas of opposite magnetic polarity, in contrast to
active region filaments that follow the polarity inversion line. We
used the GREGOR Infrared Spectrograph (GRIS) to obtain the full
Stokes vector in the spectral lines Si I λ1082.7 nm, He I λ1083.0
nm, and Ca I λ1083.9 nm. We focus on the near-infrared calcium line
to investigate the photospheric magnetic field and velocities, and
use the line core intensities and velocities of the helium line to
study the chromospheric plasma. The individual fibrils of the arch
filament system connect the sunspot with patches of magnetic polarity
opposite to that of the spot. These patches do not necessarily coincide
with pores, where the magnetic field is strongest. Instead, areas are
preferred not far from the polarity inversion line. These areas exhibit
photospheric downflows of moderate velocity, but significantly higher
downflows of up to 30 km s-1 in the chromospheric helium
line. Our findings can be explained with new emerging flux where the
matter flows downward along the field lines of rising flux tubes,
in agreement with earlier results.
Title: Magnetic fields of opposite polarity in sunspot penumbrae
Authors: Franz, M.; Collados, M.; Bethge, C.; Schlichenmaier, R.;
Borrero, J. M.; Schmidt, W.; Lagg, A.; Solanki, S. K.; Berkefeld,
T.; Kiess, C.; Rezaei, R.; Schmidt, D.; Sigwarth, M.; Soltau, D.;
Volkmer, R.; von der Luhe, O.; Waldmann, T.; Orozco, D.; Pastor Yabar,
A.; Denker, C.; Balthasar, H.; Staude, J.; Hofmann, A.; Strassmeier,
K.; Feller, A.; Nicklas, H.; Kneer, F.; Sobotka, M.
Bibcode: 2016A&A...596A...4F
Altcode: 2016arXiv160800513F
Context. A significant part of the penumbral magnetic field returns
below the surface in the very deep photosphere. For lines in the
visible, a large portion of this return field can only be detected
indirectly by studying its imprints on strongly asymmetric and
three-lobed Stokes V profiles. Infrared lines probe a narrow layer
in the very deep photosphere, providing the possibility of directly
measuring the orientation of magnetic fields close to the solar
surface.
Aims: We study the topology of the penumbral magnetic
field in the lower photosphere, focusing on regions where it returns
below the surface.
Methods: We analyzed 71 spectropolarimetric
datasets from Hinode and from the GREGOR infrared spectrograph. We
inferred the quality and polarimetric accuracy of the infrared data
after applying several reduction steps. Techniques of spectral
inversion and forward synthesis were used to test the detection
algorithm. We compared the morphology and the fractional penumbral
area covered by reversed-polarity and three-lobed Stokes V profiles for
sunspots at disk center. We determined the amount of reversed-polarity
and three-lobed Stokes V profiles in visible and infrared data of
sunspots at various heliocentric angles. From the results, we computed
center-to-limb variation curves, which were interpreted in the context
of existing penumbral models.
Results: Observations in visible
and near-infrared spectral lines yield a significant difference in the
penumbral area covered by magnetic fields of opposite polarity. In
the infrared, the number of reversed-polarity Stokes V profiles is
smaller by a factor of two than in the visible. For three-lobed Stokes
V profiles the numbers differ by up to an order of magnitude.
Title: Horizontal flow fields in and around a small active region. The
transition period between flux emergence and decay
Authors: Verma, M.; Denker, C.; Balthasar, H.; Kuckein, C.; González
Manrique, S. J.; Sobotka, M.; Bello González, N.; Hoch, S.; Diercke,
A.; Kummerow, P.; Berkefeld, T.; Collados, M.; Feller, A.; Hofmann,
A.; Kneer, F.; Lagg, A.; Löhner-Böttcher, J.; Nicklas, H.; Pastor
Yabar, A.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Schubert,
M.; Sigwarth, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier,
K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T.
Bibcode: 2016A&A...596A...3V
Altcode: 2016arXiv160507462V
Context. The solar magnetic field is responsible for all aspects
of solar activity. Thus, emergence of magnetic flux at the surface
is the first manifestation of the ensuing solar activity.
Aims: Combining high-resolution and synoptic observations aims to
provide a comprehensive description of flux emergence at photospheric
level and of the growth process that eventually leads to a mature
active region.
Methods: The small active region NOAA 12118
emerged on 2014 July 17 and was observed one day later with the 1.5-m
GREGOR solar telescope on 2014 July 18. High-resolution time-series
of blue continuum and G-band images acquired in the blue imaging
channel (BIC) of the GREGOR Fabry-Pérot Interferometer (GFPI) were
complemented by synoptic line-of-sight magnetograms and continuum
images obtained with the Helioseismic and Magnetic Imager (HMI) onboard
the Solar Dynamics Observatory (SDO). Horizontal proper motions and
horizontal plasma velocities were computed with local correlation
tracking (LCT) and the differential affine velocity estimator (DAVE),
respectively. Morphological image processing was employed to measure
the photometric and magnetic area, magnetic flux, and the separation
profile of the emerging flux region during its evolution.
Results: The computed growth rates for photometric area, magnetic
area, and magnetic flux are about twice as high as the respective
decay rates. The space-time diagram using HMI magnetograms of five days
provides a comprehensive view of growth and decay. It traces a leaf-like
structure, which is determined by the initial separation of the two
polarities, a rapid expansion phase, a time when the spread stalls,
and a period when the region slowly shrinks again. The separation
rate of 0.26 km s-1 is highest in the initial stage, and
it decreases when the separation comes to a halt. Horizontal plasma
velocities computed at four evolutionary stages indicate a changing
pattern of inflows. In LCT maps we find persistent flow patterns such
as outward motions in the outer part of the two major pores, a diverging
feature near the trailing pore marking the site of upwelling plasma and
flux emergence, and low velocities in the interior of dark pores. We
detected many elongated rapidly expanding granules between the two
major polarities, with dimensions twice as large as the normal granules.
Title: Upper chromospheric magnetic field of a sunspot penumbra:
observations of fine structure
Authors: Joshi, J.; Lagg, A.; Solanki, S. K.; Feller, A.; Collados,
M.; Orozco Suárez, D.; Schlichenmaier, R.; Franz, M.; Balthasar,
H.; Denker, C.; Berkefeld, T.; Hofmann, A.; Kiess, C.; Nicklas, H.;
Pastor Yabar, A.; Rezaei, R.; Schmidt, D.; Schmidt, W.; Sobotka, M.;
Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.; von der Lühe,
O.; Waldmann, T.
Bibcode: 2016A&A...596A...8J
Altcode: 2016arXiv160801988J
Aims: The fine-structure of the magnetic field in a sunspot
penumbra in the upper chromosphere is to be explored and compared
to that in the photosphere.
Methods: Spectropolarimetric
observations with high spatial resolution were recorded with the 1.5-m
GREGOR telescope using the GREGOR Infrared Spectrograph (GRIS). The
observed spectral domain includes the upper chromospheric Hei triplet
at 10 830 Å and the photospheric Sii 10 827.1 Å and Cai 10 833.4 Å
spectral lines. The upper chromospheric magnetic field is obtained
by inverting the Hei triplet assuming a Milne-Eddington-type model
atmosphere. A height-dependent inversion was applied to the Sii 10
827.1 Å and Cai 10 833.4 Å lines to obtain the photospheric magnetic
field.
Results: We find that the inclination of the magnetic
field varies in the azimuthal direction in the photosphere and in the
upper chromosphere. The chromospheric variations coincide remarkably
well with the variations in the inclination of the photospheric field
and resemble the well-known spine and interspine structure in the
photospheric layers of penumbrae. The typical peak-to-peak variations
in the inclination of the magnetic field in the upper chromosphere
are found to be 10°-15°, which is roughly half the variation in
the photosphere. In contrast, the magnetic field strength of the
observed penumbra does not vary on small spatial scales in the upper
chromosphere.
Conclusions: Thanks to the high spatial resolution
of the observations that is possible with the GREGOR telescope at 1.08
microns, we find that the prominent small-scale fluctuations in the
magnetic field inclination, which are a salient part of the property
of sunspot penumbral photospheres, also persist in the chromosphere,
although at somewhat reduced amplitudes. Such a complex magnetic
configuration may facilitate penumbral chromospheric dynamic phenomena,
such as penumbral micro-jets or transient bright dots.
Title: Active region fine structure observed at 0.08 arcsec resolution
Authors: Schlichenmaier, R.; von der Lühe, O.; Hoch, S.; Soltau, D.;
Berkefeld, T.; Schmidt, D.; Schmidt, W.; Denker, C.; Balthasar, H.;
Hofmann, A.; Strassmeier, K. G.; Staude, J.; Feller, A.; Lagg, A.;
Solanki, S. K.; Collados, M.; Sigwarth, M.; Volkmer, R.; Waldmann,
T.; Kneer, F.; Nicklas, H.; Sobotka, M.
Bibcode: 2016A&A...596A...7S
Altcode: 2016arXiv160707094S
Context. The various mechanisms of magneto-convective energy transport
determine the structure of sunspots and active regions.
Aims: We characterise the appearance of light bridges and other
fine-structure details and elaborate on their magneto-convective
nature.
Methods: We present speckle-reconstructed images taken
with the broad-band imager (BBI) at the 1.5 m GREGOR telescope in the
486 nm and 589 nm bands. We estimate the spatial resolution from the
noise characteristics of the image bursts and obtain 0.08″ at 589
nm. We describe structure details in individual best images as well
as the temporal evolution of selected features.
Results: We
find branched dark lanes extending along thin (≈1″) light bridges
in sunspots at various heliocentric angles. In thick (≳ 2″) light
bridges the branches are disconnected from the central lane and have a Y
shape with a bright grain toward the umbra. The images reveal that light
bridges exist on varying intensity levels and that their small-scale
features evolve on timescales of minutes. Faint light bridges show
dark lanes outlined by the surrounding bright features. Dark lanes are
very common and are also found in the boundary of pores. They have a
characteristic width of 0.1″ or smaller. Intergranular dark lanes of
that width are seen in active region granulation.
Conclusions: We
interpret our images in the context of magneto-convective simulations
and findings: while central dark lanes in thin light bridges are
elevated and associated with a density increase above upflows, the dark
lane branches correspond to locations of downflows and are depressed
relative to the adjacent bright plasma. Thick light bridges with central
dark lanes show no projection effect. They have a flat elevated plateau
that falls off steeply at the umbral boundary. There, Y-shaped filaments
form as they do in the inner penumbra. This indicates the presence of
inclined magnetic fields, meaning that the umbral magnetic field is
wrapped around the convective light bridge.
Title: Probing deep photospheric layers of the quiet Sun with high
magnetic sensitivity
Authors: Lagg, A.; Solanki, S. K.; Doerr, H. -P.; Martínez González,
M. J.; Riethmüller, T.; Collados Vera, M.; Schlichenmaier, R.;
Orozco Suárez, D.; Franz, M.; Feller, A.; Kuckein, C.; Schmidt, W.;
Asensio Ramos, A.; Pastor Yabar, A.; von der Lühe, O.; Denker, C.;
Balthasar, H.; Volkmer, R.; Staude, J.; Hofmann, A.; Strassmeier,
K.; Kneer, F.; Waldmann, T.; Borrero, J. M.; Sobotka, M.; Verma, M.;
Louis, R. E.; Rezaei, R.; Soltau, D.; Berkefeld, T.; Sigwarth, M.;
Schmidt, D.; Kiess, C.; Nicklas, H.
Bibcode: 2016A&A...596A...6L
Altcode: 2016arXiv160506324L
Context. Investigations of the magnetism of the quiet Sun are hindered
by extremely weak polarization signals in Fraunhofer spectral
lines. Photon noise, straylight, and the systematically different
sensitivity of the Zeeman effect to longitudinal and transversal
magnetic fields result in controversial results in terms of the strength
and angular distribution of the magnetic field vector.
Aims:
The information content of Stokes measurements close to the diffraction
limit of the 1.5 m GREGOR telescope is analyzed. We took the effects of
spatial straylight and photon noise into account.
Methods: Highly
sensitive full Stokes measurements of a quiet-Sun region at disk center
in the deep photospheric Fe I lines in the 1.56 μm region were obtained
with the infrared spectropolarimeter GRIS at the GREGOR telescope. Noise
statistics and Stokes V asymmetries were analyzed and compared to a
similar data set of the Hinode spectropolarimeter (SOT/SP). Simple
diagnostics based directly on the shape and strength of the profiles
were applied to the GRIS data. We made use of the magnetic line ratio
technique, which was tested against realistic magneto-hydrodynamic
simulations (MURaM).
Results: About 80% of the GRIS spectra
of a very quiet solar region show polarimetric signals above a 3σ
level. Area and amplitude asymmetries agree well with small-scale
surface dynamo-magneto hydrodynamic simulations. The magnetic line ratio
analysis reveals ubiquitous magnetic regions in the ten to hundred Gauss
range with some concentrations of kilo-Gauss fields.
Conclusions:
The GRIS spectropolarimetric data at a spatial resolution of ≈0.̋4
are so far unique in the combination of high spatial resolution scans
and high magnetic field sensitivity. Nevertheless, the unavoidable
effect of spatial straylight and the resulting dilution of the weak
Stokes profiles means that inversion techniques still bear a high risk
of misinterpretating the data.
Title: Flow and magnetic field properties in the trailing sunspots
of active region NOAA 12396
Authors: Verma, M.; Denker, C.; Böhm, F.; Balthasar, H.; Fischer,
C. E.; Kuckein, C.; Bello González, N.; Berkefeld, T.; Collados,
M.; Diercke, A.; Feller, A.; González Manrique, S. J.; Hofmann, A.;
Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Pator Yabar, A.; Rezaei,
R.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.;
Sobotka, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier,
K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T.
Bibcode: 2016AN....337.1090V
Altcode:
Improved measurements of the photospheric and chromospheric
three-dimensional magnetic and flow fields are crucial for a precise
determination of the origin and evolution of active regions. We present
an illustrative sample of multi-instrument data acquired during a
two-week coordinated observing campaign in August 2015 involving,
among others, the GREGOR solar telescope (imaging and near-infrared
spectroscopy) and the space missions Solar Dynamics Observatory (SDO)
and Interface Region Imaging Spectrograph (IRIS). The observations
focused on the trailing part of active region NOAA 12396 with complex
polarity inversion lines and strong intrusions of opposite polarity
flux. The GREGOR Infrared Spectrograph (GRIS) provided Stokes IQUV
spectral profiles in the photospheric Si I λ1082.7 nm line, the
chromospheric He I λ1083.0 nm triplet, and the photospheric Ca I
λ1083.9 nm line. Carefully calibrated GRIS scans of the active region
provided maps of Doppler velocity and magnetic field at different
atmospheric heights. We compare quick-look maps with those obtained
with the ``Stokes Inversions based on Response functions'' (SIR)
code, which furnishes deeper insight into the magnetic properties
of the region. We find supporting evidence that newly emerging flux
and intruding opposite polarity flux are hampering the formation
of penumbrae, i.e., a penumbra fully surrounding a sunspot is only
expected after cessation of flux emergence in proximity to the sunspots.
Title: Three-dimensional structure of a sunspot light bridge
Authors: Felipe, T.; Collados, M.; Khomenko, E.; Kuckein, C.; Asensio
Ramos, A.; Balthasar, H.; Berkefeld, T.; Denker, C.; Feller, A.;
Franz, M.; Hofmann, A.; Joshi, J.; Kiess, C.; Lagg, A.; Nicklas, H.;
Orozco Suárez, D.; Pastor Yabar, A.; Rezaei, R.; Schlichenmaier,
R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka, M.; Solanki,
S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.;
von der Lühe, O.; Waldmann, T.
Bibcode: 2016A&A...596A..59F
Altcode: 2016arXiv161104803F
Context. Active regions are the most prominent manifestations of solar
magnetic fields; their generation and dissipation are fundamental
problems in solar physics. Light bridges are commonly present during
sunspot decay, but a comprehensive picture of their role in the
removal of the photospheric magnetic field is still lacking.
Aims: We study the three-dimensional configuration of a sunspot,
and in particular, its light bridge, during one of the last stages of
its decay.
Methods: We present the magnetic and thermodynamical
stratification inferred from full Stokes inversions of the photospheric
Si I 10 827 Å and Ca I 10 839 Å lines obtained with the GREGOR
Infrared Spectrograph of the GREGOR telescope at the Observatorio del
Teide, Tenerife, Spain. The analysis is complemented by a study of
continuum images covering the disk passage of the active region, which
are provided by the Helioseismic and Magnetic Imager on board the Solar
Dynamics Observatory.
Results: The sunspot shows a light bridge
with penumbral continuum intensity that separates the central umbra from
a smaller umbra. We find that in this region the magnetic field lines
form a canopy with lower magnetic field strength in the inner part. The
photospheric light bridge is dominated by gas pressure (high-β),
as opposed to the surrounding umbra, where the magnetic pressure
is higher. A convective flow is observed in the light bridge. This
flow is able to bend the magnetic field lines and to produce field
reversals. The field lines merge above the light bridge and become
as vertical and strong as in the surrounding umbra. We conclude that
this occurs because two highly magnetized regions approach each other
during the sunspot evolution. Movies associated to Figs. 2 and 13
are available at http://www.aanda.org
Title: Inference of magnetic fields in the very quiet Sun
Authors: Martínez González, M. J.; Pastor Yabar, A.; Lagg, A.;
Asensio Ramos, A.; Collados, M.; Solanki, S. K.; Balthasar, H.;
Berkefeld, T.; Denker, C.; Doerr, H. P.; Feller, A.; Franz, M.;
González Manrique, S. J.; Hofmann, A.; Kneer, F.; Kuckein, C.;
Louis, R.; von der Lühe, O.; Nicklas, H.; Orozco, D.; Rezaei, R.;
Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka,
M.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Verma, M.; Waldman,
T.; Volkmer, R.
Bibcode: 2016A&A...596A...5M
Altcode: 2018arXiv180410089M
Context. Over the past 20 yr, the quietest areas of the solar surface
have revealed a weak but extremely dynamic magnetism occurring
at small scales (<500 km), which may provide an important
contribution to the dynamics and energetics of the outer layers of
the atmosphere. Understanding this magnetism requires the inference
of physical quantities from high-sensitivity spectro-polarimetric
data with high spatio-temporal resolution.
Aims: We present
high-precision spectro-polarimetric data with high spatial resolution
(0.4'') of the very quiet Sun at 1.56 μm obtained with the GREGOR
telescope to shed some light on this complex magnetism.
Methods:
We used inversion techniques in two main approaches. First, we assumed
that the observed profiles can be reproduced with a constant magnetic
field atmosphere embedded in a field-free medium. Second, we assumed
that the resolution element has a substructure with either two constant
magnetic atmospheres or a single magnetic atmosphere with gradients of
the physical quantities along the optical depth, both coexisting with
a global stray-light component.
Results: Half of our observed
quiet-Sun region is better explained by magnetic substructure within
the resolution element. However, we cannot distinguish whether this
substructure comes from gradients of the physical parameters along the
line of sight or from horizontal gradients (across the surface). In
these pixels, a model with two magnetic components is preferred, and
we find two distinct magnetic field populations. The population with
the larger filling factor has very weak ( 150 G) horizontal fields
similar to those obtained in previous works. We demonstrate that the
field vector of this population is not constrained by the observations,
given the spatial resolution and polarimetric accuracy of our data. The
topology of the other component with the smaller filling factor is
constrained by the observations for field strengths above 250 G:
we infer hG fields with inclinations and azimuth values compatible
with an isotropic distribution. The filling factors are typically
below 30%. We also find that the flux of the two polarities is not
balanced. From the other half of the observed quiet-Sun area 50% are
two-lobed Stokes V profiles, meaning that 23% of the field of view
can be adequately explained with a single constant magnetic field
embedded in a non-magnetic atmosphere. The magnetic field vector and
filling factor are reliable inferred in only 50% based on the regular
profiles. Therefore, 12% of the field of view harbour hG fields with
filling factors typically below 30%. At our present spatial resolution,
70% of the pixels apparently are non-magnetised.
Title: Fitting peculiar spectral profiles in He I 10830Å absorption
features
Authors: González Manrique, S. J.; Kuckein, C.; Pastor Yabar, A.;
Collados, M.; Denker, C.; Fischer, C. E.; Gömöry, P.; Diercke, A.;
Bello González, N.; Schlichenmaier, R.; Balthasar, H.; Berkefeld, T.;
Feller, A.; Hoch, S.; Hofmann, A.; Kneer, F.; Lagg, A.; Nicklas, H.;
Orozco Suárez, D.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka,
M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Verma,
M.; Volkmer, R.; von der Lühe, O.; Waldmann, T.
Bibcode: 2016AN....337.1057G
Altcode: 2016arXiv160300679G
The new generation of solar instruments provides better
spectral, spatial, and temporal resolution for a better
understanding of the physical processes that take place on the
Sun. Multiple-component profiles are more commonly observed with these
instruments. Particularly, the He I 10830 Å triplet presents such
peculiar spectral profiles, which give information on the velocity
and magnetic fine structure of the upper chromosphere. The purpose
of this investigation is to describe a technique to efficiently fit
the two blended components of the He I 10830 Å triplet, which are
commonly observed when two atmospheric components are located within
the same resolution element. The observations used in this study were
taken on 2015 April 17 with the very fast spectroscopic mode of the
GREGOR Infrared Spectrograph (GRIS) attached to the 1.5-m GREGOR solar
telescope, located at the Observatorio del Teide, Tenerife, Spain. We
apply a double-Lorentzian fitting technique using Levenberg-Marquardt
least-squares minimization. This technique is very simple and much
faster than inversion codes. Line-of-sight Doppler velocities can
be inferred for a whole map of pixels within just a few minutes. Our
results show sub- and supersonic downflow velocities of up to 32 km
s-1 for the fast component in the vicinity of footpoints of
filamentary structures. The slow component presents velocities close
to rest.
Title: Flows in and around Active Region NOAA12118 Observed with
the GREGOR Solar Telescope and SDO/HMI
Authors: Verma, M.; Denker, C.; Balthasar, H.; Kuckein, C.; González
Manrique, S. J.; Sobotka, M.; Bello González, N.; Hoch, S.; Diercke,
A.; Kummerow, P.; Berkefeld, T.; Collados, M.; Feller, A.; Hofmann,
A.; Kneer, F.; Lagg, A.; Löhner-Böttcher, J.; Nicklas, H.; Pastor
Yabar, A.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Schubert,
M.; Sigwarth, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier,
K.; Volkmer, R.; von der Lühe, O.; Waldmann, T.
Bibcode: 2016ASPC..504...29V
Altcode: 2016arXiv160301109V
Accurate measurements of magnetic and velocity fields in and around
solar active regions are key to unlocking the mysteries of the
formation and the decay of sunspots. High spatial resolution images
and spectral sequences with a high cadence obtained with the GREGOR
solar telescope give us an opportunity to scrutinize 3-D flow fields
with local correlation tracking and imaging spectroscopy. We present
GREGOR early science data acquired in 2014 July - August with the GREGOR
Fabry-Pérot Interferometer and the Blue Imaging Channel. Time-series
of blue continuum (λ 450.6 nm) images of the small active region
NOAA 12118 were restored with the speckle masking technique to derive
horizontal proper motions and to track the evolution of morphological
changes. In addition, high-resolution observations are discussed in
the context of synoptic data from the Solar Dynamics Observatory.
Title: A distinct magnetic property of the inner penumbral
boundary. Formation of a stable umbra-penumbra boundary in a sunspot
Authors: Jurčák, J.; Bello González, N.; Schlichenmaier, R.;
Rezaei, R.
Bibcode: 2015A&A...580L...1J
Altcode:
Context. A sunspot emanates from a growing pore or protospot. In order
to trigger the formation of a penumbra, large inclinations at the
outskirts of the protospot are necessary. The penumbra develops and
establishes by colonising both umbral areas and granulation. Evidence
for a unique stable boundary value for the vertical component of the
magnetic field strength, Bstablever, was found
along the umbra-penumbra boundary of developed sunspots.
Aims: We
study the changing value of Bver as the penumbra forms and as
it reaches a stable state. We compare this with the corresponding value
in fully developed penumbrae.
Methods: We use broadband G-band
images and spectropolarimetric GFPI/VTT data to study the evolution
of and the vertical component of the magnetic field on a forming
umbra-penumbra boundary. For comparison with stable sunspots, we also
analyse the two maps observed by Hinode/SP on the same spot after the
penumbra formed.
Results: The vertical component of the magnetic
field, Bver, at the umbra-penumbra boundary increases during
penumbra formation owing to the incursion of the penumbra into umbral
areas. After 2.5 h, the penumbra reaches a stable state as shown
by the GFPI data. At this stable stage, the simultaneous Hinode/SP
observations show a Bver value comparable to that of
umbra-penumbra boundaries of fully fledged sunspots.
Conclusions:
We confirm that the umbra-penumbra boundary, traditionally defined by
an intensity threshold, is also characterised by a distinct canonical
magnetic property, namely by Bverstable. During
the penumbra formation process, the inner penumbra extends
into regions where the umbra previously prevailed. Hence, in
areas where Bver<Bstablever,
the magneto-convection mode operating in the umbra turns into a
penumbral mode. Eventually, the inner penumbra boundary settles at
Bverstable, which hints toward the role of
Bverstable as inhibitor of the penumbral mode
of magneto-convection.
Title: A distinct magnetic property of the inner penumbral boundary
Authors: Jurčák, Jan; Bello Gonzalez, Nazaret; Schlichenmaier,
Rolf; Rezaei, Reza
Bibcode: 2015arXiv150608574J
Altcode:
A sunspot emanates from a growing pore or protospot. In order to
trigger the formation of a penumbra, large inclinations at the
outskirts of the protospot are necessary. The penumbra develops and
establishes by colonising both umbral areas and granulation. Evidence
for a unique stable boundary value for the vertical component of the
magnetic field strength, $B^{\rm stable}_{\rm ver}$, was found along
the umbra-penumbra boundary of developed sunspots. We use broadband
G-band images and spectropolarimetric GFPI/VTT data to study the
evolution of and the vertical component of the magnetic field on a
forming umbra-penumbra boundary. For comparison with stable sunspots,
we also analyse the two maps observed by Hinode/SP on the same spot
after the penumbra formed. The vertical component of the magnetic
field, $B_{\rm ver}$, at the umbra-penumbra boundary increases
during penumbra formation owing to the incursion of the penumbra into
umbral areas. After 2.5 hours, the penumbra reaches a stable state
as shown by the GFPI data. At this stable stage, the simultaneous
Hinode/SP observations show a $B_{\rm ver}$ value comparable to that of
umbra-penumbra boundaries of fully fledged sunspots. We confirm that
the umbra-penumbra boundary, traditionally defined by an intensity
threshold, is also characterised by a distinct canonical magnetic
property, namely by $B^{\rm stable}_{\rm ver}$. During the penumbra
formation process, the inner penumbra extends into regions where the
umbra previously prevailed. Hence, in areas where $B_{\rm ver} <
B^{\rm stable}_{\rm ver}$, the magneto-convection mode operating in
the umbra turns into a penumbral mode. Eventually, the inner penumbra
boundary settles at $B^{\rm stable}_{\rm ver}$, which hints toward the
role of $B_{\rm ver}^{\rm stable}$ as inhibitor of the penumbral mode
of magneto-convection.
Title: Evolution of magnetic field inclination in a forming penumbra
Authors: Jurčák, Jan; Bello González, Nazaret; Schlichenmaier,
Rolf; Rezaei, Reza
Bibcode: 2014PASJ...66S...3J
Altcode: 2014PASJ..tmp...93J
As a sunspot penumbra forms, the magnetic field vector at the outer
boundary of the protospot undergoes a transformation. We study the
changes of the magnetic field vector at this boundary as a penumbral
segment forms. We analyze a set of spectropolarimetric maps covering
2 hr during the formation of a sunspot in NOAA 11024. The data were
recorded with the GFPI instrument attached to the German VTT. We
observe a stationary umbra/quiet Sun boundary, where the magnetic
field becomes more horizontal with time. The magnetic field inclination
increases by 5°, reaching a maximum value of about 59°. The maximum
inclination coincides with the onset of filament formation. In time,
the penumbra filaments become longer and the penumbral bright grains
protrude into the umbra, where the magnetic field is stronger and
more vertical. Consequently, we observe a decrease in the magnetic
field inclination at the boundary as the penumbra grows. In summary,
in order to initiate the formation of the penumbra, the magnetic
field at the umbral (protospot) boundary becomes more inclined. As
the penumbra grows, the umbra/penumbra boundary migrates inwards,
and at this boundary the magnetic field turns more vertical again,
while it remains inclined in the outer penumbra.
Title: Stray-light correction in 2D spectroscopy
Authors: Schlichenmaier, R.; Franz, M.
Bibcode: 2013A&A...555A..84S
Altcode:
Context. In solar physics, spectropolarimeters based on Fabry-Pérot
interferometers are commonly used for high spatial resolution
observations. In the data pipeline, corrections for scattered light may
be performed on each narrow band image.
Aims: We elaborate on
the effects of stray-light corrections on Doppler maps.
Methods:
First, we demonstrate the basic correction effect in a simplified
situation of two profiles that suffer from stray light. Then, we study
the correction effects on velocity maps by transforming a Hinode SP
map into a two-dimensional spectroscopic data set with i(x,y) at each
wavelength point, which mimicks narrow band images. Velocity maps are
inferred from line profiles of original and stray-light corrected
data.
Results: The correction of scattered light in narrow
band images affects the inferred Doppler velocity maps: relative red
shifts always become more red, and relative blue shifts become more
blue. This trend is independent of whether downflows have dark or bright
intensities. As a result, the effects of overcorrection produce both
downflows and upflows.
Conclusions: In 2D spectropolarimetry,
corrections for scattered light can improve the image intensity and
velocity contrast but inherently produce downflow signatures in the
penumbra. Hence, such corrections are justified only if the properties
of the stray light (seeing, telescope, and instrument) are well known.
Title: Correlations between sunspots and their moat flows
Authors: Löhner-Böttcher, J.; Schlichenmaier, R.
Bibcode: 2013A&A...551A.105L
Altcode: 2013arXiv1301.2434L
Context. The presence of the moat flow around sunspots is intimately
linked to the mere existence of sunspots.
Aims: We characterize
the moat flow (MF) and Evershed flow (EF) in sunspots to enhance our
knowledge of sunspot structures and photospheric flow properties.
Methods: We calibrated HMI synoptic Doppler maps and used them
to analyze 3 h time averages of 31 circular, stable, and fully
developed sunspots at heliocentric angles of some 50°. Assuming
axially symmetrical flow fields, we infer the azimuthally averaged
horizontal velocity component of the MF and EF from 51 velocity maps. We
studied the MF properties (velocity and extension) and elaborate on
how these components depend on sunspot parameters (sunspot size and
EF velocity). To explore the weekly and monthly evolution of MFs, we
compare spots rotating from the eastern to western limbs and spots that
reappear on the eastern limb.
Results: Our calibration procedure
of HMI Doppler maps yields reliable and consistent results. In 3 h
averages, we find the MF decreases on average from some 1000 ± 200 m/s
just outside the spot boundary to 500 m/s after an additional 4 Mm. The
average MF extension lies at 9.2 ± 5 Mm, where the velocity drops
below some 180 m/s. Neither the MF velocity nor its extension depend
significantly on the sunspot size or EF velocity. But, the EF velocity
does show a tendency to be enhanced with sunspot size. On a time scale
of a week and a month, we find decreasing MF extensions and a tendency
for the MF velocity to increase for strongly decaying sunspots, whereas
the changing EF velocity has no impact on the MF.
Conclusions:
On 3 h averages, the EF velocity scales with the size of sunspots, while
the MF properties show no significant correlation with the EF or with
the sunspot size. This we interpret as a hint that the physical origins
of EF and MF are distinct. Appendix A is available in electronic
form at http://www.aanda.org
Title: The velocity field of sunspot penumbrae. II. Return flow and
magnetic fields of opposite polarity
Authors: Franz, M.; Schlichenmaier, R.
Bibcode: 2013A&A...550A..97F
Altcode: 2012arXiv1212.4732F
Aims: We search for penumbral magnetic fields of opposite
polarity and for their correspondence with downflows.
Methods:
We used spectropolarimetric HINODE data of a spot very close to
disk center to suppress the horizontal velocity components as much
as possible. We focus our study on 3-lobe Stokes V profiles.
Results: From forward modeling and inversions, we show that 3-lobe
profiles testify to the presence of opposite magnetic fields. They occur
predominately in the mid and outer penumbra and are associated with
downflows in the deep layers of the photosphere.
Conclusions:
Standard magnetograms show that only 4% of the penumbral area harbors
magnetic fields of opposite polarity. If 3-lobe profiles are included
in the analysis, this number increases to 17%.
Title: 2nd ATST-EAST Workshop in Solar Physics: Magnetic Fields from
the Photosphere to the Corona
Authors: Rimmele, T. R.; Tritschler, A.; Wöger, F.; Collados Vera,
M.; Socas-Navarro, H.; Schlichenmaier, R.; Carlsson, M.; Berger, T.;
Cadavid, A.; Gilbert, P. R.; Goode, P. R.; Knölker, M.
Bibcode: 2012ASPC..463.....R
Altcode:
No abstract at ADS
Title: Comparing Simultaneous Measurements of two High-Resolution
Imaging Spectropolarimeters: The `Göttingen' FPI@VTT and CRISP@SST
Authors: Bello González, N.; Bellot Rubio, L. R.; Ortiz, A.; Rezaei,
R.; Rouppe van der Voort, L.; Schlichenmaier, R.
Bibcode: 2012ASPC..463..251B
Altcode: 2012arXiv1204.1023B
In July 2009, the leading spot of the active region NOAA11024 was
observed simultaneously and independently with the ‘Göttingen’
FPI at VTT and CRISP at SST, i.e., at two different sites,
telescopes, instruments and using different spectral lines. The data
processing and data analysis have been carried out independently
with different techniques. Maps of physical parameters retrieved
from 2D spectro-polarimetric data observed with ‘Göttingen’
FPI and CRISP show an impressive agreement. In addition, the
‘Göttingen’ FPI maps also exhibit a notable resemblance with
simultaneous TIP (spectrographic) observations. The consistency in the
results demonstrates the excellent capabilities of these observing
facilities. Besides, it confirms the solar origin of the detected
signals and the reliability of FPI-based spectro-polarimeters.
Title: GRIS: The GREGOR Infrared Spectrograph
Authors: Collados, M.; López, R.; Páez, E.; Hernández, E.; Reyes,
M.; Calcines, A.; Ballesteros, E.; Díaz, J. J.; Denker, C.; Lagg,
A.; Schlichenmaier, R.; Schmidt, W.; Solanki, S. K.; Strassmeier,
K. G.; von der Lühe, O.; Volkmer, R.
Bibcode: 2012AN....333..872C
Altcode:
This paper describes the main characteristics of GRIS (GREGOR Infrared
Spectrograph), the grating spectrograph installed in the recently
inaugurated (May 2012) 1.5-meter GREGOR telescope located at the
Observatorio del Teide in Tenerife. The spectrograph has a standard
Czerny-Turner configuration with parabolic collimator and camera mirrors
that belong to the same conic surface. Although nothing prevents its
use at visible wavelengths, the spectrograph will be initially used
in combination with the infrared detector of the Tenerife Infrared
Polarimeter (TIP-II) in standard spectroscopic mode as well as for
spectropolarimetric measurements.
Title: Supersonic Magnetic Flows in the Quiet Sun Observed with
SUNRISE/IMaX
Authors: Borrero, J. M.; Pillet, V. M.; Schlichenmaier, R.; Schmidt,
W.; Berkefeld, T.; Solanki, S. K.; Bonet, J. A.; Iniesta, J. C. d. T.;
Domingo, V.; Barthol, P.; Gandorfer, A.
Bibcode: 2012ASPC..455..155B
Altcode: 2012arXiv1202.4354B
In this contribution we describe some recent observations of high-speed
magnetized flows in the quiet Sun granulation. These observations
were carried out with the Imaging Magnetograph eXperiment (IMaX)
onboard the stratospheric balloon SUNRISE, and possess an unprecedented
spatial resolution and temporal cadence. These flows were identified as
highly shifted circular polarization (Stokes V) signals. We estimate
the LOS velocity responsible for these shifts to be larger than 6 km
s-1, and therefore we refer to them as supersonic magnetic
flows. The average lifetime of the detected events is 81.3 s and
they occupy an average area of about 23 000 km2. Most of
the events occur within granular cells and correspond therefore to
upflows. However some others occur in intergranular lanes or bear no
clear relation to the convective velocity pattern. We analyze a number
of representative examples and discuss them in terms of magnetic loops,
reconnection events, and convective collapse.
Title: On the Formation of Penumbrae as Observed with the German
VTT SOHO/MDI, and SDO/HMI
Authors: Schlichenmaier, R.; Rezaei, R.; González, N. B.
Bibcode: 2012ASPC..455...61S
Altcode: 2011arXiv1102.0965S
Solar magnetic fields are generated in the solar interior and pop up
at the solar surface to form active regions. As the magnetic field
appears on the surface, it forms various structures like small magnetic
elements, pores, and sunspots. The nature of these formation processes
is largely unknown. In this contribution we elaborate on the formation
of sunspots and particularly on the formation of penumbrae. We report
on observations that we obtained at the German Vacuum Tower Telescope
(VTT) on July 4, 2009 on the formation of the spot in AR 11024. This
data set is complemented with data from the Michelson Doppler Imager
(MDI) aboard SOHO, which offers an entire time coverage. Moreover, the
evolution of AR 11024 is compared with a particular event of penumbra
formation in AR 11124 around November 13, 2010, using intensity images
from the Helioseismic and Magnetic Imager (HMI) onboard SDO. We conclude
that two processes contribute to the increase of the magnetic flux of a
sunspot: (1) merging pores, and (2) emerging bipoles of which the spot
polarity migrates towards and merges with the spot. As the penumbra
forms, the area, magnetic flux, and maximum field strength in the
umbra stay constant or increase slightly, i.e., the formation of the
penumbra is associated with flux emergence and flux increase of the
proto-spot. If two pores merge or if a pore merges with a proto-spot
a light bridge is created. This initial light bridge dissolves in the
further evolution.
Title: Shear and vortex motions in a forming sunspot . Twist
relaxation in magnetic flux ropes
Authors: Bello González, N.; Kneer, F.; Schlichenmaier, R.
Bibcode: 2012A&A...538A..62B
Altcode:
Aims: We measure proper motions of fine structures in a forming
sunspot to infer information about the dynamics of flux emergence at
the sub-photospheric level.
Methods: The active region NOAA
11024 was observed with the Vacuum Tower Telescope at Observatorio del
Teide/Tenerife over several days in July 2009. Here, we concentrate
on a two-hour sequence taken on July 4, when the leading spot was
at an early stage of its evolution. Speckle reconstructions from Ca
ii K images and polarimetric data in Fe i λ6173 allow us to study
proper motions of umbral fine structures.
Results: We detect
three prominent features: (1) A light bridge, divided by a dark lane
along its axis, shows proper motions in opposing directions on its
sides, with velocities of ~100-500 m s-1. The flows are
seen in both the Ca ii K and the broadband time sequences. (2) Umbral
dots in one umbral region outline a vortex with speeds of up to 550
m s-1. The direction of the motion of the umbral dots is
different from that in the light bridge. (3) At one rim of the umbra,
the fine structure of the magnetic field moves horizontally with typical
velocities of 250-300 m s-1, prior to the formation of the
penumbra.
Conclusions: We report on shear and vortex motions in
a forming sunspot and interpret them as tracers of twist relaxation
in magnetic flux ropes. We suggest that the forming sunspot contains
detached magnetic flux ropes that emerge at the surface with different
amounts of twist. As they merge to form a sunspot, they untwist giving
rise to the observed shear and vortex motions.
Title: The formation of sunspot penumbra. Magnetic field properties
Authors: Rezaei, R.; Bello González, N.; Schlichenmaier, R.
Bibcode: 2012A&A...537A..19R
Altcode: 2011arXiv1111.3189R
Aims: We study the magnetic flux emergence and formation of
a sunspot penumbra in the active region NOAA 11024.
Methods:
We simultaneously observed the Stokes parameters of the photospheric
iron lines at 1089.6 nm with the TIP and 617.3 nm with the GFPI
spectropolarimeters along with broad-band images using G-band and
Ca ii K filters at the German VTT. The photospheric magnetic field
vector was reconstructed from an inversion of the measured Stokes
profiles. Using the AZAM code, we converted the inclination from
line-of-sight (LOS) to the local reference frame (LRF).
Results:
Individual filaments are resolved in maps of magnetic parameters. The
formation of the penumbra is intimately related to the inclined
magnetic field. No penumbra forms in areas with strong magnetic field
strength and small inclination. Within 4.5 h observing time, the LRF
magnetic flux of the penumbra increases from 9.7 × 1020
to 18.2 × 1020 Mx, while the magnetic flux of the umbra
remains constant at ~3.8 × 1020 Mx. Magnetic flux in the
immediate surroundings is incorporated into the spot, and new flux is
supplied via small flux patches (SFPs), which on average have a flux
of 2-3 × 1018 Mx. The spot's flux increase rate of 4.2 ×
1016 Mx s-1 corresponds to the merging of one
SFP per minute. We also find that, during the formation of the spot
penumbra, a) the maximum magnetic field strength of the umbra does not
change; b) the magnetic neutral line keeps the same position relative
to the umbra; c) the new flux arrives on the emergence side of the
spot while the penumbra forms on the opposite side; d) the average
LRF inclination of the light bridges decreases from 50° to 37°;
and e) as the penumbra develops, the mean magnetic field strength
at the spot border decreases from 1.0 to 0.8 kG.
Conclusions:
The SFPs associated with elongated granules are the building blocks of
structure formation in active regions. During the sunspot formation,
their contribution is comparable to the coalescence of pores. Besides a
set of critical parameters for the magnetic field, a quiet environment
in the surroundings is important for penumbral formation. As remnants
of trapped granulation between merging pores, the light bridges are
found to play a crucial role in the formation process. They seem to
channel the magnetic flux through the spot during its formation. Light
bridges are also the locations where the first penumbral filaments form.
Title: Sunspot Modeling: From Simplified Models to Radiative MHD
Simulations
Authors: Rempel, Matthias; Schlichenmaier, Rolf
Bibcode: 2011LRSP....8....3R
Altcode:
We review our current understanding of sunspots from the scales of their
fine structure to their large scale (global) structure including the
processes of their formation and decay. Recently, sunspot models have
undergone a dramatic change. In the past, several aspects of sunspot
structure have been addressed by static MHD models with parametrized
energy transport. Models of sunspot fine structure have been relying
heavily on strong assumptions about flow and field geometry (e.g.,
flux-tubes, "gaps", convective rolls), which were motivated in
part by the observed filamentary structure of penumbrae or the
necessity of explaining the substantial energy transport required
to maintain the penumbral brightness. However, none of these models
could self-consistently explain all aspects of penumbral structure
(energy transport, filamentation, Evershed flow). In recent years,
3D radiative MHD simulations have been advanced dramatically to the
point at which models of complete sunspots with sufficient resolution
to capture sunspot fine structure are feasible. Here, overturning
convection is the central element responsible for energy transport,
filamentation leading to fine structure, and the driving of strong
outflows. On the larger scale these models are also in the progress
of addressing the subsurface structure of sunspots as well as sunspot
formation. With this shift in modeling capabilities and the recent
advances in high resolution observations, the future research will be
guided by comparing observation and theory.
Title: The formation of a penumbra as observed with the German VTT
and SoHO/MDI
Authors: Schlichenmaier, Rolf; González, Nazaret Bello; Rezaei, Reza
Bibcode: 2011IAUS..273..134S
Altcode: 2010arXiv1009.4457S
The generation of magnetic flux in the solar interior and its transport
to the outer solar atmosphere will be in the focus of solar physics
research for the next decades. One key-ingredient is the process
of magnetic flux emergence into the solar photosphere, and the
reorganization to form the magnetic phenomena of active regions
like sunspots and pores. On July 4, 2009, we observed a region
of emerging magnetic flux, in which a proto-spot without penumbra
forms a penumbra within some 4.5 hours. This process is documented
by multi-wavelength observations at the German VTT: (a) imaging, (b)
data with high resolution and temporal cadence acquired in Fe I 617.3
nm with the 2D imaging spectropolarimter GFPI, and (c) scans with the
slit based spectropolarimeter TIP in Fe I 1089.6 nm. MDI contiuum maps
and magnetograms are used to follow the formation of the proto-spot, and
the subsequent evolution of the entire active region. During the
formation of the penumbra, the area and the magnetic flux of the spot
increases. The additional magnetic flux is supplied by the adjacent
region of emerging magnetic flux: As emerging bipole separate, the
poles of the spot polarity migrate towards the spot, and finally merge
with it. As more and more flux is accumulated, a penumbra forms. From
inversions we infer maps for the magnetic field and the Doppler velocity
(being constant along the line-of-sight). We calculate the magnetic flux
of the forming spot and of the bipole footpoints that merge with the
proto-spot. We witness the onset of the Evershed flow and the associated
enhance of the field inclination as individual penumbral filaments
form. Prior to the formation of individual penumbral sectors we detect
the existence of `counter' Evershed flows. These in-flows turn into
the classical radial Evershed outflows as stable penumbra segments form.
Title: Searching for Overturning Convection in Penumbral Filaments:
Slit Spectroscopy at 0farcs2 Resolution
Authors: Bellot Rubio, L. R.; Schlichenmaier, R.; Langhans, K.
Bibcode: 2010ApJ...725...11B
Altcode: 2010arXiv1009.5650B
Recent numerical simulations of sunspots suggest that overturning
convection is responsible for the existence of penumbral filaments
and the Evershed flow, but there is little observational evidence
of this process. Here, we carry out a spectroscopic search for
small-scale convective motions in the penumbra of a sunspot located
5° away from the disk center. The position of the spot is very
favorable for the detection of overturning downflows at the edges
of penumbral filaments. Our analysis is based on measurements of
the Fe I 709.0 nm line taken with the Littrow spectrograph of the
Swedish 1 m Solar Telescope under excellent seeing conditions. We
compute line bisectors at different intensity levels and derive
Doppler velocities from them. The velocities are calibrated using
a nearby telluric line, with systematic errors smaller than 150 m
s-1. Deep in the photosphere, as sampled by the bisectors
at the 80%-88% intensity levels, we always observe blueshifts or
zero velocities. The maximum blueshifts reach 1.2 km s-1
and tend to be cospatial with bright penumbral filaments. In the line
core, we detect blueshifts for the most part, with small velocities
not exceeding 300 m s-1. Redshifts also occur, but at the
level of 100-150 m s-1, and only occasionally. The fact
that they are visible in high layers casts doubts on their convective
origin. Overall, we do not find indications of downflows that could be
associated with overturning convection at our detection limit of 150
m s-1. Either no downflows exist, or we have been unable
to observe them because they occur beneath τ = 1 or the spatial
resolution/height resolution of the measurements is still insufficient.
Title: Supersonic Magnetic Upflows in Granular Cells Observed with
SUNRISE/IMAX
Authors: Borrero, J. M.; Martínez-Pillet, V.; Schlichenmaier, R.;
Solanki, S. K.; Bonet, J. A.; del Toro Iniesta, J. C.; Schmidt, W.;
Barthol, P.; Gandorfer, A.; Domingo, V.; Knölker, M.
Bibcode: 2010ApJ...723L.144B
Altcode: 2010arXiv1009.1227B
Using the IMaX instrument on board the SUNRISE stratospheric balloon
telescope, we have detected extremely shifted polarization signals
around the Fe I 5250.217 Å spectral line within granules in the solar
photosphere. We interpret the velocities associated with these events
as corresponding to supersonic and magnetic upflows. In addition, they
are also related to the appearance of opposite polarities and highly
inclined magnetic fields. This suggests that they are produced by the
reconnection of emerging magnetic loops through granular upflows. The
events occupy an average area of 0.046 arcsec2 and last for
about 80 s, with larger events having longer lifetimes. These supersonic
events occur at a rate of 1.3 × 10-5 occurrences per second
per arcsec2.
Title: Modeling the Subsurface Structure of Sunspots
Authors: Moradi, H.; Baldner, C.; Birch, A. C.; Braun, D. C.; Cameron,
R. H.; Duvall, T. L.; Gizon, L.; Haber, D.; Hanasoge, S. M.; Hindman,
B. W.; Jackiewicz, J.; Khomenko, E.; Komm, R.; Rajaguru, P.; Rempel,
M.; Roth, M.; Schlichenmaier, R.; Schunker, H.; Spruit, H. C.;
Strassmeier, K. G.; Thompson, M. J.; Zharkov, S.
Bibcode: 2010SoPh..267....1M
Altcode: 2009arXiv0912.4982M; 2010SoPh..tmp..171M
While sunspots are easily observed at the solar surface, determining
their subsurface structure is not trivial. There are two main
hypotheses for the subsurface structure of sunspots: the monolithic
model and the cluster model. Local helioseismology is the only means
by which we can investigate subphotospheric structure. However, as
current linear inversion techniques do not yet allow helioseismology to
probe the internal structure with sufficient confidence to distinguish
between the monolith and cluster models, the development of physically
realistic sunspot models are a priority for helioseismologists. This
is because they are not only important indicators of the variety of
physical effects that may influence helioseismic inferences in active
regions, but they also enable detailed assessments of the validity of
helioseismic interpretations through numerical forward modeling. In
this article, we provide a critical review of the existing sunspot
models and an overview of numerical methods employed to model wave
propagation through model sunspots. We then carry out a helioseismic
analysis of the sunspot in Active Region 9787 and address the serious
inconsistencies uncovered by Gizon et al. (2009a, 2009b). We find that
this sunspot is most probably associated with a shallow, positive
wave-speed perturbation (unlike the traditional two-layer model)
and that travel-time measurements are consistent with a horizontal
outflow in the surrounding moat.
Title: The role of emerging bipoles in the formation of a sunspot
penumbra
Authors: Schlichenmaier, R.; Bello González, N.; Rezaei, R.; Waldmann,
T. A.
Bibcode: 2010AN....331..563S
Altcode: 2010arXiv1003.1313S
The generation of magnetic flux in the solar interior and its transport
from the convection zone into the photosphere, the chromosphere,
and the corona will be in the focus of solar physics research for
the next decades. With 4 m class telescopes, one plans to measure
essential processes of radiative magneto-hydrodynamics that are needed
to understand the nature of solar magnetic fields. One key-ingredient
to understand the behavior of solar magnetic field is the process
of flux emergence into the solar photosphere, and how the magnetic
flux reorganizes to form the magnetic phenomena of active regions
like sunspots and pores. Here, we present a spectropolarimetric and
imaging data set from a region of emerging magnetic flux, in which a
proto-spot without penumbra forms a penumbra. During the formation of
the penumbra the area and the magnetic flux of the spot increases. First
results of our data analysis demonstrate that the additional magnetic
flux, which contributes to the increasing area of the penumbra, is
supplied by the region of emerging magnetic flux. We observe emerging
bipoles that are aligned such that the spot polarity is closer to the
spot. As an emerging bipole separates, the pole of the spot polarity
migrates towards the spot, and finally merges with it. We speculate
that this is a fundamental process, which makes the sunspot accumulate
magnetic flux. As more and more flux is accumulated a penumbra forms
and transforms the proto-spot into a full-fledged sunspot.
Title: Center to limb variation of penumbral Stokes V profiles
Authors: Franz, M.; Schlichenmaier, R.
Bibcode: 2010AN....331..570F
Altcode: 2010arXiv1008.2346F
We investigated the horizontal and the vertical component of the
Evershed flow (EF). To this end, we computed average Stokes V profiles
for various velocity classes in penumbrae at different heliocentric
angles. Our results show that for blueshifted profiles an additional
lobe with the same polarity as the spot is present in the blue side
of the average Stokes V profile. The amplitude of the additional
lobe grows with increasing blueshift and with increasing heliocentric
angle. For small redshifts, the profiles show an additional lobe with
the opposite polarity as the spot on the red side of the average Stokes
V profile. Even at disk center, the original polarity of the average
Stokes V profile is reversed for strong redshifts. The transition
between the different types of Stokes V profiles is continuous and
indicates that not only the vertical, but also the horizontal EF is
a magnetized stream of plasma in a magnetic background field.
Title: The formation of a sunspot penumbra
Authors: Schlichenmaier, R.; Rezaei, R.; Bello González, N.; Waldmann,
T. A.
Bibcode: 2010A&A...512L...1S
Altcode:
Context. The formation of a penumbra is crucial for our understanding
of solar magnetism, but it has not been observed in detail.
Aims: We aim to enhance our knowledge of how a sunspot penumbra forms
and how sunspots grow in size.
Methods: We present a data
set of the active region NOAA 11024 acquired at the German VTT with
speckle-reconstructed images in the G-band and Ca ii K. The data set
includes spectropolarimetric profiles from GFPI in Fe i 617.3 nm and
TIP in Fe i 1089.6 nm.
Results: On 2009 July 4, at 08:30 UT,
a leading spot without penumbra and pores of opposite polarity were
present in the active region. For the next 4:40 h, we observed the
formation of a penumbra in the leading spot at a cadence of 5 images per
second. We produced speckle reconstructed images of 0.3 arcsec spatial
resolution or better, interrupted by one large gap of 35 min and a
few more small gaps of about 10 min. The leading spot initially has a
size of 230 arcsec2 with only a few penumbral filaments and
then grows to a size of 360 arcsec2. The penumbra forms in
segments, and it takes about 4 h until it encircles half of the umbra,
on the side opposite the following polarity. On the side towards the
following polarity, elongated granules mark a region of magnetic flux
emergence.
Conclusions: This ongoing emergence appears to prevent
a steady penumbra from forming on this side. While the penumbra forms,
the umbral area is constant; i.e., the increase in the total spot
area is caused exclusively by the growth of the penumbra. From this
we conclude that the umbra has reached an upper size limit and that
any new magnetic flux that joins the spot is linked to the process of
penumbral formation. Movies are only available in electronic form
at http://www.aanda.org
Title: Small-Scale Velocities in Sunspot Penumbrae
Authors: Franz, M.; Schlichenmaier, R.; Schmidt, W.
Bibcode: 2010ASSP...19..510F
Altcode: 2010mcia.conf..510F
To investigate the penumbral plasma flow at small scales, we used
spectropolarimetric data of sunspots recorded by Hinode at 07:00UT
and 16:00UT on 14 November 2008.We computed maps of apparent Doppler
velocities by comparing the spectral position of the Fe I 630.15nm line
with the position of the line core of an average quiet Sun profile. We
evaluated the bisector of the line wing to investigate the flow pattern
in the deep photosphere.
Title: Spectral Analysis of Sunspot Penumbrae Observed with Hinode
Authors: Franz, M.; Schlichenmaier, R.
Bibcode: 2009ASPC..415..369F
Altcode: 2010arXiv1008.2391F
To investigate the penumbral plasma flow on a small scale,
spectropolarimetric data of sunspots recorded by Hinode was used. Maps
of Doppler velocities were created by evaluating the bisector in
the line-wing, thereby visualizing the flow pattern in the low
photosphere where the Evershed effect is most pronounced. In
penumbrae close to the disk center, the vertical component of the
Evershed flow dominates. The latter consists of a series of elongated
up-flow patterns extending radially through the entire center-side
penumbra at a constant azimuth. Along this structure, strong up-flows
appear in concentrated patches separated by weaker up-flows or even
down-flows. The strong up-flows appear at the bright heads and the
umbral side of the dark-core of the filament, while the down-flows
are rather located at the penumbral side of the filament. Projection
effects lead to an overall red-shift of the limb-side penumbra, but
the described pattern of up- and down-flows is still ascertainable.
Title: The Horizontal Magnetic Field of the Quiet Sun: Numerical
Simulations in Comparison to Observations with Hinode
Authors: Steiner, O.; Rezaei, R.; Schlichenmaier, R.; Schaffenberger,
W.; Wedemeyer-Böhm, S.
Bibcode: 2009ASPC..415...67S
Altcode: 2009arXiv0904.2030S
Three-dimensional magnetohydrodynamic simulations of the surface layers
of the Sun intrinsically produce a predominantly horizontal magnetic
field in the photosphere. This is a robust result in the sense that it
arises from simulations with largely different initial and boundary
conditions for the magnetic field. While the disk-center synthetic
circular and linear polarization signals agree with measurements from
Hinode, their center-to-limb variation sensitively depends on the
height variation of the horizontal and the vertical field component
and they seem to be at variance with the observed behavior.
Title: The velocity field of sunspot penumbrae. I. A global view
Authors: Franz, M.; Schlichenmaier, R.
Bibcode: 2009A&A...508.1453F
Altcode: 2009arXiv0909.4744F
Aims. We investigated the vertical penumbral plasma flow on small
spatial scales using data recorded by the spectropolarimeter of the
solar optical telescope onboard Hinode.
Methods: We computed maps
of apparent Doppler velocities by comparing the spectral positions
of the Fe I 630.15 nm & Fe I 630.25 nm lines with the averaged
line profiles of the quiet Sun. To visualize the flow pattern in the
low photosphere, we used a bisector of the wing of the absorption
lines. The small heliocentric angle (3° ≤ Θ ≤ 9°) of our data
sets means that the horizontal component of the Evershed flow (EF)
does not contribute significantly to the line shift.
Results: We
found that in the quiet Sun (QS), the area showing upflows is always
larger than the one exhibiting downflows. In the penumbra, upflows
dominate only at low velocities |v_dop| ≤ 0.4 km s-1,
while at higher velocities |v_dop| ≥ 0.6 km s-1 downflows
prevail. Additionally, the maximal upflow velocity in penumbrae is
lower, while the maximal downflow velocity is larger with respect to
the QS velocities. Furthermore, on a spatial average, the penumbra
shows a redshift, corresponding to a downflow of more than 0.1 km
s-1. Upflows are elongated and appear predominately in
the inner penumbra. Strong downflows with velocities of up to 9 km
s-1 are concentrated at the penumbra-QS boundary. They are
magnetized and are rather round. The inner penumbra shows an average
upflow, which turns into a mean downflow in the outer penumbra. The
upflow patches in the inner penumbra and the downflow locations in the
outer penumbra could be interpreted as the sources and the sinks of
the EF. We did not find any indication of roll-type convection within
penumbral filaments.
Title: Temporal Evolution of Magnetic Elements
Authors: Rezaei, R.; Schlichenmaier, R.; Schmidt, W.; Beck, C.
Bibcode: 2009ASPC..405..195R
Altcode: 2007arXiv0712.0234R
We study the structure and evolution of the magnetic field of the quiet
Sun by investigating weak spectro-polarimetric signals. To this end,
we observed a quiet region close to the disk center with the German VTT
in Tenerife, July 07, 2006. We recorded 38 scans of the same area. Each
scan was eight arcsec wide and observed within about 100 seconds. We
used POLIS to simultaneously observe Stokes profiles of the neutral
iron lines at 630.15 and 630.25 nm, the Stokes-I profile of the Ca
II H line at 396.8 nm, and a continuum speckle channel at 500 nm. We
witness two examples of magnetic flux cancellation of small-scale
opposite-polarity patches, followed by an enhanced chromospheric
emission. In each case, the two opposite-polarity patches gradually
became smaller and, within a few minutes, the smaller one completely
disappeared. The larger patch also diminished significantly. We provide
evidence for a cancellation scenario in the photosphere which leaves
minor traces at the chromospheric level.
Title: Sunspots: From Small-Scale Inhomogeneities Towards a Global
Theory
Authors: Schlichenmaier, Rolf
Bibcode: 2009SSRv..144..213S
Altcode: 2008SSRv..tmp..187S; 2008arXiv0811.2747S
The penumbra of a sunspot is a fascinating phenomenon featuring complex
velocity and magnetic fields. It challenges both our understanding
of radiative magneto-convection and our means to measure and derive
the actual geometry of the magnetic and velocity fields. In this
contribution we attempt to summarize the present state-of-the-art
from an observational and a theoretical perspective. We describe
spectro-polarimetric measurements which reveal that the penumbra is
inhomogeneous, changing the modulus and the direction of the velocity,
and the strength and the inclination of the magnetic field with
depth, i.e., along the line-of-sight, and on spatial scales below
0.5 arcsec. Yet, many details of the small-scale geometry of the
fields are still unclear such that the small scale inhomogeneities
await a consistent explanation. A simple model which relies on
magnetic flux tubes evolving in a penumbral “background”
reproduces some properties of sunspot inhomogeneities, like its
filamentation, its strong (Evershed-) outflows, and its uncombed
geometry, but it encounters some problems in explaining the penumbral
heat transport. Another model approach, which can explain the heat
transport and long bright filaments, but fails to explain the Evershed
flow, relies on elongated convective cells, either field-free as in
the gappy penumbra or filled with horizontal magnetic field as in
Danielson’s convective rolls. Such simplified models fail to give a
consistent picture of all observational aspects, and it is clear that
we need a more sophisticated description of the penumbra, that must
result from simulations of radiative magneto-convection in inclined
magnetic fields. First results of such simulations are discussed. The
understanding of the small-scales will then be the key to understand
the global structure and the large-scale stability of sunspots.
Title: Sunspots: From Small-Scale Inhomogeneities Towards a Global
Theory
Authors: Schlichenmaier, Rolf
Bibcode: 2009odsm.book..213S
Altcode:
The penumbra of a sunspot is a fascinating phenomenon featuring complex
velocity and magnetic fields. It challenges both our understanding
of radiative magneto-convection and our means to measure and derive
the actual geometry of the magnetic and velocity fields. In this
contribution we attempt to summarize the present state-of-the-art from
an observational and a theoretical perspective.
Title: Reversal-free Ca II H Profiles: a Challenge for Solar
Chromosphere Modeling in Quiet Inter-Network
Authors: Rezaei, R.; Bruls, J.; Beck, C.; Schmidt, W.; Kalkofen, W.;
Schlichenmaier, R.
Bibcode: 2008ESPM...12.2.13R
Altcode:
There is no agreement on the thermal structure of the solar
chromosphere. While results of the CO observations and 3D MHD
simulations suggest very cool structures in the upper atmosphere,
SUMER observations of UV spectral lines is interpreted as signature
of a full-time hot chromosphere. We tried to look for cool structures
in the solar chromosphere. We observed the intensity profile of the
Ca II H line in a quiet Sun region close to the disk center at the
German Vacuum Tower Telescope. We analyze over 10^5 line profiles from
inter-network regions. For comparison with the observed profiles, we
synthesize spectra for a variety of model atmospheres with a non local
thermodynamic equilibrium(NLTE) radiative transfer code. A fraction of
about 25% of the observed Ca II H line profiles do not show a measurable
emission peak in H2v and H2r wavelength bands (reversal-free). All of
the chosen model atmospheres with a temperature rise fail to reproduce
such profiles. On the other hand, the synthetic calcium profile of a
model atmosphere that has a monotonic decline of the temperature with
height shows a reversal-free profile that has much lower intensities
than any observed line profile. The observed reversal-free profiles,
at a spatial resolution of 1 arcs and a temporal resolution of 5 s,
indicate the existence of cool patches in the interior of chromospheric
network cells, at least for short time intervals. Our finding is not
only in conflict with a full-time hot chromosphere (e.g., FALC), but
also with a very cool chromosphere as found in some dynamic simulations.
Title: The Small Scale Flow Field of a Sunspot Penumbra
Authors: Schlichenmaier, R.; Franz, M.
Bibcode: 2008ESPM...12.2.28S
Altcode:
A sunspot is a coherent phenomenon on large spatial and temporal
scales, but it consists of an ensemble of small-scale and dynamic
features. It seems crucial to realize that a sunspot is not static,
but manifests a dynamic equilibrium: The dynamic fine structure forms
a globally stable sunspot, and it is the goal of sunspot physics to
understand how an ensemble of short living features on small scales is
organized to form a coherently large and long living sunspot. To this
end it is a necessity to investigate the nature of the fine structure:
its small-scale flow field, the topology of the magnetic field, and the
radiative interactions that form the intensity fine structure. We know
that the energy transport has to be of convective nature, but up to now
the underlying convective processes remain unclear. Is it convection in
magnetic field free gaps that exist in the space separating a strong
magnetic field which is more or less static? Is it convective flows
that are channeled by magnetic flux tube? Or is it dissipative turbulent
magneto-convection? In this contribution we study the small-scale
flow field in order to learn about the convective flow pattern. We
take advantage of the high spatial and high spectral resolution data
from Hinode. The satellite has acquired spectroscopic data from disk
passages of about 10 sunspots in 2007. Taking into account the varying
viewing angle according to the location of the sunspot on the disk,
we investigate the small-scale flow pattern. Thereby, we aim at an
understanding of the type of convection that is responsible for the
Evershed flow, for the small-scale penumbral structure, and for the
energy transport in penumbrae.
Title: Reversal-free Ca II H profiles: a challenge for solar
chromosphere modeling in quiet inter-network
Authors: Rezaei, R.; Bruls, J. H. M. J.; Schmidt, W.; Beck, C.;
Kalkofen, W.; Schlichenmaier, R.
Bibcode: 2008A&A...484..503R
Altcode: 2008arXiv0804.2325R
Aims: We study chromospheric emission to understand the temperature
stratification in the solar chromosphere.
Methods: We observed
the intensity profile of the Ca II H line in a quiet Sun region close
to the disk center at the German Vacuum Tower Telescope. We analyze
over 105 line profiles from inter-network regions. For
comparison with the observed profiles, we synthesize spectra for a
variety of model atmospheres with a non local thermodynamic equilibrium
(NLTE) radiative transfer code.
Results: A fraction of about
25% of the observed Ca II H line profiles do not show a measurable
emission peak in H2v and H2r wavelength bands
(reversal-free). All of the chosen model atmospheres with a temperature
rise fail to reproduce such profiles. On the other hand, the synthetic
calcium profile of a model atmosphere that has a monotonic decline of
the temperature with height shows a reversal-free profile that has much
lower intensities than any observed line profile.
Conclusions:
The observed reversal-free profiles indicate the existence of cool
patches in the interior of chromospheric network cells, at least
for short time intervals. Our finding is not only in conflict with a
full-time hot chromosphere, but also with a very cool chromosphere as
found in some dynamic simulations.
Title: Fine Structure of the Net Circular Polarization in a Sunspot
Penumbra
Authors: Tritschler, A.; Müller, D. A. N.; Schlichenmaier, R.;
Hagenaar, H. J.
Bibcode: 2007ApJ...671L..85T
Altcode: 2007arXiv0710.4545T
We present novel evidence for fine structure observed in the
net circular polarization (NCP) of a sunspot penumbra based on
spectropolarimetric measurements utilizing the Zeeman-sensitive Fe
I 630.2 nm line. For the first time we detect filamentary organized
fine structure of the NCP on spatial scales that are similar to the
inhomogeneities found in the penumbral flow field. We also observe an
additional property of the visible NCP, a zero-crossing of the NCP
in the outer parts of the center-side penumbra, which has not been
recognized before. In order to interpret the observations we solve the
radiative transfer equations for polarized light in a model penumbra
with embedded magnetic flux tubes. We demonstrate that the observed
zero-crossing of the NCP can be explained by an increased magnetic
field strength inside magnetic flux tubes in the outer penumbra combined
with a decreased magnetic field strength in the background field. Our
results strongly support the concept of the uncombed penumbra.
Title: Hinode observations reveal boundary layers of magnetic elements
in the solar photosphere
Authors: Rezaei, R.; Steiner, O.; Wedemeyer-Böhm, S.; Schlichenmaier,
R.; Schmidt, W.; Lites, B. W.
Bibcode: 2007A&A...476L..33R
Altcode: 2007arXiv0711.0408R
Aims:We study the structure of the magnetic elements in network-cell
interiors.
Methods: A quiet Sun area close to the disc centre was
observed with the spectro-polarimeter of the Solar Optical Telescope
on board the Hinode space mission, which yielded the best spatial
resolution ever achieved in polarimetric data of the Fe I 630 nm line
pair. For comparison and interpretation, we synthesize a similar data
set from a three-dimensional magneto-hydrodynamic simulation.
Results: We find several examples of magnetic elements, either
roundish (tube) or elongated (sheet), which show a central area of
negative Stokes-V area asymmetry framed or surrounded by a peripheral
area with larger positive asymmetry. This pattern was predicted
some eight years ago on the basis of numerical simulations. Here,
we observationally confirm its existence for the first time.
Conclusions: We gather convincing evidence that this pattern of
Stokes-V area asymmetry is caused by the funnel-shaped boundary of
magnetic elements that separates the flux concentration from the
weak-field environment. On this basis, we conclude that electric
current sheets induced by such magnetic boundary layers are common in
the photosphere.
Title: Magnetic properties of G-band bright points in a sunspot moat
Authors: Beck, C.; Bellot Rubio, L. R.; Schlichenmaier, R.;
Sütterlin, P.
Bibcode: 2007A&A...472..607B
Altcode: 2007arXiv0707.1232B
We present simultaneous spectropolarimetric observations of four
visible (630 nm) and three infrared (1565 nm) spectral lines from the
German Vacuum Tower Telescope, together with speckle-reconstructed
filtergrams in the G-band and the Ca II H line core from the Dutch
Open Telescope. After alignment of the data sets, we used the G-band
intensity to locate bright points (BPs) in the moat of a regular
sunspot. With the cospatial and cotemporal information provided by the
polarimetric data, we characterize the magnetic, kinematic, and thermal
properties of the BPs. We find that (a) 94% of the BPs are associated
with magnetic fields; (b) their field strengths range between 500 and
1400 G, with a rather flat distribution; (c) the contrast of BPs in the
G-band depends on the angle between the vector magnetic field and the
line of sight; (d) the BPs harbor downflows of magnetized plasma and
exhibit Stokes V profiles with large area and amplitude asymmetries;
(e) the magnetic interior of BPs is hotter than the immediate field-free
surroundings by about 1000 K at equal optical depth; and (f) the mean
effective diameter of BPs in our data set is 150 km, with very few
BPs larger than 300 km. Most of these properties can be explained
by the classical magnetic flux tube model. However, the wide range
of BP parameters found in this study indicates that not all G-band
BPs are identical to stable long-lived flux tubes or sheets of kG
strength. Appendices A-C are only available in electronic form
at http://www.aanda.org
Title: Variation of the Stokes-V area asymmetry across magnetic
elements
Authors: Rezaei, R.; Steiner, O.; Wedemeyer-Böhm, S.; Schlichenmaier,
R.; Lites, B. W.
Bibcode: 2007AN....328..706R
Altcode:
No abstract at ADS
Title: Opposite magnetic polarity of two photospheric lines in single
spectrum of the quiet Sun
Authors: Rezaei, R.; Schlichenmaier, R.; Schmidt, W.; Steiner, O.
Bibcode: 2007A&A...469L...9R
Altcode: 2007arXiv0704.3135R
Aims:We study the structure of the photospheric magnetic field of the
quiet Sun by investigating weak spectro-polarimetric signals.
Methods: We took a sequence of Stokes spectra of the Fe I 630.15
nm and 630.25 nm lines in a region of quiet Sun near the disk
center, using the POLIS spectro-polarimeter at the German VTT on
Tenerife. The line cores of these two lines form at different heights
in the atmosphere. The 3σ noise level of the data is about 1.8
× 10-3 I_c.
Results: We present co-temporal and
co-spatial Stokes-V profiles of the Fe I 630 nm line pair, where
the two lines show opposite polarities in a single spectrum. We
compute synthetic line profiles and reproduce these spectra with a
two-component model atmosphere: a non-magnetic component and a magnetic
component. The magnetic component consists of two magnetic layers with
opposite polarity: the upper one moves upwards while the lower one moves
downward. In-between, there is a region of enhanced temperature.
Conclusions: The Stokes-V line pair of opposite polarity in a single
spectrum can be understood as a magnetic reconnection event in the
solar photosphere. We demonstrate that such a scenario is realistic,
but the solution may not be unique.
Title: Relation between photospheric magnetic field and chromospheric
emission
Authors: Rezaei, R.; Schlichenmaier, R.; Beck, C. A. R.; Bruls,
J. H. M. J.; Schmidt, W.
Bibcode: 2007A&A...466.1131R
Altcode: 2007astro.ph..1896R
Aims: We investigate the relationship between the photospheric
magnetic field and the emission of the mid chromosphere of the
Sun.
Methods: We simultaneously observed the Stokes parameters
of the photospheric iron line pair at 630.2 nm and the intensity
profile of the chromospheric Ca II H line at 396.8 nm in a quiet
Sun region at a heliocentric angle of 53°. Various line parameters
have been deduced from the Ca II H line profile. The photospheric
magnetic field vector has been reconstructed from an inversion of
the measured Stokes profiles. After alignment of the Ca and Fe maps,
a common mask has been created to define network and inter-network
regions. We perform a statistical analysis of network and inter-network
properties. The H-index is the integrated emission in a 0.1 nm band
around the Ca core. We separate a non-magnetically, Hnon,
and a magnetically, Hmag, heated component from a
non-heated component, Hco in the H-index.
Results:
The average network and inter-network H-indices are equal to 12 and
10 pm, respectively. The emission in the network is correlated with
the magnetic flux density, approaching a value of H ≈ 10 pm for
vanishing flux. The inter-network magnetic field is dominated by weak
field strengths with values down to 200 G and has a mean absolute
flux density of about 11 Mx cm-2.
Conclusions:
We find that a dominant fraction of the calcium emission caused by
the heated atmosphere in the magnetic network has non-magnetic origin
(Hmag≈2 pm, Hnon≈3 pm). Considering the effect
of straylight, the contribution from an atmosphere with no temperature
rise to the H-index (Hco≈6 pm) is about half of the
observed H-index in the inter-network. The H-index in the inter-network
is not correlated to any property of the photospheric magnetic field,
suggesting that magnetic flux concentrations have a negligible role
in the chromospheric heating in this region. The height range of the
thermal coupling between the photosphere and low/mid chromosphere
increases in presence of magnetic field. In addition, we demonstrate
that a poor signal-to-noise level in the Stokes profiles leads to a
significant over-estimation of the magnetic field strength.
Title: Disentangling The Magnetic Field Structure Of Sunspots -
Stereoscopic Polarimetry With Solar Orbiter
Authors: Müller, D. A. N.,; Schlichenmaier, R.; Fleck, B.; Fritz, G.
Bibcode: 2007ESASP.641E..32M
Altcode:
Sunspots exhibit complex, highly structured magnetic fields and
flows. Disentangling the atmospheric structure of sunspots is a
great challenge, and can only be achieved by the combination of
spectropolarimetry at high spatial resoultion and detailed modeling
efforts. We use a generalized 3D the embeds magnetic flux tuber in
a stratified atmosphere and calculates the emerging polarization
of spectrail lines for arbitrary viewing angles. The resulting
polarization maps are a very efficient tool to distinguish between
different atmospheric scenarios and determine the 3D structure of the
magnetic field and the flow field. In this contribution, we present
synthetic maps of the net circular polarication (NCP) as a function of
the heliocentric angle for different spectral lines of interest. Among
these are the Fe I 617.3 nm line which would be observed by the VIM
instrument abard Solar Orbiter and the Fe I 630.2 nm line which will
be observed by Hinode (formerly known as Solar-B).
Title: Photospheric magnetic field and chromospheric emission
Authors: Rezaei, R.; Schlichenmaier, R.; Beck, C.; Schmidt, W.
Bibcode: 2007msfa.conf..169R
Altcode: 2007astro.ph..1681R
We present a statistical analysis of network and internetwork
properties in the photosphere and the chromosphere. For the first
time we simultaneously observed (a) the four Stokes parameters of the
photospheric iron line pair at 630.2 nm and (b) the intensity profile
of the Ca H line at 396.8 nm. The vector magnetic field was inferred
from the inversion of the iron lines. We aim at an understanding of the
coupling between photospheric magnetic field and chromospheric emission.
Title: On the inhomogeneities of the sunspot penumbra
Authors: Schlichenmaier, R.; Müller, D. A. N.; Beck, C.
Bibcode: 2007msfa.conf..233S
Altcode: 2007astro.ph..3021S
The penumbra is ideally suited to challenge our understanding
of magnetohydrodynamics. The energy transport takes place as
magnetoconvection in inclined magnetic fields under the effect of
strong radiative cooling at the surface. The relevant processes
happen at small spatial scales. In this contribution we describe
and elaborate on these small-scale inhomogeneities of a sunspot
penumbra. We describe the penumbral properties inferred from imaging,
spectroscopic and spectropolarimetric data, and discuss the question
of how these observations can be understood in terms of proposed models
and theoretical concepts.
Title: Magnetic properties of G-band bright points
Authors: Beck, C.; Mikurda, K.; Bellot Rubio, L. R.; Schlichenmaier,
R.; Sütterlin, P.
Bibcode: 2007msfa.conf..165B
Altcode:
Bright points (BPs) visible in the G band at 430 nm are commonly used
as tracers of magnetic fields, indicating the location of kG flux
concentrations. To study the actual magnetic properties of G-band BPs,
we took observations in 2003 and 2005, employing simultaneously a
speckle setup in the G band and vector spectropolarimetry to derive
the magnetic field vector. From the analysis of the co-aligned
polarimetric data we find that the BPs show a broad range of field
strengths, magnetic fluxes, and field inclinations. Many G-band
BPs are not co-spatial with the central part of the nearby flux
concentrations. Even at the small heliocentric angle of only 12°,
the BPs appear projected on adjacent granules, whereas the magnetic
field is concentrated in the intergranular lanes. Our findings support
the view that the G-band BPs are a result of the "hot wall effect". The
downward shift of the optical depth scale in the presence of magnetic
fields allows to see deeper and hotter layers in the hot granules next
to the field concentrations, where CH dissociates. Thus, information
drawn from imaging observations of BPs has limited use to investigate
the actual magnetic field structure, when the BPs are not co-spatial
with the central part of the flux concentrations.
Title: Polarimetric Observations of the Formation of a G-Band
Bright Point
Authors: Beck, C.; Schmidt, W.; Bellot Rubio, L. R.; Schlichenmaier,
R.; Sütterlin, P.; Lites, B. W.
Bibcode: 2006ASPC..358...72B
Altcode:
We investigate the kinematic and magnetic properties of G-band bright
points in the moat of a regular sunspot. The analysis is based on vector
polarimetric measurements made at the German Vacuum Tower Telescope in
visible (630 nm) and infrared (1565 nm) spectral lines, complemented
by high-resolution filtergrams in the G-band at 430.6 nm and the core
of the Ca II H line at 396.7 nm from the Dutch Open Telescope. The
spectro-polarimetric data has been inverted to derive the magnetic
field properties of the observed region. We witness the formation of a
G-band bright point from a patch of diffuse flux with an initial field
strength of 0.4 kG. The magnetic field strength increases to 0.9 kG in
the course of several minutes, accompanied by a downflow of magnetized
plasma. A few minutes after the field intensification, a G-band bright
point is seen at the location of the flux concentration. The formation
of the bright point shows the signatures of convective collapse.
Title: The multi-component field topology of sunspot penumbrae. A
diagnostic tool for spectropolarimetric measurements
Authors: Müller, D. A. N.; Schlichenmaier, R.; Fritz, G.; Beck, C.
Bibcode: 2006A&A...460..925M
Altcode: 2006astro.ph..9632M
Context: .Sunspot penumbrae harbor highly structured magnetic fields
and flows. The moving flux tube model offers an explanation for several
observed phenomena, e.g. the Evershed effect and bright penumbral
grains.
Aims: .A wealth of information can be extracted from
spectropolarimetric observations. In order to deduce the structure of
the magnetic field in sunspot penumbrae, detailed forward modeling is
necessary. On the one hand, it gives insight into the sensitivity of
various spectral lines to different physical scenarios. On the other
hand, it is a very useful tool to guide inversion techniques. In this
work, we present a generalized 3D geometrical model that embeds an
arbitrarily shaped flux tube in a stratified magnetized atmosphere.
Methods: .The new semi-analytical geometric model serves as a
frontend for a polarized radiative transfer code. The advantage of
this model is that it preserves the discontinuities of the physical
parameters across the flux tube boundaries. This is important for the
detailed shape of the emerging Stokes Profiles and the resulting net
circular polarization (NCP).
Results: .(a) The inclination of
downflows in the outer penumbra must be shallower than approximately
15° (b) observing the limb-side NCP of sunspots in the Fe I 1564.8
nm line offers a promising way to identify a reduced magnetic field
strength in flow channels; (c) the choice of the background atmosphere
can significantly influence the shape of the Stokes profiles, but does
not change the global characteristics of the resulting NCP curves for
the tested atmospheric models.
Title: The flow field in the sunspot canopy
Authors: Rezaei, R.; Schlichenmaier, R.; Beck, C.; Bellot Rubio, L. R.
Bibcode: 2006A&A...454..975R
Altcode: 2006astro.ph..4301R
Aims.We investigate the flow field in the sunspot canopy using
simultaneous Stokes vector spectropolarimetry of three sunspots
(θ=27°, 50°, 75°) and their surroundings in visible (630.15 and
630.25 nm) and near infrared (1564.8 and 1565.2 nm) neutral iron
lines.
Methods: .To calibrate the Doppler shifts, we compare
an absolute velocity calibration using the telluric O_2-line at
630.20 nm and a relative velocity calibration using the Doppler
shift of Stokes V profiles in the umbra under the assumption that
the umbra is at rest. Both methods yield the same result within the
calibration uncertainties (~150 m s-1). We study the radial
dependence of Stokes V profiles in the directions of disk center and
limb side.
Results: .Maps of Stokes V profile shifts, polarity,
amplitude asymmetry, field strength and magnetic field azimuth provide
strong evidence for the presence of a magnetic canopy and for the
existence of a radial outflow in the canopy.
Conclusions: .Our
findings indicate that the Evershed flow does not cease abruptly at the
white-light spot boundary, but that at least a part of the penumbral
Evershed flow continues into the magnetic canopy.
Title: The fine-structure of a Sunspot penumbra
Authors: Schlichenmaier, R.
Bibcode: 2006IAUJD...3E..88S
Altcode:
The fine-structure of the penumbra is an ideal target to study the
effects of radiative magnetoconvection in inclined magnetic field,
and to compare theoretical models with observed properties. I will
present spectroscopic and spectro-polarimetric measurements, which are
interpreted to reconstruct the thermal stratification and the complex
topology of the magnetic field and the flow field. Such measurements
reveal that the line-of-sight velocity and the magnetic field depend on
depth in the atmosphere and exhibit gradients or discontinuities. The
magnetic field is found to be uncombed, with an essentially horizontal
component that carries a radial outward flow (Evershed flow), and a
less inclined magnetic field component. I will report on the discovery
of dark-cored bright filaments, and present spectroscopic measurements,
which demonstrate that the dark cores are associated with the Evershed
flow. The observational findings will be interpreted in the framework
of theoretical models and ideas.
Title: Net Circular Polarization of Sunspot Penumbrae- A Versatile
Tool for Diagnosing Magnetic Field Structure
Authors: Müller, D. A. N.; Schlichenmaier, R.; Fritz, G.; Beck, C.
Bibcode: 2006ESASP.617E..72M
Altcode: 2006soho...17E..72M
No abstract at ADS
Title: Two-dimensional spectroscopy of a sunspot. III. Thermal and
kinematic structure of the penumbra at 0.5 arcsec resolution
Authors: Bellot Rubio, L. R.; Schlichenmaier, R.; Tritschler, A.
Bibcode: 2006A&A...453.1117B
Altcode: 2006astro.ph..1423B
We investigate the thermal and kinematic configuration of a sunspot
penumbra using high spectral and spatial resolution intensity profiles
of the non-magnetic Fe I 557.6 nm line. The data set was acquired
with the 2D solar spectrometer TESOS. The profiles are inverted using
a one-component model atmosphere with gradients of the physical
quantities. From this inversion we obtain the stratification with
depth of temperature, line-of-sight velocity, and microturbulence
across the penumbra. Our results suggest that the physical mechanism(s)
responsible for the penumbral filaments operate preferentially in the
lower photosphere. The spot, located at an heliocentric angle of 23°,
exhibits larger continuum intensities in the center-side penumbra
as compared with the limb side, which translates into an average
temperature difference of 100-150 K at log τ500 = 0. We
investigate the nature of the bright ring that appears in the inner
penumbra when sunspots are observed in the wing of spectral lines. It
is suggested that the bright ring does not reflect a temperature
enhancement in the mid photospheric layers. The line-of-sight velocities
retrieved from the inversion are used to determine the flow geometry
at different heights in the photosphere. Both the flow speed and
flow angle increase with optical depth and radial distance. Downflows
are detected in the mid and outer penumbra, but only in deep layers
(log τ500 ≥ -1.4). We demonstrate that the velocity
stratifications retrieved from the inversion are consistent with the
idea of penumbral flux tubes channeling the Evershed flow. Finally, we
show that larger Evershed flows are associated with brighter continuum
intensities in the inner center-side penumbra. Dark structures,
however, are also associated with significant Evershed flows. This
leads us to suggest that the bright and dark filaments seen at 0.5
arcsec resolution are not individual flow channels, but a collection
of them. Our analysis highlights the importance of very high spatial
resolution spectroscopic and spectropolarimetric measurements for a
better understanding of sunspot penumbrae.
Title: Net Circular Polarization Of Sunspot Penumbrae - A Versatile
Model For Diagnosing Magnetic Field Structure
Authors: Mueller, Daniel; Schlichenmaier, R.; Fritz, G.; Beck, C.
Bibcode: 2006SPD....37.0707M
Altcode: 2006BAAS...38..229M
Sunspot penumbrae harbor highly structured magnetic fields and
flows. The moving flux tube model offers an explanation for several
observed phenomena, e.g. the Evershed effect and bright penumbral
grains. In this work, we present a generalized 3D model that embeds an
arbitrarily shaped flux tube in a stratified magnetized atmosphere. The
new model is a versatile tool to calculate the spectral signature of
flux tubes in the penumbra and especially make predictions about the
flow speed and tube inclination from observed maps of the net circular
polarization (NCP). As a first result, we find that the inclination
of downflows in the outer penumbra must be shallower than approx. 15°.
Title: Prospects of Solar Physics from the Ground
Authors: Schlichenmaier, Rolf
Bibcode: 2006IAUS..233..427S
Altcode:
The solar magnetism, its origin, and its impact on the earth are of
primary interest for solar physicists. The understanding of the solar
dynamo in the convection zone and the coupling of the magnetic fields
up to the corona and the heliosphere calls for synoptic as well as
for high spatial resolution observations of the Sun. Understanding
the interactions between radiative and magneto-convective processes at
the interface between the solar interior and the atmosphere requires
spectro-polarimetric observations at high spatial and spectral
resolution with high polarimetric accuracy. Thus large-aperture
telescopes are needed to resolve the small scales and to collect
enough photons to study the evolution of the magnetic processes. For
assembling the mosaic of the solar dynamo and its magnetic coupling
out to the heliosphere, large scale properties and hence synoptic
observations play a crucial role. I present my personal perspective
of the prospects in ground-based solar physics, and comment on the
planned and upcoming new facilities including SOLIS, GREGOR, NST,
SUNRISE, and ATST, as well as ALMA and FASR, but also mention the
upcoming space missions HMI@SDO and SOLAR-B.
Title: A polarization model for the German Vacuum Tower Telescope
from in situ and laboratory measurements
Authors: Beck, C.; Schlichenmaier, R.; Collados, M.; Bellot Rubio,
L.; Kentischer, T.
Bibcode: 2005A&A...443.1047B
Altcode:
It is essential to properly calibrate the polarimetric properties of
telescopes, if one wants to take advantage of the capabilities of high
precision spectro-polarimeters. We have constructed a model for the
German Vacuum Tower Telescope (VTT) that describes its time-dependent
polarization properties. Since the coelostat of the telescope changes
the polarization state of the light by introducing cross talk among
different polarization states, such a model is necessary to correct the
measurements, in order to retrieve the true polarization as emitted
from the Sun. The telescope model is quantified by a time-dependent
Mueller matrix that depends on the geometry of the light beam through
the telescope, and on material properties: the refractive indices of the
coelostat mirrors, and the birefringence of the entrance window to the
vacuum tube. These material properties were determined experimentally
in-situ by feeding the telescope with known states of polarization
(including unpolarized light) and by measuring its response, and from
measurements of an aluminum-coated sample in the laboratory. Accuracy
can in our case be determined only for the combination of telescope
and spectro-polarimeter used; for the instrument POLIS at the VTT,
we estimate an accuracy of ±4-5× 10-3 for the cross talk
correction coefficients.
Title: Multi-line spectroscopy of dark-cored penumbral filaments
Authors: Bellot Rubio, L. R.; Langhans, K.; Schlichenmaier, R.
Bibcode: 2005A&A...443L...7B
Altcode:
Dark-cored filaments could be the basic building blocks of sunspot
penumbrae. Yet, their nature and physical conditions are unknown. In an
attempt to improve this situation, we present the first high-resolution
spectra of dark-cored penumbral filaments. Several such filaments
were observed near the umbra/penumbra boundary of a sunspot located at
heliocentric angles of 5° and 20°. Our data reveal (a) significantly
larger Doppler shifts in the dark cores as compared to their lateral
brightenings; (b) Doppler shifts that increase with depth in the
photosphere, up to 1.5 km s-1; and (c) Doppler shifts that
increase with increasing heliocentric distance. The Doppler velocities
measured in the dark cores are almost certainly produced by upflows. In
addition, dark-cored penumbral filaments exhibit weaker fields than
their surroundings (by 100-300 G). These results provide new constraints
for models of dark-cored penumbral filaments.
Title: Flow filaments linking bright and dark filaments in a sunspot
penumbra
Authors: Tritschler, A.; Schlichenmaier, R.; Bellot Rubio, L. R.
Bibcode: 2005AGUSMSP11A..08T
Altcode:
We present two-dimensional spectroscopic sunspot observations of
high spatial (≍ 0.5 arcsec) and high spectral resolution (λ/Δλ
= 250000). The observations were taken with the Telecentric Solar
Spectrometer (TESOS) operated at the German Vacuum Tower Telescope on
Tenerife. We examine a single scan taken in the popular non-magnetic
neutral iron line at 557.6 nm and concentrate our analysis on the
unsettled issue of the relation between the Evershed flow and the
intensity structure in a sunspot penumbra. At the end of the 20th
century, observers concluded that the highest flow velocities are
connected to the dark filaments which harbour more horizontal magnetic
fields than the bright filaments. Based on a correlation analysis we
find that the correlation between flows and intensity varies from the
inner to the outer penumbra, from the center-side to the limb-side
penumbra, and depends on the length of the trace used to perform
the correlation. The line-of-sight velocity maps reveal that the
Evershed flow on the center-side penumbra appears highly organised in
narrow flow filaments, while the flows in the red-shifted limb-side
penumbra do not show a filamentary fine-structure. A high correlation
between flow speed and intensity is only observed over small spatial
scales, i.e. considering short traces cutting individual features. The
correlation is positive in the inner centre and limb-side penumbra, and
tends to be negative in the outer penumbra. Our results imply that the
Evershed flow is present in bright and dark filaments. In individual
cases we find that flow filaments connect bright and dark filaments
supporting the moving tube model for the penumbral fine structure.
Title: On the relation between penumbral intensity and flow filaments
Authors: Schlichenmaier, R.; Bellot Rubio, L. R.; Tritschler, A.
Bibcode: 2005AN....326..301S
Altcode:
Taking advantage of high spatial (≈ 0.5 arcsec) and high spectral
(λ/δλ = 250 000) resolution observations obtained with the 2D
spectrometer TESOS, we analyze a sunspot located at a heliocentric angle
of 23o. We elaborate on the issue of a correlation between
dark filaments and the Evershed flow in sunspot penumbrae. Controversies
on the existence of such a correlation are resolved: It varies from
the inner to the outer penumbra, from the center-side to the limb-side
penumbra, and depends on the length of the trace which is used to
perform the correlation. The flow map exhibits flow filaments in the
center-side penumbra while the red-shifted limb-side penumbra does not
show filamentary fine-structure. High correlation coefficients, |C|
≈ 0.9 are only found if small scales, i.e. short traces cutting
individual features are considered. C is positive in the inner
center and limb-side penumbra, and tends to be negative in the outer
penumbra. Our results imply that the Evershed flow is present in bright
and dark filaments. In individual cases we find that bright and dark
intensities are connected by a flow filament supporting the moving
tube model for the penumbral fine structure.
Title: Penumbral Line Asymmetries Using KAOS
Authors: Soltau, Dirk; Berkefeld, Thomas; Schlichenmaier, Rolf;
Tritschler, Alexandra; Rubio, Luis Ramon Bellot
Bibcode: 2005sao..conf..129S
Altcode:
We analyse and interpret the line asymmetries in a sunspot penumbra
at a heliocentric angle of 23°. The data, acquired with TESOS [1]
and KAOS [2] at the VTT, is of high spatial (0.5") and high spectral (
λ/∆λ = 250 000) resolution. We observed in Fe I 557.6 nm (g=0) [3].
Title: Asymmetrical appearance of dark-cored filaments in sunspot
penumbrae
Authors: Sütterlin, P.; Bellot Rubio, L. R.; Schlichenmaier, R.
Bibcode: 2004A&A...424.1049S
Altcode:
Recent sunspot observations at unprecedented resolution have led to the
discovery of dark cores in the bright filaments that form the penumbra
(\citealt{scharmer02_Nat420}). The discovery paper considered spots
at disk center only, so the properties of the dark-cored filaments
remain largely unknown. Here we analyze a speckle-reconstructed time
series of G-band and blue continuum images of a sunspot acquired with
the Dutch Open Telescope. The target was located at an heliocentric
angle of 27 deg. We confirm the existence of dark-cored penumbral
filaments also in spots outside the disk center, and report on distinct
differences between the center and limb-side penumbra. In the inner
center-side penumbra, filaments are detected as two narrow bright
streaks separated by a central obscuration. These structures move
together as a single entity. On the limb side, dark cores are hardly
seen. The time series is used to determine the sizes (∼200-250 km),
proper motions (∼280 m s-1), and lifetimes (⪉45 min)
of typical dark-cored filaments.
Title: Two-dimensional spectroscopy of a sunspot. II. Penumbral
line asymmetries
Authors: Schlichenmaier, R.; Bellot Rubio, L. R.; Tritschler, A.
Bibcode: 2004A&A...415..731S
Altcode:
We present, analyse, and interpret line asymmetries from Fe I 557.6
nm of a sunspot penumbra at a heliocentric angle of 23o
with high spatial (0.5 arcsec) and spectral (λ/\triangleλ=250 000)
resolution. The data set is described and presented in the first paper
of this series \citep{tritschler+etal2003}. Line bisectors are used
to quantify the line asymmetries. Our findings are: (1) For averaged
limb and center side bisectors the shift increases linearly with the
bisector intensity level, but the limb side bisector is more inclined
than the center side bisector. (2) Individual bisectors exhibit kinks,
such that the bisector at high intensity levels is shifted towards the
red for both, limb and center side bisectors. Some of the kinks produce
bisector reversals in the outer center side penumbra. The bisector
properties and their intriguing differences between center and limb side
can be explained if one assumes downflows in deep atmospheric layers
(\log τ > -1). This is demonstrated by synthetic bisectors. The
differences between the two penumbral sides are due to projection
effects of non-horizontal flow channels. Our findings also imply that
bisectors reversals are not due to elevated channels, but due to the
presence of downflows. Along a specific center side flow filament the
bisector shift is found to be largest in the line wing, except for the
outer end of the filament, where a kink at high bisector intensities
toward the red is found. This is consistent with an upflow at the
inner footpoint, a deep lying horizontal flow, and, after a spatial
distance of 4 arcsec, with a downflow at the end of the flow filament.
Title: Two-dimensional spectroscopy of a sunspot. I. Properties of
the penumbral fine structure
Authors: Tritschler, A.; Schlichenmaier, R.; Bellot Rubio, L. R.;
KAOS Team; Berkefeld, T.; Schelenz, T.
Bibcode: 2004A&A...415..717T
Altcode:
We investigate the properties of the fine structure of a sunspot
penumbra based on spectroscopic measurements with high spectral
(λ/δλ=250 000) and high spatial (≈0.5 arcsec) resolution. The
magnetically insensitive Fe I 557.6 nm line is used to probe the
penumbral atmosphere. The data was taken at the German Vacuum Tower
Telescope with the 2D-spectrometer TESOS, taking advantage of the
recently installed Kiepenheuer Adaptive Optics System (KAOS). The field
of view covers a sunspot located at 23o off the disk center
and its immediate surroundings. The penumbral structure is studied
by means of maps computed for the line-of-sight velocity, the line
width, the equivalent width and the line depression. Line-of-sight
velocities are derived from the Doppler shifts at different bisector
levels. From these maps we infer the flow field geometry and study the
azimuthal and radial dependences of the line parameters. Our findings
can be summarized as follows: (a) the flow pattern has a conspicuous
filamentary structure in the deep photospheric layers and is rather
diffuse in the high layers. (b) The flow field slightly spreads and
fans out with height. (c) The flow geometry confirms the presence of
an upflow component in the inner penumbra and a downflow component in
the middle and outer penumbra. (d) We find an enhanced brightness of
the mid-penumbra (``bright ring'') in the line wings, but not in the
continuum or line core. (e) The azimuthal average of the equivalent
width, the line width and the absolute flow velocity increase with
radial distance within the penumbra. (f) Small-scale variations of
the equivalent width and the line width on the center-side penumbra
are co-spatial and correlated with (blue-shifted) fluctuations in
the line-of-sight velocity. (g) Inner limb-side penumbral grains are
associated with blue-shifts of v≤-400 m s-1, indicating
upflows. (h) One umbral dot in our sample is associated with a
blue-shift of v=-200 m s-1.
Title: On the heat transport in a sunspot penumbra
Authors: Schlichenmaier, R.; Solanki, S. K.
Bibcode: 2003A&A...411..257S
Altcode:
The penumbra radiates an energy flux that is roughly 75% of the
quiet-sun value. One mechanism proposed to bring this flux to the
surface is interchange convection of magnetic flux tubes according
to which hot flux tubes rise to the surface, cool off their heat
by radiation and sink down again. Another way to deposit heat in
the penumbral photosphere is by steady upflows along magnetic flux
tubes. We discuss these two mechanisms and elaborate on consequences
that can be compared with and constrained by observations. We estimate
the time scales for variations of the intensity and the magnetic field
pattern. By comparing them with the corresponding observed time scales,
we find that pure interchange convection is unable to account for the
observed penumbral heat flux. Heating the penumbra by steady upflows
along magnetic flux tubes, however, turns out to be sufficient to
explain the penumbral brightness, under the condition that significant
magnetic return flux is present within the penumbra. Associated with
the magnetic return flux, downflows within the penumbra should be
present, in accordance with recent observational findings of such
downflows. Exploring other possible heating mechanisms, we find that
dissipation of magnetic energy is negligible, while dissipation of the
kinetic energy of the Evershed flow could contribute significantly to
the brightness of the penumbra.
Title: Thermal Kinematic Structure of a Sunspot at 0.5 arcsec
Resolution
Authors: Bellot Rubio, L. R.; Schlichenmaier, R.; Tritschler, A.
Bibcode: 2003ANS...324..104B
Altcode: 2003ANS...324..P10B
No abstract at ADS
Title: 2D Spectroscopy with a Triple Gabry-Perot Spectrometer and
Adaptive Optics
Authors: Tritschler, A.; Schlichenmaier, R.; Bellot Rutbio, L.
Bibcode: 2003ANS...324...21T
Altcode: 2003ANS...324..C02T
No abstract at ADS
Title: Field-aligned Evershed flows in the photosphere of a sunspot
penumbra
Authors: Bellot Rubio, L. R.; Balthasar, H.; Collados, M.;
Schlichenmaier, R.
Bibcode: 2003A&A...403L..47B
Altcode:
We determine the inclinations of the vector magnetic field and flow
velocity in a sunspot penumbra by interpreting full Stokes profiles of
three infrared lines observed with the Tenerife Infrared Polarimeter. It
is shown that analyses based on one-component atmospheres deliver flow
velocities which are more horizontal than the average magnetic field
by up to 10 deg. This apparent violation of the concept of frozen-in
magnetic fields is solved as soon as two magnetic atmospheres are
allowed to coexist in the resolution element. The magnetic field and
velocity in the atmospheric component carrying the Evershed flow are
found to be aligned to within +/- 2 deg all the way from the inner
to the outer penumbra. This is the first observational confirmation
of magnetic fields being frozen into the plasma in sunspots. Our
results indicate that sunspot penumbrae can be understood in terms of
inclined flux tubes embedded in a more vertical background field. The
flux tubes carry most of the Evershed flows and return to the solar
surface in the middle penumbra and beyond. The background atmosphere
is essentially at rest in the inner penumbra, and harbors small flows
in the outer penumbra.
Title: 2D Solar Spectroscopy with a Triple Fabry-Perot Filtergraph
Authors: Bellot Rubio, Luis R.; Tritschler, Alexandra; Schlichenmaier,
Rolf
Bibcode: 2003IAUJD..20E..27B
Altcode:
TESOS is a tunable narrow-band filter based on three Fabry-Perot etalons
operated in a telecentric configuration. Installed at the German Vacuum
Tower Telescope (VTT) of Teide Observatory (Spain) it is used regularly
for 2D spectroscopy of the solar atmosphere. Its spectral resolution
of 250000 is similar to that of grating spectrographs. In the high
resolution mode the field of view of TESOS is 42 arcsec in diameter and
the image scale is 0.09 arcsec/pixel. Due to the high quantum efficiency
of the detector above 500 nm exposure times of a few tens of msec can
be used. As a result TESOS is able to measure the intensity profiles
of a given spectral line across the field of view in less than 40 sec
(assuming 100 wavelength positions). Recently TESOS has been combined
with KAOS the adaptive optics system of the VTT to improve the spatial
resolution of the observations up to 0.4 arcsec. Here we present the
first science results of TESOS+KAOS based on observations of a sunspot
penumbra. Maps of various spectroscopic parameters are computed and
the line asymmetries induced by the Evershed flows are investigated
by means of a bisector analysis of the FeI line at 557.6 nm.
Title: The Sunspot Penumbra: New Developments (Invited review)
Authors: Schlichenmaier, R.
Bibcode: 2003ASPC..286..211S
Altcode: 2003ctmf.conf..211S
No abstract at ADS
Title: Spectral signature of magnetic flux tubes in sunspot penumbrae
Authors: Müller, D. A. N.; Schlichenmaier, R.; Steiner, O.; Stix, M.
Bibcode: 2002A&A...393..305M
Altcode:
We study the polarization of spectral lines in the penumbra by
integrating the radiative transfer equation of polarized light for a
three-dimensional model atmosphere of a sunspot. In this model, the
Evershed flow is confined to magnetic flux tubes which are embedded
in a static background atmosphere, in accordance with the moving tube
model of Schlichenmaier et al. (\cite{Schlichenmaier1998apjl},b). The
gradients and/or discontinuities in the fluid velocity and the
magnetic field at the flux tube boundaries give rise to asymmetric
Stokes profiles. We concentrate on the Stokes-V profiles and study the
net circular polarization (NCP) of two photospheric spectral lines of
neutral iron, Fe I 630.25 nm and Fe I 1564.8 nm. The different behavior
of these two lines, which are exemplary for atomic spectral lines with
a large Landé factor and significantly different wavelength, is based
on the difference in excitation potential of the corresponding atomic
transitions and the fact that the wavelength dependence of the Doppler
shift is linear, while that of the Zeeman splitting is quadratic. We
find that the azimuthal variation of the NCP, N(psi, is a predominantly
antisymmetric function of psi with respect to the line connecting
disk center and spot center (line-of-symmetry) for the infrared line
of Fe I 1564.8 nm, while the variation is predominantly symmetric for
Fe I 630.25 nm. We show that the antisymmetric variation is caused
by anomalous dispersion (Faraday pulsation) and the discontinuity
in the azimuthal angle of the magnetic field, which is due to the
relative inclination between flux tube and background field. We
then compute synthetic NCP maps of a sunspot and compare them with
observational results. Finally, the center-to-limb variation of the
NCP, N(theta ), of these spectral lines is investigated. We show
that the location of the zero-crossing point of N(theta ) on the
center side of the line-of-symmetry represents a diagnostic tool to
determine the inclination angle of the Evershed flow: A vanishing NCP
on the center-side of the line-of-symmetry is an indirect evidence of
downflows in the penumbra.
Title: Penumbral fine structure: Theoretical understanding
Authors: Schlichenmaier, R.
Bibcode: 2002AN....323..303S
Altcode:
To understand the dynamic fine structure and the Evershed effect of
the sunspot penumbra, we have carried out time-dependent simulations
of a thin magnetic flux tube embedded in a static sunspot model
(Schlichenmaier et al. 1998a,b). Here, we present new simulation results
of the moving tube model which reveal flux tubes that, instead of lying
horizontal in the penumbral photosphere, develop waves (photospheric
serpent) that reproduce not only the inward migration of penumbral
grains, but also their observed outward migration, and which exhibit
downflow arches in the outer penumbra. During its evolution the
flux tube exhibits quasi-steady states. These states are compared to
time-independent solutions of siphon flow models, which have been used
to explain the Evershed flow (Meyer & Schmidt 1968, Degenhardt 1991,
Montesinos & Thomas 1997).
Title: Net circular polarization of sunspot penumbrae - symmetry
breaking by anomalous dispersion
Authors: Müller, D. A. N.; Schlichenmaier, R.; Steiner, O.; Stix, M.
Bibcode: 2002ESASP.508..141M
Altcode: 2002soho...11..141M
We examine the polarization of spectral lines in the penumbra of
sunspots by solving the radiative transfer equation of polarized
light for a three-dimensional axially symmetric model atmosphere
of a sunspot. The Evershed flow is confined to horizontal magnetic
flux tubes obtained from MHD calculations. These are embedded in
an inclined background magnetic field. In this work, we concentrate
on the Stokes-V profiles and examine the net circular polarization
(NCP), N = ∫V(λ)dλ, of two photospheric spectral lines of neutral
iron, Fe I 630.25 nm and Fe I 1564.8 nm. Analyzing spectra at a fixed
distance from the spot center, we find that the azimuthal variation
of N, N(ψ), is an antisymmetric function of ψ w.r.t. to the line
connecting disk center and spot center for Fe I 1564.8 nm, while
the variation is predominantly symmetric for Fe I 630.25 nm. We show
that the antisymmetric variation is caused by anomalous dispersion
(rotation of the polarization vector in a magnetized plasma). The
different inclination angles lead to a discontinuity in the azimuth
of the magnetic field along the line-of-sight. We show that this
discontinuity together with the effect of anomalous dispersion produced
an antisymmetric component in N(ψ) which outweighs the symmetric
component from the discontinuity for Fe I 1564.8 nm, while it is
negligible for Fe I 630.25 nm. We finally compute synthetic NCP maps of
a sunspot which offer an explanation for recent observational results.
Title: Spectropolarimetry in a sunspot penumbra. Spatial dependence
of Stokes asymmetries in Fe I 1564.8 nm
Authors: Schlichenmaier, R.; Collados, M.
Bibcode: 2002A&A...381..668S
Altcode:
Stokes profiles of sunspot penumbrae show distinct asymmetries, which
point to gradients in the velocity field and in the magnetic field. We
present spectropolarimetric measurements of the Stokes vector in the
neutral iron triplet at 1564.8 nm taken with the Tenerife Infrared
Polarimeter (TIP) at the German Vacuum Tower Telescope (VTT) in
Tenerife. We report on the peculiarities of the profiles of circularly
and linearly polarized light for spots at different heliocentric
angles. We elaborate on the spatial dependence of Stokes asymmetries
within the penumbra and find for profiles of circularly polarized light:
(1) In the center-side penumbra the amplitude difference of Stokes-V
exhibits a sign reversal on a radial cut, i.e., in the inner (outer)
penumbra the red (blue) lobe is broader and shows a smaller amplitude
than the blue (red) lobe. (2) In the outer limb-side penumbra (beyond
the magnetic neutral line) the red lobe is broader and of less amplitude
than the blue lobe. (3) Along the magnetic neutral line we find abnormal
Stokes-V profiles, which consist of more than 2 lobes. This indicates
the presence of two polarities. For small heliocentric angles abnormal
profiles are also seen beyond the magnetic neutral line in the outer
penumbra. (4) Maps of the net circular polarization have the tendency to
be antisymmetric with respect to the axis that connects disk center with
spot center. This finding is striking, because corresponding maps for Fe
I 630.25 are symmetric. For linearly polarized profiles we extract the
following features: (5) On the center-side penumbra at a heliocentric
angle of 56o a Doppler-shift as high as 5 km s-1
can be directly measured by the splitting of the pi -component of
the linearly polarized component. (6) In limb-side penumbrae, the
profiles of the pi -component show the typical asymmetry properties of
the Evershed flow as observed in Stokes-I of magnetically insensitive
lines. (7) In the outer center- and limb-side penumbrae the center of
the pi -component is blue-shifted relative to the zero-crossing of
the V-profile. Motivated by the moving tube model of Schlichenmaier
et al. (\cite{schlichenmaier+jahn+schmidt1998b}), we construct simple
model atmospheres featuring hot upflows and cool outflows and calculate
corresponding synthetic V-profiles. These profiles are compared with
our measured ones and with observed V-profiles in Fe I 630.25 from
other authors. We find that the synthetic V-profiles can reproduce
all essential characteristics of observed V-profiles for both lines.
Title: Net circular polarization of sunspot penumbrae. Symmetry
breaking through anomalous dispersion
Authors: Schlichenmaier, R.; Müller, D. A. N.; Steiner, O.; Stix, M.
Bibcode: 2002A&A...381L..77S
Altcode:
The net circular polarization, N, is used as a measure for the
asymmetry of Stokes-V profiles: Nequiv int V(lambda ) d lambda ,
integrated over an absorption line. Exemplary for Fe I 630.2 nm and
Fe I 1564.8 nm, we synthesize penumbral V-profiles that stem from a
model atmosphere in which the Evershed flow is confined to horizontal
flux tubes which are embedded in a magnetic field that has the same
magnetic field strength as the flow channel, but is less inclined
w.r.t. the surface normal. At the two points where a line-of-sight
enters and leaves the flow channel, discontinuities in the inclination,
gamma , the velocity v, and the azimuth, phi , of the magnetic field
vector w.r.t. the plane perpendicular to the line-of-sight produce
V-asymmetries. Assuming an axially symmetric penumbra, we investigate
the azimuthal dependence N(psi ) for a mid-penumbral radius. We find:
(1) Without including anomalous dispersion, N(psi ) is symmetric
w.r.t. the line that connects disk center to the center of the spot. (2)
Including anomalous dispersion, this symmetry is broken. We demonstrate
that this is due to the difference in azimuth, triangle phi (psi ),
between the flow channel and the background that varies along the
penumbral circle. For Fe I 630.2 nm this effect is found to be of
minor relevance leading to essentially symmetric N-maps, whereas strong
asymmetries are predicted for Fe I 1564.8 nm. Our results provide an
explanation for recent observational findings.
Title: Time-slice diagrams of solar granulation
Authors: Müller, D. A. N.; Steiner, O.; Schlichenmaier, R.; Brandt,
P. N.
Bibcode: 2001SoPh..203..211M
Altcode:
From a series of 1400 white-light images of solar granulation spanning
a time period of 8.2 hours, skeletal plots of time-slice diagrams are
derived showing intergranular lane positions as a function of time. The
diagrams permit to automatically track, classify, and relate 42 186
granules. Recurrently fragmenting granules are found that survive
by means of their descendants for more than 3 hours. Such long-lived
active granules tend to have a mean spatial distance along the slice
of about 10 Mm. This distance decreases with decreasing minimal
required lifetime. Since active granules are expected to generate a
steadily divergent flow over a long period of time, it is suggested
to identify them as a source of the mesogranular flow. Deficiencies
of the time-slice analysis are discussed. The relative frequency of
different types of granules and the granule decay time as derived
from the time-slice diagrams are compared with corresponding results
of previous works.
Title: Small-Scale Flow Field in a Sunspot Penumbra
Authors: Schlichenmaier, R.; Schmidt, W.
Bibcode: 2001ASPC..236..289S
Altcode: 2001aspt.conf..289S
No abstract at ADS
Title: Penumbral Stokes-V Asymmetries of Fe I 1564.8 nm
Authors: Schlichenmaier, R.; Soltau, D.; Lühe, O. V. D.; Collados, M.
Bibcode: 2001ASPC..236..579S
Altcode: 2001aspt.conf..579S
No abstract at ADS
Title: Small-scale flow field in a sunspot penumbra
Authors: Schmidt, W.; Schlichenmaier, R.
Bibcode: 2000A&A...364..829S
Altcode:
We have measured velocity maps of a round sunspot near the center of
the solar disk. We derive the small-scale velocity field of the penumbra
in the deep photosphere. Superimposed on a radial outflow, we find up-
and downflows, which are associated with bright and dark features: The
flow in the dark component of the penumbra has a larger inclination
angle with respect to the surface normal than the bright component,
everywhere in the penumbra. The maximum inclination difference is
8°. In the inner and mid penumbra, bright features (penumbral grains)
are associated with an upflow of about 0.6 km s-1. At the
outer spot boundary, the flow is inclined downwards predominantly in
the dark component (-7degr ).
Title: Flow geometry in a sunspot penumbra
Authors: Schlichenmaier, R.; Schmidt, W.
Bibcode: 2000A&A...358.1122S
Altcode:
We have measured the material flow in the penumbra of a large symmetric
sunspot during the passage of the spot across the solar meridian. The
line-of-sight velocity field has been obtained from Doppler measurements
in a Fe II line using a filter spectrometer with a large field of
view. From data sets taken on different days, i.e. at different view
angles, we have reconstructed the magnitude and orientation of the
penumbral flow field in the deep photosphere. We find upflows near
the inner and downflows at the outer boundary of the penumbra with
nearly horizontal outflow in between. From our measurements we derive
the following flow geometry: narrow upflow channels rise at different
penumbral radii, they bend outwards with a nearly horizontal outflow,
and are finally tilted a few degrees downwards at the outer penumbral
boundary, but still inside the spot. The flow reaches its maximum speed
of about 3.5 km s-1 in the outer part of the penumbra. Our
findings are a significant step towards an understanding of the mass
balance of the Evershed flow. The proposed geometry is consistent with
recent numerical models of penumbral filaments.
Title: Radiative cooling of a hot flux tube in the solar photosphere
Authors: Schlichenmaier, R.; Bruls, J. H. M. J.; Schüssler, M.
Bibcode: 1999A&A...349..961S
Altcode:
Radiative energy transport is of key importance for the dynamics of
slender magnetic flux tubes in the solar atmosphere, particularly
so in connection with the filamentation of the sunspot penumbra. In
investigations using the thin-flux-tube approximation of the MHD
equations, the radiative exchange with the surrounding atmosphere has
hitherto been described by the relaxation-time approach, also called
`Newton's law of cooling'. The strongly nonlinear temperature-dependence
of the radiative absorption coefficient and large temperature
differences between the tube and its environment render this concept
questionable. As a simple model of a bright penumbral filament we
consider the cooling of a hot horizontal flux tube with a longitudinal
flow, embedded in a non-stratified, homogeneous atmosphere at 4 800
K. We compare the results of the relaxation-time approach and of
a nonlinear diffusion approximation with the numerical solution of
the equation of (grey) radiative transfer. We find that the cooling
times given by the relaxation-time method compare well with the
results from radiative transfer as long as the initial temperature
of the tube is below 7 500 K and its lateral optical depth does not
exceed unity. Under these conditions, the tube cools more or less
homogeneously over its cross section. For hotter and optically thick
tubes, the strong temperature-dependence of the absorption coefficient
leads to the formation of a cooling front, which migrates radially
inward at approximately constant speed. Such inhomogeneous cooling
is well represented by the nonlinear diffusion approximation. The
self-similar evolution of the cooling front permits an analytical
estimate of the cooling time, which provides a reasonable approximation
of the result of the radiative transfer calculation. This estimate can
be used to derive an improved radiative cooling term in the framework
of the thin-flux-tube approximation, so that both optically thin and
optically thick flux tubes can be treated adequately. The results of
the radiative transfer calculations are applied to obtain an estimate
of the length and brightness of penumbral bright grains.
Title: Vertical mass flux in a sunspot penumbra
Authors: Schlichenmaier, R.; Schmidt, W.
Bibcode: 1999A&A...349L..37S
Altcode:
We present the first direct measurement of vertical motion in the
deepest atmospheric layers of a penumbra, obtained at hitherto
unreached spatial resolution. Isolated hot upflows in the inner
penumbra feed the horizontal Evershed flow that is observed in the
mid and outer penumbra. We discover cool downflows which surround the
hot upflows in the inner penumbra. At the outer edge of the penumbra,
the Evershed flow terminates in a ring of downflow channels. The
upflows transport a sufficient amount of energy to account for the
observed brightness of the penumbra. These measurements have therefore
significant implications for understanding the mass balance and the
energy transport in a sunspot penumbra.
Title: A Model for Penumbral Phenomena
Authors: Schlichenmaier, R.
Bibcode: 1999ASPC..183...91S
Altcode: 1999hrsp.conf...91S
No abstract at ADS
Title: Vertical and horizontal mass flux in a sunspot penumbra.
Authors: Schlichenmaier, R.; Schmidt, W.
Bibcode: 1999AGAb...15....8S
Altcode: 1999AGM....15..A06S
We present direct measurements of vertical and horizontal motions in
the deepest atmospheric layers of a sunspot penumbra with a spatial
resolution of about 0.5 arcsec. The observations have been made with
the Vacuum Tower Telescope of the Kiepenheuer-Institut, operated
at the Observatorio del Teide on Tenerife, using the Telecentric
Solar Spectrometer (TESOS), which is based on two Fabry-Perot
interferometers. >From two-dimensional spectra we infer the
line-of-sight velocities for the weak C I line at 538.0 nm, which forms
in the deepest photospheric layers, and for the Fe II 542.5 nm line,
a rather temperature sensitive line which forms in the mid-photosphere
at a height of about 100 km above the continuum level. We have observed
a fairly round sunspot (NOAA 8578) with a diameter of 32 000 km on its
passage through the central meridian on four consecutive days. We find
that isolated hot upflows in the inner penumbra feed the horizontal
Evershed flow that is observed in the mid and outer penumbra. At the
outer edge of the penumbra, the Evershed flow terminates in a ring of
downflow channels. Measurements at various position angles allow us
to derive the inclination angles of these flows. Based on our measured
upflow velocities, we estimate that the heat flux that emerges from our
brightest feature compares with the observed brightness of penumbral
grains. These measurements have therefore profound implications for
understanding the mass balance and the energy transport in a sunspot
penumbra.
Title: Radiative cooling of a hot flux tube in the solar photosphere
Authors: Schlichenmaier, R.; Bruls, J. H. M. J.; Schüssler, M.
Bibcode: 1999AGAb...15Q..75S
Altcode: 1999AGM....15..J16S
Radiative energy transport is of key importance for the dynamics of
slender magnetic flux tubes in the solar atmosphere. In investigations
using the thin-flux-tube approximation of the MHD equations, the
radiative exchange with the surrounding atmosphere has hitherto been
described by the relaxation-time approach, also called `Newton's law of
cooling'. The strongly nonlinear temperature-dependence of the radiative
absorption coefficient and large temperature differences between
the tube and its environment render this concept questionable. As a
simple model of a bright penumbral filament we consider the cooling
of a hot horizontal flux tube with a longitudinal flow, embedded in
a non-stratified, homogeneous atmosphere at 4 800 K. We compare the
results of the relaxation-time approach and of a nonlinear diffusion
approximation with the numerical solution of the equation of (grey)
radiative transfer. We find that the cooling times given by the
relaxation-time method compare well with the results from radiative
transfer as long as the initial temperature of the tube is below 7
500 K and its lateral optical depth does not exceed unity. Under these
conditions, the tube cools homogeneously over its cross section. For
hotter and optically thick tubes, the strong temperature-dependence of
the absorption coefficient leads to the formation of a cooling front,
which migrates radially inward at approximately constant speed. Such
inhomogeneous cooling is well represented by the nonlinear diffusion
approximation. The self-similar evolution of the cooling front permits
an analytical estimate of the cooling time, which provides a reasonable
approximation of the result of the radiative transfer calculation. This
estimate can be used to derive an improved radiative cooling term in
the framework of the thin-flux-tube approximation.
Title: Magnetic flux tubes evolving in sunspots. A model for the
penumbral fine structure and the Evershed flow
Authors: Schlichenmaier, R.; Jahn, K.; Schmidt, H. U.
Bibcode: 1998A&A...337..897S
Altcode: 1998astro.ph..7036S
Assuming that the interchange convection of magnetic flux elements
is the physical cause for the existence of filamentary penumbrae in
sunspots, we investigate the behavior of an individual fibril embedded
in the deep penumbra. The fibril is approximated by a thin magnetic flux
tube which evolves dynamically in the environment given by the global
magnetostatic model of a sunspot. Our simulation shows that the flux
tube, initially positioned at the penumbra-quiet Sun boundary in the
model, will rise through its deep penumbra developing a flow along the
tube that points upward beneath the photosphere, and radially outward
above the photosphere. Our results suggest that a bright filament may
be formed by an extended tail of a penumbral grain. Such filaments
are optically thick, hotter than the surroundings, and elevated above
a darker background. An upflow in penumbral grains bends horizontally
outwards above the photosphere and gradually cools down due to radiative
losses leading to a tail that gradually darkens. The plasma flow inside
the flux tube then becomes transparent and the tube constitutes a
thin elevated flow channel, that can reproduce the observed features
of the Evershed effect. We present also a new acceleration mechanism
for the Evershed flow. It is demonstrated that a local surplus of gas
pressure develops inside the tube as it rises through the specific
(superadiabatic and magnetized) penumbral background. The resulting
gradient of the gas pressure can drive the flow along the tube.
Title: A Dynamical Model for the Penumbral Fine Structure and the
Evershed Effect in Sunspots
Authors: Schlichenmaier, R.; Jahn, K.; Schmidt, H. U.
Bibcode: 1998ApJ...493L.121S
Altcode: 1997astro.ph.12029S
Relying on the assumption that the interchange convection of magnetic
flux tubes is the physical cause for the existence of sunspot penumbrae,
we propose a model in which the dynamical evolution of a thin magnetic
flux tube reproduces the Evershed effect and the penumbral fine
structure such as bright and dark filaments and penumbral grains. According to our model, penumbral grains are the manifestation of
the footpoints of magnetic flux tubes, along which hot subphotospheric
plasma flows upward with a few km s-1. Above the photosphere
the hot plasma inside the tube is cooled by radiative losses as it
flows horizontally outward. As long as the flowing plasma is hotter
than the surroundings, it constitutes a bright radial filament. The
flow confined to a thin elevated channel reaches the temperature
equilibrium with the surrounding atmosphere and becomes optically
thin near the outer edge of the penumbra. Here the tube has a height
of approximately 100 km above the continuum, and the flow velocity
reaches up to 14 km s-1. Such a flow channel can reproduce
the observed signatures of the Evershed effect.
Title: Die Dynamik magnetischer Flußröhren im Sonnenfleck : ein
Modell für den Evershed-Effekt und die penumbrale Feinstruktur Title:
Die Dynamik magnetischer Flußröhren im Sonnenfleck : ein Modell
für den Evershed-Effekt und die penumbrale Feinstruktur Title:
The dynamics of magnetic flux tubes in sunspots : a model for the
Evershed effect and penumbral fine structure;
Authors: Schlichenmaier, Rolf
Bibcode: 1997PhDT........34S
Altcode:
No abstract at ADS
Title: Dynamics of a Thin Magnetic Flux Tube in the Penumbra
Authors: Schlichenmaier, R.; Jahn, K.; Schmidt, H. U.
Bibcode: 1997ASPC..118..140S
Altcode: 1997fasp.conf..140S
The concept of the interchange convection has been proposed as a
hypothetic explanation of peculiarities of sunspot penumbrae. In order
to investigate this concept, we performed a numerical simulation in
which a thin magnetic flux tube embedded in a sunspot model evolves
dynamically. The simulations offer a consistent explanation for
penumbral fine structures such as bright filaments, penumbral grains,
and probably for the Evershed effect.
Title: Energy transport in the penumbra.
Authors: Schlichenmaier, R.; Jahn, K.; Schmidt, H. U.
Bibcode: 1996NAWG.1996..202S
Altcode:
The problem of energy transport in sunspot penumbrae is addressed. This
paper presents the mathematical tools to investigate numerically the
dynamics of a thin flux tube, which evolves in a 2D background formed
by a magneto-static sunspot model. The goal is to study the role of
interchange convection in penumbral energy transport and to examine
the usefullness of thin flux tubes as a physical entity to explain
observed penumbral features such as penumbral grains, bright and dark
filaments and the Evershed flow.
Title: Evolution of a Magnetic Flux Tube in a Sunspot Penumbra
Authors: Jahn, K.; Schlichenmaier, R.; Schmidt, H. U.
Bibcode: 1996ApL&C..34...59J
Altcode: 1995astro.ph..8128J
The motion of an individual magnetic flux tube inside the penumbra
of a sunspot is studied numerically. Here, we present preliminary
results. The thin flux tube approximation together with a simplified
radiative heat exchange with the surroundings is used to study the
evolution of a flux tube embedded into a background given by a global
magneto-static sunspot model. The investigation is undertaken in order
to verify the conjecture that convection in sunspot penumbrae occurs
by an interchange of magnetic flux tubes. The code being developed
can be used to study dynamic aspects of filamentary structure in the
penumbra: the temporal and spatial fluctuations of the temperature
and the magnetic field, the motion of bright penumbral grains, or the
Evershed effect. Here we present the evolution of a wave formed by the
tube whose fragment emerges in the penumbral photosphere and migrates
towards the umbra. The properties of this wave show qualitative
features of the observed bright penumbral grains with corresponding
upward velocity and its correlation with brightness and the inclination
of the magnetic field, and also of the Evershed effect.
Title: Dynamics of a magnetic flux tube in the penumbra.
Authors: Schlichenmaier, R.; Jahn, K.; Schmidt, H. U.
Bibcode: 1996AGAb...12...90S
Altcode:
No abstract at ADS
Title: Numerical Study of a Thin Magnetic Flux Tubes' Migration in
a Sunspots' Penumbra
Authors: Jahn, K.; Schlichenmaier, R.; Schmidt, H. U.
Bibcode: 1996pas..meet...73J
Altcode:
No abstract at ADS
Title: The phase of the radial mean field in the solar dynamo.
Authors: Schlichenmaier, R.; Stix, M.
Bibcode: 1995A&A...302..264S
Altcode:
Observations indicate that the radial and azimuthal components of the
mean solar magnetic field oscillate with a phase shift of approximately
180deg during the 22-year cycle. In order to calculate such phase shifts
we construct a simple two-dimensional, nonlinear α^2OMEGA
dynamo, which operates in the overshoot region beneath the convection
zone. Like previous models, our model predicts an almost in-phase
oscillation for most parameter choices. Special configurations, in which
the two components of the mean field have different distributions in
latitude, may resolve the dilemma. Alternative conclusions are that our
knowledge of the α effect is insufficient, or that the observational
result is not reliable.
Title: The phase of the radial field in the solar dynamo
Authors: Schlichenmaier, R.; Stix, M.
Bibcode: 1994smf..conf..107S
Altcode:
No abstract at ADS