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Author name code: svanda
ADS astronomy entries on 2022-09-14
author:Svanda, Michal
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Title: The New Composite Solar Flare Index from Solar Cycle 17 to
Cycle 24 (1937 - 2020)
Authors: Velasco Herrera, Victor Manuel; Soon, Willie; Knoška,
Štefan; Perez-Peraza, Jorge Alberto; Cionco, Rodolfo G.; Kudryavtsev,
Sergey M.; Qiu, Shican; Connolly, Ronan; Connolly, Michael; Švanda,
Michal; Acosta Jara, José; Gregori, Giovanni Pietro
2022SoPh..297..108V Altcode:
The chromosphere is a highly dynamic outer plasma layer of the
Sun. Its physical processes accounting for the variability are poorly
understood. We reconstructed the solar chromospheric flare index (SFI)
to study the solar chromospheric variability from 1937 to 2020. The
new SFI database is a composite record of the Astronomical Institute
Ondřejov Observatory of the Czech Academy of Sciences from 1937 -
1976 and the records of the Kandilli Observatory of Istanbul, Turkey
from 1977 - 2020. The SFI records are available in daily, monthly, and
yearly resolutions. We carried out the time-frequency analyses of the
new 84-year long SFI records using the wavelet transform. We report
the periodicities of 21.88 (Hale cycle), 10.94 (Schwabe cycle), 5.2
(quasi-quinquennial cycle), 3.5, 1.7, 1, 0.41 (or 149.7 days, Rieger
cycle), 0.17 (62.1 days), 0.07 (25.9 days, solar rotational modulation)
years. All these periodicities seem always present and persistent
throughout the observational interval. Thus, we suggest that there is
no reason to assume these solar periodicities are absent from other
solar cycles. Time variations of the amplitude of each oscillation or
periodicity were also studied using the inverse wavelet transform. We
found that for the SFI the most active flare cycles over the record were
Cycles 17, 19, and 21, while Cycles 20, 22, 23, and 24 were the weakest
ones with Cycle 18 was intermediate in flare activity. This shows
several differences to the equivalent relationships for solar activity
implied by sunspot number records. Furthermore, this confirms that
solar activity trends and variability in the chromosphere as captured
by SFI are not necessarily the same as those of the Sun's photosphere,
as implied by the sunspot number activity records, for instance. We
have also introduced a new signal/noise wavelet coherence metric
to analyze two different chromospheric indices available (i.e. the
SFI and the disk-integrated chromospheric Ca II K activity indices)
and to quantify the differences and similarities of the oscillations
within the solar chromosphere. Our findings suggest the importance of
carrying out additional co-analyses with other solar activity records
to find physical inter-relations and connections between the different
solar layers from the photosphere, the chromosphere to the corona.
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Title: Modelling of geomagnetically induced currents in the Czech
transmission grid
Authors: Švanda, Michal; Smičková, Anna; VýbošÅ¥oková, Tatiana
2021EP&S...73..229S Altcode: 2021arXiv211202438S
We investigate the maximum expected magnitudes of the geomagnetically
induced currents (GICs) in the Czech transmission power network. We
compute a model utilising the Lehtinen-Pirjola method, considering the
plane-wave model of the geoelectric field, and using the transmission
network parameters kindly provided by the operator. We find that the
maximum amplitudes expected in the nodes of the Czech transmission
grid during the Halloween storm-like event are about 15 A. For the
"extreme-storm" conditions with a 1-V/km geoelectric field, the
expected maxima do not exceed 40 A. We speculate that the recently
proven statistical correlation between the increased geomagnetic
activity and anomaly rate in the power grid may be due to the repeated
exposure of the devices to the low-amplitude GICs.
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Title: One-sided arc averaging geometries in time-distance local
helioseismology
Authors: Korda, David; Švanda, Michal; Roudier, Thierry
2021A&A...654A..84K Altcode: 2021arXiv210800872K
Context. The study of solar oscillations (helioseismology) has been a
very successful method of researching the Sun. Helioseismology teaches
us about the structure and mean properties of the Sun. Together
with mid-resolution data, the local properties were uncovered in
quiet-Sun regions. However, magnetic fields affect the oscillations
and prevent us from studying the properties of magnetically active
regions with helioseismology. <BR /> Aims: We aim to create a new
methodology to suppress the negative effects of magnetic fields on
solar oscillations and measure plasma properties close to active
regions. <BR /> Methods: The methodology consists of new averaging
geometries, a non-linear approach to travel-time measurements, and a
consistent inversion method that combines plasma flows and sound-speed
perturbations. <BR /> Results: We constructed the one-sided arc
averaging geometries and applied them to the non-linear approach
of travel-time measurements. Using the one-sided arc travel times,
we reconstructed the annulus travel times in a quiet-Sun region. We
tested the methodology against the validated helioseismic inversion
pipeline. We applied the new methodology for an inversion for surface
horizontal flows in a region with a circular H-type sunspot. The
inverted surface horizontal flows are comparable with the output of
the coherent structure tracking, which is not strongly affected by
the presence of the magnetic field. We show that the new methodology
suppresses the negative effects of magnetic fields up to outer
penumbra. We measure divergent flows with properties comparable to
the moat flow. <BR /> Conclusions: The new methodology can teach us
about the depth structure of active regions and physical conditions
that contribute to the evolution of the active regions.
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Title: IRIS observations of chromospheric heating by acoustic waves
in solar quiet and active regions
Authors: Abbasvand, V.; Sobotka, M.; Švanda, M.; Heinzel, P.; Liu,
W.; Mravcová, L.
2021A&A...648A..28A Altcode: 2021arXiv210208678A
<BR /> Aims: To study the heating of solar chromospheric magnetic and
nonmagnetic regions by acoustic and magnetoacoustic waves, the deposited
acoustic-energy flux derived from observations of strong chromospheric
lines is compared with the total integrated radiative losses. <BR />
Methods: A set of 23 quiet-Sun and weak-plage regions were observed in
the Mg II k and h lines with the Interface Region Imaging Spectrograph
(IRIS). The deposited acoustic-energy flux was derived from Doppler
velocities observed at two different geometrical heights corresponding
to the middle and upper chromosphere. A set of scaled nonlocal
thermodynamic equilibrium 1D hydrostatic semi-empirical models -
obtained by fitting synthetic to observed line profiles - was applied
to compute the radiative losses. The characteristics of observed waves
were studied by means of a wavelet analysis. <BR /> Results: Observed
waves propagate upward at supersonic speed. In the quiet chromosphere,
the deposited acoustic flux is sufficient to balance the radiative
losses and maintain the semi-empirical temperatures in the layers under
study. In the active-region chromosphere, the comparison shows that
the contribution of acoustic-energy flux to the radiative losses is
only 10−30%. <BR /> Conclusions: Acoustic and magnetoacoustic waves
play an important role in the chromospheric heating, depositing a main
part of their energy in the chromosphere. Acoustic waves compensate
for a substantial fraction of the chromospheric radiative losses in
quiet regions. In active regions, their contribution is too small to
balance the radiative losses and the chromosphere has to be heated by
other mechanisms.
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Title: Evolution and motions of magnetic fragments during the active
region formation and decay: A statistical study
Authors: Švanda, Michal; Sobotka, Michal; Mravcová, Lucia;
Výbošťoková, Tatiana
2021A&A...647A.146S Altcode: 2021arXiv210202807S
Context. The evolution of solar active regions is still not fully
understood. The growth and decay of active regions have mostly been
studied in case-by-case studies. <BR /> Aims: Instead of studying the
evolution of active regions case by case, we performed a large-scale
statistical study to find indications for the statistically most
frequent scenario. <BR /> Methods: We studied a large sample of active
regions recorded by the Helioseismic and Magnetic Imager instrument. The
sample was split into two groups: forming (367 members) and decaying
(679 members) active regions. We tracked individual dark features
(i.e. those that are assumed to be intensity counterparts of magnetised
fragments from small objects to proper sunspots) and followed their
evolution. We investigated the statistically most often locations
of fragment merging and splitting as well as their properties. <BR
/> Results: Our results confirm that statistically, sunspots form
by merging events of smaller fragments. The coalescence process is
driven by turbulent diffusion in a process similar to random-walk,
where supergranular flows seem to play an important role. The number
of appearing fragments does not seem to significantly correlate with
the number of sunspots formed. The formation seems to be consistent
with the magnetic field accumulation. Statistically, the merging occurs
most often between a large and a much smaller object. The decay of the
active region seems to take place preferably by a process similar to
the erosion.
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Title: Photospheric downflows observed with SDO/HMI, HINODE, and an
MHD simulation
Authors: Roudier, T.; Švanda, M.; Malherbe, J. M.; Ballot, J.; Korda,
D.; Frank, Z.
2021A&A...647A.178R Altcode: 2021arXiv210303077R
Downflows on the solar surface are suspected to play a major role in
the dynamics of the convection zone, at least in its outer part. We
investigate the existence of the long-lasting downflows whose effects
influence the interior of the Sun but also the outer layers. We study
the sets of Dopplergrams and magnetograms observed with Solar Dynamics
Observatory and Hinode spacecrafts and an magnetohydrodynamic (MHD)
simulation. All of the aligned sequences, which were corrected from
the satellite motions and tracked with the differential rotation,
were used to detect the long-lasting downflows in the quiet-Sun at
the disc centre. To learn about the structure of the flows below the
solar surface, the time-distance local helioseismology was used. The
inspection of the 3D data cube (x, y, t) of the 24 h Doppler sequence
allowed us to detect 13 persistent downflows. Their lifetimes lie in
the range between 3.5 and 20 h with a sizes between 2″ and 3″ and
speeds between −0.25 and −0.72 km s<SUP>−1</SUP>. These persistent
downflows are always filled with the magnetic field with an amplitude of
up to 600 Gauss. The helioseismic inversion allows us to describe the
persistent downflows and compare them to the other (non-persistent)
downflows in the field of view. The persistent downflows seem to
penetrate much deeper and, in the case of a well-formed vortex, the
vorticity keeps its integrity to the depth of about 5 Mm. In the MHD
simulation, only sub-arcsecond downflows are detected with no evidence
of a vortex comparable in size to observations at the surface of the
Sun. The long temporal sequences from the space-borne allows us to show
the existence of long-persistent downflows together with the magnetic
field. They penetrate inside the Sun but are also connected with the
anchoring of coronal loops in the photosphere, indicating a link between
downflows and the coronal activity. A links suggests that EUV cyclones
over the quiet Sun could be an effective way to heat the corona.
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Title: Plasma flows and sound-speed perturbations in the average
supergranule
Authors: Korda, David; Švanda, Michal
2021A&A...646A.184K Altcode: 2021arXiv210105731K
Context. Supergranules create a peak in the spatial spectrum
of photospheric velocity features. Even though they have some
properties of convection cells, their origin is still being
debated in the literature. The time-distance helioseismology
constitutes a method that is suitable for investigating the deep
structure of supergranules. <BR /> Aims: Our aim is to construct
the model of the flows in the average supergranular cell using
fully consistent time-distance inverse methodology. <BR /> Methods:
We used the Multi-Channel Subtractive Optimally Localised Averaging
inversion method with regularisation of the cross-talk. We combined
the difference and the mean travel-time averaging geometries. We
applied this methodology to travel-time maps averaged over more than
10<SUP>4</SUP> individual supergranular cells. These cells were detected
automatically in travel-time maps computed for 64 quiet days around the
disc centre. The ensemble averaging method allows us to significantly
improve the signal-to-noise ratio and to obtain a clear picture
of the flows in the average supergranule. <BR /> Results: We found
near-surface divergent horizontal flows which quickly and monotonously
weakened with depth; they became particularly weak at the depth of
about 7 Mm, where they even apparently switched sign. The amplitude
of the `reversed' flow was comparable to the background flows. The
inverted vertical flows and sound-speed perturbations were spoiled
by unknown systematic errors. To learn about the vertical component,
we integrated the continuity equation from the surface. The derived
estimates of the vertical flow depicted a sub-surface increase from
about 5 m s<SUP>-1</SUP> at the surface to about 35 m s<SUP>-1</SUP>
at the depth of about 3 Mm followed by a monotonous decrease to greater
depths. The vertical flow remained positive (an upflow) and became
indistinguishable from the background at the depth of about 15 Mm. We
further detected a systematic flow in the longitudinal direction. The
course of this systematic flow with depth agrees well with the model
of the solar rotation in the sub-surface layers.
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Title: Observational study of chromospheric heating by acoustic waves
Authors: Abbasvand, V.; Sobotka, M.; Švanda, M.; Heinzel, P.;
García-Rivas, M.; Denker, C.; Balthasar, H.; Verma, M.; Kontogiannis,
I.; Koza, J.; Korda, D.; Kuckein, C.
2020A&A...642A..52A Altcode: 2020arXiv200802688A
<BR /> Aims: Our aim is to investigate the role of acoustic and
magneto-acoustic waves in heating the solar chromosphere. Observations
in strong chromospheric lines are analyzed by comparing the deposited
acoustic-energy flux with the total integrated radiative losses. <BR
/> Methods: Quiet-Sun and weak-plage regions were observed in the Ca
II 854.2 nm and Hα lines with the Fast Imaging Solar Spectrograph
(FISS) at the 1.6-m Goode Solar Telescope on 2019 October 3 and
in the Hα and Hβ lines with the echelle spectrograph attached
to the Vacuum Tower Telescope on 2018 December 11 and 2019 June
6. The deposited acoustic energy flux at frequencies up to 20 mHz
was derived from Doppler velocities observed in line centers and
wings. Radiative losses were computed by means of a set of scaled
non-local thermodynamic equilibrium 1D hydrostatic semi-empirical
models obtained by fitting synthetic to observed line profiles. <BR />
Results: In the middle chromosphere (h = 1000-1400 km), the radiative
losses can be fully balanced by the deposited acoustic energy flux in
a quiet-Sun region. In the upper chromosphere (h > 1400 km), the
deposited acoustic flux is small compared to the radiative losses in
quiet as well as in plage regions. The crucial parameter determining
the amount of deposited acoustic flux is the gas density at a given
height. <BR /> Conclusions: The acoustic energy flux is efficiently
deposited in the middle chromosphere, where the density of gas is
sufficiently high. About 90% of the available acoustic energy flux in
the quiet-Sun region is deposited in these layers, and thus it is a
major contributor to the radiative losses of the middle chromosphere. In
the upper chromosphere, the deposited acoustic flux is too low, so that
other heating mechanisms have to act to balance the radiative cooling.
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Title: Exploiting Solar Visible-Range Observations by Inversion
Techniques: From Flows in the Solar Subsurface to a Flaring Atmosphere
Authors: Švanda, Michal; Jurčák, Jan; Korda, David; Kašparová,
Jana
2020rfma.book..349S Altcode:
Observations of the Sun in the visible spectral range belong to standard
measurements obtained by instruments both on the ground and in the
space. Nowadays, both nearly continuous full-disc observations with
medium resolution and dedicated campaigns of high spatial, spectral
and/or temporal resolution constitute a holy grail for studies that
can capture (both) the long- and short-term changes in the dynamics
and energetics of the solar atmosphere. Observations of photospheric
spectral lines allow us to estimate not only the intensity at small
regions, but also various derived data products, such as the Doppler
velocity and/or the components of the magnetic field vector. We show
that these measurements contain not only direct information about the
dynamics of solar plasmas at the surface of the Sun but also imprints
of regions below and above it. Here, we discuss two examples: First,
the local time-distance helioseismology as a tool for plasma dynamic
diagnostics in the near subsurface and second, the determination of
the solar atmosphere structure during flares. The methodology in both
cases involves the technique of inverse modelling.
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Title: Immediate and delayed responses of power lines and transformers
in the Czech electric power grid to geomagnetic storms
Authors: Švanda, Michal; Mourenas, Didier; Žertová, Karla;
Výbošt'oková, Tatiana
2020JSWSC..10...26S Altcode: 2020arXiv200514448S
Eruptive events of solar activity often trigger abrupt variations of the
geomagnetic field. Through the induction of electric currents, human
infrastructures are also affected, namely the equipment of electric
power transmission networks. It was shown in past studies that the
rate of power-grid anomalies may increase after an exposure to strong
geomagnetically induced currents. We search for a rapid response of
devices in the Czech electric distribution grid to disturbed days of
high geomagnetic activity. Such disturbed days are described either
by the cumulative storm-time Dst or d(SYM-H)/dt low-latitude indices
mainly influenced by ring current variations, by the cumulative
AE high-latitude index measuring substorm-related auroral current
variations, or by the cumulative ap mid-latitude index measuring
both ring and auroral current variations. We use superposed epoch
analysis to identify possible increases of anomaly rates during and
after such disturbed days. We show that in the case of abundant series
of anomalies on power lines, the anomaly rate increases significantly
immediately (within 1 day) after the onset of geomagnetic storms. In
the case of transformers, the increase of the anomaly rate is generally
delayed by 2-3 days. We also find that transformers and some electric
substations seem to be sensitive to a prolonged exposure to substorms,
with a delayed increase of anomalies. Overall, we show that in the 5-day
period following the commencement of geomagnetic activity there is an
approximately 5-10% increase in the recorded anomalies in the Czech
power grid and thus this fraction of anomalies is probably related to
an exposure to GICs.
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Title: Evolution of photospheric flows under an erupting filament
in the quiet-Sun region
Authors: Wollmann, Jiří; Švanda, Michal; Korda, David; Roudier,
Thierry
2020A&A...636A.102W Altcode: 2020arXiv200312515W
Context. We studied the dynamics of the solar atmosphere in the region
of a large quiet-Sun filament, which erupted on 21 October 2010. The
filament eruption started at its northern end and disappeared from
the Hα line-core filtergrams line within a few hours. The very fast
motions of the northern leg were recorded in ultraviolet light by
the Atmospheric Imaging Assembly (AIA) imager. <BR /> Aims: We aim to
study a wide range of available datasets describing the dynamics of
the solar atmosphere for five days around the filament eruption. This
interval covers three days of the filament evolution, one day before
the filament growth and one day after the eruption. We search for
possible triggers that lead to the eruption of the filament. <BR />
Methods: The surface velocity field in the region of the filament
were measured by means of time-distance helioseismology and
coherent structure tracking. The apparent velocities in the higher
atmosphere were estimated by tracking the features in the 30.4 nm
AIA observations. To capture the evolution of the magnetic field,
we extrapolated the photospheric line-of-sight magnetograms and also
computed the decay index of the magnetic field. <BR /> Results: We found
that photospheric velocity fields showed some peculiarities. Before the
filament activation, we observed a temporal increase of the converging
flows towards the filament's spine. In addition, the mean squared
velocity increased temporarily before the activation and peaked just
before it, followed by a steep decrease. We further see an increase
in the average shear of the zonal flow component in the filament's
region, followed by a steep decrease. The photospheric line-of-sight
magnetic field shows a persistent increase of induction eastward from
the filament spine. The decay index of the magnetic field at heights
around 10 Mm shows a value larger than critical one at the connecting
point of the northern filament end. The value of the decay index
increases monotonically there until the filament activation. Then,
it decreased sharply. <P />AIA30.4 nm movie is available at <A
href="https://www.aanda.org/10.1051/0004-6361/202037525/olm">https://www.aanda.org</A>
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Title: Chromospheric Heating by Acoustic Waves Compared to Radiative
Cooling. II. Revised Grid of Models
Authors: Abbasvand, Vahid; Sobotka, Michal; Heinzel, Petr; Švanda,
Michal; Jurčák, Jan; del Moro, Dario; Berrilli, Francesco
2020ApJ...890...22A Altcode: 2020arXiv200103413A
Acoustic and magnetoacoustic waves are considered to be possible
agents of chromospheric heating. We present a comparison of deposited
acoustic energy flux with total integrated radiative losses in the
middle chromosphere of the quiet Sun and a weak plage. The comparison
is based on a consistent set of high-resolution observations acquired
by the Interferometric Bidimensional Spectrometer instrument in the
Ca II 854.2 nm line. The deposited acoustic-flux energy is derived
from Doppler velocities observed in the line core and a set of 1737
non-local thermodynamic equilibrium 1D hydrostatic semi-empirical
models, which also provide the radiative losses. The models are obtained
by scaling the temperature and column mass of five initial models by
Vernazza et al. (1981; VAL) B-F to get the best fit of synthetic to
observed profiles. We find that the deposited acoustic-flux energy in
the quiet-Sun chromosphere balances 30%-50% of the energy released by
radiation. In the plage, it contributes by 50%-60% in locations with
vertical magnetic field and 70%-90% in regions where the magnetic
field is inclined more than 50° to the solar surface normal.
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Title: Exploiting solar visible-range observations by inversion
techniques: from flows in the solar subsurface to a flaring atmosphere
Authors: Švanda, Michal; Jurčák, Jan; Korda, David; Kašparová,
Jana
2020arXiv200103874S Altcode:
Observations of the Sun in the visible spectral range belong to standard
measurements obtained by instruments both on the ground and in the
space. Nowadays, both nearly continuous full-disc observations with
medium resolution and dedicated campaigns of high spatial, spectral
and/or temporal resolution constitute a holy grail for studies that
can capture (both) the long- and short-term changes in the dynamics
and energetics of the solar atmosphere. Observations of photospheric
spectral lines allow us to estimate not only the intensity at small
regions, but also various derived data products, such as the Doppler
velocity and/or the components of the magnetic field vector. We show
that these measurements contain not only direct information about the
dynamics of solar plasmas at the surface of the Sun but also imprints
of regions below and above it. Here, we discuss two examples: First,
the local time-distance helioseismology as a tool for plasma dynamic
diagnostics in the near subsurface and second, the determination of
the solar atmosphere structure during flares. The methodology in both
cases involves the technique of inverse modelling.
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Title: Comparison of time-distance inversion methods applied to
SDO/HMI Dopplergrams
Authors: Korda, David; Švanda, Michal; Zhao, Junwei
2019A&A...629A..55K Altcode: 2019arXiv190803950K
Context. The Helioseismic and Magnetic Imager (HMI) onboard the Solar
Dynamics Observatory (SDO) satellite has been observing the Sun since
2010. The uninterrupted series of Dopplergrams are ideal for studying
the dynamics of the upper solar convection zone. Within the Joint
Science Operations Center (JSOC) the time-distance inversions for
flows and sound-speed perturbations were introduced. The automatic
pipeline has produced flow and sound-speed maps every 8 h. We verify
the results of JSOC inversions by comparing the data products to
equivalent results from inverse modelling obtained by an independent
inversion pipeline. <BR /> Aims: We compared the results from the
JSOC pipeline for horizontal flow components and the perturbations of
the speed of sound at set of depths with equivalent results from an
independently implemented pipeline using a different time-distance
inversion scheme. Our inversion pipeline allows inversion for all
quantities at once while allowing minimisation of the crosstalk between
them. This gives us an opportunity to discuss the possible biases
present in the JSOC data products. <BR /> Methods: For the tests we
used the subtractive optimally localised averaging (SOLA) method with a
minimisation of the cross-talk. We compared three test inversions for
each quantity at each target depth. At first, we used the JSOC setup
to reproduce the JSOC results. Subsequently, we used the extended
pipeline to improve these results by incorporating more independent
travel-time measurements but keeping the JSOC-indicated localisation
in the Sun. Finally, we inverted for flow components and sound-speed
perturbations using a localisation kernel with properties advertised
in the JSOC metadata. <BR /> Results: We successfully reproduced the
horizontal flow components. The sound-speed perturbations are strongly
affected by the high level of the cross-talk in JSOC products. This
leads to larger amplitudes in the inversions for the sound-speed
perturbations. Different results were obtained when a target function
localised around the target depth was used. This is a consequence of
non-localised JSOC averaging kernels. We add that our methodology also
allows inversion for the vertical flow.
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Title: A New Look into Putative Duplicity and Pulsations of the Be
Star β CMi
Authors: Harmanec, P.; Švanda, M.; Korčáková, D.; Chini, R.;
Nasseri, A.; Yang, S.; Božić, H.; Šlechta, M.; Vanzi, L.
2019ApJ...875...13H Altcode: 2019arXiv190207450H
Bright Be star β CMi has been identified as a nonradial pulsator on the
basis of space photometry with the Microvariability and Oscillations
of Stars (MOST) satellite and also as a single-line spectroscopic
binary with a period of 170.ͩ4. The purpose of this study is to
re-examine both these findings using numerous electronic spectra
from the Dominion Astrophysical Observatory, Ondřejov Observatory,
Universitätssterwarte Bochum, archival electronic spectra from
several observatories, as well as the original MOST satellite
photometry. We measured the radial velocity of the outer wings of
the double Hα emission in all spectra at our disposal, and were not
able to confirm significant radial-velocity changes. We also discuss
the problems related to the detection of very small radial-velocity
changes and conclude that while it is still possible that the star
is a spectroscopic binary, there is currently no convincing proof
of it from the radial-velocity measurements. Wavelet analysis of the
MOST photometry shows that there is only one persistent (and perhaps
slightly variable) periodicity of 0.ͩ617 of the light variations,
with a double-wave light curve; all other short periods having only
transient character. Our suggestion that this dominant period is the
star’s rotational period agrees with the estimated stellar radius,
projected rotational velocity, and with the orbital inclination derived
by two teams of investigators. New spectral observations obtained
in the whole-night series would be needed to find out whether some
possibly real, very small radial-velocity changes cannot, in fact, be
due to rapid line-profile changes. <P />Based on spectral observations
obtained at the Dominion Astrophysical Observatory, NRC Herzberg,
Programs in Astronomy and Astrophysics, National Research Council of
Canada, Ondřejov Observatory and Universitätssternwarte Bochum, and
on photometry from the Canadian MOST satellite and UBV observations
from the Hvar Observatory.
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Title: Combined helioseismic inversions for 3D vector flows and
sound-speed perturbations
Authors: Korda, David; Švanda, Michal
2019A&A...622A.163K Altcode: 2019arXiv190101293K
Context. Time-distance helioseismology is the method of the study of the
propagation of waves through the solar interior via the travel times
of those waves. The travel times of wave packets contain information
about the conditions in the interior integrated along the propagation
path of the wave. The travel times are sensitive to perturbations of
a variety of quantities. The usual task is to invert for the vector of
plasma flows or the sound-speed perturbations separately. The separate
inversions may be polluted by systematic bias, for instance, originating
in the leakage of vector flows into the sound-speed perturbations and
vice versa (called a cross-talk). Information about the cross-talk
is necessary for a proper interpretation of results. <BR /> Aims: We
introduce an improved methodology of the time-distance helioseismology
which allows us to invert for a full 3D vector of plasma flows and
the sound-speed perturbations at once. Using this methodology one
can also derive the mean value of the vertical component of plasma
flows and the cross-talk between the plasma flows and the sound-speed
perturbations. <BR /> Methods: We used the Subtractive Optimally
Localised Averaging method with a minimisation of the cross-talk
as a tool for inverse modelling. In the forward model, we use Born
approximation travel-time sensitivity kernels with the Model S as
a background. The methodology was validated using forward-modelled
travel times with both mean and difference point-to-annulus averaging
geometries applied to a snapshot of fully self-consistent simulation of
the convection. <BR /> Results: We tested the methodology on synthetic
data. We demonstrate that we are able to recover flows and sound-speed
perturbations in the near-surface layers. We have taken the advantage
of the sensitivity of our methodology to entire vertical velocity, and
not only to its variations as in other available methodologies. The
cross-talk from both the vertical flow component and the sound-speed
perturbation has only a negligible effect for inversions for the
horizontal flow components. Furthermore, this cross-talk can be
minimised if needed. The inversions for the vertical component of
the vector flows or for the sound-speed perturbations are affected
by the cross-talk from the horizontal components, which needs to be
minimised in order to provide valid results. It seems that there is
a nearly constant cross-talk between the vertical component of the
vector flows and the sound-speed perturbations.
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Title: Heating of the solar photosphere during a white-light flare
Authors: Jurčák, Jan; Kašparová, Jana; Švanda, Michal; Kleint,
Lucia
2018A&A...620A.183J Altcode: 2018arXiv181107794J
Context. The Fe I lines observed by the Hinode/SOT spectropolarimeter
were always seen in absorption, apart from the extreme solar limb. Here
we analyse a unique dataset capturing these lines in emission during
a solar white-light flare. <BR /> Aims: We analyse the temperature
stratification in the solar photosphere during a white-light flare and
compare it with the post-white-light flare state. <BR /> Methods: We
used two scans of the Hinode/SOT spectropolarimeter to infer, by means
of the LTE inversion code Stokes Inversion based on Response function
(SIR), the physical properties in the solar photosphere during and
after a white-light flare. The resulting model atmospheres are compared
and the changes are related to the white-light flare. <BR /> Results:
We show that the analysed white-light flare continuum brightening is
probably not caused by the temperature increase at the formation height
of the photospheric continuum. However, the photosphere is heated
by the flare approximately down to log τ = -0.5 and this results
in emission profiles of the observed Fe I lines. From the comparison
with the post-white-light flare state of the atmosphere, we estimate
that the major contribution to the increase in the continuum intensity
originates in the heated chromosphere.
---------------------------------------------------------
Title: Understanding the HMI Pseudocontinuum in White-light Solar
Flares
Authors: Švanda, Michal; Jurčák, Jan; Kašparová, Jana; Kleint,
Lucia
2018ApJ...860..144S Altcode: 2018arXiv180503369S
We analyze observations of the X9.3 solar flare (SOL2017-09-06T11:53)
observed by SDO/HMI and Hinode/Solar Optical Telescope. Our aim is to
learn about the nature of the HMI pseudocontinuum I <SUB>c</SUB> used as
a proxy for the white-light continuum. From model atmospheres retrieved
by an inversion code applied to the Stokes profiles observed by the
Hinode satellite, we synthesize profiles of the Fe I 617.3 nm line and
compare them to HMI observations. Based on a pixel-by-pixel comparison,
we show that the value of I <SUB>c</SUB> represents the continuum level
well in quiet-Sun regions only. In magnetized regions, it suffers from
a simplistic algorithm that is applied to a complex line shape. During
this flare, both instruments also registered emission profiles in the
flare ribbons. Such emission profiles are poorly represented by the
six spectral points of HMI and the MDI-like algorithm does not account
for emission profiles in general; thus, the derived pseudocontinuum
intensity does not approximate the continuum value properly.
---------------------------------------------------------
Title: Large-scale photospheric motions determined from granule
tracking and helioseismology from SDO/HMI data
Authors: Roudier, Th.; Švanda, M.; Ballot, J.; Malherbe, J. M.;
Rieutord, M.
2018A&A...611A..92R Altcode: 2017arXiv171205255R
Context. Large-scale flows in the Sun play an important role in the
dynamo process linked to the solar cycle. The important large-scale
flows are the differential rotation and the meridional circulation
with an amplitude of km s<SUP>-1</SUP> and few m s<SUP>-1</SUP>,
respectively. These flows also have a cycle-related components,
namely the torsional oscillations. Aim. Our attempt is to determine
large-scale plasma flows on the solar surface by deriving horizontal
flow velocities using the techniques of solar granule tracking,
dopplergrams, and time-distance helioseismology. <BR /> Methods:
Coherent structure tracking (CST) and time-distance helioseismology
were used to investigate the solar differential rotation and meridional
circulation at the solar surface on a 30-day HMI/SDO sequence. The
influence of a large sunspot on these large-scale flows with a specific
7-day HMI/SDO sequence has been also studied. <BR /> Results: The
large-scale flows measured by the CST on the solar surface and the
same flow determined from the same data with the helioseismology in
the first 1 Mm below the surface are in good agreement in amplitude
and direction. The torsional waves are also located at the same
latitudes with amplitude of the same order. We are able to measure
the meridional circulation correctly using the CST method with only
3 days of data and after averaging between ± 15° in longitude. <BR
/> Conclusions: We conclude that the combination of CST and Doppler
velocities allows us to detect properly the differential solar rotation
and also smaller amplitude flows such as the meridional circulation
and torsional waves. The results of our methods are in good agreement
with helioseismic measurements.
---------------------------------------------------------
Title: Differential rotation in magnetic chemically peculiar stars
Authors: Mikulášek, Z.; Krtička, J.; Paunzen, E.; Švanda, M.;
Hummerich, S.; Bernhard, K.; Jagelka, M.; Janík, J.; Henry, G. W.;
Shultz, M. E.
2018CoSka..48..203M Altcode:
Magnetic chemically peculiar (mCP) stars constitute about 10% of
upper-main-sequence stars and are characterized by strong magnetic
fields and abnormal photospheric abundances of some chemical
elements. Most of them exhibit strictly periodic light, magnetic,
radio, and spectral variations that can be fully explained by a rigidly
rotating main-sequence star with persistent surface structures and
a stable global magnetic field. Long-term observations of the phase
curves of these variations enable us to investigate possible surface
differential rotation with unprecedented accuracy and reliability. The
analysis of the phase curves in the best-observed mCP stars indicates
that the location and the contrast of photometric and spectroscopic
spots as well as the geometry of the magnetic field remain constant for
at least many decades. The strict periodicity of mCP variables supports
the concept that the outer layers of upper-main-sequence stars do not
rotate differentially. However, there is a small, inhomogeneous group
consisting of a few mCP stars whose rotation periods vary on timescales
of decades. The period oscillations may reflect real changes in the
angular velocity of outer layers of the stars which are anchored by
their global magnetic fields. In CU Vir, V901 Ori, and perhaps BS Cir,
the rotational period variation indicates the presence of vertical
differential rotation; however, its exact nature has remained elusive
until now. The incidence of mCP stars with variable rotational periods
is currently investigated using a sample of fifty newly identified
Kepler mCP stars.
---------------------------------------------------------
Title: Testing the Wavelet Analysis on the Evolution of the Polaris
Pulsation Period using the SMEI Photometry
Authors: Švanda, Michal; Harmanec, Petr
2017RNAAS...1...39S Altcode: 2017RNAAS...1a..39S
No abstract at ADS
---------------------------------------------------------
Title: Automatic detection of white-light flare kernels in SDO/HMI
intensitygrams
Authors: Mravcová, Lucia; Švanda, Michal
2017NewA...57...14M Altcode: 2017arXiv170600988M
Solar flares with a broadband emission in the white-light range
of the electromagnetic spectrum belong to most enigmatic phenomena
on the Sun. The origin of the white-light emission is not entirely
understood. We aim to systematically study the visible-light emission
connected to solar flares in SDO/HMI observations. We developed a
code for automatic detection of kernels of flares with HMI intensity
brightenings and study properties of detected candidates. The code
was tuned and tested and with a little effort, it could be applied
to any suitable data set. By studying a few flare examples, we found
indication that HMI intensity brightening might be an artefact of the
simplified procedure used to compute HMI observables.
---------------------------------------------------------
Title: Estimate of the regularly gridded 3D vector flow field from
a set of tomographic maps
Authors: Švanda, Michal; Kozoň, Marek
2017A&A...600A.117S Altcode: 2016arXiv161208275S
Time-distance inversions usually provide tomographic maps of
the interesting plasma properties (we focus on flows) at various
depths. These maps, however, do not correspond directly to the flow
field, but rather to the true flow field smoothed by the averaging
kernels. We introduce a method to derive a regularly gridded estimate of
the true velocity field from a set of tomographic maps. We mainly aim to
reconstruct the flow on a uniform grid in the vertical domain. We derive
the algorithm, implement it and validate using synthetic data. The
use of the synthetic data allows us to investigate the influence of
random noise and to develop the methodology to deal with it properly.
---------------------------------------------------------
Title: Differential rotation, flares and coronae in A to M stars
Authors: Balona, L. A.; Švanda, M.; Karlický, M.
2016MNRAS.463.1740B Altcode: 2016MNRAS.tmp.1226B
Kepler data are used to investigate flares in stars of all spectral
types. There is a strong tendency across all spectral types for
the most energetic flares to occur among the most rapidly rotating
stars. Differential rotation could conceivably play an important
role in enhancing flare energies. This idea was investigated,
but no correlation could be found between rotational shear and the
incidence of flares. Inspection of Kepler light curves shows that
rotational modulation is very common over the whole spectral type
range. Using the rotational light amplitude, the size distribution
of star-spots was investigated. Our analysis suggests that stars with
detectable flares have spots significantly larger than non-flare stars,
indicating that flare energies are correlated with the size of the
active region. Further evidence of the existence of spots on A stars
is shown by the correlation between the photometric period and the
projected rotational velocity. The existence of spots indicates the
presence of magnetic fields, but the fact that A stars lack coronae
implies that surface convection is a necessary condition for the
formation of the corona.
---------------------------------------------------------
Title: Flares on A-type Stars: Evidence for Heating of Solar Corona
by Nanoflares?
Authors: Švanda, Michal; Karlický, Marian
2016ApJ...831....9S Altcode: 2016arXiv160803494S
We analyzed the occurrence rates of flares on stars of spectral types
K, G, F, and A, observed by Kepler. We found that the histogram of
occurrence frequencies of stellar flares is systematically shifted
toward a high-energy tail for A-type stars compared to stars of cooler
spectral types. We extrapolated the fitted power laws toward flares
with smaller energies (nanoflares) and made estimates for total energy
flux to stellar atmospheres by flares. We found that, for A-type stars,
the total energy flux density was at least four-times smaller than for
G stars. We speculate that this deficit in energy supply may explain
the lack of hot coronae on A-type stars. Our results indicate the
importance of nanoflares for heating and formation of the solar corona.
---------------------------------------------------------
Title: Chromospheric Heating by Acoustic Waves Compared to Radiative
Cooling
Authors: Sobotka, M.; Heinzel, P.; Švanda, M.; Jurčák, J.; del Moro,
D.; Berrilli, F.
2016ApJ...826...49S Altcode: 2016arXiv160504794S
Acoustic and magnetoacoustic waves are among the possible candidate
mechanisms that heat the upper layers of the solar atmosphere. A weak
chromospheric plage near the large solar pore NOAA 11005 was observed
on 2008 October 15, in the Fe I 617.3 nm and Ca II 853.2 nm lines of
the Interferometric Bidimemsional Spectrometer attached to the Dunn
Solar Telescope. In analyzing the Ca II observations (with spatial
and temporal resolutions of 0.″4 and 52 s) the energy deposited by
acoustic waves is compared to that released by radiative losses. The
deposited acoustic flux is estimated from the power spectra of Doppler
oscillations measured in the Ca II line core. The radiative losses
are calculated using a grid of seven one-dimensional hydrostatic
semi-empirical model atmospheres. The comparison shows that the
spatial correlation of the maps of radiative losses and acoustic flux
is 72%. In a quiet chromosphere, the contribution of acoustic energy
flux to radiative losses is small, only about 15%. In active areas
with a photospheric magnetic-field strength between 300 and 1300 G
and an inclination of 20°-60°, the contribution increases from 23%
(chromospheric network) to 54% (a plage). However, these values have
to be considered as lower limits and it might be possible that the
acoustic energy flux is the main contributor to the heating of bright
chromospheric network and plages.
---------------------------------------------------------
Title: Polar cap magnetic field reversals during solar grand minima:
could pores play a role?
Authors: Švanda, Michal; Brun, Allan Sacha; Roudier, Thierry;
Jouve, Laurène
2016A&A...586A.123S Altcode: 2015arXiv151106894S
We study the magnetic flux carried by pores located outside active
regions with sunspots and investigate their possible contribution to
the reversal of the global magnetic field of the Sun. We find that they
contain a total flux of comparable amplitude to the total magnetic flux
contained in polar caps. The pores located at distances of 40-100 Mm
from the closest active region systematically have the correct polarity
of the magnetic field to contribute to the polar cap reversal. These
pores can be found predominantly in bipolar magnetic regions. We propose
that during grand minima of solar activity, such a systematic polarity
trend, which is akin to a weak magnetic (Babcock-Leighton-like) source
term, could still be operating but was missed by the contemporary
observers because of the limited resolving power of their telescopes.
---------------------------------------------------------
Title: Issues with time-distance inversions for supergranular flows
Authors: Švanda, Michal
2015A&A...575A.122S Altcode: 2015arXiv150104160S
<BR /> Aims: Recent studies have shown that time-distance inversions
for flows start to be dominated by a random noise at a depth of only
a few Mm. It was proposed that the ensemble averaging might be a
solution for learning about the structure of the convective flows,
e.g. about the depth structure of supergranulation. <BR /> Methods:
Time-distance inversion is applied to the statistical sample of ∼
10<SUP>4</SUP> supergranules, which allows the inversion cost function
to be regularised weakly about the random-noise term and thus provides
a much better localisation in space. We compare these inversions
at four depths (1.9, 2.9, 4.3, and 6.2 Mm) when using different
spatio-temporal filtering schemes in order to gain confidence about
these inferences. <BR /> Results: The flows inferred by using different
spatio-temporal filtering schemes are different (even by the sign)
even though the formal averaging kernels and the random-noise levels
are very similar. The inverted flows changes its sign several times
with depth. I suggest that this is due to the inaccuracies in the
forward problem that are possibly amplified by the inversion. It is
also possible that other time-distance inversions are affected by this.
---------------------------------------------------------
Title: The PLATO 2.0 mission
Authors: Rauer, H.; Catala, C.; Aerts, C.; Appourchaux, T.; Benz,
W.; Brandeker, A.; Christensen-Dalsgaard, J.; Deleuil, M.; Gizon,
L.; Goupil, M. -J.; Güdel, M.; Janot-Pacheco, E.; Mas-Hesse,
M.; Pagano, I.; Piotto, G.; Pollacco, D.; Santos, Ċ.; Smith, A.;
Suárez, J. -C.; Szabó, R.; Udry, S.; Adibekyan, V.; Alibert, Y.;
Almenara, J. -M.; Amaro-Seoane, P.; Eiff, M. Ammler-von; Asplund, M.;
Antonello, E.; Barnes, S.; Baudin, F.; Belkacem, K.; Bergemann, M.;
Bihain, G.; Birch, A. C.; Bonfils, X.; Boisse, I.; Bonomo, A. S.;
Borsa, F.; Brandão, I. M.; Brocato, E.; Brun, S.; Burleigh, M.;
Burston, R.; Cabrera, J.; Cassisi, S.; Chaplin, W.; Charpinet, S.;
Chiappini, C.; Church, R. P.; Csizmadia, Sz.; Cunha, M.; Damasso, M.;
Davies, M. B.; Deeg, H. J.; Díaz, R. F.; Dreizler, S.; Dreyer, C.;
Eggenberger, P.; Ehrenreich, D.; Eigmüller, P.; Erikson, A.; Farmer,
R.; Feltzing, S.; de Oliveira Fialho, F.; Figueira, P.; Forveille,
T.; Fridlund, M.; García, R. A.; Giommi, P.; Giuffrida, G.; Godolt,
M.; Gomes da Silva, J.; Granzer, T.; Grenfell, J. L.; Grotsch-Noels,
A.; Günther, E.; Haswell, C. A.; Hatzes, A. P.; Hébrard, G.; Hekker,
S.; Helled, R.; Heng, K.; Jenkins, J. M.; Johansen, A.; Khodachenko,
M. L.; Kislyakova, K. G.; Kley, W.; Kolb, U.; Krivova, N.; Kupka, F.;
Lammer, H.; Lanza, A. F.; Lebreton, Y.; Magrin, D.; Marcos-Arenal,
P.; Marrese, P. M.; Marques, J. P.; Martins, J.; Mathis, S.; Mathur,
S.; Messina, S.; Miglio, A.; Montalban, J.; Montalto, M.; Monteiro,
M. J. P. F. G.; Moradi, H.; Moravveji, E.; Mordasini, C.; Morel, T.;
Mortier, A.; Nascimbeni, V.; Nelson, R. P.; Nielsen, M. B.; Noack,
L.; Norton, A. J.; Ofir, A.; Oshagh, M.; Ouazzani, R. -M.; Pápics,
P.; Parro, V. C.; Petit, P.; Plez, B.; Poretti, E.; Quirrenbach, A.;
Ragazzoni, R.; Raimondo, G.; Rainer, M.; Reese, D. R.; Redmer, R.;
Reffert, S.; Rojas-Ayala, B.; Roxburgh, I. W.; Salmon, S.; Santerne,
A.; Schneider, J.; Schou, J.; Schuh, S.; Schunker, H.; Silva-Valio,
A.; Silvotti, R.; Skillen, I.; Snellen, I.; Sohl, F.; Sousa, S. G.;
Sozzetti, A.; Stello, D.; Strassmeier, K. G.; Švanda, M.; Szabó,
Gy. M.; Tkachenko, A.; Valencia, D.; Van Grootel, V.; Vauclair,
S. D.; Ventura, P.; Wagner, F. W.; Walton, N. A.; Weingrill, J.;
Werner, S. C.; Wheatley, P. J.; Zwintz, K.
2014ExA....38..249R Altcode: 2014ExA...tmp...41R; 2013arXiv1310.0696R
PLATO 2.0 has recently been selected for ESA's M3 launch opportunity
(2022/24). Providing accurate key planet parameters (radius, mass,
density and age) in statistical numbers, it addresses fundamental
questions such as: How do planetary systems form and evolve? Are there
other systems with planets like ours, including potentially habitable
planets? The PLATO 2.0 instrument consists of 34 small aperture
telescopes (32 with 25 s readout cadence and 2 with 2.5 s candence)
providing a wide field-of-view (2232 deg <SUP>2</SUP>) and a large
photometric magnitude range (4-16 mag). It focusses on bright (4-11
mag) stars in wide fields to detect and characterize planets down to
Earth-size by photometric transits, whose masses can then be determined
by ground-based radial-velocity follow-up measurements. Asteroseismology
will be performed for these bright stars to obtain highly accurate
stellar parameters, including masses and ages. The combination of
bright targets and asteroseismology results in high accuracy for
the bulk planet parameters: 2 %, 4-10 % and 10 % for planet radii,
masses and ages, respectively. The planned baseline observing strategy
includes two long pointings (2-3 years) to detect and bulk characterize
planets reaching into the habitable zone (HZ) of solar-like stars
and an additional step-and-stare phase to cover in total about 50 %
of the sky. PLATO 2.0 will observe up to 1,000,000 stars and detect
and characterize hundreds of small planets, and thousands of planets
in the Neptune to gas giant regime out to the HZ. It will therefore
provide the first large-scale catalogue of bulk characterized planets
with accurate radii, masses, mean densities and ages. This catalogue
will include terrestrial planets at intermediate orbital distances,
where surface temperatures are moderate. Coverage of this parameter
range with statistical numbers of bulk characterized planets is unique
to PLATO 2.0. The PLATO 2.0 catalogue allows us to e.g.: - complete
our knowledge of planet diversity for low-mass objects, - correlate the
planet mean density-orbital distance distribution with predictions from
planet formation theories,- constrain the influence of planet migration
and scattering on the architecture of multiple systems, and - specify
how planet and system parameters change with host star characteristics,
such as type, metallicity and age. The catalogue will allow us to study
planets and planetary systems at different evolutionary phases. It
will further provide a census for small, low-mass planets. This will
serve to identify objects which retained their primordial hydrogen
atmosphere and in general the typical characteristics of planets
in such low-mass, low-density range. Planets detected by PLATO 2.0
will orbit bright stars and many of them will be targets for future
atmosphere spectroscopy exploring their atmosphere. Furthermore,
the mission has the potential to detect exomoons, planetary rings,
binary and Trojan planets. The planetary science possible with PLATO
2.0 is complemented by its impact on stellar and galactic science via
asteroseismology as well as light curves of all kinds of variable stars,
together with observations of stellar clusters of different ages. This
will allow us to improve stellar models and study stellar activity. A
large number of well-known ages from red giant stars will probe the
structure and evolution of our Galaxy. Asteroseismic ages of bright
stars for different phases of stellar evolution allow calibrating
stellar age-rotation relationships. Together with the results of ESA's
Gaia mission, the results of PLATO 2.0 will provide a huge legacy to
planetary, stellar and galactic science.
---------------------------------------------------------
Title: Moat Flow System around Sunspots in Shallow Subsurface Layers
Authors: Švanda, Michal; Sobotka, Michal; Bárta, Tomáš
2014ApJ...790..135S Altcode: 2014arXiv1406.2482S
We investigate the subsurface moat flow system around McIntosh
H-type symmetrical sunspots and compare it to the flow system within
supergranular cells. Representatives of both types of flows are
constructed by means of the statistical averaging of flow maps obtained
by time-distance helioseismic inversions. We find that moat flows around
H-type sunspots replace supergranular flows but there are two principal
differences between the two phenomena: the moat flow is asymmetrical,
probably due to the proper motion of sunspots with respect to the
local frame of rest, while the flow in the supergranular cell is highly
symmetrical. Furthermore, the whole moat is a downflow region, while the
supergranule contains the upflow in the center, which turns into the
downflow at about 60% of the cell radius from its center. We estimate
that the mass downflow rate in the moat region is at least two times
larger than the mass circulation rate within the supergranular cell.
---------------------------------------------------------
Title: Structure and evolution of solar supergranulation using
SDO/HMI data
Authors: Roudier, Th.; Švanda, M.; Rieutord, , M.; Malherbe, J. M.;
Burston, R.; Gizon, L.
2014A&A...567A.138R Altcode: 2014arXiv1407.0196R
Context. Studying the motions on the solar surface is fundamental
for understanding how turbulent convection transports energy and how
magnetic fields are distributed across the solar surface. <BR /> Aims:
From horizontal velocity measurements all over the visible disc of the
Sun and using data from the Solar Dynamics Observatory/Helioseismic and
Magnetic Imager (SDO/HMI), we investigate the structure and evolution
of solar supergranulation. <BR /> Methods: Horizontal velocity fields
were measured by following the proper motions of solar granules
using a newly developed version of the coherent structure tracking
code. With this tool, maps of horizontal divergence were computed. We
then segmented and identified supergranular cells and followed their
histories by using spatio-temporal labelling. With this data set we
derived the fundamental properties of supergranulation, including their
motion. <BR /> Results: We find values of the fundamental parameters
of supergranulation similar to previous studies: a mean lifetime of
1.5 days and a mean diameter of 25 Mm. The tracking of individual
supergranular cells reveals the solar differential rotation and a
poleward circulation trend of the meridional flow. The shape of the
derived differential rotation and meridional flow does not depend on
the cell size. If there is a background magnetic field, the diverging
flows in supergranules are weaker. <BR /> Conclusions: This study
confirms that supergranules are suitable tracers that may be used to
investigate the large-scale flows of the solar convection as long as
they are detectable enough on the surface.
---------------------------------------------------------
Title: An Estimate of Chromospheric Heating by Acoustic Waves
Authors: Sobotka, M.; Švanda, M.; Jurčak, J.; Heinzel, P.; Del Moro,
D.; Berrilli, F.
2014CEAB...38...53S Altcode:
Several mechanisms may heat the solar chromosphere: acoustic waves,
magnetoacoustic waves (slow, fast, and Alfvén waves), and small-scale
magnetic reconnections. Based on observations in the Ca II 854.2 nm
line, the contribution of acoustic waves to the heating of quiet and
plage regions in the chromosphere is discussed. The energy released
by radiative losses is compared with the energy deposited by acoustic
waves. Radiative losses are computed using a grid of six semi-empirical
models VAL A--F. The deposited acoustic flux is calculated using power
spectra of Doppler oscillations measured in the Ca~II line core. The
comparison shows that the spatial correlation of maps of radiative
losses and acoustic flux is 70 %. The deposited acoustic flux provides
at least 25--30~% of the energy radiated in the quiet chromosphere
and 50~% in plage regions.
---------------------------------------------------------
Title: Dynamics of the solar atmosphere above a pore with a light
bridge
Authors: Sobotka, M.; Švanda, M.; Jurčák, J.; Heinzel, P.; Del Moro,
D.; Berrilli, F.
2013A&A...560A..84S Altcode: 2013arXiv1309.7790S
Context. Solar pores are small sunspots lacking a penumbra that have
a prevailing vertical magnetic-field component. They can include light
bridges at places with locally reduced magnetic field. Like sunspots,
they exhibit a wide range of oscillatory phenomena. <BR /> Aims:
A large isolated pore with a light bridge (NOAA 11005) is studied
to obtain characteristics of a chromospheric filamentary structure
around the pore, to analyse oscillations and waves in and around
the pore, and to understand the structure and brightness of the light
bridge. <BR /> Methods: Spectral imaging observations in the line Ca II
854.2 nm and complementary spectropolarimetry in Fe I lines, obtained
with the DST/IBIS spectrometer and HINODE/SOT spectropolarimeter,
were used to measure photospheric and chromospheric velocity fields,
oscillations, waves, the magnetic field in the photosphere, and
acoustic energy flux and radiative losses in the chromosphere. <BR />
Results: The chromospheric filamentary structure around the pore has
all important characteristics of a superpenumbra: it shows an inverse
Evershed effect and running waves, and has a similar morphology and
oscillation character. The granular structure of the light bridge in
the upper photosphere can be explained by radiative heating. Acoustic
waves leaking up from the photosphere along the inclined magnetic
field in the light bridge transfer enough energy flux to balance
the entire radiative losses of the light-bridge chromosphere. <BR />
Conclusions: A penumbra is not a necessary condition for the formation
of a superpenumbra. The light bridge is heated by radiation in the
photosphere and by acoustic waves in the chromosphere.
---------------------------------------------------------
Title: Tomography of Plasma Flows in the Upper Solar Convection
Zone Using Time-Distance Inversion Combining Ridge and Phase-speed
Filtering
Authors: Švanda, Michal
2013ApJ...775....7S Altcode: 2013arXiv1307.4836S
The consistency of time-distance inversions for horizontal
components of the plasma flow on supergranular scales in the upper
solar convection zone is checked by comparing the results derived
using two k-ω filtering procedures—ridge filtering and phase-speed
filtering—commonly used in time-distance helioseismology. I show that
both approaches result in similar flow estimates when finite-frequency
sensitivity kernels are used. I further demonstrate that the performance
of the inversion improves (in terms of a simultaneously better averaging
kernel and a lower noise level) when the two approaches are combined
together in one inversion. Using the combined inversion, I invert for
horizontal flows in the upper 10 Mm of the solar convection zone. The
flows connected with supergranulation seem to be coherent only for
the top ~5 Mm deeper down there is a hint of change of the convection
scales toward structures larger than supergranules.
---------------------------------------------------------
Title: Comparison of Solar Surface Flows Inferred from Time-Distance
Helioseismology and Coherent Structure Tracking Using HMI/SDO
Observations
Authors: Švanda, Michal; Roudier, Thierry; Rieutord, Michel; Burston,
Raymond; Gizon, Laurent
2013ApJ...771...32S Altcode: 2013arXiv1305.0875S
We compare measurements of horizontal flows on the surface of the
Sun using helioseismic time-distance inversions and coherent structure
tracking of solar granules. Tracking provides two-dimensional horizontal
flows on the solar surface, whereas the time-distance inversions
estimate the full three-dimensional velocity flows in the shallow
near-surface layers. Both techniques use Helioseismic and Magnetic
Imager observations as input. We find good correlations between the
various measurements resulting from the two techniques. Further, we
find a good agreement between these measurements and the time-averaged
Doppler line-of-sight velocity, and also perform sanity checks on the
vertical flow that resulted from the three-dimensional time-distance
inversion.
---------------------------------------------------------
Title: Time-distance inversions for horizontal and vertical flows
on supergranular scales applied to MDI and HMI data
Authors: Švanda, M.; Schunker, H.; Burston, R.
2013JPhCS.440a2024S Altcode: 2013arXiv1302.0790S
We study the possibility of consistent extension of MDI full-disc
helioseismic campaigns with the growing data set of HMI observations. To
do so, we down-sample and filter the HMI Dopplegrams so that the
resulting spatial power spectrum is similar to the spatial power
spectrum of MDI full-disc Dopplergrams. The set of co-spatial and
co-temporal datacube pairs from both instruments containing no missing
and no bad frames were processed using the same codes and inverted
independently for all three components of the plasma flow in the
near surface layers. The results from the two instruments are highly
correlated, however systematically larger (by ~ 20%) flow magnitudes are
derived from HMI. We comment that this may be an effect of the different
formation depth of the Doppler signal from the two instruments.
---------------------------------------------------------
Title: Atmosphere above a large solar pore
Authors: Sobotka, M.; Švanda, M.; Jurčák, J.; Heinzel, P.; Del
Moro, D.
2013JPhCS.440a2049S Altcode: 2013arXiv1302.4893S
A large solar pore with a granular light bridge was observed on October
15, 2008 with the IBIS spectrometer at the Dunn Solar Telescope and a
69-min long time series of spectral scans in the lines Ca II 854.2 nm
and Fe I 617.3 nm was obtained. The intensity and Doppler signals in the
Ca II line were separated. This line samples the middle chromosphere
in the core and the middle photosphere in the wings. Although no
indication of a penumbra is seen in the photosphere, an extended
filamentary structure, both in intensity and Doppler signals, is
observed in the Ca II line core. An analysis of morphological and
dynamical properties of the structure shows a close similarity to a
superpenumbra of a sunspot with developed penumbra. A special attention
is paid to the light bridge, which is the brightest feature in the
pore seen in the Ca II line centre and shows an enhanced power of
chromospheric oscillations at 3-5 mHz. Although the acoustic power
flux in the light bridge is five times higher than in the "quiet"
chromosphere, it cannot explain the observed brightness.
---------------------------------------------------------
Title: Comparison of solar horizontal velocity fields from SDO/HMI
and Hinode data
Authors: Roudier, Th.; Rieutord, M.; Prat, V.; Malherbe, J. M.; Renon,
N.; Frank, Z.; Švanda, M.; Berger, T.; Burston, R.; Gizon, L.
2013A&A...552A.113R Altcode: 2013arXiv1303.4271R
Context. The measurement of the Sun's surface motions with a high
spatial and temporal resolution is still a challenge. <BR /> Aims:
We wish to validate horizontal velocity measurements all over the
visible disk of the Sun from Solar Dynamics Observatory/ Helioseismic
and Magnetic Imager (SDO/HMI) data. <BR /> Methods: Horizontal velocity
fields are measured by following the proper motions of solar granules
using a newly developed version of the coherent structure tracking
(CST) code. The comparison of the surface flows measured at high
spatial resolution (Hinode, 0.1 arcsec) and low resolution (SDO/HMI,
0.5 arcsec) allows us to determine corrections to be applied to
the horizontal velocity measured from HMI white light data. <BR />
Results: We derive horizontal velocity maps with spatial and temporal
resolutions of respectively 2.5 Mm and 30 min. From the two components
of the horizontal velocity v<SUB>x</SUB> and v<SUB>y</SUB> measured
in the sky plane and the simultaneous line of sight component from
SDO/HMI dopplergrams v<SUB>D</SUB>, we derive the spherical velocity
components (v<SUB>r</SUB>, v<SUB>θ</SUB>, v<SUB>ϕ</SUB>). The
azimuthal component v<SUB>ϕ</SUB> gives the solar differential rotation
with a high precision (± 0.037 km s<SUP>-1</SUP>) from a temporal
sequence of only three hours. <BR /> Conclusions: By following the
proper motions of the solar granules, we can revisit the dynamics of
the solar surface at high spatial and temporal resolutions from hours
to months and years with the SDO data.
---------------------------------------------------------
Title: An Average Supergranule: Much Larger Vertical Flows Than
Expected
Authors: Švanda, M.
2013CEAB...37..447S Altcode: 2013arXiv1301.1821S
Supergranules are believed to be an evidence for large-scale subsurface
convection. The vertical component of the supergranular flow field
is very hard to measure, but it is considered to be only a few mps in
and below the photosphere. Here I present the results of the analysis
using three-dimensional inversion for time-distance helioseismology
that indicates existence of large-magnitude vertical upflow in the
near sub-surface layers. Possible issues and consequences of this
inference are also discussed.
---------------------------------------------------------
Title: A New Look into the Spectral and Light Variations of
\varepsilon Aur
Authors: Harmanec, P.; Božić, H.; Korčáková, D.; Kotková, L.;
Škoda, P.; Šlechta, M.; Švanda, M.; Votruba, V.; Wolf, M.; Zasche,
P.; Henden, A.; Ribeiro, J.
2013CEAB...37...99H Altcode: 2013arXiv1303.6401H
Investigating long series of spectral and photometric observations,
we found that the orbital elements of \varepsilon Aur are subject
to much larger uncertainties than usually believed. The Hα emission
is found to move basically with the F primary but its exact location
should still be investigated. We also find strong additional absorption
and large reddening of the object near the third contact during the
eclipse. Episodic atmospheric mass transfer from the F primary towards
its companion is tentatively suggested.
---------------------------------------------------------
Title: Inversions for Average Supergranular Flows Using
Finite-frequency Kernels
Authors: Švanda, Michal
2012ApJ...759L..29S Altcode: 2012arXiv1209.6147S
I analyze the maps recording the travel-time shifts caused by averaged
plasma anomalies under an "average supergranule," constructed by
means of statistical averaging over 5582 individual supergranules
with large divergence signals detected in two months of Helioseismic
and Magnetic Imager Dopplergrams. By utilizing a three-dimensional
validated time-distance inversion code, I measure a peak vertical
velocity of 117 ± 2 m s<SUP>-1</SUP> at depths around 1.2 Mm in the
center of the supergranule and a root-mean-square vertical velocity of
21 m s<SUP>-1</SUP> over the area of the supergranule. A discrepancy
between this measurement and the measured surface vertical velocity
(a few m s<SUP>-1</SUP>) can be explained by the existence of the
large-amplitude vertical flow under the surface of supergranules with
large divergence signals, recently suggested by Duvall & Hanasoge.
---------------------------------------------------------
Title: Optimisation of solar synoptic observations
Authors: Klvaña, Miroslav; Sobotka, Michal; Švanda, Michal
2012SPIE.8448E..0AK Altcode:
The development of instrumental and computer technologies is
connected with steadily increasing needs for archiving of large data
volumes. The current trend to meet this requirement includes the
data compression and growth of storage capacities. This approach,
however, has technical and practical limits. A further reduction of
the archived data volume can be achieved by means of an optimisation
of the archiving that consists in data selection without losing the
useful information. We describe a method of optimised archiving of
solar images, based on the selection of images that contain a new
information. The new information content is evaluated by means of the
analysis of changes detected in the images. We present characteristics
of different kinds of image changes and divide them into fictitious
changes with a disturbing effect and real changes that provide a new
information. In block diagrams describing the selection and archiving,
we demonstrate the influence of clouds, the recording of images during
an active event on the Sun, including a period before the event onset,
and the archiving of long-term history of solar activity. The described
optimisation technique is not suitable for helioseismology, because it
does not conserve the uniform time step in the archived sequence and
removes the information about solar oscillations. In case of long-term
synoptic observations, the optimised archiving can save a large amount
of storage capacities. The actual capacity saving will depend on the
setting of the change-detection sensitivity and on the capability to
exclude the fictitious changes.
---------------------------------------------------------
Title: Some Dynamic Analysis of the Photosphere from Hinode/SOT and
SDO/HMI Observations
Authors: Roudier, T.; Malherbe, J.; Rieutord, M.; Berger, T.; Frank,
Z.; Prat, V.; Renon, N.; Gizon, L.; Svanda, M.
2012ASPC..456...65R Altcode:
We first present the important role played by the families of granule
(or Tree of Fragmenting granules) in the formation of the photospheric
network. Then, we describe the occurence and characteristics of
acoustic events (AE), defined as spatially concentrated energy flux,
in the quiet Sun. Finally, we present how horizontal velocities obtained
from SDO/HMI data are calibrated by using Hinode/SOT observations.
---------------------------------------------------------
Title: Quasi full-disk maps of solar horizontal velocities using
SDO/HMI data
Authors: Roudier, Th.; Rieutord, M.; Malherbe, J. M.; Renon, N.;
Berger, T.; Frank, Z.; Prat, V.; Gizon, L.; Švanda, M.
2012A&A...540A..88R Altcode: 2012arXiv1203.0514R
<BR /> Aims: For the first time, the motion of granules (solar plasma
on the surface on scales larger than 2.5 Mm) has been followed over
the entire visible surface of the Sun, using SDO/HMI white-light
data. <BR /> Methods: Horizontal velocity fields are derived from image
correlation tracking using a new version of the coherent structure
tracking algorithm. The spatial and temporal resolutions of the
horizontal velocity map are 2.5 Mm and 30 min, respectively. <BR
/> Results: From this reconstruction, using the multi-resolution
analysis, one can obtain to the velocity field at different scales
with its derivatives such as the horizontal divergence or the vertical
component of the vorticity. The intrinsic error on the velocity is
~0.25 km s<SUP>-1</SUP> for a time sequence of 30 min and a mesh size
of 2.5 Mm. This is acceptable compared to the granule velocities, which
range between 0.3 km s<SUP>-1</SUP> and 1.8 km s<SUP>-1</SUP>. A high
correlation between velocities computed from Hinode and SDO/HMI has
been found (85%). From the data we derive the power spectrum of the
supergranulation horizontal velocity field, the solar differential
rotation, and the meridional velocity.
---------------------------------------------------------
Title: Multichannel Three-Dimensional SOLA Inversion for Local
Helioseismology
Authors: Jackiewicz, J.; Birch, A. C.; Gizon, L.; Hanasoge, S. M.;
Hohage, T.; Ruffio, J. -B.; Švanda, M.
2012SoPh..276...19J Altcode: 2011arXiv1109.2712J
Inversions for local helioseismology are an important and necessary step
for obtaining three-dimensional maps of various physical quantities
in the solar interior. Frequently, the full inverse problems that
one would like to solve prove intractable because of computational
constraints. Due to the enormous seismic data sets that already
exist and those forthcoming, this is a problem that needs to be
addressed. To this end, we present a very efficient linear inversion
algorithm for local helioseismology. It is based on a subtractive
optimally localized averaging (SOLA) scheme in the Fourier domain,
utilizing the horizontal-translation invariance of the sensitivity
kernels. In Fourier space the problem decouples into many small
problems, one for each horizontal wave vector. This multichannel
SOLA method is demonstrated for an example problem in time-distance
helioseismology that is small enough to be solved both in real and
Fourier space. We find that both approaches are successful in solving
the inverse problem. However, the multichannel SOLA algorithm is much
faster and can easily be parallelized.
---------------------------------------------------------
Title: Solar synoptic telescope. Characteristics, possibilities,
and limits of design
Authors: Klvaňa, M.; Sobotka, M.; Švanda, M.
2011CoSka..41...92K Altcode:
A rapid evolution of electronics and information technologies makes
it possible to use new original designs of synoptic telescopes for
solar observations, to increase the demands on their functions, and
to fully automate the observation. However, there are hardware and
software limits that strongly influence the working capabilities of
synoptic telescopes. In this contribution, we analyze relationships
between the synoptic telescope's characteristics, the parameters of
image digitization, the control, the achievable degree of automation of
observations, and the possibilities to implement functions connected
with the solar activity monitoring and image archiving. The principles
listed above serve as a basis for the design study of the Auxiliary
Full-Disc Telescope for the European Solar Telescope (EST), a
pan-European project of a large 4-meter solar telescope.
---------------------------------------------------------
Title: Validated helioseismic inversions for 3D vector flows
Authors: Švanda, M.; Gizon, L.; Hanasoge, S. M.; Ustyugov, S. D.
2011A&A...530A.148S Altcode: 2011arXiv1104.4083S
Context. According to time-distance helioseismology, information
about internal fluid motions is encoded in the travel times of solar
waves. The inverse problem consists of inferring three-dimensional
vector flows from a set of travel-time measurements. While only few
tests of the inversions have been done, it is known that the retrieval
of the small-amplitude vertical flow velocities is problematic. A
thorough study of biases and noise has not been carried out in
realistic conditions. <BR /> Aims: Here we investigate the potential
of time-distance helioseismology to infer three-dimensional convective
velocities in the near-surface layers of the Sun. We developed a new
subtractive optimally localised averaging (SOLA) code suitable for
pipeline pseudo-automatic processing. Compared to its predecessor,
the code was improved by accounting for additional constraints in
order to get the right answer within a given noise level. The main
aim of this study is to validate results obtained by our inversion
code. <BR /> Methods: We simulate travel-time maps using a snapshot
from a numerical simulation of solar convective flows, realistic Born
travel-time sensitivity kernels, and a realistic model of travel-time
noise. These synthetic travel times are inverted for flows and the
results compared with the known input flow field. Additional constraints
are implemented in the inversion: cross-talk minimization between flow
components and spatial localization of inversion coefficients. <BR />
Results: Using modes f, p<SUB>1</SUB> through p<SUB>4</SUB>, we show
that horizontal convective flow velocities can be inferred without
bias, at a signal-to-noise ratio greater than one in the top 3.5 Mm,
provided that observations span at least four days. The vertical
component of velocity (v<SUB>z</SUB>), if it were to be weak, is
more difficult to infer and is seriously affected by cross-talk from
horizontal velocity components. We emphasise that this cross-talk
must be explicitly minimised in order to retrieve v<SUB>z</SUB>
in the top 1 Mm. We also show that statistical averaging over many
different areas of the Sun allows for reliably measuring of average
properties of all three flow components in the top 5.5 Mm of the
convection zone. <P />Figures 16-28 are available in electronic form
at <A href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: Space-time segmentation method for study of the vertical
structure and evolution of solar supergranulation from data provided
by local helioseismology
Authors: Žlebčík, R.; Švanda, M.; Klvaňa, M.
2011NewA...16....1Z Altcode: 2010arXiv1006.0617Z
Solar supergranulation remains a mystery in spite of decades of
intensive studies. Most of the papers about supergranulation deal with
its surface properties. Local helioseismology provides an opportunity
to look below the surface and see the vertical structure of this
convective feature. We present a concept of a (3 + 1)-D segmentation
algorithm capable of recognising individual supergranules in a
sequence of helioseismic 3-D flow maps. As an example, we applied
this method to the state-of-the-art data and derived descriptive
statistical properties of segmented supergranules - typical size of
20-30 Mm, characteristic lifetime of 18.7 h, and estimated depth of
15-20 Mm. We present preliminary results obtained on the topic of the
three-dimensional structure and evolution of supergranulation. The
method has great potential in analysing the better data expected from
the helioseismic inversions, which are being developed.
---------------------------------------------------------
Title: Dynamics of Active Regions Revealed by Tracking of Doppler
Features
Authors: Švanda, M.; Sobotka, M.; Klvaňa, M.; Bumba, V.
2010ASSP...19..410S Altcode: 2010mcia.conf..410S
We investigate the large-scale horizontal dynamics of active regions in
the 23rd solar cycle. The large-scale horizontal velocity fields were
measured applying the local correlation tracking (LCT) algorithm to the
processed high-cadence full-resolution full-disc MDI Dopplergrams. We
performed the selection of NOAA active regions in the available dataset
and followed their individual evolution in time. The statistical study
of this sample gives us a unique opportunity to study the dynamics
of active regions at various stages of their evolution. In few cases,
we found behavior that is consistent with the dynamical disconnection
of sunspots from the magnetic roots.
---------------------------------------------------------
Title: Transport of Supergranules and their Vertical Coherence
Authors: Švanda, M.; Kosovichev, A. G.; Klvaňa, M.; Sobotka, M.;
Duvall, T. L., Jr.
2009ASPC..416..547S Altcode:
In recent papers, we have introduced a method for measuring the
photospheric flow field that is based on the tracking of supergranular
structures. Here, in combination with helioseismic data, we are
able to estimate the depth in the solar convection envelope to
which the detected large-scale flow field is coherent. We show that
the upper 10 Mm in the convection zone depicts similar features in
horizontal velocity. Our interpretation of this observation is that
the supergranulation is a coherent structure 10 Mm deep and is subject
to large-scale transport by the underlying velocity field.
---------------------------------------------------------
Title: Large-scale horizontal flows in the solar
photosphere. V. Possible evidence for the disconnection of bipolar
sunspot groups from their magnetic roots
Authors: Švanda, M.; Klvaňa, M.; Sobotka, M.
2009A&A...506..875S Altcode: 2009arXiv0908.3183S
In a recent paper (Švanda et al. 2008, A&A, 477, 285) we pointed
out that, based on the tracking of Doppler features in the full-disc
MDI Dopplergrams, the active regions display two dynamically different
regimes. We speculated that this could be a manifestation of the
sudden change in the active regions dynamics, caused by the dynamic
disconnection of sunspots from their magnetic roots as proposed by
Schüssler & Rempel (2005, A&A, 441, 337). Here we investigate
the dynamic behaviour of the active regions recorded in the high-cadence
MDI data over the last solar cycle in order to confirm the predictions
in the Schüssler's & Rempel's paper. We find that, after drastic
reduction of the sample, which is done to avoid disturbing effects,
a large fraction of active regions displays a sudden decrease in the
rotation speed, which is compatible with the mechanism of the dynamic
disconnection of sunspots from their parental magnetic structures.
---------------------------------------------------------
Title: Large-scale horizontal flows in the solar photosphere IV. On
the vertical structure of large-scale horizontal flows
Authors: Švanda, M.; Klvaňa, M.; Sobotka, M.; Kosovichev, A. G.;
Duvall, T. L.
2009NewA...14..429S Altcode: 2008arXiv0812.1971S
In the recent papers, we introduced a method utilised to measure
the flow field. The method is based on the tracking of supergranular
structures. We did not precisely know, whether its results represent
the flow field in the photosphere or in some subphotospheric
layers. In this paper, in combination with helioseismic data, we
are able to estimate the depths in the solar convection envelope,
where the detected large-scale flow field is well represented by
the surface measurements. We got a clear answer to question what
kind of structures we track in full-disc Dopplergrams. It seems that
in the quiet Sun regions the supergranular structures are tracked,
while in the regions with the magnetic field the structures of the
magnetic field are dominant. This observation seems obvious, because
the nature of Doppler structures is different in the magnetic regions
and in the quiet Sun. We show that the large-scale flow detected by
our method represents the motion of plasma in layers down to ∼10
Mm. The supergranules may therefore be treated as the objects carried
by the underlying large-scale velocity field.
---------------------------------------------------------
Title: Photospheric flows around a quiescent filament at Large and
small scale and their ffects on filament destabilization
Authors: Roudier, Th.; Malherbe, J. M.; Švanda, M.; Molodij, G.;
Keil, S.; Sütterlin, P.; Schmieder, B.; Bommier, V.; Aulanier, G.;
Meunier, N.; Rieutord, M.; Rondi, S.
2008sf2a.conf..569R Altcode:
We study the influence of large and small scales photospheric
motions on the destabilization of an eruptive filament, observed
on October 6, 7, and 8, 2004 as part of an international observing
campaign (JOP 178). Large-scale horizontal flows are invetigated
from a series of MDI/SOHO full-disc Dopplergrams and magnetograms
from THEMIS. Small-scale horizontal flows were derived using local
correlation tracking on TRACE satellite, Dutch Open Telescope (DOT)
and The Dunn Solar telescope (DST) data. The topology of the flow field
changed significantly during the filament eruptive phase, suggesting
a possible coupling between the surface flow field and the coronal
magnetic field. We measured an increase of the shear below the point
where the eruption starts and a decrease in shear after the eruption. We
conclude that there is probably a link between changes in surface flow
and the disappearance of the eruptive filament.
---------------------------------------------------------
Title: Tracking of Supergranules - Does It Make Any Sense?
Authors: Svanda, M.; Klvaòa, M.; Sobotka, M.
2008ESPM...12.2.10S Altcode: 2008arXiv0809.4757S
The motions of the plasma and structures in and below the solar
photosphere is not well understood. The results obtained using various
methods cannot be in general considered as consistent, especially in
details. In this contribution we show a summary of the results obtained
by the method we have developed recently. <P />To study the photospheric
dynamics we apply the local correlation tracking algorithm to the
series of full-disc Dopplergrams obtained by Michelson Doppler Imager
(MDI) on-board the SoHO observatory. The dominant structures recorded
in the Dopplergrams are supergranules. Under the assumtion that the
supergranules are carried by the flow field of the larger scale, we
study properties of this underlying velocity field. The methodology
consists of an extensive data processing of primary data in order to
suppress disturbing effects such as p-modes of solar oscillations
or instrumental issues. Aditional coordinate transformations
are also needed to make the data suitable for tracking. <P />We
perform comparative tests with synthetic data with known properties
and with results of time-distance helioseismology with a great
success. Correlation coeficients of the comparison of mean components
of the flow field are larger than 0.8, for the comparison of details
in the vector velocity field the correlation coeficient is larger than
0.6. <P />The results of the method applied to the real data agree
well with well-known features detected in the photospheric velocity
fields and reported by many other authors. With the proposed method
we detect differential rotation, meridional circulation, torsional
oscillations, and other features. A few case studies are shown to
demonstrate the performance of the method. <P />As a conclusion
let's answer the question in the title. We believe that tracking of
supergranules makes a perfect sense when studying the large-scale
flows in the solar photosphere. The method we demonstrate is suitable
to detect large-scale velocity field with effective resolution of 60"
and random error of 15 m/s. We believe that our method may provide a
powerful tool for studies related to the dynamic behaviour of plasmas
in the solar photosphere.
---------------------------------------------------------
Title: The Conception of the Full-disc Telescope for EST Instrument
Authors: Klvana, M.; Sobotka, M.; Svanda, M.
2008ESPM...12.2.73K Altcode:
In year 2008 European Association for Solar Telescopes (EAST)
started to carry out the project of the European Solar Telescope (EST)
instrument. The main telescope of EST is expected to have a diameter of
4 m and therefore it will become one of the biggest solar telescopes
in the world. The main telescope will be connected to the smaller
full-disc solar telescope, the development of which is carried out by
our group. <P />In this contribution we present optical and mechanical
demands of the telescope, its optical design and the description of
the basic operation regimes. We analyse the possibilities of inclusion
of this full-disc telescope in the general EST controlling system. We
show that the small telescope can be used for alignment of the main
telescope and for the determination of the changing instrumental
constants of the instrument during a long-term run.
---------------------------------------------------------
Title: Effects of Solar Active Regions on Meridional Flows
Authors: Švanda, Michal; Kosovichev, Alexander G.; Zhao, Junwei
2008ApJ...680L.161S Altcode: 2008arXiv0805.1789S
The aim of this Letter is to extend our previous study of the
solar-cycle variations of meridional flows and to investigate their
latitudinal and longitudinal structure in the subphotospheric layer,
especially their variations in magnetic regions. Helioseismology
observations indicate that mass flows around active regions are
dominated by inflows into those regions. On average, those local
flows are more important around the leading magnetic polarities of
active regions than around the following polarities and depend on
the evolutionary stage of particular active regions. We present a
statistical study based on MDI/SOHO observations of 1996-2002 and show
that this effect explains a significant part of the cyclic change
of meridional flows in near-equatorial regions, but not at higher
latitudes. A different mechanism driving solar-cycle variations of
the meridional flow probably operates.
---------------------------------------------------------
Title: Large-scale horizontal flows in the solar
photosphere. III. Effects on filament destabilization
Authors: Roudier, T.; Švanda, M.; Meunier, N.; Keil, S.; Rieutord,
M.; Malherbe, J. M.; Rondi, S.; Molodij, G.; Bommier, V.; Schmieder, B.
2008A&A...480..255R Altcode: 2007arXiv0711.3112R
Aims:We study the influence of large-scale photospheric motions on
the destabilization of an eruptive filament, observed on October 6,
7, and 8, 2004, as part of an international observing campaign (JOP
178). <BR />Methods: Large-scale horizontal flows were investigated
from a series of MDI full-disc Dopplergrams and magnetograms. From
the Dopplergrams, we tracked supergranular flow patterns using the
local correlation tracking (LCT) technique. We used both LCT and manual
tracking of isolated magnetic elements to obtain horizontal velocities
from magnetograms. <BR />Results: We find that the measured flow
fields obtained by the different methods are well-correlated on large
scales. The topology of the flow field changed significantly during
the filament eruptive phase, suggesting a possible coupling between
the surface flow field and the coronal magnetic field. We measured
an increase in the shear below the point where the eruption starts
and a decrease in shear after the eruption. We find a pattern in the
large-scale horizontal flows at the solar surface that interact with
differential rotation. <BR />Conclusions: We conclude that there is
probably a link between changes in surface flow and the disappearance
of the eruptive filament.
---------------------------------------------------------
Title: Large-scale horizontal flows in the solar
photosphere. II. Long-term behaviour and magnetic activity response
Authors: Švanda, M.; Klvaňa, M.; Sobotka, M.; Bumba, V.
2008A&A...477..285S Altcode: 2007arXiv0710.2693S
We have developed a method to map large-scale horizontal velocity
fields in the solar photosphere. The method was developed, tuned, and
calibrated using synthetic data. Now, we apply the method to the series
of Michelson Doppler Imager (MDI) Dopplergrams covering almost one
solar cycle to generate the information about the long-term behaviour
of surface flows. Our method clearly reproduces the widely accepted
properties of mean flow field components, such as torsional oscillations
and a pattern of meridional circulation. We also performed a period
analysis, however due to the data series length and large gaps we did
not detect any significant periods. The relation between the magnetic
activity and the mean zonal motion is studied. We found evidence
that the emergence of compact magnetic regions locally accelerates
the rotation of the supergranular pattern in their vicinity and that
the presence of magnetic fields generally decelerates the rotation in
the equatorial region. Our results show that active regions in the
equatorial region emerge, exhibiting a constant velocity (faster by
60 ± 9 m s<SUP>-1</SUP> than the Carrington rate), suggesting that
they emerge from the base of the surface radial shear at 0.95 R_⊙,
disconnect from their magnetic roots, and slow down during their
evolution.
---------------------------------------------------------
Title: Surface velocity network with anti-solar differential rotation
on the active K-giant σ Geminorum
Authors: Kovári, Zs.; Bartus, J.; Švanda, M.; Vida, K.; Strassmeier,
K. G.; Oláh, K.; Forgács-Dajka, E.
2007AN....328.1081K Altcode: 2007arXiv0711.0771K
We demonstrate the power of the local correlation tracking technique
on stellar data for the first time. We recover the spot migration
pattern of the long-period RS CVn-type binary σ Gem from a set of six
Doppler images from 3.6 consecutive rotation cycles. The resulting
surface flow map suggests a weak anti-solar differential rotation
with {α≈-0.0022±0.0016}, and a coherent poleward spot migration
with an average velocity of 220±10 m s<SUP>-1</SUP>. This result
agrees with our recent findings from another study and could also be
confirmed theoretically.
---------------------------------------------------------
Title: Velocity Fields in the Solar Photosphere
Authors: Svanda, Michal
2007arXiv0712.1958S Altcode:
Large-scale velocity fields in the solar photosphere remain a mystery
in spite of many years of intensive studies. In this thesis, the new
method of the measurements of the solar photospheric flow fields is
proposed. It is based on local correlation tracking algorithm applied
to full-disc dopplergrams obtained by Michelson Doppler Images (MDI)
on-board the Solar and Heliospheric Observatory (SoHO). The method
is tuned and tested on synthetic data, it is shown that the method is
capable of measuring of horizontal velocity fields with an accuracy of
15 \mps. It is also shown that the method provides the measurements
comparable with time-distance local helioseismology. The method is
applied to real data sets. It reproduces well known properties of
solar photospheric velocity fields. Moreover, the case studies show
an evidence about the influence of the changes in the flow field
topology on the stability of the eruptive filament and support the
theory of the dynamical disconnection of bipolar sunspots from their
magnetic roots. The method has a great perspective in the future
use. The meridional flux transportation speed is also studied and it is
shown that the direct measurement may differ from time-distance local
helioseimology in the areas occupied by the strong magnetic field. This
result has an impact to the flux transport dynamo models, which use
the meridional speed as the essential observational input parameter.
---------------------------------------------------------
Title: Anti-solar differential rotation and surface flow pattern on
UZ Libræ
Authors: Vida, K.; Kovári, Zs.; Švanda, M.; Oláh, K.; Strassmeier,
K. G.; Bartus, J.
2007AN....328.1078V Altcode: 2007arXiv0711.1187V
We re-investigate UZ Libræ spectra obtained at KPNO in 1998 and
2000. From the 1998 data we compose 11 consecutive Doppler images using
the Ca I-6439, Fe I-6393 and Fe I-6411 lines. Applying the method
of average cross-correlation of contiguous Doppler images we find
anti-solar differential rotation with a surface shear of α≈-0.03. The
pilot application of the local correlation tracking technique for the
same data qualitatively confirms this result and indicates complex flow
pattern on the stellar surface. From the cross-correlation of the two
available Doppler images in 2000 we also get anti-solar differential
rotation but with a much weaker shear of α≈-0.004.
---------------------------------------------------------
Title: Speed of Meridional Flows and Magnetic Flux Transport on
the Sun
Authors: Švanda, Michal; Kosovichev, Alexander G.; Zhao, Junwei
2007ApJ...670L..69S Altcode: 2007arXiv0710.0590S
We use the magnetic butterfly diagram to determine the speed of the
magnetic flux transport on the solar surface toward the poles. The
manifestation of the flux transport is clearly visible as elongated
structures extended from the sunspot belt to the polar regions. The
slopes of these structures are measured and interpreted as meridional
magnetic flux transport speed. Comparison with the time-distance
helioseismology measurements of the mean speed of the meridional flows
at a depth of 3.5-12 Mm shows a generally good agreement, but the
speeds of the flux transport and the meridional flow are significantly
different in areas occupied by the magnetic field. The local circulation
flows around active regions, especially the strong equatorward flows
on the equatorial side of active regions, affect the mean velocity
profile derived by helioseismology but do not influence the magnetic
flux transport. The results show that the mean longitudinally averaged
meridional flow measurements by helioseismology may not be used directly
in solar dynamo models for describing the magnetic flux transport, and
that it is necessary to take into account the longitudinal structure
of these flows.
---------------------------------------------------------
Title: Anti-solar differential rotation on the active K-giant σ
Geminorum
Authors: Kovári, Zs.; Bartus, J.; Strassmeier, K. G.; Vida, K.;
Švanda, M.; Oláh, K.
2007A&A...474..165K Altcode:
The active K1 giant σ Gem and its differential surface rotation is
revisited. We refine our previous inconclusive result by recovering
the spot migration pattern of this long-period RS CVn-type binary
through application of the technique of “average cross-correlation
of contiguous Doppler images” to a set of six Doppler images from
3.6 consecutive rotation cycles. We find an anti-solar differential
rotation law with a shear of α≈ -0.022±0.006. We also find evidence
of a poleward migration trend of spots with an average velocity of
≈300 m s<SUP>-1</SUP>.
---------------------------------------------------------
Title: Comparison of Large-Scale Flows on the Sun Measured by
Time-Distance Helioseismology and Local Correlation Tracking
Authors: Švanda, Michal; Zhao, Junwei; Kosovichev, Alexander G.
2007SoPh..241...27S Altcode: 2007astro.ph..1717S
We present a direct comparison between two different techniques:
time-distance helioseismology and a local correlation tracking
method for measuring mass flows in the solar photosphere and in
a near-surface layer. We applied both methods to the same dataset
(MDI high-cadence Dopplergrams covering almost the entire Carrington
rotation 1974) and compared the results. We found that, after necessary
corrections, the vector flow fields obtained by these techniques are
very similar. The median difference between directions of corresponding
vectors is 24°, and the correlation coefficients of the results for
mean zonal and meridional flows are 0.98 and 0.88, respectively. The
largest discrepancies are found in areas of small velocities where the
inaccuracies of the computed vectors play a significant role. The good
agreement of these two methods increases confidence in the reliability
of large-scale synoptic maps obtained by them.
---------------------------------------------------------
Title: Large-scale horizontal flows in the solar
photosphere. I. Method and tests on synthetic data
Authors: Švanda, M.; Klvaňa, M.; Sobotka, M.
2006A&A...458..301S Altcode: 2006astro.ph..8443S
We propose a useful method for mapping large-scale velocity
fields in the solar photosphere. It is based on the local <P
/>correlation tracking algorithm when tracing supergranules in
full-disc dopplergrams. The method was developed using synthetic
data. <P />The data are transformed during the data processing into
a suitable coordinate system, the noise is removed, and finally the
velocity field is <P />calculated. Resulting velocities are compared
with the model velocities and the calibration is done. From our results
it becomes clear that <P />this method could be applied to full-disc
dopplergrams acquired by the Michelson Doppler Imager (MDI) onboard
the Solar and Heliospheric Observatory (SoHO).
---------------------------------------------------------
Title: Doppler Velocity Fields in Magnetic Structures and their
Surroundings
Authors: Klvaňa, M.; Bumba, V.; Švanda, M.
2006CEAB...30...21K Altcode:
We demonstrate the effect of a velocity shift of photospheric velocity
fields in regions filled by magnetic field compared to the velocity
fields of non-magnetic regions surrounding them. We analyse this effect
on five different types of measurements in the active region NOAA 8086
obtained by SOLMAG (Ondřejov magnetograph) in four spectral lines
on September 18, 1997. The mean value of the Doppler velocity field
in regions filled by magnetic field shows a red-shift of 40--190 m
s<SUP>-1</SUP> with respect to the neighbouring non-magnetic regions.
---------------------------------------------------------
Title: Mapping of Large-Scale Photospheric Velocity Fields
Authors: Švanda, M.; Klvaňa, M.; Sobotka, M.
2005ESASP.600E..71S Altcode: 2005ESPM...11...71S; 2005dysu.confE..71S
No abstract at ADS
---------------------------------------------------------
Title: Temporal Changes of the Photospheric Velocity Fields
Authors: Klvaňa, M.; Švanda, M.; Bumba, V.
2005HvaOB..29...89K Altcode:
We analyse the influence of the planets Mercury, Venus, Earth and
Jupiter on the Doppler velocity field in the solar photosphere, using
the theory of tidal forces. We compare the measured Doppler velocity
fields estimated in a zonal area along the solar equator with the
results of the dynamical calculations. From this comparison it follows
that we do not succeed to demonstrate the presence of a velocity field,
caused by the tidal forces, in the measured data. If tidal waves in
the solar photosphere do exist, they are lost in the noise and their
horizontal velocity field probably will be under the limit of ±20
m s^{-1}.
---------------------------------------------------------
Title: Motions of Supergranular Structures on the Solar Surface
Authors: Švanda, M.; Klnaňa, M.; Sobotka, M.
2005HvaOB..29...39S Altcode:
We present a method used to study motions of supergranular structures
in the solar photosphere. It is based on the local correlation
tracking method applied to full-disk dopplergrams measured by
SoHO/MDI. In primary dopplergrams, there is a plenty of noise
originating mostly from oscillations and morphological changes of
the shape of supergranules. We describe a method used to suppress
such noise. To demonstrate the suggested technique and properties of
computed velocity fields, we processed a few-day period during the
time of minimum of solar activity. The obtained vector velocity fields
are drawn using streamlines in the Carrington's coordinate system. We
discuss the reproductivity of the obtained results and the connection
of the velocity field we found to the large-scale velocity field in
the upper part of the convection zone.
---------------------------------------------------------
Title: Do tidal waves exist in the solar photosphere?
Authors: Klvaňa, M.; Švanda, M.; Krivtsov, A.; Bumba, V.
2004HvaOB..28..157K Altcode:
The temporal coincidences between solar activity regularities and the
motion of some planets show that there could exist a mutual physical
relationship between both processes. One of the possible causal agents
could be the action of the gravitational field. Although the altitude
of planetary tidal waves on the Sun, following many authors, is of
the order of only one millimetre, in the case of resonance it could be
enlarged appreciably. Therefore, we decided to verify the possibility
of detection of such waves in the solar atmosphere using the earlier
developed dynamical theory of tidal waves. In this paper we present
and discuss the preliminary results, obtained from the comparison
of the velocity fields measured with the MDI instrument of the SOHO
satellite and of the velocity fields of a tidal wave calculated on
the basis of the dynamical theory.
---------------------------------------------------------
Title: Dynamics of motions in the quiet photosphere
Authors: Svanda, M.; Klvana, M.; Sobotka, M.; Bumba, V.
2003ESASP.535..149S Altcode: 2003iscs.symp..149S
We determine the vector velocity fields describing the plasma motions
in the quiet solar photosphere, using the motions of the supergranular
structures, obtained by the analysis of series of Doppler measurements
of velocity fields of the whole solar disk. It turned out that
the studied vector velocity fields can be submerged under the noise
level, originating due to the strong variability of the supergranular
structures during their life time. We describe the method we used for
the suppressing of such noise, and we bring the criterions used by
the election of free parameters. We demonstrate examples of obtained
vector velocity fields and of the resulting motions of matter on the
visible photospheric surface. We discuss different factors influencing
the reproductivness of obtained results. We construct a graph of the
dependence of the differential rotation on the heliographic latitude
from the mean vector velocity field, we got for the solar disk without
expressive magnetic fields.
---------------------------------------------------------
Title: Comparison of Coronal Structures 11.8.1999 on the Long
Observation Base
Authors: Belík, M.; Markova, E.; Brandejsova, E.; Kucera, J.; Kares,
J.; Svanda, M.
2000ESASP.463..587B Altcode: 2000sctc.proc..587B
No abstract at ADS
