Author name code: choudhary
ADS astronomy entries on 2022-09-14
author:"Choudhary, Debi Prasad"
------------------------------------------------------------------------
Title: The magnetic topology of the inverse Evershed flow
Authors: Prasad, A.; Ranganathan, M.; Beck, C.; Choudhary, D. P.;
Hu, Q.
Bibcode: 2022A&A...662A..25P
Altcode: 2022arXiv220302702P
Context. The inverse Evershed flow (IEF) is a mass motion towards
sunspots at chromospheric heights.
Aims: We combined
high-resolution observations of NOAA 12418 from the Dunn Solar
Telescope and vector magnetic field measurements from the Helioseismic
and Magnetic Imager (HMI) to determine the driver of the IEF.
Methods: We derived chromospheric line-of-sight (LOS) velocities from
spectra of Hα and Ca II IR. The HMI data were used in a non-force-free
magnetic field extrapolation to track closed field lines near the
sunspot in the active region. We determined their length and height,
located their inner and outer foot points, and derived flow velocities
along them.
Results: The magnetic field lines related to the
IEF reach on average a height of 3 megameter (Mm) over a length
of 13 Mm. The inner (outer) foot points are located at 1.2 (1.9)
sunspot radii. The average field strength difference ΔB between inner
and outer foot points is +400 G. The temperature difference ΔT is
anti-correlated with ΔB with an average value of −100 K. The pressure
difference Δp is dominated by ΔB and is primarily positive with a
driving force towards the inner foot points of 1.7 kPa on average. The
velocities predicted from Δp reproduce the LOS velocities of 2-10 km
s−1 with a square-root dependence.
Conclusions:
We find that the IEF is driven along magnetic field lines connecting
network elements with the outer penumbra by a gas pressure difference
that results from a difference in field strength as predicted by the
classical siphon flow scenario.
Title: Velocities of an Erupting Filament
Authors: Wang, Shuo; Jenkins, Jack M.; Muglach, Karin; Martinez Pillet,
Valentin; Beck, Christian; Long, David M.; Choudhary, Debi Prasad;
McAteer, James
Bibcode: 2022ApJ...926...18W
Altcode: 2021arXiv211107830W
Solar filaments exist as stable structures for extended periods of
time before many of them form the core of a coronal mass ejection
(CME). We examine the properties of an erupting filament on 2017 May
29-30 with high-resolution He I 10830 Å and Hα spectra from the
Dunn Solar Telescope, full-disk Dopplergrams of He I 10830 Å from
the Chromospheric Telescope, and EUV and coronograph data from SDO
and STEREO. Pre-eruption line-of-sight velocities from an inversion
of He I with the HAZEL code exhibit coherent patches of 5 Mm extent
that indicate counter-streaming and/or buoyant behavior. During the
eruption, individual, aligned threads appear in the He I velocity
maps. The distribution of velocities evolves from Gaussian to strongly
asymmetric. The maximal optical depth of He I 10830 Å decreased from
τ = 1.75 to 0.25, the temperature increased by 13 kK, and the average
speed and width of the filament increased from 0 to 25 km s-1
and 10 to 20 Mm, respectively. All data sources agree that the filament
rose with an exponential acceleration reaching 7.4 m s-2
that increased to a final velocity of 430 km s-1 at 22:24
UT; a CME was associated with this filament eruption. The properties
during the eruption favor a kink/torus instability, which requires
the existence of a flux rope. We conclude that full-disk chromospheric
Dopplergrams can be used to trace the initial phase of on-disk filament
eruptions in real time, which might potentially be useful for modeling
the source of any subsequent CMEs.
Title: Heating of the solar atmosphere by electric currents
Authors: Choudhary, Debi Prasad; Louis, Rohan; Prasad, Avijeet; Beck,
Christian; Yalim, Mehmet
Bibcode: 2021AGUFMSH12B..07C
Altcode:
We present direct evidence of Ohmic dissipation of electric currents
that leads to heating of the solar chromosphere above a light bridge
in a sunspot by combining high-resolution spectroscopic Ca II IR data
from the Dunn Solar Telescope with vector magnetic field observations
from HMI. An extrapolation of the photospheric magnetic field from
HMI to the corona using a non-force-free field technique provided the
three-dimensional distribution of electric currents at locations of
magnetic discontinuities, while the inversion of the chromospheric
Ca II IR line spectra with the CAISAR code delivered the temperature
stratifications from the photosphere to the chromosphere. The comparison
of these results clearly shows that the light bridge is a site of
strong electric currents of about 0.3 A m2 at the bottom boundary, which
extend to about 0.7 Mm while decreasing monotonically with height. The
dissipation of these currents produces a chromospheric temperature
excess of about 600800 K relative to the umbra. Only the light bridge,
where relatively weak and highly inclined magnetic fields emerge
over a duration of 13 hrs, shows a spatial coincidence of thermal
enhancements and electric currents. The temperature enhancements
and the Cowling heating are primarily confined to a height range
of 0.40.7 Mm above the light bridge. The corresponding increase in
internal energy of 200 J m3 can be supplied by the heating in about
10 min. This heating process by Ohmic dissipation can happen at any
place in the solar atmosphere where large electric currents co-exist
with non-zero resistivity. Large-scale corona loops embedded in a more
vertical background magnetic field could experience the same effect
leading to a localized heating of coronal plasma.
Title: Heating of the solar chromosphere in a sunspot light bridge
by electric currents
Authors: Louis, Rohan E.; Prasad, Avijeet; Beck, Christian; Choudhary,
Debi P.; Yalim, Mehmet S.
Bibcode: 2021A&A...652L...4L
Altcode: 2021arXiv210712066L
Context. Resistive Ohmic dissipation has been suggested as a mechanism
for heating the solar chromosphere, but few studies have established
this association.
Aims: We aim to determine how Ohmic dissipation
by electric currents can heat the solar chromosphere.
Methods:
We combine high-resolution spectroscopic Ca II data from the Dunn Solar
Telescope and vector magnetic field observations from the Helioseismic
and Magnetic Imager (HMI) to investigate thermal enhancements in a
sunspot light bridge. The photospheric magnetic field from HMI was
extrapolated to the corona using a non-force-free field technique that
provided the three-dimensional distribution of electric currents, while
an inversion of the chromospheric Ca II line with a local thermodynamic
equilibrium and a nonlocal thermodynamic equilibrium spectral archive
delivered the temperature stratifications from the photosphere to the
chromosphere.
Results: We find that the light bridge is a site of
strong electric currents, of about 0.3 A m−2 at the bottom
boundary, which extend to about 0.7 Mm while decreasing monotonically
with height. These currents produce a chromospheric temperature excess
of about 600−800 K relative to the umbra. Only the light bridge,
where relatively weak and highly inclined magnetic fields emerge over a
duration of 13 h, shows a spatial coincidence of thermal enhancements
and electric currents. The temperature enhancements and the Cowling
heating are primarily confined to a height range of 0.4−0.7 Mm above
the light bridge. The corresponding increase in internal energy of 200 J
m−3 can be supplied by the heating in about 10 min.
Conclusions: Our results provide direct evidence for currents heating
the lower solar chromosphere through Ohmic dissipation.
Title: Measuring and modeling the variability of solar Balmer lines
Authors: Criscuoli, S.; Marchenko, S.; Deland, M.; Choudhary, D.;
Kopp, G.
Bibcode: 2021AAS...23811312C
Altcode:
We investigate the variability of solar Balmer lines (H-alpha,
beta, gamma, delta) observed by space-borne radiometers (SORCE,
SCIAMACHY, GOME-2, OMI, and TROPOMI), combining these precise,
long-term observations with abundant, high-resolution data from the
ground-based NSO/ISS spectrograph. We relate the detected variability
to the appearance of magnetic features on the solar disk. We find that
on solar-rotation timescales Balmer line activity indices (defined as
line-core to line-wing ratios) closely follow variations in the total
solar irradiance (which is predominantly photospheric), thus frequently
(specifically, during passages of big sunspot groups) deviates from
behavior of the line-activity indices that track chromospheric activity
levels. At longer timescales (years), the correlation with chromospheric
indices increases, with periods of low- or even anti- correlation found
at intermediate timescales. Comparisons with Balmer-line variability
patterns obtained from a semi-empirical model indicate that it is
unlikely that the periods of low/anti correlations can be ascribed to
the presence of filaments, in contradiction to some previous studies.
Title: Spectropolarimetric Measurements Of The Inverse Evershed Flow
Authors: Choudhary, D. P.; Beck, C.; Prasad, A.; Monankrishna, R.;
Dhara, S.
Bibcode: 2021AAS...23831319C
Altcode:
We use high spatial resolution observations with spectropolarimeters
at the Dunn Solar Telescope in multiple spectral lines originating
in the chromosphere to study inverse Evershed flow channels that
connect the outer penumbra and the moat region around sunspots at
various heliocentric positions. The measurements were combined with
extrapolated magnetic field lines to determine the three-dimensional
topology of the flow channels. The magnetic field lines guiding the
flows reach on average a height of 2-5 Mm over a length of 10-20 Mm,
with cold inner (hot outer) foot points located at 1.2 (1.9) sunspot
radii. The average difference in field strength between the inner and
outer foot point is about +400 G, while the temperature difference is
about -100 K. This configuration can drive and sustain a siphon flow
from the moat boundary towards the penumbra over the lifetime of the
flow channels of about one hour. This study clearly demonstrates the
inverse Evershed flow to be a siphon flow along arched chromospheric
loops.
Title: Solar activity and responses observed in Balmer lines
Authors: Marchenko, S.; Criscuoli, S.; DeLand, M. T.; Choudhary,
D. P.; Kopp, G.
Bibcode: 2021A&A...646A..81M
Altcode:
Context. Many stars show Sun-like magnetic activity cycles, which
are frequently observed by tracking changes in the chromospherically
sensitive CaII H&K doublet. However, relationships between the
line profile changes related to the magnetic activity seen in strong
spectral transitions in other portions of a stellar spectrum are
yet to be understood.
Aims: We follow variability patterns in
various solar lines in order to relate them to the emergence, passage,
and decay of active solar regions.
Methods: The line activity
indices (core-to-wing ratio) for the upper Balmer lines - Hβ, Hγ,
and Hδ - are constructed from the near-daily solar measurements
acquired by the Ozone Monitoring Instrument and the TROPOspheric
Monitoring Instrument.
Results: On solar rotation timescales,
the upper Balmer line activity indices closely follow variations in
the total solar irradiance, r ∼ -(0.6 - 0.7), and thus frequently
deviate from the behavior of the line activity indices that track
chromospheric activity levels (e.g., the CH 430 nm band used in this
study), specifically during passages of big sunspot groups.
Title: A Machine Learning Approach to Identify Solar Stokes Profiles
in Flaring and Non-Flaring Active Regions
Authors: Venkatesan, V.; Cadavid, A. C.; Romich, K.; Choudhary, D. P.
Bibcode: 2020AGUFMNG0040001V
Altcode:
Solar flares are explosive events on the surface of the Sun that
release electromagnetic radiation, which can disrupt the earth's
atmosphere and cause havoc in our communication system. Models for
flare forecasting use properties of active region (AR) magnetic fields
as predictors of flare occurrence. The magnetic field properties are
obtained using inversion models that decode the information contained
in Stokes Profiles (SP) as the radiation passes through the solar
atmosphere. The inversion techniques ignore the rich information
contained in the SP since they tend to use line fitting methods
and derive average magnetic field properties. The line parameters
can give better information on the magnetic field complexity of
the AR atmosphere. We apply a modified K-means clustering method to
Hinode spectropolarimetric data to identify and classify the Stokes V
profiles in flaring and non-flaring ARs. The modified K-means method
leads to a stable result, in which random initializations converge to
comparable clustering. The profiles which characterize the centroids
of the clusters are used to calculate three-line parameters: the
amplitude asymmetry, the area asymmetry (associated with the degree
of non-linear polarization), and the percentage of atypical profiles
inside and outside the polarity inversion lines (PIL). We find that
the amplitude asymmetry is higher in non-flaring vs. flaring regions;
the area asymmetry is greater in flaring ARs vs. non-flaring ARs,
and inside the PIL vs. outside. Our results indicate that flaring ARs,
harbor a higher percentage of atypical profiles compared to non-flaring
ARs & outside the PIL. These results are compatible with those
found using the individual pixel profiles in the calculations. They
indicate that the three parameters can be used to distinguish flaring
from non-flaring ARs.
Title: Four Solar Cycle Spectrum Variation of the Sun-as-a-Star
Authors: Choudhary, D. P.; Chapman, G. A.; Cadavid, A. C.; Cookson, A.
Bibcode: 2020AGUFMA227.0008C
Altcode:
The chromospheric activity of the Sun is governed by the magnetic
field anchored in the photosphere. The solar cycle 23 was a smaller
cycle compared to the recent cycles, in terms of sunspot number and
total disk integrated magnetic field. Comparison of the chromospheric
lines in past cycles may provide insight of the effect of magnetic
field on solar atmosphere. In this paper, we study the dependence
of chromospheric activity on magnetic field of the Sun-as-a-star
in four solar cycles during 1977-2018. The study is conducted by
merging the data obtained by Dr. W. Livingston and the observations by
Integrated Sunlight Spectrometer (ISS) and Vector Spectromagnetograph
(VSM) of Synoptic Optical Long-term Investigations of the Sun (SOLIS)
instrument. The chromospheric activity is measured as the line depth
and equivalent width (EW) of spectral lines in Hα, He I 10830 nm,
Ca II 854.2 nm, Ca II H and K, and Na D I 589.6 nm obtained with the
ISS. The full disk mean total magnetic flux (FDMTMF) observed with
the VSM is used as the measure of magnetic activity of the Sun. The
equivalent width of Ca II K and He I 10830 nm measured by Livingston
along with the Magnetic Plage Strength Index (MPSI) value and a Mount
Wilson Sunspot Index (MWSI) obtained with 150-Foot Solar Tower in
Mt. Wilson Observatory are used to further study the relationship
between the magnetic field and chromospheric activity.
Title: The Formation of an Atypical Sunspot Light Bridge as a Result
of Large-scale Flux Emergence
Authors: Louis, Rohan E.; Beck, Christian; Choudhary, Debi P.
Bibcode: 2020ApJ...905..153L
Altcode: 2020arXiv201014085L
We use a combination of full-disk data from the Solar Dynamics
Observatory and high-resolution data from the Dunn Solar Telescope (DST)
to study the formation, structure, and evolution of an atypical light
bridge (LB) in a regular sunspot. The LB results from the emergence of
magnetic flux with one footpoint rooted in a pore outside the parent
sunspot that appears about 17 hr before the LB. The pore has a polarity
opposite to that of the sunspot and recedes from it at a speed of
about 0.4 km s-1. This is accompanied by the development of
an elongated magnetic channel in the outer penumbra that triggers the
formation of the LB when it reaches the inner penumbral boundary. The
LB is a nearly horizontal structure with a field strength of about
1.2 kG that exhibits long-lived photospheric blueshifts of about 0.85
km s-1 along its entire length. The emergence of the LB
leads to dynamic surges in the chromosphere and transition region
about 13 minutes later. We derive the photospheric and chromospheric
structure of the LB in the DST data from spectral line parameters and
inversions of He I at 1083 nm, Si I at 1082.7 nm, Ca II IR at 854
nm, and Hα at 656 nm and speckle-reconstructed imaging
at 700 and 430 nm. The LB shows an elongated filamentary shape in
the photosphere without lateral extrusions. The thermal inversion
of Ca II IR reveals the LB to be about 600-800 K hotter than the
umbra. Different sections of the LB are elevated to heights between 400
and 700 km. Our results indicate that LB formation is part of a flux
emergence event with the LB envelope reaching a height of about 29 Mm
before dissolving after about 13 hr. We conclude that the existence
of persistent, large-scale photospheric blueshifts in LBs is the most
likely criterion for distinguishing between flux emergence events and
overturning convection in field-free umbral intrusions.
Title: The Effect of Sunspot Umbrae on the Total Solar Irradiance
Authors: Chapman, G. A.; Choudhary, D. P.; Cookson, A. M.
Bibcode: 2020AGUFMA227.0007C
Altcode:
Most studies of the contribution of sunspots on the total solar
irradiance (TSI) are based on the area of sunspots of constant average
darkness. However, the average darkness (contrast) depends on the
relative area of the umbra, the dark "core" of many sunspots. We will
present results from a study of photometric observations, that measure
the actual contrast of sunspots and their effect on TSI. This work is
partially supported by NASA grant 80NSSC18K1328.
Title: Center-to-limb Variation of the Inverse Evershed Flow
Authors: Beck, C.; Choudhary, D. P.; Ranganathan, M.
Bibcode: 2020ApJ...902...30B
Altcode: 2020arXiv200812748B
We present the properties of the inverse Evershed flow (IEF)
based on the center-to-limb variation of the plasma speed and loop
geometry of chromospheric superpenumbral fibrils in eleven sunspots
that were located at a wide range of heliocentric angles from 12° to
79°. The observations were acquired at the Dunn Solar Telescope in the
spectral line of Hα at 656 nm to determine chromospheric flows and the
photospheric Si I line at 1082.7 nm to estimate the photospheric umbral
magnetic field strength. All sunspots display opposite line-of-sight
(LOS) velocities on the limb and center side with a distinct shock
signature near the outer penumbral edge. We developed a simplified
flexible sunspot model assuming axisymmetry and prescribing the radial
flow speed profile at a known loop geometry to replicate the observed
two-dimensional IEF patterns under different viewing angles. The
simulated flow maps match the observations for chromospheric loops with
10-20 Mm length starting at 0.8-1.1 sunspot radii, an apex height of
1-3 Mm, and a flow speed of 2-9 km s-1. We find on average
a good agreement of the simulated velocities and the observations
on elliptical annuli around the sunspot. Individual IEF channels
show a significant range of variation in their properties and reach
maximal LOS speeds of up to 12 km s-1. Upwards or downwards
directed flows do not show a change of sign in the LOS velocities for
heliocentric angles above 30°. Our results are consistent with the IEF
being caused by a siphon flow mechanism driving a flow at about sonic
speed along elevated loops with a flattened top in the chromosphere.
Title: Temporal Evolution of the Inverse Evershed Flow
Authors: Beck, C.; Choudhary, D. P.
Bibcode: 2020ApJ...891..119B
Altcode: 2020arXiv200204560B
The inverse Evershed flow (IEF) is an inflow of material into the
penumbra of sunspots in the solar chromosphere that occurs along dark,
elongated super-penumbral fibrils extending from about the outer edge
of the moat cell to the sunspot. The IEF channels exhibit brightenings
in the penumbra, where the supersonic IEF descends to the photosphere
causing shock fronts with localized heating. We used an 1 hr time series
of spectroscopic observations of the chromospheric spectral lines of
Ca II IR at 854 nm and Hα at 656 nm taken with the Interferometric
Bidimensional Spectrometer at the Dunn Solar Telescope to investigate
the temporal evolution of IEF channels. Complementary information on
the photospheric magnetic field was obtained from observations with the
Facility Infrared Spectropolarimeter at 1083 nm and the Helioseismic
and Magnetic Imager. We find that individual IEF channels are long-lived
(10-60 minutes) and only show minor changes in position and flow speed
during their lifetime. Initiation and termination of IEF channels
takes several minutes. The IEF channels with line-of-sight velocities
of about 10 km s-1 show no lasting impact from transient or
oscillatory phenomena with maximal velocity amplitudes of only about 1
km s-1 that run along them. We could not detect any clear
correlation of the location and evolution of IEF channels to local
magnetic field properties in the photosphere in the penumbra or moving
magnetic features in the sunspot moat. Our results support a picture
of the IEF as a field-aligned siphon flow along arched loops. From our
data we cannot determine if their evolution is controlled by events
at the outer end in the moat or at the inner end in the penumbra.
Title: Variability in Irradiance and Photometric Indices During the
Last Two Solar Cycles
Authors: Choudhary, Debi Prasad; Cadavid, Ana Cristina; Cookson,
Angela; Chapman, Gary A.
Bibcode: 2020SoPh..295...15C
Altcode:
The Total Solar Irradiance (TSI) primarily varies on an 11-year time
scale and is governed by features such as sunspots and associated
decay products such as plage and faculae. These short-lived physical
features can also modulate the solar irradiance at intermediate and
short temporal scales. Here we investigate the periodic variations,
at solar-surface-rotation time scales, of photometric indices
derived from images obtained at the San Fernando Observatory (SFO),
and we compare them to the properties of the contemporaneous TSI
as measured by the Total Irradiance Monitor (TIM) onboard the SOlar
Radiation and Climate Experiment (SORCE) spacecraft. Both the daily
ground- and space-based data, which span from early 2003 to late 2018,
present missing pixels. We use an autoregressive gap-filling method to
construct continuous time series to be analyzed via Fourier and wavelet
spectral techniques. Lomb-Scargle periodograms, which can handle time
series with missing data, are used for comparison. Both the Fourier
spectral power and the periodograms yield compatible results with
statistically significant periodicities in the range 25 - 35 days. All
of the time series have maximum power at 27 days. Significant secondary
periods are found at 29 - 30 days and 34 - 35 days. Wavelet analyses
of the full time series show that the photometric index resulting
from the red-continuum photometric sum [Σr] and the
TSI exhibit common high power at surface-solar-rotation scales
during the active part of the solar cycle. The phase relation at
the surface-solar-rotation scales is not definite. During the solar
minimum interval between Solar Cycles (SCs) 23 and 24, variations in
the TSI are found to be related to variations both in the photometric
index ΣK, calculated from Ca II K-line photometric sums
and in the magnetic flux in the solar activity latitudinal band (as
found in previous work). This suggests that the TSI changes during the
minimum are caused by the reduced line-blanketing effect of diffused
magnetic field.
Title: Stokes Line Parameters as Possible Indicators of Flaring
Activity: A Comparison of Flaring and Non-Flaring Active Regions
Authors: Romich, K.; Cadavid, A. C.; Choudhary, D. P.; Beck, C.
Bibcode: 2019AGUFMSH31D3338R
Altcode:
While the association between solar active regions (ARs) and solar
flares is well-established, there is currently no reliable means
of determining when (or if) a given AR will flare. Much of flare
forecasting is based on the application of machine-learning statistical
techniques that use parameters derived from the local magnetic field
as predictors; these are commonly obtained through spectropolarimetric
inversions using Stokes profiles from the observed radiation. The
standard inversion codes, such as those based on the Milne-Eddington
approximation, yield the average magnetic field values at or near flare
locations. However, this fails to utilize the rich information contained
in the shape of Stokes profiles, such as the existence of line-of-sight
(LOS) magnetic and velocity gradients and multiple magnetic components
along the LOS or within the resolving element. The resulting loss
of information can potentially lead to inaccurate forecasts. We
propose a novel approach, in which line parameters derived from
spectrally-resolved Stokes profiles are considered as possible
precursors to flare events. Using data from the spectropolarimeter
onboard the Hinode satellite, we examine the amplitude asymmetry, net
circular polarization, and degree of complexity of Stokes V profiles
from several flaring and non-flaring ARs. Particular attention is given
to regions near the polarity inversion line (PIL) due to its documented
role in flare initiation. We define the PIL using the magnetic field
vector relative to the solar surface, which we calculate from the Stokes
Q, U, and V profiles; this corrects for projection effects arising
from LOS observations of magnetic flux from ARs off disc center. In
light of the shortcomings of existing methods, we hope to expand the
set of viable indicators of flaring activity with the long-term goal
of improving flare forecasting models.
Title: Time dependent properties of Inverse Evershed Flow and
Perspectives with Daniel K. Inouye Solar Telescope (DKIST)
Authors: Choudhary, D. P.; Beck, C.
Bibcode: 2019AGUFMSH41F3334C
Altcode:
We observed isolated leading sunspot, located at a heliocentric angle
of 43 degrees, of a decaying active region (AR) NOAA 12418 on 16
September 2015 with the Interferometric Bidimensional, the Facility
InfraRed Spectropolarimeter at the Dunn Solar Telescope to study the
time dependent chromospheric flow properties along the fibril structure,
which is widely known as Inverse Evershed Flow. The observations were
carried out in spectral lines of H_alpha at 656 nm and Ca ii IR at
854.2 nm, photospheric Zeeman-sensitive Si i line at 1082.7 nm and the
chromospheric He i line at 1083 nm. Our data is complemented with the
Milne-Eddington inversion results for the photospheric magnetic field
obtained with the derived from observations by the Helioseismic and
Magnetic Imager on-board the Solar Dynamics Observatory. We find that
all three chromospheric lines show a very similar behavior in their
line-core intensity and the LOS velocity with matching spatial and
temporal properties. We find that individual IEF channels persist for a
few ten minutes to more than one hour. IEF channels that disappear are
often rapidly replaced by a new channel at the about the same location
after a short time. The IEF channels show little radial or lateral
motion and usually end in the mid to outer penumbra. Initiation of
the flow takes about 10min, while the termination is faster and takes
only about 5min. The IEF channels seem to appear at preferred azimuth
angles that are spaced at about 10◦ distance. The transient events
were found to have almost no effect on the overall Inverse Evershed
Flow pattern. In this paper, we shall present the results from our
current study and perspectives for further observational study with
upcoming 4 meter Daniel K. Inouye Solar Telescope.
Title: Temporal relations between total solar irradiance and
photometric indices during the last two solar cycles.
Authors: Cadavid, A. C.; Choudhary, D. P.; Chapman, G. A.; Cookson, A.
Bibcode: 2019AGUFMSH11D3391C
Altcode:
Our present understanding, from both empirical and semi-empirical
models, indicates that the variations in the Total Solar Irradiance
(TSI) on time scales of days to the solar cycle are primarily
associated with solar surface magnetic activity, which encompasses
sunspots, faculae, and the network. In previous work, approximately
seven years of TSI measurements from the Total Irradiance Monitor
(TIM) on board the SOlar Radiation and Climate Experiment (SORCE)
spacecraft were compared with photometric indices derived from red and
K-line images obtained on a daily basis at the San Fernando Observatory
(SFO), California State University Northridge (CSUN). The best linear
regression model yielded a coefficient of multiple determination,
R2, of 0.9495. Expanding on this earlier work and employing
additional analysis techniques not previously used, we consider 16
years of SORCE and SFO data, from early 2003 to late 2018. We use
an autoregressive gap filling method to construct continuous series
which can be analyzed via Fourier and wavelet spectral techniques in
order to investigate the characteristics of the time signals on short
temporal scales. Lomb-Scargle periodograms, which can handle time
series with missing data, are used for comparison. Both the Fourier
spectral power and the periodograms yield compatible results with
significant periodicities on the solar rotation time scales. For both
active and quiet Sun periods, cross-wavelet transforms between the
TSI and the photometric indices signals are used to identify regions
of high common power in the time-frequency maps. The wavelet transform
coherence indicates local periods and times during which the photometric
indices signals and TSI have significant coherence and phase locking,
independent of the power.
Title: An Erupting Solar Filament Observed at the DST
Authors: Wang, S.; Jenkins, J. M.; Pillet, V. M.; Beck, C.; Long,
D.; Choudhary, D. P.; McAteer, J.
Bibcode: 2019AGUFMSH33B3376W
Altcode:
An erupting filament that lead to a coronal mass ejection (CME) was
observed at the Dunn Solar Telescope (DST). We present HAZEL inversions
of spectropolarimetric observations of the quiescent filament acquired
with the Facility Infrared Spectropolarimeter at the DST. This study
includes three observations of the He I triplet at 10830 Å on May 29
and 30, 2017. The filament was stable on May 29, and was observed in
the process of rising at speeds of 20-30 km/s during the two spatial
maps taken on May 30. Vector magnetic fields along the filament were
obtained that show an inverse configuration indicative of a flux rope
topology, including co-aligned threads. To take advantage of the better
spatial and temporal resolution of the Daniel K. Inouye Solar Telescope
(DKIST), future collaborations of the DKIST and the DST to study solar
filaments are discussed.
Title: 2D non-LTE modelling of a filament observed in the Hα line
with the DST/IBIS spectropolarimeter
Authors: Schwartz, P.; Gunár, S.; Jenkins, J. M.; Long, D. M.;
Heinzel, P.; Choudhary, D. P.
Bibcode: 2019A&A...631A.146S
Altcode: 2019arXiv191003607S
Context. We study a fragment of a large quiescent filament observed on
May 29, 2017 by the Interferometric BIdimensional Spectropolarimeter
(IBIS) mounted at the Dunn Solar Telescope. We focus on its quiescent
stage prior to its eruption.
Aims: We analyse the spectral
observations obtained in the Hα line to derive the thermodynamic
properties of the plasma of the observed fragment of the filament.
Methods: We used a 2D filament model employing radiative transfer
computations under conditions that depart from the local thermodynamic
equilibrium. We employed a forward modelling technique in which we
used the 2D model to produce synthetic Hα line profiles that we
compared with the observations. We then found the set of model input
parameters, which produces synthetic spectra with the best agreement
with observations.
Results: Our analysis shows that one part
of the observed fragment of the filament is cooler, denser, and
more dynamic than its other part that is hotter, less dense, and more
quiescent. The derived temperatures in the first part range from 6000 K
to 10 000 K and in the latter part from 11 000 K to 14 000 K. The gas
pressure is 0.2-0.4 dyn cm-2 in the first part and around
0.15 dyn cm-2 in the latter part. The more dynamic nature
of the first part is characterised by the line-of-sight velocities with
absolute values of 6-7 km s-1 and microturbulent velocities
of 8-9 km s-1. On the other hand, the latter part exhibits
line-of-sight velocities with absolute values 0-2.5 km s-1
and microturbulent velocities of 4-6 km s-1.
Title: Filament Magnetic Fields at the DST and DKIST
Authors: Wang, Shuo; Jenkins, Jack; Pillet, Valentin; Beck, Christian;
Long, David; Choudhary, Debi Prasad; McAteer, James
Bibcode: 2019AAS...23422603W
Altcode:
Observations from the 0.8-m Dunn Solar Telescope (DST) are
qualitatively similar to data that will be produced by the 4-m
Daniel K. Inouye Solar Telescope (DKIST), albeit at a lower spatial
resolution and polarimetric sensitivity. We present HAZEL inversions
of spectropolarimetric observations of a quiescent filament acquired
with the Facility Infrared Spectropolarimeter at the DST. This study
includes three observations of the He I triplet at 10830 Å on May 29
and 30, 2017. The filament was stable on May 29, and was observed in
the process of rising at speeds of 20-30 km/s during the two spatial
maps taken on May 30. Vector magnetic fields along the filament were
obtained that show an inverse configuration indicative of a flux rope
topology, including co-aligned threads. To take advantage of the better
spatial and temporal resolution of the DKIST, future collaborations
of the DKIST and the DST to study solar filaments are discussed. We
propose to further study the evolution of solar filaments that erupt
and lead to Coronal Mass Ejections using interspaced observations from
the DKIST and DST spectropolarimeters. While the DST observations will
give information about the global evolution of physical properties
leading to the destabilization, the DKIST observations will provide the
information about the physical conditions in the small-scale structures
that support the filament material.
Title: Classification of Active-Region Stokes Profiles with Possible
Applications for Flare Forecasting
Authors: Romich, Kristine; Cadavid, Ana Cristina; Choudhary, Debi
Prasad
Bibcode: 2019shin.confE.164R
Altcode:
While the association between solar active regions (ARs) and
solar flares is well-established, there is currently no reliable
means of determining when (or if) a given AR will flare. Much of
flare forecasting is based on the application of machine-learning
statistical techniques that use quantities derived from the local
magnetic field as predictors. These properties, in turn, are derived
from dopplergrams obtained at different polarizations or through
spectropolarimetric inversions using Stokes profiles from the observed
radiation. The standard inversion techniques, such as those based on
the Milne-Eddington approximation, give the average magnetic field
values of flare locations. However, this fails to utilize the rich
information contained in the shape of Stokes profiles, such as the
existence of line-of-sight (LOS) magnetic and velocity gradients and
multiple magnetic components along the LOS or within the resolving
element. This results in loss of information and potentially inaccurate
forecasts. Here we outline a novel approach, in which parameters
derived from the raw Stokes profiles are examined as possible
precursors to flare events. Using data from the spectropolarimeter
onboard the Hinode satellite, we examine the amplitude asymmetry, net
circular polarization, and degree of complexity of Stokes profiles
from flaring and non-flaring ARs. Particular attention is given to
regions near the polarity inversion line due to its documented role in
flare initiation. In light of the shortcomings of existing methods,
we hope to expand the set of viable indicators of flaring activity
with the long-term goal of improving flare forecasting models.
Title: Magnetic Structures of a Quiecent Filament
Authors: Wang, Shuo; Jenkins, Jack; Pillet, Valentin; Beck, Christian;
Long, David; Choudhary, Debi; McAteer, James
Bibcode: 2019shin.confE..52W
Altcode:
A quiecent filament is observed from the Dunn Solar Telescope (DST) on
May 29 and 30, 2017. We present HAZEL inversions of spectropolarimetric
observations acquired with the Facility Infrared Spectropolarimeter
at the DST. This study includes three observations of the He I triplet
at 1083.0 nm.
Title: Magnetic Properties and Flow Angle of the Inverse Evershed
Flow at Its Downflow Points
Authors: Beck, C.; Choudhary, D. P.
Bibcode: 2019ApJ...874....6B
Altcode: 2019arXiv190204660B
We determined the direction and strength of the photospheric and lower
chromospheric magnetic field in the umbra and penumbra of a sunspot from
inversions of spectropolarimetric observations of photospheric lines
at 617 nm and 1565 nm and the chromospheric Ca II IR line at 854 nm,
respectively. We compare the magnetic field vector with the direction
of 75 flow channels that harbor the chromospheric inverse Evershed
effect (IEF) near their downflow points (DFPs) in the sunspot’s
penumbra. The azimuth and inclination of the IEF channels to the line
of sight (LOS) were derived from spatial maps of the LOS velocity and
line-core intensity of the Ca II IR line and a thermal inversion of the
Ca II IR spectra to obtain temperature cubes. We find that the flow
direction of the IEF near the DFPs is aligned with the photospheric
magnetic field to within about ±15°. The IEF flow fibrils make
an angle of 30°-90° to the local vertical with an average value
of about 65°. The average field strength at the DFPs is about 1.3
kG. Our findings suggest that the IEF in the lower chromosphere is a
field-aligned siphon flow, where the larger field strength at the inner
footpoints together with the lower temperature in the penumbra causes
the necessary gas pressure difference relative to the outer footpoints
in the hotter quiet Sun with lower magnetic field strength. The IEF
connects to magnetic field lines that are not, like in the case of the
regular Evershed flow, but which continue upward into the chromosphere,
indicating an “uncombed” penumbral structure.
Title: Thermodynamic Properties of the Inverse Evershed Flow at Its
Downflow Points
Authors: Choudhary, D. P.; Beck, C.
Bibcode: 2018ApJ...859..139C
Altcode:
We used spectropolarimetric observations of a sunspot in the active
region NOAA 11809 in the Ca II line at 854.2 nm taken with the
SpectroPolarimeter for Optical and Infrared Regions at the Dunn Solar
Telescope to infer thermodynamic parameters along 100 super-penumbral
fibrils that harbor the inverse Evershed flow. The fibrils were
identified in line-of-sight (LOS) velocity and line-core intensity
maps. The chromospheric LOS velocity abruptly decreases from 3 to 15 km
s-1 to zero at the inner footpoints of the fibrils that are
located from the mid penumbra to about 1.4 spot radii. The spectra often
show multiple absorption components, indicating spatially or vertically
unresolved structures. Synthetic spectra with a 100% fill factor
of a flow channel in the upper atmosphere yield strongly asymmetric
profiles but no multiple separate components. The line-core intensity
always peaks slightly closer to the umbra than the LOS velocity. Using
the CAlcium Inversion using a Spectral ARchive code, we find that the
fibrils make an angle of 30°-60° to the local vertical away from the
umbra. The temperature near the downflow points is enhanced by 200 K
at log τ ∼ -2 and up to 2000 K at log τ ∼ (-6) compared to the
quiet Sun, without any signature in the low photosphere. Our results are
consistent with a critical, i.e., sonic, or supersonic siphon flow along
super-penumbral flux tubes in which accelerating plasma abruptly attains
subcritical velocity through a standing shock in or near the penumbra.
Title: High-resolution Observations of Hα Spectra with a Subtractive
Double Pass
Authors: Beck, C.; Rezaei, R.; Choudhary, D. P.; Gosain, S.;
Tritschler, A.; Louis, R. E.
Bibcode: 2018SoPh..293...36B
Altcode: 2017arXiv171207077B
High-resolution imaging spectroscopy in solar physics has relied on
Fabry-Pérot interferometers (FPIs) in recent years. FPI systems,
however, become technically challenging and expensive for telescopes
larger than the 1 m class. A conventional slit spectrograph with a
diffraction-limited performance over a large field of view (FOV) can
be built at much lower cost and effort. It can be converted into an
imaging spectro(polari)meter using the concept of a subtractive double
pass (SDP). We demonstrate that an SDP system can reach a similar
performance as FPI-based systems with a high spatial and moderate
spectral resolution across a FOV of 100″×100″
with a spectral coverage of 1 nm. We use Hα spectra taken with an SDP
system at the Dunn Solar Telescope and complementary full-disc data to
infer the properties of small-scale superpenumbral filaments. We find
that the majority of all filaments end in patches of opposite-polarity
fields. The internal fine-structure in the line-core intensity of Hα
at spatial scales of about 0.″5 exceeds that in other parameters
such as the line width, indicating small-scale opacity effects in a
larger-scale structure with common properties. We conclude that SDP
systems in combination with (multi-conjugate) adaptive optics are a
valid alternative to FPI systems when high spatial resolution and a
large FOV are required. They can also reach a cadence that is comparable
to that of FPI systems, while providing a much larger spectral range
and a simultaneous multi-line capability.
Title: High Resolution Chromospheric Observations of a Sunspot
Authors: Choudhary, D. P.; Slijepcevic, M.
Bibcode: 2016AGUFMSH42B..03C
Altcode:
We observed a round sunspot in the active region NOAA 12553 using
1.6 meter New Solar Telescope of Big Bear Solar Observatory during
15-21 June 2016. The observations were carried out using narrow band
Fabry-Perot Filter in 656.3 nm Halpha and 705.4 nm TiO lines. We
study the dynamical phenomena in Umbra and Penumbra using the time
lapse movie obtained with rapid time cadence. In this paper, we shall
present preliminary results.
Title: A Comparison of Sunspot and Umbral Area from the San Fernando
Obervatory and SDO
Authors: Chapman, G. A.; Cookson, A.; Choudhary, D. P.
Bibcode: 2016AGUFMSH31B2559C
Altcode:
Sunspot area is an important and basic datum for determining the level
of solar activity. We report on a study of spot total and umbral areas
determined from images obtained by the San Fernando Observatory (CSUN)
and the Solar Dynamics Observatory spacecraft. This research has been
supported by grants from NASA and NSF.
Title: The association of filament eruptions to coronal mass ejections
Authors: Slijepcevic, M.; Choudhary, D. P.
Bibcode: 2016AGUFMSH11C2248S
Altcode:
We analyze the filament eruption events and CMEs using data from
LASCO, SDO and other sources in order to find their association. A
CME event is considered associated to solar filament eruptions if
they occur within a radially projected area that is an appropriate
distance from the surface eruption, with a time difference between
events to take account of the propagation. The strength and frequency
of filament eruptions should play a role in the dynamical properties
of a CME event. To discern this relationship, a catalog of highest
rated filament eruptions by the Heliophysics Event Knowledgebase was
observed in relation to CME events. Additionally, a case study of
the solar storm of July 2012 in relation to filament eruptions that
occurred simultaneously is presented.
Title: Photoelectric instability and debris disk rings: One theory
to rule them all
Authors: Lyra, Wladimir; Cadavid, Ana Cristina; Choudhary, Debi Prasad;
Christian, Damian; Kuchner, Marc Jason; Richert, Alexander John Wolf
Bibcode: 2016hst..prop14572L
Altcode:
HST images of circumstellar debris disks have helped advance
tremendously our understanding of these disks, thought to represent
planetary systems during the late stages of planet formation as the
gas clears and the system becomes optically thin. These systems are
analogs of the Kuiper belt in the solar system, and show a variety
of non-trivial structures attributed to planetary perturbations and
utilized to constrain the properties of the planets. However, analyses
of these systems have largely ignored the fact that, increasingly,
debris disks are found to contain small quantities of gas. We have
recently shown that dust-gas interactions with photoelectric heating
can produce some of the key patterns seen in debris disks that were
previously attributed to planets. We propose to code software and run
a suite of models to develop the theory of photoelectric instability in
gas-rich optically thin disks in light of the observational constrains
set by HST observations and its interaction with other dynamically
important processes such as hydromagnetic turbulence, radiation forces,
planetary perturbers, and stellar flares.
Title: <p> Three Dimensional Chromosphere Thermal Structure
of Sunspot
Authors: Choudhary, D. P.
Bibcode: 2015AGUFMSH13D2458C
Altcode:
We have observed sunspots using the Spectropolarimeter for infrared
and optical wavelength ranges at the Dunn Solar Telescope during
29 July to 4 August 2013. The data consists of full Stokes profiles
in the Ca II 854.2 nm and Fe I 1.56 micron lines. The inversion of
these Stokes spectra provides the magnetic, thermal and velocity
structure at photospheric and chromospheric heights of sunspots. In
this contribution, we present the results on the 3D thermal structure
in the super-penumbral canopy of a well rounded sunspot, derived
by a novel approach for the inversion of Ca II IR spectra. Tracing
individual fibrils in the super-penumbral canopy, we find that about
half of them form only short loops of a a few Mm length that return
to the photosphere in the close surroundings of the sunspot instead
of connecting to more remote magnetic network at the outer end of the
moat flow.
Title: Modeling SSI Variations using Ground-Based Images from the
San Fernando Observatory
Authors: Chapman, G. A.; Choudhary, D. P.
Bibcode: 2015AGUFMSH32A..05C
Altcode:
Full-Disk photometric images are obtained on a daily basis at the San
Fernando Observatory. The images are at wavelengths of 672, 472, and
393 nm. From these images, relative irradiance indices are calculated
and compared with SSI variations at selected wavelengths. We will
present results of modeling spacecraft SSI variations with our indices.
Title: A comparative study of solar facula during cycle 23 and 24
Authors: Chowdhury, P.; Choudhary, D. P.; Moon, Y. J.
Bibcode: 2015AGUFMSH23B2436C
Altcode:
The solar activity minimum between the end of cycle 23 and beginning of
cycle 24 was the longest and deepest since the modern satellite era of
20th century. In this paper, we have investigated statistical properties
of solar facula and sunspot area (and their ratio) covering entire solar
cycle 23 and the ascending phase of cycle 24. The facular area has been
considered from the K-line composite at the San Fernando Observatory
and is a direct measurement of the strength of solar cycle activity. It
is found that solar facular area decreased during minimum phase of
cycle 23/24 compared to maximum phase and also during rising phase
of cycle 24. However, the ratio of facula to sunspot area increased
during minimum epoch of cycle 23. Power spectrum analysis shows that
along with other periods, the solar rotational periods 22 -31 days and
Rieger type periods are both prominent during maxima, minima of cycle 23
and ascending branch of cycle 24. During the decline phase of cycle 23,
the period ~ 27 days is more prominent whereas ~ 14 days and ~ 31 days
periods are dominant during activity maxima. During maximum phase of
cycle 23 and 24, there was no phase lag between sunspot and facular
area, but a phase lag ~ 3 months has been detected during activity
minima of cycle 23. These results indicate that the distribution of
active regions during the activity maximum years is quite different
from that in the minimum years. We shall present discussion of our
results in this paper.
Title: Observations of the Solar Faculae at San Fernando Observatory
and Solar Dynamics Observatory
Authors: Choudhary, D. P.; Cookson, A.
Bibcode: 2015AGUFMSH23B2437C
Altcode:
In this paper we compare the full disk images of the Sun obtained
in 393.4 nm Ca II K line from Cartesian Full Disk Telescopes (CFDT)
of San Fernando Observatory (SFO) and 1600Å and 1700Å images from
Solar Dynamic Telescope (SDO). The facular excess and facular area
are determined for these two types of images using the data reduction
procedure developed at SFO. We find strong correlation between the
derived quantities from SFO and SDO images. Also, the facular excess and
facular area show a very good correlation with the sunspot numbers. The
sunspot numbers derived from the SDO images from our model agrees well
with tabulated values.
Title: Short-term periodicities in interplanetary, geomagnetic and
solar phenomena during solar cycle 24
Authors: Chowdhury, Partha; Choudhary, D. P.; Gosain, S.; Moon, Y. -J.
Bibcode: 2015Ap&SS.356....7C
Altcode:
In this paper we study the quasi-periodic variations of sunspot
area/number, 10.7 cm solar radio flux, Average Photospheric Magnetic
Flux, interplanetary magnetic field ( B z ) and the
geomagnetic activity index A p during the ascending phase
of the current solar cycle 24. We use both Lomb-Scargle periodogram
and wavelet analysis technique and find evidence for a multitude of
quasi-periodic oscillations in all the data sets. In high frequency
range (10 days to 100 days), both methods yield similar significance
periodicities around 20-35 days and 45-60 days in all data sets. In
the case of intermediate range, the significant periods were around
100-130 days, 140-170 days and 180-240 days The Morlet wavelet power
spectrum shows that all of the above-mentioned periods are intermittent
in nature. We find that the well-known "Rieger period" of (150-160
days) and near Rieger periods (130-190 days) were significant in both
solar, interplanetary magnetic field and geomagnetic activity data sets
during cycle 24. The geomagnetic activity is the result of the solar
wind-magnetosphere interaction. Thus the variations in the detected
periodicity in variety of solar, interplanetary and geomagnetic indices
could be helpful to improve our knowledge of the inter-relationship
between various processes in the Sun-Earth-Heliosphere system.
Title: Fast Inversion of Solar Ca II Spectra
Authors: Beck, C.; Choudhary, D. P.; Rezaei, R.; Louis, R. E.
Bibcode: 2015ApJ...798..100B
Altcode: 2014arXiv1410.8451B
We present a fast (Lt1 s per profile) inversion code for solar Ca
II lines. The code uses an archive of spectra that are synthesized
prior to the inversion under the assumption of local thermodynamic
equilibrium (LTE). We show that it can be successfully applied to
spectrograph data or more sparsely sampled spectra from two-dimensional
spectrometers. From a comparison to a non-LTE inversion of the same
set of spectra, we derive a first-order non-LTE correction to the
temperature stratifications derived in the LTE approach. The correction
factor is close to unity up to log τ ~ -3 and increases to values of
2.5 and 4 at log τ = -6 in the quiet Sun and the umbra, respectively.
Title: Three Dimensional Chromospheric Temperature Structure of
Sunspot
Authors: Choudhary, D. P.; Beck, C.; Rezaei, R.
Bibcode: 2014AGUFMSH41B4132C
Altcode:
We have observed sunspots using the Spectropolarimeter for infrared
and optical wavelength ranges at the Dunn Solar Telescope during
29 July to 4 August 2013. The data consists of full Stokes profiles
in the Ca II 854.2 nm and Fe I 1.56 micron lines. The inversion of
these Stokes spectra provides the magnetic, thermal and velocity
structure at photospheric and chromospheric heights of sunspots. In
this contribution, we present the results on the 3D thermal structure
in the super-penumbral canopy of a well rounded sunspot, derived
by a novel approach for the inversion of Ca II IR spectra. Tracing
individual fibrils in the super-penumbral canopy, we find that about
half of them form only short loops of a a few Mm length that return
to the photosphere in the close surroundings of the sunspot instead
of connecting to more remote magnetic network at the outer end of the
moat flow.
Title: Homologous flare-CME events and their metric type II radio
burst association
Authors: Yashiro, S.; Gopalswamy, N.; Mäkelä, P.; Akiyama, S.;
Uddin, W.; Srivastava, A. K.; Joshi, N. C.; Chandra, R.; Manoharan,
P. K.; Mahalakshmi, K.; Dwivedi, V. C.; Jain, R.; Awasthi, A. K.;
Nitta, N. V.; Aschwanden, M. J.; Choudhary, D. P.
Bibcode: 2014AdSpR..54.1941Y
Altcode:
Active region NOAA 11158 produced many flares during its disk
passage. At least two of these flares can be considered as homologous:
the C6.6 flare at 06:51 UT and C9.4 flare at 12:41 UT on February
14, 2011. Both flares occurred at the same location (eastern edge of
the active region) and have a similar decay of the GOES soft X-ray
light curve. The associated coronal mass ejections (CMEs) were slow
(334 and 337 km/s) and of similar apparent widths (43° and 44°), but
they had different radio signatures. The second event was associated
with a metric type II burst while the first one was not. The COR1
coronagraphs on board the STEREO spacecraft clearly show that the
second CME propagated into the preceding CME that occurred 50 min
before. These observations suggest that CME-CME interaction might be
a key process in exciting the type II radio emission by slow CMEs.
Title: A Three-dimensional View of the Thermal Structure in a
Super-penumbral Canopy
Authors: Beck, C.; Choudhary, D. P.; Rezaei, R.
Bibcode: 2014ApJ...788..183B
Altcode: 2014arXiv1405.1473B
We investigate the three-dimensional (3D) thermal topology in a
super-penumbral canopy of an active region (AR). We derive temperature
stratifications in the AR by an inversion of the Ca II IR line at 854.2
nm, assuming local thermal equilibrium. We find that about half of the
radially oriented fibrils in the super-penumbral canopy form short,
low-lying (h < 1 Mm) loops in the 3D temperature cube. These closed
loops connect from bright grains in or close to the penumbra to the
photosphere a few Mms away from the sunspot. The other half of the
fibrils monotonically rise with distance from the sunspot. Many of
the fibrils show a central dark core and two lateral brightenings in
line-core intensity images. The corresponding velocity image shows
fibrils that are as wide as the fibrils seen in intensity without a
lateral substructure. Additionally, we study a feature from a different
class of structures without prominent mass flows. Its 3D topology is
formed by two parallel, closed loops that connect patches of opposite
polarity. We present evidence that the inverse Evershed flow into the
sunspot in the lower chromosphere is the consequence of siphon flows
along short loops that connect photospheric foot points. The dark-cored
structure of the chromospheric fibrils cannot have a convective origin
because of their location above regular granulation. The dark core
most likely results from an opacity difference between the central
axis and the lateral edges caused by the significant flow speed along
the fibrils.
Title: Different Periodicities in the Sunspot Area and the Occurrence
of Solar Flares and Coronal Mass Ejections in Solar Cycle 23 - 24
Authors: Choudhary, D. P.; Lawrence, J. K.; Norris, M.; Cadavid, A. C.
Bibcode: 2014SoPh..289..649C
Altcode:
In order to investigate the relationship between magnetic-flux
emergence, solar flares, and coronal mass ejections (CMEs), we study
the periodicity in the time series of these quantities. It has been
known that solar flares, sunspot area, and photospheric magnetic flux
have a dominant periodicity of about 155 days, which is confined to
a part of the phase of the solar cycle. These periodicities occur at
different phases of the solar cycle during successive phases. We present
a time-series analysis of sunspot area, flare and CME occurrence during
Cycle 23 and the rising phase of Cycle 24 from 1996 to 2011. We find
that the flux emergence, represented by sunspot area, has multiple
periodicities. Flares and CMEs, however, do not occur with the same
period as the flux emergence. Using the results of this study, we
discuss the possible activity sources producing emerging flux.
Title: Multiwavelength diagnostics of the precursor and main phases
of an M1.8 flare on 2011 April 22
Authors: Awasthi, A. K.; Jain, R.; Gadhiya, P. D.; Aschwanden, M. J.;
Uddin, W.; Srivastava, A. K.; Chandra, R.; Gopalswamy, N.; Nitta,
N. V.; Yashiro, S.; Manoharan, P. K.; Choudhary, D. P.; Joshi, N. C.;
Dwivedi, V. C.; Mahalakshmi, K.
Bibcode: 2014MNRAS.437.2249A
Altcode: 2013arXiv1310.6029A; 2013MNRAS.tmp.2720A
We study the temporal, spatial and spectral evolution of the M1.8 flare,
which occurred in the active region 11195 (S17E31) on 2011 April 22,
and explore the underlying physical processes during the precursor
phase and their relation to the main phase. The study of the source
morphology using the composite images in 131 Å wavelength observed by
the Solar Dynamics Observatory/Atmospheric Imaging Assembly and 6-14
keV [from the Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI)] revealed a multiloop system that destabilized systematically
during the precursor and main phases. In contrast, hard X-ray emission
(20-50 keV) was absent during the precursor phase, appearing only from
the onset of the impulsive phase in the form of foot-points of emitting
loops. This study also revealed the heated loop-top prior to the loop
emission, although no accompanying foot-point sources were observed
during the precursor phase. We estimate the flare plasma parameters,
namely temperature (T), emission measure (EM), power-law index (γ)
and photon turn-over energy (ɛto), and found them to be
varying in the ranges 12.4-23.4 MK, 0.0003-0.6 × 1049
cm-3, 5-9 and 14-18 keV, respectively, by forward fitting
RHESSI spectral observations. The energy released in the precursor
phase was thermal and constituted ≈1 per cent of the total energy
released during the flare. The study of morphological evolution of
the filament in conjunction with synthesized T and EM maps was carried
out, which reveals (a) partial filament eruption prior to the onset of
the precursor emission and (b) heated dense plasma over the polarity
inversion line and in the vicinity of the slowly rising filament during
the precursor phase. Based on the implications from multiwavelength
observations, we propose a scheme to unify the energy release during
the precursor and main phase emissions in which the precursor phase
emission was originated via conduction front that resulted due to the
partial filament eruption. Next, the heated leftover S-shaped filament
underwent slow-rise and heating due to magnetic reconnection and finally
erupted to produce emission during the impulsive and gradual phases.
Title: Solar Chromosphere Flare Spectrograph
Authors: Choudhary, Debi Prasad
Bibcode: 2014cosp...40E.543C
Altcode:
This paper describes develop of a two channel echelle spectrograph,
Solar Chromospheric Flare Spectreograph (SCFC), to observe the
optical spectra at the locations of ares and explosive events on
the Sun. The SCFS is designed to record spectra in two channels in
the wavelength range of 350-890 nm, which has several chromospheric
spectral lines. The SCFS will have a multi-fiber based slit capable of
observing at 100 locations of the active region magnetic field polarity
inversion lines. The field of view of SCFS will be 80 x 80 arc sec
with spatial resolution of 8 arc sec. The spectral resolution of 60,000
will be adequate for measuring Doppler velocities of about 5 km/s. The
instrument is designed using off-the-shelves optical and mechanical
parts with minimum fabrication at an in-house machine shop. We propose
to integrate the SCFS with the full-disk Halpha telescope at the Big
Bear Solar Observatory that is operating semi-automatically a round
the year except for weather interruptions. The SCFS observations will
also be mainly used to study the physics of ares, but part of the time
will be devoted to classroom educational activities.
Title: Chromospheric Magnetic Field of Exploding Solar Active Regions
Authors: Choudhary, Debi Prasad
Bibcode: 2014cosp...40E.542C
Altcode:
How changes in the three-dimensional magnetic field of solar active
region are related to Coronal Mass Ejections (CME) is an important
question for contemporary solar physics. Complex active regions are
the predominant source of powerful high-speed CMEs, which can result
in strong geomagnetic storms. In this paper we present the properties
of chromospheric magnetic field of active regions that produced solar
flares and CMEs using observations of the Synoptic Optical Long-term
Investigations of the Sun (SOLIS) facility operated by the National
Solar Observatory. Currently, the SOLIS Vector Spectromagnetograph
(VSM) is the only instrument that is capable of obtaining full Stokes
profiles in both the photospheric Fe I 630.2 nm and chromospheric Ca
II 854.2 nm lines on a daily basis. VSM also has the capability of
making rapid scans covering an area sufficiently large to contain an
active region. We shall present the Stokes profile characteristics of
photospheric and chromospheric lines of few CME source regions.
Title: Three Dimensional Chromospheric Structure of Sunspot
Authors: Choudhary, Debi Prasad; Rezaei, Reza; Beck, Christian
Bibcode: 2014cosp...40E.544C
Altcode:
We have observed sunspots using the Spectropolarimeter for infrared
and optical wavelength ranges at the Dunn Solar Telescope during 29
July to 4 August 2013. The data consists of full Stokes profiles
in the Ca II 854.2 nm and Fe I 1.56 micron lines. The inversion
of these Stokes spectra provides the magnetic, thermal and velocity
structure at photospheric and chromospheric heights of sunspots. In this
contribution, we present the first results on the 3D thermal structure
in the super-penumbral canopy of a well rounded sunspot, derived
by a novel approach for the inversion of Ca II IR spectra. Tracing
individual fibrils in the super-penumbral canopy, we find that about
half of them form only short loops of a a few Mm length that return
to the photosphere in the close surroundings of the sunspot instead
of connecting to more remote magnetic network at the outer end of the
moat flow.
Title: Solar energetic particle events during the rise phases of
solar cycles 23 and 24
Authors: Chandra, R.; Gopalswamy, N.; Mäkelä, P.; Xie, H.; Yashiro,
S.; Akiyama, S.; Uddin, W.; Srivastava, A. K.; Joshi, N. C.; Jain,
R.; Awasthi, A. K.; Manoharan, P. K.; Mahalakshmi, K.; Dwivedi, V. C.;
Choudhary, D. P.; Nitta, N. V.
Bibcode: 2013AdSpR..52.2102C
Altcode:
We present a comparative study of the properties of coronal mass
ejections (CMEs) and flares associated with the solar energetic particle
(SEP) events in the rising phases of solar cycles (SC) 23 (1996-1998)
(22 events) and 24 (2009-2011) (20 events), which are associated
with type II radio bursts. Based on the SEP intensity, we divided the
events into three categories, i.e. weak (intensity < 1 pfu), minor
(1 pfu < intensity < 10 pfu) and major (intensity ⩾ 10 pfu)
events. We used the GOES data for the minor and major SEP events and
SOHO/ERNE data for the weak SEP event. We examine the correlation
of SEP intensity with flare size and CME properties. We find that
most of the major SEP events are associated with halo or partial halo
CMEs originating close to the sun center and western-hemisphere. The
fraction of halo CMEs in SC 24 is larger than the SC 23. For the minor
SEP events one event in SC23 and one event in SC24 have widths <
120° and all other events are associated with halo or partial halo
CMEs as in the case of major SEP events. In case of weak SEP events,
majority (more than 60%) of events are associated with CME width <
120°. For both the SC the average CMEs speeds are similar. For major
SEP events, average CME speeds are higher in comparison to minor and
weak events. The SEP event intensity and GOES X-ray flare size are
poorly correlated. During the rise phase of solar cycle 23 and 24,
we find north-south asymmetry in the SEP event source locations: in
cycle 23 most sources are located in the south, whereas during cycle
24 most sources are located in the north. This result is consistent
with the asymmetry found with sunspot area and intense flares.
Title: Sunspot Bright Points
Authors: Choudhary, Debi Prasad; Shimizu, Toshifumi
Bibcode: 2013SoPh..288..171C
Altcode: 2010arXiv1001.2354P
We used the flux-calibrated images from the Broad-band Filter
Imager and Stokes Polarimeter data obtained with the Solar Optical
Telescope onboard the Hinode spacecraft to study the properties of
bright points in and around sunspots. The selected bright points are
smaller in diameter than 150 km with contrasts exceeding about 3 %
in the ratio of sunspot images obtained with the G-band (430.5 nm)
and Ca II H (396.85 nm) filters. The bright points are classified as
umbral dot, peripheral umbral dot, penumbral grains, and G-band bright
point depending on their location. The bright points are preferentially
located around the penumbral boundary and in the fast decaying parts of
the umbra. The color temperature of the bright points is in the range
of 4600 K to 6600 K with cooler ones located in the central part of the
umbra. The temperature increases as a function of distance from the
center outward. The G-band, CN-band (388.35 nm), and Ca II H fluxes
of the bright points as a function of their blue-band (450.55 nm)
brightness increase continuously in a nonlinear fashion unlike their
red (668.4 nm) and green (555.05 nm) counterparts. This is consistent
with a model in which the localized heating of the flux tube depletes
the molecular concentration, resulting in the reduced opacity that
leads to the exposition of deeper and hotter layers. The light curve
of the bright points shows that the enhanced brightness at these
locations lasts for about 15 to 60 min with the least contrast for
the points outside the sunspot. The umbral dots near the penumbral
boundary are associated with elongated filamentary structures. The
spectropolarimeter observations show that the filling factor decreases
as the G-band brightness increases. We discuss the results using the
model in which the G-band bright points are produced in the cluster
of flux tubes that a sunspot consists of.
Title: Flux emergence, flux imbalance, magnetic free energy and
solar flares
Authors: Choudhary, Debi Prasad; Gosain, Sanjay; Gopalswamy, Nat;
Manoharan, P. K.; Chandra, R.; Uddin, W.; Srivastava, A. K.; Yashiro,
S.; Joshi, N. C.; Kayshap, P.; Dwivedi, V. C.; Mahalakshmi, K.;
Elamathi, E.; Norris, Max; Awasthi, A. K.; Jain, R.
Bibcode: 2013AdSpR..52.1561C
Altcode:
Emergence of complex magnetic flux in the solar active regions lead
to several observational effects such as a change in sunspot area
and flux embalance in photospheric magnetograms. The flux emergence
also results in twisted magnetic field lines that add to free energy
content. The magnetic field configuration of these active regions
relax to near potential-field configuration after energy release
through solar flares and coronal mass ejections. In this paper,
we study the relation of flare productivity of active regions with
their evolution of magnetic flux emergence, flux imbalance and free
energy content. We use the sunspot area and number for flux emergence
study as they contain most of the concentrated magnetic flux in the
active region. The magnetic flux imbalance and the free energy are
estimated using the HMI/SDO magnetograms and Virial theorem method. We
find that the active regions that undergo large changes in sunspot
area are most flare productive. The active regions become flary when
the free energy content exceeds 50% of the total energy. Although,
the flary active regions show magnetic flux imbalance, it is hard to
predict flare activity based on this parameter alone.
Title: A multiwavelength study of eruptive events on January 23,
2012 associated with a major solar energetic particle event
Authors: Joshi, N. C.; Uddin, W.; Srivastava, A. K.; Chandra, R.;
Gopalswamy, N.; Manoharan, P. K.; Aschwanden, M. J.; Choudhary, D. P.;
Jain, R.; Nitta, N. V.; Xie, H.; Yashiro, S.; Akiyama, S.; Mäkelä,
P.; Kayshap, P.; Awasthi, A. K.; Dwivedi, V. C.; Mahalakshmi, K.
Bibcode: 2013AdSpR..52....1J
Altcode: 2013arXiv1303.1251J
We use multiwavelength data from space and ground based instruments
to study the solar flares and coronal mass ejections (CMEs) on January
23, 2012 that were responsible for one of the largest solar energetic
particle (SEP) events of solar cycle 24. The eruptions consisting of two
fast CMEs (≈1400 km s-1 and ≈2000 km s-1) and
M-class flares that occurred in active region 11402 located at ≈N28
W36. The two CMEs occurred in quick successions, so they interacted
very close to the Sun. The second CME caught up with the first one
at a distance of ≈11-12 Rsun. The CME interaction may be
responsible for the elevated SEP flux and significant changes in the
intensity profile of the SEP event. The compound CME resulted in a
double-dip moderate geomagnetic storm (Dst∼-73nT). The two dips are
due to the southward component of the interplanetary magnetic field in
the shock sheath and the ICME intervals. One possible reason for the
lack of a stronger geomagnetic storm may be that the ICME delivered
a glancing blow to Earth.
Title: Chromospheric Magnetic Field of Exploding Solar Active Regions
Authors: Choudhary, Debi P.
Bibcode: 2013SPD....44...07C
Altcode:
How changes in the three-dimensional magnetic field of solar active
region are related to Coronal Mass Ejections (CME) is an important
question for contemporary solar physics. Complex active regions are
the predominant source of powerful high-speed CMEs, which can result
in strong geomagnetic storms. In this paper we present the properties
of chromospheric magnetic field of active regions that produced solar
flares and CMEs using observations of the Synoptic Optical Long-term
Investigations of the Sun (SOLIS) facility operated by the National
Solar Observatory. Currently, the SOLIS Vector Spectromagnetograph
(VSM) is the only instrument that is capable of obtaining full Stokes
profiles in both the photospheric Fe I λ630.2 nm and chromospheric
Ca II λ854.2 nm lines on a daily basis. VSM also has the capability
of making rapid scans covering an area sufficiently large to contain
an active region. We shall present the Stokes profile characteristics
of photospheric and chromospheric lines of few CME source regions.
Title: He I D3 Observation of the 1984 May 22 M6.3 Solar Flare
Authors: Liu, Chang; Xu, Y.; Deng, N.; Lee, J.; Zhang, J.; Choudhary,
D. P.; Wang, H.
Bibcode: 2013SPD....44...54L
Altcode:
He I D3 line has a unique response to the flare impact on the low solar
atmosphere and can be a powerful diagnostic tool for energy transport
processes. Using high-resolution and high-cadence images obtained
from the recently digitized films of Big Bear Solar Observatory,
we report D3 observation of the M6.3 flare on 1984 May 22, which
occurred in an active region with a circular magnetic polarity
inversion line (PIL). The impulsive phase of the flare starts with
a main elongated source that darkens in D3, inside of which bright
emission kernels appear at the time of the initial small peak in
hard X-rays (HXRs). These flare cores subsequently evolve into a
sharp emission strand lying within the dark halo simultaneously
with the main peak in HXRs, reversing the overall source contrast
from -5% to 5%. The radiated energy in D3 during the main peak is
estimated to be about 10^30 ergs, which is comparable to that carried
by nonthermal electrons above 20 keV. Afterwards the flare proceeds
along the circular PIL in the counterclockwise direction to form a dark
circular ribbon in D3, which apparently mirrors the bright ribbons in
Halpha and He I 10830 A. All these ribbons last for over one hour in
the late gradual phase. We suggest that the present event resembles
the so-called black-light flare that is proposed based on continuum
images, and that D3 darkening and brightening features herein may be
due to, respectively, the thermal conduction heating and the direct
precipitation of high-energy electrons.
Title: Chromospheric Magnetic Field of Exploding Solar Active Regions
Authors: Choudhary, Debi Prasad; Deng, Na
Bibcode: 2013shin.confE..21C
Altcode:
How changes in the three-dimensional magnetic field of solar active
region are related to Coronal Mass Ejections (CME) is an important
question for contemporary solar physics. Complex active regions are
the predominant source of powerful high-speed CMEs, which can result
in strong geomagnetic storms. In this paper we present the properties
of chromospheric magnetic field of active regions that produced solar
flares and CMEs using observations of the Synoptic Optical Long-term
Investigations of the Sun (SOLIS) facility operated by the National
Solar Observatory. Currently, the SOLIS Vector Spectromagnetograph
(VSM) is the only instrument that is capable of obtaining full Stokes
profiles in both the photospheric Fe I 630.2 nm and chromospheric Ca
II 854.2 nm lines on a daily basis. VSM also has the capability of
making rapid scans covering an area sufficiently large to contain an
active region. We shall present the Stokes profile characteristics of
photospheric and chromospheric lines of few CME source regions.
Title: Height of shock formation in the solar corona inferred from
observations of type II radio bursts and coronal mass ejections
Authors: Gopalswamy, N.; Xie, H.; Mäkelä, P.; Yashiro, S.; Akiyama,
S.; Uddin, W.; Srivastava, A. K.; Joshi, N. C.; Chandra, R.; Manoharan,
P. K.; Mahalakshmi, K.; Dwivedi, V. C.; Jain, R.; Awasthi, A. K.;
Nitta, N. V.; Aschwanden, M. J.; Choudhary, D. P.
Bibcode: 2013AdSpR..51.1981G
Altcode: 2013arXiv1301.0893G
Employing coronagraphic and EUV observations close to the solar surface
made by the Solar Terrestrial Relations Observatory (STEREO) mission,
we determined the heliocentric distance of coronal mass ejections
(CMEs) at the starting time of associated metric type II bursts. We
used the wave diameter and leading edge methods and measured the CME
heights for a set of 32 metric type II bursts from solar cycle 24. We
minimized the projection effects by making the measurements from a
view that is roughly orthogonal to the direction of the ejection. We
also chose image frames close to the onset times of the type II bursts,
so no extrapolation was necessary. We found that the CMEs were located
in the heliocentric distance range from 1.20 to 1.93 solar radii (Rs),
with mean and median values of 1.43 and 1.38 Rs, respectively. We
conclusively find that the shock formation can occur at heights
substantially below 1.5 Rs. In a few cases, the CME height at type
II onset was close to 2 Rs. In these cases, the starting frequency
of the type II bursts was very low, in the range 25-40 MHz, which
confirms that the shock can also form at larger heights. The starting
frequencies of metric type II bursts have a weak correlation with the
measured CME/shock heights and are consistent with the rapid decline
of density with height in the inner corona.
Title: A Study of the Hemispheric Asymmetry of Sunspot Area during
Solar Cycles 23 and 24
Authors: Chowdhury, Partha; Choudhary, D. P.; Gosain, Sanjay
Bibcode: 2013ApJ...768..188C
Altcode:
Solar activity indices vary over the Sun's disk, and various activity
parameters are not considered to be symmetric between the northern
and southern hemispheres of the Sun. The north-south asymmetry of
different solar indices provides an important clue to understanding
subphotospheric dynamics and solar dynamo action, especially with
regard to nonlinear dynamo models. In the present work, we study
the statistical significance of the north-south asymmetry of sunspot
areas for the complete solar cycle 23 (1996-2008) and rising branch of
cycle 24 (first 45 months). The preferred hemisphere in each year of
cycles 23 and 24 has been identified by calculating the probability of
hemispheric distribution of sunspot areas. The statistically significant
intermediate-term periodicities of the north-south asymmetry of
sunspot area data have also been investigated using Lomb-Scargle and
wavelet techniques. A number of short- and mid-term periods including
the best-known Rieger one (150-160 days) are detected in cycle 23 and
near Rieger-type periods during cycle 24, and most of them are found
to be time variable. We present our results and discuss their possible
explanations with the help of theoretical models and observations.
Title: Chromospheric Properties of Sun as a Star
Authors: Contreras, Luis D.; Choudhary, D. P.
Bibcode: 2013AAS...22115904C
Altcode:
The chromosphere of the sun and stars are primarily governed by
the magnetic field, which can be observed in the photosphere. The
synoptic magnetograms and integrated sunlight spectra, observed with
Synoptic Optical Long-term Investigations of the Sun (SOLIS), are
used to study the relationship of magnetic field and chromospheric
line properties. We use the synoptic full disk magnetograms observed
using of photospheric and chromospheric lines to characterize the solar
magnetic field. Comparing several chromospheric spectral lines ratios
of Integrated Sunlight Spectra, we study the effect of photospheric
magnetic field on chromosphere.
Title: Photospheric and Chromospheric Measurements of a High-Speed
Flow near the Light Bridge of a δ-Spot
Authors: Choudhary, D. P.; Deng, N.
Bibcode: 2012ASPC..463...43C
Altcode:
We present the results of spectropolarimetry observations of active
region NOAA 9664 in photospheric Fe I 6302 Å and chromospheric Mg I
b2 5872 Å spectral lines simultaneously. The active region
was situated near the disk center at the time of our observations. Among
several interesting features, we observe six compact, down-flow regions,
each about two arc seconds in size near the magnetic neutral line. The
Stokes V profiles at these locations are normal in the chromosphere
but anomalous in the photosphere. The corresponding Stokes I profiles
are symmetric in the chromosphere and highly asymmetric in the
photosphere. We discuss possible origin of these features as a result
of bending of penumbral Evershed flow channels as neutral line light
bridges are formed in δ-spots.
Title: Periodicities in the occurrence of Solar Flare, Coronal Mass
Ejection and Sunspot Area in Solar Cycle 23-24
Authors: Choudhary, D. P.
Bibcode: 2012AGUFMSH43B2174C
Altcode:
In order to investigate the relationship between magnetic flux
emergence, solar flares and Coronal Mass Ejection (CME), we study the
periodicity in the time series of these quantities. It has been known
that solar flares, sunspot area and photospheric magnetic flux have
a dominant periodicity of about 155 days. However, the time-period
relationship obtained using a Morlet wavelet shows that the periodicity
is confined to a part of solar cycle and is non repeating. We present
the time series analysis of sunspot area, flare and CME occurrence
during cycle 23 and the rising phase of cycle 24 from 1996 to 2011. We
find that the flux emergence has multiple periodicities. Flares
and CMEs, however, do not occur with the same period as the flux
emergence. Using the results of this study, we discuss the possible
activity sources producing emerging flux.
Title: Diagnosis of Flow and Magnetic Fields Using Simultaneous
Spectro-Polarimetry of Photospheric Fe I and Chromospheric Mg I lines
Authors: Deng, N.; Choudhary, D. P.; Balasubramaniam, K. S.
Bibcode: 2012ASPC..454..229D
Altcode:
We present a study of active region (AR) NOAA 9661 using simultaneous
spectro-polarimetric observations of photospheric Fe I (630.25 and
630.15 nm) and chromospheric Mg I b2 (517.27 nm) lines
obtained with the HAO/NSO Advanced Stokes Polarimeter (ASP). SIR
(Stokes Inversion based on Response function) code was applied to
the Stokes spectra of Fe I line pair and Mg I line, thus providing
magnetic field vectors at the photosphere and low chromosphere. We
quantitatively compared the magnetic field at the two heights and
obtained reasonable results. Doppler velocities were extracted from
both Stokes I and V profiles of the three spectral lines, which reveal
strong red shifts in the penumbra near the magnetic neutral line.
Title: Properties of 16 Sunspots Observed with Hinode Solar Optical
Telescope
Authors: Choudhary, D. P.; Toshifumi, S.; MacDonald, G. A.
Bibcode: 2012ASPC..454..197C
Altcode:
We studied 16 sunspots with different sizes and shapes using the
observations with the Hinode Solar Optical Telescope. The ratio of
G-band and Ca II H images reveal rich structures both within the umbra
and penumbra of most spots. The striking features are the compact
blob at the foot point of the umbra side of the penumbral fibrils with
disk center-limb side asymmetry. In this paper, we present properties
of these features using the spectropolarimetry and images in G-band,
Ca II and blue filters. We discuss the results using the contemporary
models of the sunspots.
Title: Stokes Profiles at the Narrow Magnetic Lanes of Sunspots
Authors: MacDonald, G. A.; Yassin, K. A.; Choudhary, D. P.
Bibcode: 2012ASPC..454..201M
Altcode:
We examine the narrow lanes of transverse and longitudinal magnetic
field with opposite polarity of active region NOAA 10930 using the
stokes profiles observed using the Spectropolarometer on board the
Hinode mission. We compare the observations of the active region
just before, during and after an X-class flare, which occurred during
December 13, 2006 from 02:20 to 06:18 UT. A static fibril interacting
with a rotating penumbra of opposite polarity in the hours prior to
the flare is conspicuous in the temporal evolution of active region
magnetic structure. Above the fibril were several small sites of hot
gas in the chromosphere. During and after the flare, the fibril and
its corresponding flow and profiles were much less pronounced. Our
results suggest that the rotation of the spot lead to the building
of energy in the active region magnetic structure resulting eventual
breakdown and explosion.
Title: Photospheric flows in the active regions (asymmetric and
localized Doppler velocities)
Authors: Choudhary, Debi Prasad
Bibcode: 2012AdSpR..49..416C
Altcode:
We observed 10 active regions through their disk passage during June
25-August 25, 1988, with the Tower Vector Magnetograph (TVM) of Marshall
Space Flight Center. The TVM was used in scanning mode to measure
the photospheric Doppler velocities with the Line-Center-Magnetogram
(LCM) technique in the spectral line of FeI 5250.2 Å. In this paper
we present the result of a subset of observations obtained while
the active regions were situated away from the solar limb. A wide
range of magnetic complexity and associated chromospheric activity
characterized these active regions. It was found that the value
of zero-crossing wavelength of the integrated Stokes-V profile of
two opposite magnetic polarities were different, corresponding to
Doppler velocities ranging from ∼100 m s-1 to ∼1475
m s-1. The measurements of relative velocities between
different locations, connected by magnetic flux tubes as inferred from
YOHKOH soft X-ray and TRACE 171 Å Fe IX images, showed widely different
values of dominant localized flows. The region of parasite polarity,
which showed recurrent chromospheric activity, was blue shifted with
respect to the main "magnetic element" of the same polarity. Some of
them were also the sites of sheared magnetic field configuration. The
magnitude of the relative velocity between the leading and following
polarity is more for the active regions of higher "field asymmetry".
Title: Fast Up-flows Observed on Granules with Sunrise
Authors: Macdonald, G. A.; Hirzberger, J.; Solanki, S.; Choudhary,
D. P.
Bibcode: 2011AGUFMSH13B1985M
Altcode:
We present results from a rigorous study of fast granular up flows
observed with the Imaging Magnetograph eXperiment (IMaX) aboard
the SUNRISE balloon-borne observatory. A time series ∼ 23 minutes
long made June 9, 2009 with a cadence of ∼ 33sec and resolution
of 0.15--0.18'' was used. Our study concentrates on up
flows with a LOS speeds in excess of 2km/s. These flows occur most
frequently on the bright areas of the smallest granules, and less
so on the bright edges of larger granules. The maximum up flow speed
tends to scale with its area which scales with the size of the host
granule. The longest-lived up flows are located on larger granules,
while shorter-lived ones tend to be located on smaller granules. Results
from simulations carried out in other studies suggest that the fastest
granular up flows occur chiefly during two scenarios: 1) on the edges
of granules when their centers cool, before the granules subsequently
split and 2) when a granule is forced under the surface by powerful
down flows from adjacent granules. Our observations provide evidence
for these results.
Title: Chromospheric Magnetic Field of Solar Active Regions
Authors: Choudhary, D. P.; Sakurai, T.
Bibcode: 2011AGUFMSH31A1987C
Altcode:
The three dimensional magnetic field of solar active regions is studied
by comparing the observed and computed chromospheric magnetograms
obtained at National Solar Observatory with SOLIS and NASA/NSO
Spectromagnetograph. The model chromospheric field is obtained by
extrapolating the observed photospheric field in to chromosphere
with a potential (current-free) magnetic field model in Cartesian
geometry. The long lived active regions display the strong-field
non-potentiality during their initial phase and converse to a potential
field configuration later.
Title: Research and Education in Solar Physics at CSUN
Authors: Choudhary, D. P.
Bibcode: 2011AGUFMED43B0544C
Altcode:
The CAREER award to PI Choudhary at the Department of Physics and
Astronomy of California State University Northridge (CSUN) has not
only helped to establish a robust research group in solar magnetism, it
helped several students to choose research career. The CSUN is hispanic
serving campus where most students work while studying. It is specially
challenging for the students belonging to minority community. Here we
shall present the achievements of this project at our campus.
Title: Structure of sunspots observed with Hinode Solar Optical
Telescope
Authors: Choudhary, Debi Prasad; MacDonald, Gordon A.; Deng, Na;
Toshifumi, Shimizu
Bibcode: 2011IAUS..273..478C
Altcode:
We studied 16 sunspots with different sizes and shapes using the
observations with the Hinode Solar Optical Telescope. The ratio of
G-band and CaII H images reveal rich structures both within the umbra
and penumbra of most spots. The striking features are the compact
blob at the foot point of the umbra side of the penumbral fibrils with
disk center-limb side asymmetry. In this paper, we present properties
of these features using the spectropolarimetry and images in G-band,
CaII and blue filters. We discuss the results using the contemporary
models of the sunspots.
Title: The solar active region magnetic field and energetics
Authors: Hu, Qiang; Deng, Na; Choudhary, Debi P.; Dasgupta, B.;
Su, Jiangtao
Bibcode: 2011IAUS..273..369H
Altcode:
Motivated by increasingly more advanced solar observations, we recently
develop a method of coronal magnetic field extrapolation, especially
for an active region (sunspot region). Based on a more complex
variational principle, the principle of minimum (energy) dissipation
rate (MDR), we adopt and solve a more complex equation governing the
coronal magnetic field that is non-force-free in general. We employ
the vector magnetograms from multiple instruments, including Hinode,
NSO, and HSOS, and particularly observations at both photospheric and
chromospheric levels for one active region. We discuss our results
in the context of quantitative characterization of active region
magnetic energy and magnetic topology. These quantitative analyses
aid in better understanding and developing prediction capability of
the solar activity that is largely driven by the solar magnetic field.
Title: What determines the penumbral size and Evershed flow speed?
Authors: Deng, Na; Shimizu, Toshifumi; Choudhary, Debi Prasad;
Wang, Haimin
Bibcode: 2011IAUS..273..216D
Altcode: 2011arXiv1102.3164D
Using Hinode SP and G-band observations, we examined the relationship
between magnetic field structure and penumbral length as well as
Evershed flow speed. The latter two are positively correlated with
magnetic inclination angle or horizontal field strength within 1.5
kilogauss, which is in agreement with recent magnetoconvective
simulations of Evershed effect. This work thus provides direct
observational evidence supporting the magnetoconvection nature of
penumbral structure and Evershed flow in the presence of strong and
inclined magnetic field.
Title: Rapid Enhancement of Sheared Evershed Flow Along the Neutral
Line Associated with an X6.5 Flare Observed by Hinode
Authors: Deng, Na; Liu, C.; Choudhary, D.; Wang, H.
Bibcode: 2011SPD....42.2232D
Altcode: 2011BAAS..43S.2232D
We present G-band and Ca II H observations of NOAA AR 10930 obtained by
Hinode/SOT on 2006 December 6 covering an X6.5 flare. Local Correlation
Tracking (LCT) technique was applied to the foreshortening-corrected
G-band image series to acquire horizontal proper motions in this complex
beta-gamma-delta active region. With the continuous high quality,
spatial and temporal resolution G-band data, we not only confirm the
rapid decay of outer penumbrae and darkening of the central structure
near the flaring neutral line, but also unambiguously detect for
the first time the enhancement of the sheared Evershed flow (average
horizontal flow speed increased from 330+-3.1 to 403+-4.6 m/s) along
the neutral line right after the eruptive white-light flare. Post-flare
Ca II H images indicate that the originally fanning out field lines
at the two sides of the neutral line get connected. Since penumbral
structure and Evershed flow are closely related to photospheric
magnetic inclination or horizontal field strength, we interpret the
rapid changes of sunspot structure and surface flow as the result of
flare-induced magnetic restructuring down to the photosphere. The
magnetic fields turn from fanning out to inward connection causing
outer penumbrae decay, meanwhile those near the flaring neutral line
become more horizontal leading to stronger Evershed flow there. The
inferred enhancement of horizontal magnetic field near the neutral
line is consistent with recent magnetic observations and theoretical
predictions of flare-invoked photospheric magnetic field change.
Title: The Relationship Among Magnetic Field Configuration, Penumbral
Size, and Evershed Flow Speed
Authors: Deng, Na; Shimizu, T.; Choudhary, D.
Bibcode: 2010AAS...21640308D
Altcode: 2010BAAS...41..879D
Recent observations (e.g., Wang et al. 2004; Deng et al. 2005) have
shown that there is a sudden penumbral decay associated with major
flares during which the overall magnetic field inclination in penumbra
changes due to magnetic reconnection. We propose that the size (i.e.,
length) of sunspot penumbra is related to the localized magnetic field
configuration. In order to test this hypothesis, we study 11 sunspots
close to disk center with different sizes, which were observed by
Hinode/SOT at the late phase of solar cycle 23. We notice that even
for typical alpha sunspots near the disc center, the penumbral length
is different in different sectors in the same spot. Since the Evershed
flow is coupled with penumbra, we also study the properties of Evershed
flow, such as its speed, at different location of a sunspot. The SP
data is used to study the magnetic field parameters (e.g., strength,
inclination) in different penumbral sectors that show distinct penumbral
length. The Evershed flow is measured both by Doppler shift from SP
observation and by Local Correlation Tracking based on time series of
BFI data. We aim to find the relationship among magnetic field topology,
penumbral size, and Evershed flow speed.
Title: Net Circular Polarization Maps of Stokes-Profiles with Hinode
Observations
Authors: Choudhary, D. P.; Yassin, Kemal A.
Bibcode: 2009SPD....40.0914C
Altcode:
We use the Hinode spectropolarimetry observation of solar active
regions in FeI 630.2 nm wavelength to compute net circular polarization
(NCP). The Hinode observations do not have blending that are often
encountered in ground based observations. In this paper, we present our
preliminary results and compare them with our previous results using
ground based data. Acknowledgements: This work is supported
by NSF under grant ATM 05-48952 and by NASA under grant NNX08AQ32G.
Title: The Three-Dimensional Structure of Magnetic Fields and Electric
Currents in an Active Region
Authors: Deng, Na; Choudhary, D.; Lagg, A.; Jing, J.; Wang, H.
Bibcode: 2009SPD....40.0904D
Altcode:
Active Region NOAA 9661 consisting of various magnetic features (alpha
spot, delta spot, pores, and plages) was observed simultaneously
in three spectral lines (FeI 630.25, 630.15, and MgI 517.27) using
full Stokes spectro-polarimetry with Advanced Stokes Polarimeter at
NSO/SP. The formation heights of the three spectral lines span from
photosphere to the low chromosphere. The magnetic field vectors are
inverted from Stokes parameters using advanced inversion codes. In
particular, HeLIx code is used to invert MgI's observation because
it is dedicated for chromospheric Stokes inversion. The inverted
magnetic field vectors in multiple atmospheric layers enable us to
derive the three dimensional structure of the magnetic fields, electric
current densities, current helicities, and their height gradients. Acknowledgments: This work is supported by NASA under grants
NNX08AQ32G and NNX08AQ89G, and by NSF under grants ATM 05-48952 and
ATM 07-16950.
Title: Height of Quiet Solar Chromosphere at the Limb
Authors: Tejomoortula, U.; Choudhary, D.
Bibcode: 2008AGUFMSH41A1616T
Altcode:
We present the result of observation of solar limb during the solar
quiet phase in three different spectral lines: (i) He I 1083.0 nm,
(ii) Hydrogen Paschen alpha 1281.8nm, and (iii) hydrogen Bracket Gamma
2166.1 nm. These spectral lines were observed using McMath-Pierce
Telescope and near Infrared Camera. We focus our investigation to
study the variation of the chromospheric height in the spectral lines
at different positions on the limb.
Title: Dynamics of Quiet Solar Chromosphere at the Limb
Authors: Choudhary, D.; Tejomoortula, U.; Penn, M. J.
Bibcode: 2008AGUFMSH23A1622C
Altcode:
We have observed the solar limb using 1024 × 1024 InSb Near Infrared
Camera and Spectroheliograph at McMath-Pierce telescope during the
solar minimum period of April 29 to May 1, 2008. A 120 micron slit,
corresponding to 0.3 arc second was aligned perpendicular to the solar
limb for the observations. The slit spectrum with a resolution of
0.036 Å corresponding to about 05 to 1.0 km/s were obtained in the
wavelength regions of HeI 10830 Å, Hydogen Paschen α 12818 Å and
Hydrogen Brackett γ 21661 Å. Excellent seeing conditions and the use
of adaptive optics produced stable observing conditions during most
of the observations. We present the results of line width variation
as a function of chromospheric height around the solar limb.
Title: Intensity and Magnetic Field Distribution of Sunspots
Authors: Leonard, Trevor; Choudhary, Debi Prasad
Bibcode: 2008SoPh..252...33L
Altcode: 2008SoPh..tmp..159L
We study the relationship between the brightness (I) and magnetic
field (B) distributions of sunspots using 272 samples observed at
the San Fernando Observatory and the National Solar Observatory,
Kitt Peak, whose characteristics varied widely. We find that the I -
B relationship has a quadratic form for the spots with magnetic field
less than about 2000 G. The slope of the linear part of the I - B curve
varies by about a factor of three for different types of spots. In
general the slope increases as the spot approaches disk center. The I
- B slope does not have a clear dependency on the spot size but the
lower limit appears to increase as a function of the ratio of umbra
and penumbra area. The I - B slope changes as a function of age of
the sunspots. We discuss various sunspot models using these results.
Title: Sunspot Group Decay
Authors: Hathaway, David H.; Choudhary, Debi Prasad
Bibcode: 2008SoPh..250..269H
Altcode: 2008SoPh..tmp..126H
We examine daily records of sunspot group areas (measured in millionths
of a solar hemisphere or μHem) for the last 130 years to determine
the rate of decay of sunspot group areas. We exclude observations
of groups when they are more than 60° in longitude from the central
meridian and only include data when at least three days of observations
are available following the date of maximum area for a group's disk
passage. This leaves data for over 18 000 measurements of sunspot group
decay. We find that the decay rate increases linearly from 28 μHem
day−1 to about 140 μHem day−1 for groups with
areas increasing from 35 μHem to 1000 μHem. The decay rate tends to
level off for groups with areas larger than 1000 μHem. This behavior is
very similar to the increase in the number of sunspots per group as the
area of the group increases. Calculating the decay rate per individual
sunspot gives a decay rate of about 3.65 μHem day−1
with little dependence upon the area of the group. This suggests
that sunspots decay by a Fickian diffusion process with a diffusion
coefficient of about 10 km2 s−1. Although the
18 000 decay rate measurements are lognormally distributed, this can
be attributed to the lognormal distribution of sunspot group areas
and the linear relationship between area and decay rate for the vast
majority of groups. We find weak evidence for variations in decay
rates from one solar cycle to another and for different phases of each
sunspot cycle. However, the strongest evidence for variations is with
latitude and the variations with cycle and phase of each cycle can be
attributed to this variation. High latitude spots tend to decay faster
than low latitude spots.
Title: Multiheight Analysis of Asymmetric Stokes Profiles in a Solar
Active Region
Authors: Deng, N.; Choudhary, D.; Solanki, S. K.; Lagg, A.
Bibcode: 2008AGUSMSP51D..06D
Altcode:
Parameters characterizing Stokes asymmetries are derived from full
Stokes I,Q,U,V spectra of FeI λλ 630.15, 630.25~nm line pair
(formed at two different heights in the photosphere) and MgI b
517.27~nm line (formed at lower chromosphere) in a solar active
region near disc center. The spectropolarimetric observations were
taken with the National Solar Observatory/High Altitude Observatory
Advanced Stokes Polarimeter. The observed active region consists
of a α sunspot, a δ sunspot, several pores and granulation. The
line center shifts and bi-sectors derived from Stokes-I profiles
describe the line-of-sight Doppler velocity and Stokes-I asymmetry,
respectively. Stokes-V amplitude and area asymmetries are defined by
the normalized difference of respective quantities between blue and
red lobes of circular polarization profiles. The same can be derived
from linear polarization profiles ([Q2+U2]1/2). The Stokes asymmetries
are compared for different regions and at multiple heights. Neutral
line regions of the δ spot and outer penumbral regions show distinct
large Stokes asymmetries. Both Stokes-V amplitude and area asymmetries
become larger from lower to higher atmosphere in neutral line regions
that have strong transverse field and mixed polarities. The Stokes-V
area asymmetry of outer edge of penumbrae changes from positive in
the photosphere to negative in lower chromosphere. Detailed results
and interpretation will be presented.
Title: Dual Height Spectropolarimetry Observations of Active Region
NOAA 9662
Authors: Choudhary, D.; Deng, N.; Balasubramaniam, K.
Bibcode: 2008AGUSMSP31B..06C
Altcode:
We present the results of the spectropolarimetry observations of
active region NOAA 9664 observed in spectral lines FeI 6302 and
Mgb2 5872 A. These spectral lines originate at photospheric and
lower chromospheric heights of solar atmosphere, respectively. The
active region was situated near the disk center at the time of our
observations. Among several interesting features, we observe three
compact, down-flow regions (each about two seconds in size) which
are near the magnetic neutral line. The Stokes V profiles at these
locations are normal in chromosphere but anomalous in photosphere. The
corresponding Stokes I profiles are symmetric in chromosphere and
highly asymmetric in the photosphere. We present the results of
bisector analysis of these profiles and discuss their significance in
the evolution of sunspot structure.
Title: A Practical Approach to Coronal Magnetic Field Extrapolation
Based on the Principle of Minimum Dissipation Rate
Authors: Hu, Qiang; Dasgupta, B.; Choudhary, D. P.; Büchner, J.
Bibcode: 2008ApJ...679..848H
Altcode: 2007arXiv0711.1376H
We present a newly developed approach to solar coronal magnetic field
extrapolation from vector magnetograms, based on the principle of
minimum dissipation rate (MDR). The MDR system was derived from a
variational problem that is more suitable for an open and externally
driven system, like the solar corona. The resulting magnetic field
equation is more general than force-free. Its solution can be expressed
as the superposition of two linear (constant-α) force-free fields
(LFFFs) with distinct α parameters, and one potential field. Thus,
the original extrapolation problem is decomposed into three LFFF
extrapolations, utilizing boundary data. The full MDR-based approach
requires two layers of vector magnetograph measurements on the solar
surface, while a slightly modified practical approach only requires
one. We test both approaches against three-dimensional MHD simulation
data in a finite volume. Both yield quantitatively good results. The
errors in the magnetic energy estimate are within a few percent. In
particular, the main features of relatively strong perpendicular
current density structures, representative of the non-force-freeness
of the solution, are well recovered.
Title: On the Nature of the High-Speed Plasma Flows in the 2005
September 13 Flare
Authors: Liu, C.; Choudhary, D. P.; Deng, N.; Wang, H.
Bibcode: 2008AGUSMSP51C..19L
Altcode:
A long-duration, successive flaring event accompanied by fast CMEs
occurred on 2005 September 13 in the NOAA AR 10808 and was classified
as 2B/X1.5 with peak time at 19:27 UT. In this study, we report direct
and unambiguous observations of high-speed hot plasma flows associated
with the second major peak in soft X-ray that reached X1.4 on the GOES
scale at 20:05 UT. The flows are seen as streams of enhanced density
in extreme-UV traveling above and toward arcades of the secondary
compact-loop flare at the main δ spot with an apparent speed as high
as ~350~km~s-1, and the times when they are initiated correspond to
those of bursts of nonthermal emissions in hard X-rays (HXRs) and
microwaves. In Hα, the flows appear to become emission later on when
approaching the lower atmosphere nearby the flaring magnetic polarity
inversion line and subsequently trigger a subflare with propagating
kernels. It is particularly notable that the flows are spatially
and temporally related to HXR sources detected by RHESSI and a large
erupting flux rope. We scrutinize several scenarios to investigate
the nature of the observed high-speed flows. We conclude that the
observations could be interpreted in terms of materials braking away
from a preceding filament eruption and falling gravitationally back
into the flaring region. A separate scenario is that the observed
flow motion could be a manifestation of sunward reconnection outflow
supporting the standard reconnection model.
Title: Flow Field Evolution of a Decaying Sunspot
Authors: Deng, Na; Choudhary, Debi Prasad; Tritschler, Alexandra;
Denker, Carsten; Liu, Chang; Wang, Haimin
Bibcode: 2007ApJ...671.1013D
Altcode: 2007arXiv0709.3340D
We study the evolution of the flows and horizontal proper motions
in and around a decaying follower sunspot based on time sequences
of two-dimensional spectroscopic observations in the visible and
white-light imaging data obtained over 6 days from 2005 June 7 to
12. During this time period the sunspot decayed gradually to a pore. The
spectroscopic observations were obtained with the Fabry-Pérot-based
Visible-Light Imaging Magnetograph (VIM) in conjunction with the
high-order adaptive optics (AO) system operated at the 65 cm vacuum
reflector of the Big Bear Solar Observatory (BBSO). We apply local
correlation tracking (LCT) to the speckle-reconstructed time sequences
of white-light images around 600 nm to infer horizontal proper motions,
while the Doppler shifts of the scanned Fe I line at 630.15 nm are
used to calculate line-of-sight (LOS) velocities with subarcsecond
resolution. We find that the dividing line between radial inward and
outward proper motions in the inner and outer penumbra, respectively,
survives the decay phase. In particular the moat flow is still
detectable after the penumbra disappeared. Based on our observations,
three major processes removed flux from the sunspot: (1) fragmentation
of the umbra, (2) flux cancelation of moving magnetic features (MMFs; of
the same polarity as the sunspot) that encounter the leading opposite
polarity network and plages areas, and (3) flux transport by MMFs
(of the same polarity as the sunspot) to the surrounding network and
plage regions that have the same polarity as the sunspot.
Title: Multiheight Properties of Moving Magnetic Features
Authors: Choudhary, Debi Prasad; Balasubramaniam, K. S.
Bibcode: 2007ApJ...664.1228C
Altcode:
We report on spectropolarimetric and dynamical properties of a moving
magnetic feature (MMF) around a disk-center sunspot observed using
photospheric (Fe I λλ6301.5 and 6302.5) and lower chromospheric
(Mg b2 λ5172.7) lines. We find that there are 33%
fewer MMFs at the lower chromosphere compared to the photosphere,
implying a sophisticated magnetic field geometry of tight low-level
loops. A majority of bipolar MMFs are oriented with their neutral
line perpendicular to the radial direction. Their ``spot-ward''
component has the same polarity as the sunspot. The magnetic filling
factor is larger for all types of MMFs situated closer to the spot
than those situated further away. Bipolar MMFs have a larger filling
factor compared to the unipolar ones. Comparison of dI/dλ and Stokes V
profile suggests a large magnetic filling factor within the MMFs in the
photosphere. Traversing individual MMFs, the Stokes V profiles vary from
normal antisymmetric structures to multilobed anomalous profiles. The
chromospheric counterpart of multilobed and anomalous photospheric MMF
Stokes V profiles are normal and antisymmetric. This suggests that
magnetic loops corresponding to MMFs in the lower atmosphere are of
mixed polarity and perhaps twisted while they are relatively relaxed
in the corresponding upper atmosphere. The temporal evolution of the
MMFs shows a transition between anomalous and normal Stokes V profiles.
Title: Joint Discussion 3 Solar active regions and 3D magnetic
structure
Authors: Choudhary, Debi Prasad; Sobotka, Michal
Bibcode: 2007HiA....14..139C
Altcode:
Keeping in view of the modern powerful observing tools, among
others Hinode (formerly SOLAR-B), STEREO and Frequency-Agile Solar
Radiotelescope, and sophisticated modelling techniques, Joint Discussion
3 during the IAU General Assembly 2006 focused on the properties of
magnetic field of solar active regions starting in deep interior of
the Sun, from where they buoyantly rise to the coronal heights where
the site of most explosive events are located. Intimately related with
the active regions, the origin and evolution of the magnetic field
of quiet Sun, the large scale chromospheric structures were also the
focal point of the Joint Discussion. The theoretical modelling of the
generation and dynamics of magnetic field in solar convective zone show
that the interaction of the magnetic field with the Coriolis force
and helical turbulent convection results in the tilts and twists in
the emerging flux. In the photosphere, some of these fluxes appear in
sunspots with field strengths up to about 6100 G. Spectro-polarimetric
measurements reveal that the line of sight velocities and magnetic
field of these locations are found to be uncombed and depend on depth in
the atmosphere and exhibit gradients or discontinuities. The inclined
magnetic fields beyond penumbra appear as moving magnetic features
that do not rise above upper photospheric heights. As the flux rises,
the solar chromosphere is the most immediate and intermediary layer
where competitive magnetic forces begin to dominate their thermodynamic
counterparts. The magnetic field at these heights is now measured
using several diagnostic lines such as Ca II 854.2 nm, H I 656.3 nm,
and He I 1083.0 nm. The radio observations show that the coronal
magnetic field of post flare loops are of the order of 30 G, which
might represent the force-free magnetic state of active region in the
corona. The temperatures at these coronal heights, derived from the
line widths, are in the range from 2.4 to 3.7 million degree. The same
line profile measurements indicate the existence of asymmetric flows
in the corona. The theoretical extrapolation of photospheric field
into coronal heights and their comparison with the observations show
that there exists a complex topology with separatrices associated to
coronal null points. The interaction of these structures often lead to
flares and coronal mass ejections. The current MHD modelling of active
region field shows that for coronal mass ejection both local active
region magnetic field and global magnetic field due to the surrounding
magnetic flux are important. Here, we present an extended summary of
the papers presented in Joint Discussion 03 and open questions related
to the solar magnetic field that are likely to be the prime issue with
the modern observing facilities such as Hinode and STEREO missions.
Title: Application of the principle of minimum dissipation rate to
solar coronal magnetic field extrapolation
Authors: Hu, Qiang; Dasgupta, Brahmananda; Choudhary, D. P.
Bibcode: 2007AIPC..932..376H
Altcode:
We develop an approach to deriving the 3D coronal magnetic field
configuration from vector magnetograms, based on the Principle of
Minimum Dissipation Rate (MDR). A solution to the system resulted from
MDR is generally non-force free and is expressed as linear superposition
of three linear force-free fields (one is potential) with distinct α
parameters. The final solution is thus decomposed into three linear
force-free extrapolations from bottom boundary conditions derived from
available vector magnetograms, at both photospheric and chromospheric
levels. We illustrate the approach with an analytic test case, and
discuss its advantages and limitations.
Title: International Conference on Challenges for Solar Cycle 24
Authors: Choudhary, Debi Prasad
Bibcode: 2007EOSTr..88..239C
Altcode:
Physical Research Laboratory, Ahmedabad, India, 22-25 January 2007
What will be the nature of magnetic fields at various spatial and
temporal scales on the Sun during the next activity cycle? This
and other outstanding questions of solar physics were the focus
of discussion at the International Conference on Challenges for
Solar Cycle 24. Observational solar research is equipped with ever
advanced instrumentation during each solar cycle. The use of modern
instrumentation for solar observations will yield better results
when planned with prior knowledge gained during previous activity
cycles. With this motivation, the goal of the conference was to consider
the most effective strategies for studying and understanding the solar
energetic events of cycle 24.
Title: Spectropolarimetric Signatures of Down-flows About Magnetic
Boundaries
Authors: Choudhary, Debi P.; Balasubramaniam, K.
Bibcode: 2007AAS...210.4504C
Altcode: 2007BAAS...39..160C
We analyze spectropolarimetric signatures across magnetic boundaries
of opposite polarities at the photosphere. HAO/NSO Advanced Stokes
Polarimeter measurements were made at the NSO/Dunn Solar Telescope
of photospheric spectral lines (FeI 6301.2 A and 6302.3 A). These
observations (NOAA 9662; 2001 October 16-17) about a corridor of a
magnetic inversion line near the edge of a disk-center sunspot show
strong red-asymmetry. Several (at least six) such discrete and compact
sites of both polarities, near these magnetic neural lines, show such
red-asymmetry. Strong down-flows associated with these unusual Stokes
polarization profiles are associated with these asymmetries. We discuss
detailed properties and possible origin of the observed profiles.
Title: Three-Dimensional Structure of Evershed Flows
Authors: Deng, Na; Choudhary, D. P.; Denker, C.; Verdoni, A.;
Tritschler, A.
Bibcode: 2007AAS...210.9506D
Altcode: 2007BAAS...39..224D
The Evershed flow is known as the unceasing radially outward mass flow
in sunspot penumbrae observed in the photosphere, which is most clearly
seen near the solar limb. The Evershed flow reverses its direction in
the chromosphere, where an inward mass flow is observed. In order to
quantitatively explore the three-dimensional structure of the Evershed
flow from photosphere to chromosphere, we observed a sunspot group
(NOAA 10892) at the limb using the Horizontal Spectrograph (HSG)
at the National Solar Observatory/Sacramento Peak (NSO/SP) on 2006
June 04. Spectral data sets were obtained simultaneously at four solar
absorbtion lines: Fe 630.15 nm and 630.25 nm (photosphere), NaD 588.99
nm (lower chromosphere), and Hα 656.28 nm (chromosphere). Spectral
analysis and different methods of determining Doppler velocities will
be presented and compared. This work is supported by NSF under
grant ATM 02-36945, ATM 03-42560, ATM 05-48260 and MRI AST 00-79482
and by NASA under grant NAG 5-13661.
Title: Continuum Intensity and Magnetic Field Relationship in Sunspots
Authors: Leonard, Trevor W.; Choudhary, D. P.
Bibcode: 2007AAS...210.2520L
Altcode: 2007BAAS...39..134L
The brightness (I) and magnetic field (B) distributions (I-B
relationship) of sunspots are closely related. Several previous studies
show that the relationship depends on the spot size, disk position,
life time, and phase of the solar cycle in which it appears. The
I-B relationship might also change from cycle to cycle. Many of the
previous studies were conducted by single position measurements over the
spot. Here, we present the results of a study of the I-B relationship
using a uniform set of imaging observations of about 272 sunspots
over the last two solar cycles at the San Fernando Observatory. The
corresponding full disk magnetograms are from MDI-SOHO. The first result
to be presented is the I-B relationship of sunspots. The second result
is from an examination of the linear part of the I-B relationship,
which shows that larger spots have a stiffer dependency compared to
small spots. Another such dependency is more pronounced for the spots
with a large umbra-penumbra ratio. Furthermore, the spots closer to the
disk center have a stiffer I-B slope compared to the spots situated
towards the limb. Finally, we explore the I-B properties of spots
in relation to their corresponding solar hemispheres, as a function
of their evolution history, dependence on their magnetic polarity,
and any dependency on solar cycle. This work was partially supported
by Cottrell College Science Award CC6496.
Title: Non-force Free Coronal Magnetic Field Extrapolation from
Vector Magnetograms
Authors: Janaki, M.; Hu, Q.; Dasgupta, B.; Choudhary, D. P.
Bibcode: 2006AGUFMSH33B0403J
Altcode:
We developed a new approach to deriving three-dimensional non-force free
coronal magnetic field structures from vector photospheric magnetogram,
based on the principle of Minimum Dissipation Rate (MDR). In analogy to
the principle of minimum energy, which yields a linear (constant α)
force-free magnetic field, the MDR gives a more general non-force
free magnetic field with flow. Its solution can be expressed as a
linear superposition of two linear force-free fields. The parameter,
α, for each of the two linear force free fields, can be determined
by optimizing the requirement that the recovered transverse magnetic
field components as the superposition of the corresponding components
of the two linear force free fields agree with the observed ones at
the photospheric level. A preliminary test case study of an analytic
non-force free field model illustrated the feasibility of the method. We
further apply it to real magnetograph measurements of solar active
regions to assess the relevant magnetic energy and relative helicity
content.
Title: Dynamical properties of moving magnetic features of sunspots
Authors: Choudhary, Debi P.; Balasubramaniam, K. S.
Bibcode: 2006IAUJD...3E..74C
Altcode:
We have observed the active region NOAA 9662 on 16 and 17 October 2001
using the Advanced Stokes Polarimeter (ASP) at Richard B. Dunn Solar
Telescope of National Solar Observatory. The ASP recorded all the full
Stokes profiles in photospheric FeI 630.2 nm and Chromospheric Mgb2
517.2 nm lines simultaneously. During our observations, the active
region was located near the centre of solar disk. Combining the high
resolution magnetograms obtained with the Michelson Doppler Imager
on-board the Solar Heliospheric observatory; we determine the dynamical
properties of moving magnetic features (MMF) around the prominent
leading spots of the active region. In this paper we summarize the
three dimensional structure of the MMFs and their photospheric and
chromospheric dynamical properties.
Title: Dynamical Properties Of Moving Magnetic Features Of Sunspots
Authors: Choudhary, Debi P.; Balasubramaniam, K.
Bibcode: 2006SPD....37.3404C
Altcode: 2006BAAS...38..259C
We have observed the active region NOAA 9662 on 16 and 17 October 2001
using the Advanced Stokes Polarimeter (ASP) at Richard B. Dunn Solar
Telescope of National Solar Observatory. The ASP recorded all the full
Stokes profiles in photospheric FeI 630.2 nm and Chromospheric Mgb2
517.2 nm lines simultaneously. During our observations, the active
region was located near the center of solar disk. Combining the high
resolution magnetograms obtained with the Michelson Doppler Imager
on-board the Solar Heliospheric observatory; we determine the dynamical
properties of moving magnetic features (MMF) around the prominent
leading spots of the active region. In this paper we summarize the
three dimensional structure of the MMFs and their photospheric and
chromospheric dynamical properties.
Title: Magnetic Field And Intensity Distribution In Sunspots
Authors: Leonard, Trevor W.; Choudhary, D.
Bibcode: 2006SPD....37.3501L
Altcode: 2006BAAS...38..259L
We have investigated the relationship of the magnetic field and
brightness distribution of sunspots. The sunspots are extracted from
the multi-spectral images obtained at the San Fernando Observatory and
MDI-SOHO. The corresponding magnetic field is obtained from the NSO/KP
and MDI-SOHO magnetograms. The analysis involves careful registration
of sunspot images and magnetograms. We have constructed the scatter
diagram displaying the relationship between the intensity and magnetic
field of about fifty sunspots. The results and discussion of these
scatter plots will be presented in this paper.
Title: Interactive processes of lower and upper atmosphere in search
of precursor of earthquake.
Authors: Devi, M.; Barbara, A. K.; Sarma, B.; Choudhary, D.; Depueva,
A.; Ruzhin, Yu. Y.
Bibcode: 2006cosp...36.1163D
Altcode: 2006cosp.meet.1163D
The relative role played by earthquake induced processes in modifying
ionosphere and the lower atmosphere are examined through analysis
of Total Electron Content TEC scintillation data from GPS VHF Radio
Beacon RB signals and aerosol and cloud features from LIDAR The data
for this analysis are mostly collected at Guwahati 26 0 N and 92 0 E a
low latitude seismically active location in sub Himalayan region however
fof2 data from a number of other low latitude stations from topside and
ground based ionosonde provide inputs for examining spatial and temporal
modification features at the ionosphere and then to identify the best
mode of analysis for identification of earthquake precursor Ion and
electron density variations prior and during earthquake received from
DEMETER act supporting information in our analysis The of TEC and foF2
data shows that enhancement and depletion of ionisation density at low
latitudes depend on the position of epicentre and type of earthquake
and are effected by seismically induced E field that modifies the
ExB drift Conspicuous changes in density are noticed just prior to
and after the noon hours These are discussed in the paper including
recent earthquake of 15 th February 2006 epicentre near 27 8 N and
88 5 E The paper also shows that ionospheric modification before
the earthquake could be more clearly identified by topside data in
comparison with bottom side ones and spatial mode of observation gives
better predictability than temporal mode as
Title: Magnetic Field Configuration of Flare Locations in NOAA10486
Authors: Choudhary, D.
Bibcode: 2005AGUSMSH51C..11C
Altcode:
The active region NOAA 10486 was the site of large solar flares that
were associated with the coronal mass ejections. The initial trigger
of these flares occurred at the sites where there was emergence
of new flux in addition to the large magnetic shear. Several other
large active regions also show that flux emergence is an essential
property of such locations. In this paper, we present the magnetic
field characteristics of NOAA10486 and compare them with similar active
regions of past solar cycles.
Title: Asymmetric Stokes-V Profiles at the Penumbral Boundary of
a Sunspot
Authors: Choudhary, D. P.; Balasubramaniam, K. S.; Suematsu, Y.
Bibcode: 2004ASPC..325..137C
Altcode:
We present the spectropolarimetric measurements of a sunspot
situated near central meridian disk passage. The sunspot was of
positive polarity with several opposite polarity magnetic elements
at and beyond the penumbral boundary. The Hα images of the sunspot
show bright emission regions near the penumbral boundary towards the
sun-center, which was of opposite polarity to the main spot. Doppler
velocities determined by measuring the center-of-gravity (COG) of the
Stokes-I profile, in general, agree with Evershed flow. In addition,
it shows strong flows in the penumbra. The velocities derived from
zero-crossing wavelengths of Stokes-V profiles show strong downflows
at the penumbral boundary. Double-lobed Stokes-V profiles are observed
at locations where penumbral fibrils terminate coinciding with Hα
plages. Double lobed profiles have an unshifted component similar to
the Stokes-V profiles of the sunspot penumbra and a shifted component
with a velocity of about 5 kmps. The amplitude of the second component
increases along the penumbral fibril as a function of the distance from
the center of the sunspot. In this paper we present the preliminary
results and propose to observe with the SolarB Solar Optical Telescope
(SOT) for understanding the nature of emerging flux near the sunspots.
Title: Supergranule Diffusion and Active Region Decay
Authors: Hathaway, D. H.; Choudhary, D. P.
Bibcode: 2004AAS...204.3712H
Altcode: 2004BAAS...36..711H
Models of the Sun's magnetic dynamo include turbulent diffusion to
parameterize the effects of convective motions on the evolution of
the Sun's magnetic field. Supergranules are known to dominate the
evolution of the surface magnetic field structure as evidenced by
the structure of both the active and quiet magnetic network. However,
estimates for the diffusivity attributed to supergranules differ by
an order of magnitude - from about 100 km2/s to more than
1000 km2/s. We examine this question of the diffusivity
using three different approaches. 1) We study the decay of more than
30,000 active regions by determining the rate of change in the sunspot
area of each active region from day-to-day. 2) We study the decay
of a single isolated active region near the time of solar minimum by
examining the magnetic field evolution over five solar rotations from
SOHO/MDI magnetograms obtained at 96-minute intervals. 3) We study
the characteristics of supergranules that influence the estimates of
their diffusive properties - flow speeds and lifetimes as functions
of size ∼V from SOHO/MDI Dopplergrams.
Title: Quiet-Region Filament Eruptions
Authors: Choudhary, D. P.; Moore, R. L.
Bibcode: 2004AAS...204.1805C
Altcode: 2004BAAS...36..683C
We report characteristics of quiescent filament eruptions that did
not produce coronal mass ejections (CMEs). It is known that there is
a dichotomy of quiescent filament eruptions: those that produce CMEs
and those that do not. We examined the quiescent filament eruptions,
each of which was located far from disk center (>/= 0.7 RSun) in
diffuse remnant magnetic fields of decayed active regions, was well
observed in Halpha observations and Fe XII, and had good coronagraph
coverage. We present the similarity and differences of two classes
of filament eruptions. From their lack of CME production and the
appearance of their eruptive motion in Fe XII movies, we conclude
that the non-CME-producing filament eruptions are confined eruptions
like the confined filament eruptions in active regions. We take the
similarity of the confined and eruptive quiescent filament eruptions
with their active-region counterparts to favor runaway tether-cutting
reconnection for unleashing the magnetic explosion in all these
eruptions. The results of this work have been published in Geophysical
Research Letters (Geophys. Res. Lett, 30, 2107, 2003). The work
was performed while one of the authors (DPC) held a National Research
Council NASA/MSFC Resident Research Associateship.
Title: Triggering of the Two X-class Flares of 28 and 29 October 2003
Authors: Choudhary, D. P.; Moore, R. L.; Falconer, D.; Pojoga, S.;
Tian-Sen, H.; Krucker, S.; Uddin, W.
Bibcode: 2004AAS...204.0225C
Altcode: 2004BAAS...36..982C
From H-alpha movies from Aryabhatta Research Institute of Observational
Sciences and from Prairie View Solar Observatory, hard X-ray movies
from RHESSI, line-of-sight magnetogram movies from SOHO/MDI, and
vector magnetograms from Marshal Space Flight Center, we examine the
magnetic structure and evolution of the large delta-sunspot active
region NOAA 10486 in relation to the onset and development of the
two X-class flares that occurred in this active region on 28 and 29
October 2003. We find evidence that each of these flares was triggered
by strongly sheared magnetic field via ``tether-cutting" reconnection
with adjacent/overlying strongly sheared field. In the first flare, the
initial brightening in H-alpha (1) was partly rooted in emerging sheared
magnetic field along the edge of the large positive-polarity flux
domain of the delta sunspot, and (2) consisted of four flare kernels,
two in negative magnetic flux and two in positive magnetic flux. In
the second flare, the brightening started in the core of a Z-shaped
sigmoidal sheared magnetic field and the inner two of four H-alpha
kernels were visible in 30-50 Kev hard x-ray image from RHESSI. Each
flare spread from the initial quadrupolar brightening to develop into
a much larger two-ribbon flare straddling a much more extensive swath
of strongly sheared field along the edge of the large positive-flux
domain of the delta sunspot, the first flare on the leading side and
the second flare on the trailing side of this domain. Thus, localized
internal reconnection triggered the explosion of these extensive sheared
magnetic fields. This research was supported by NASA's Office
of Space Science through the Solar and Heliospheric Physics SR&T
Program, and was done during Dr. Choudhary's tenure at MSFC/NSSTC as
an NRC Senior Resident Research Associate.
Title: Observational study of the three-dimensional magnetic field
structure and mass motion in active regions
Authors: Choudhary, Debi Prasad; Suematsu, Yoshinori, Ichimoto, Kiyoshi
Bibcode: 2004naoj.book...39C
Altcode:
No abstract at ADS
Title: Filament eruption without coronal mass ejection
Authors: Choudhary, Debi Prasad; Moore, Ronald L.
Bibcode: 2003GeoRL..30.2107C
Altcode: 2003GeoRL..30uSSC7C
We report characteristics of quiescent filament eruptions that were
not associated with coronal mass ejections (CMEs). We examined 12
quiescent filament eruptions, each of which was located far from disk
center (>=0.7 RSun) in diffuse remnant magnetic fields
of decayed active regions, was well observed in full-disk movies in
Hα and Fe XII, and had good coronagraph coverage. Of the 12 events,
9 were associated with CMEs and 3 were not. Even though the two
kinds of eruption were indistinguishable in their magnetic setting
and in the eruptive motion of the filament in the Hα movies, each
of the CME-producing eruptions produced a two-ribbon flare in Hα
and a coronal arcade and/or two-ribbon flare in Fe XII, and each of
the non-CME-producing eruptions did not. From this result, and the
appearance of the eruptive motion in the Fe XII movies, we conclude
that the non-CME-associated filament eruptions are confined eruptions
like the confined filament eruptions in active regions.
Title: Chromospheric magnetic field of solar active regions
Authors: Choudhary, Debi Prasad; Sakurai, Takashi
Bibcode: 2003naoj.book...37C
Altcode:
No abstract at ADS
Title: Magnetic and velocity fields of active regions
Authors: Choudhary, D. P.; Gosain, S.
Bibcode: 2003AN....324..362C
Altcode:
We have observed about 15 active regions on the Sun, with the Advanced
Stokes Polarimeter and Dick Dunn Telescope at NSO/SP to map the Stokes
parameters in the photospheric Fe 6302.5 Å and chromospheric Mg i
5173 Å lines, during 1999-2002. The observations are corrected for
dark current, gain, instrumental polarization and cross-talk using
ASP pipeline. The wavelength calibration is carried out using the O_2
telluric line 6302 Å which is also present in the observations. The
photospheric and chromospheric longitudinal magnetograms are made from
the Stokes V profiles, which were inter-calibrated with the Kitt Peak
magnetograms. The plasma motions are inferred from the line bisector
measurements at different positions of the spectral line. In this paper
we present the height dependence of Doppler velocity scatter plots of
a sunspot in the photospheric Fe i 6302 Å line.
Title: Study of bright points in the off-band Hα filtergrams of
active regions
Authors: Choudhary, D. P.; Gosain, S.
Bibcode: 2003AN....324..367C
Altcode:
Hα filtergrams of selected active regions are obtained by the six inch
refractor telescope at Udaipur Solar Observatory through a birefringent
filter of 0.5 Å full width at half maximum. The field of view is ~
4x5 arcmin with a spatial resolution of better than one arc sec. The
filtergrams are obtained in the wavelength range of -1 to +1 Å centered
at 6563 Å at steps of 0.1 Å. A complete scan takes about 5 seconds
(occasionally longer). The images are recorded with a cooled CCD camera
and corrected for dark current and flat field. We observe the bright
points in the filtergrams beyond 0.5 Å off line center. In general,
these features are located away from the strong field regions. We
study their property with respect to the co-temporal photospheric
magnetograms obtained with the GONG instrument at Udaipur. This might
help in understanding their relation with the magnetic flux emergence
and annihilation.
Title: Observational Study of the Three-Dimensional Magnetic Field
Structure and Mass Motion in Active Regions
Authors: Choudhary, Debi Prasad; Suematsu, Yoshinori; Ichimoto, Kiyoshi
Bibcode: 2002SoPh..209..349C
Altcode:
Spectro-polarimetric observations of active regions were carried
out in the spectral lines of Si i 10827.1 Å and He i 10830 Å to
study the three-dimensional magnetic field structure and associated
plasma flow properties. Comparison of Si i and He i magnetograms
with the potential field model shows that a large fraction of the
magnetic field is consistent with the potential field structure,
by assuming that the height difference between the origin of the two
lines is about 1200 km. The slope of the scatter plot between Si i
and He i magnetograms is 0.5, 0.76 in an emerging flux and a larger
active region, respectively. These values are lower than the scatter
plot slopes obtained from Kitt Peak photospheric and chromospheric
magnetograms, in which case the corresponding values are 0.83 and 0.9,
respectively. Considering the height difference between these two sets
of chromospheric magnetograms, this implies that the magnetic field
spreads out faster near the transition region heights. Dopplergrams
obtained by determining the centroid of the asymmetric line profiles
show that, in case of emerging flux region, the chromospheric upflow
regions are located in the magnetic neutral line areas.
Title: On Magnetic Flux Imbalance in Solar Active Regions
Authors: Choudhary, Debi Prasad; Venkatakrishnan, P.; Gosain, Sanjay
Bibcode: 2002ApJ...573..851C
Altcode:
The magnetic flux imbalance of active regions has been studied using the
longitudinal magnetograms obtained from the National Solar Observatory
at Kitt Peak. The maximum and the median value of the flux imbalance in
137 active regions situated near the disk center is found to be about
62% and 9.5%, respectively. The detailed analysis of a few selected
active regions shows that the local flux asymmetry is compensated on
global scales. For example, the NOAA Active Region 7978, which appeared
during the solar activity minimum period and evolved during five solar
rotations (1996 July-October), shows a flux imbalance of about 9.5%,
with an excess of following negative flux. However, on a global scale,
the positive and negative flux in the entire solar disk was found to be
nearly balanced during the same period. The global flux imbalance of
the Sun during a full magnetic cycle is estimated from the Carrington
maps. These maps are made by merging the solar images obtained during
a complete solar rotation (about 27 days), hence each represents the
entire surface of the Sun. The 10°-40° active latitudinal zone in the
individual hemispheres during the solar maximum shows a flux imbalance
of more than 20%. This is reduced to below 10% when the entire Sun
is considered. The present study indicates that a fraction of the
magnetic field from the localized active regions connects with far
away locations.
Title: Useful Aspects of Chromospheric Magnetic Field Data
Authors: Sakurai, T.; Choudhary, D. P.; Venkatakrishnan, P.
Bibcode: 2002stma.conf...37S
Altcode:
No abstract at ADS
Title: Chromospheric Magnetic Field of Solar Active Regions
Authors: Choudhary, Debi Prasad; Sakurai, Takashi; Venkatakrishnan, P.
Bibcode: 2001ApJ...560..439C
Altcode:
The three-dimensional magnetic field structure of 137 solar active
regions is studied by comparing the observed and computed chromospheric
magnetograms. The model chromospheric field is obtained by extrapolating
the observed photospheric field into the chromosphere with a potential
(current-free) magnetic-field model in Cartesian geometry. The
best correlation between the observed and the model chromospheric
magnetograms is found at the height of 800 km, which also corresponds
to the height of the line formation for Ca II 854.2 nm. In the weak
field range, within +/-300 G, most of the observed field is close to
the potential field. However, departures of about 50 G are observed
in a few active regions. For field values greater than +/-500 G, the
observed field does not always match the model. Whereas a part of this
could be due to the magnetogram calibration, it might also originate
from the ``nonpotentiality'' of the chromospheric field. In the case
of long-lived active regions, which make multiple disk passages, the
strong-field nonpotentiality is observed during their initial phase,
and converges to a potential field configuration later.
Title: The Photospheric Flow near the Flare Locations of Active
Regions
Authors: Choudhary, Debi Prasad
Bibcode: 2000JApA...21..249C
Altcode:
No abstract at ADS
Title: Magnetic field configuration of active region NOAA 6555 at
the time of a long-duration flare on 23 March 1991 - An Exception
to Standard Flare Reconnection Model
Authors: Choudhary, Debi Prasad; Gary, G. Allen
Bibcode: 1999SoPh..188..345C
Altcode:
The high-resolution Hα images observed during the decay phase
of a long-duration flare on 23 March 1991 are used to study the
three-dimensional magnetic field configuration of the active region NOAA
6555. Whereas all the large flares in NOAA 6555 occurred at the location
of high magnetic shear and flux emergence, this long-duration flare was
observed in the region of low magnetic shear at the photosphere. The Hα
loops activity started soon after the maximum phase of the flare. There
were a few long loop at the initial phase of the activity. Some of
these were sheared in the chromosphere at an angle of about 45° to
the east-west axis. Gradually, an increasing number of shorter loops,
oriented along the east-west axis, started appearing. The chromospheric
Dopplergrams show blue shifts at the end points of the loops. By using
different magnetic field models, we have extrapolated the photospheric
magnetograms to chromospheric heights. The magnetic field lines computed
by using the potential field model correspond to most of the observed
Hα loops. The height of the Hα loops were derived by comparing them
with the computed field lines. From the temporal evolution of the Hα
loop activity, we derive the negative rate of appearance of Hα features
as a function of height. It is found that the field lines oriented along
one of the neutral lines were sheared and low lying. The higher field
lines were mostly potential. The paper also outlines a possible scenario
for describing the post-flare stage of the observed long-duration flare.
Title: The Photospheric Flow Near the Flare Locations of Active
Regions
Authors: Choudhary, D.
Bibcode: 1999AAS...194.5907C
Altcode: 1999BAAS...31..918C
The flow pattern of flare locations in the active regions are observed
by using the Tower Vector Magnetograph (TVM) of Marshall Space Flight
Center. The line-center-magnetogram (LCM) technique (Giovanelli and
Ramsay, 1971, IAU Symp, 43, 293) has been employed to determine the
active region velocities. The LCM is based on finding the wavelength in
the line profile where two opposite circularly-polarized Zeeman-Split
components change sign. If the material in the magnetic field of
different locations have relative line of sight velocities, their
cross-over wavelength will be seen Doppler shifted. In order to use the
LCM with TVM, a series of Stoke-V images as a function of wavelength
are made and their cross-over wavelength at each pixel is determined. We
have observed 12 active regions during June 25 to August 25, 1998. Four
of these active regions (NOAA 8253, NOAA 8264, 8293 and NOAA 8307)
show flare activity associated with the flux emergence and/or changes
in magnetic shear during their disk passage. The cross-over wavelength
of the locations of activities and the leading sunspot are seen to be
different, implying the existence of the relative velocity between
them. In one of the active regions (NOAA 8253), we clearly observe
the blue shift associated with the emergence of new flux and magnetic
shear. In the present paper, we examine the nature of material flow
near the location of activity in these active regions.
Title: Emerging Flux and X-class Flares in NOAA 6555
Authors: Choudhary, Debi Prasad; Ambastha, Ashok; Ai, G.
Bibcode: 1998SoPh..179..133C
Altcode:
The active region NOAA 6555 had several locations of highly sheared
magnetic field structure, yet, only one of them was the site for all the
five X-class flares during its disk passage in March 1991. The pre-flare
observations of high-resolution Hα filtergrams, vector magnetograms and
Hβ Dopplergrams of the 2B/X5.3 flare on 25 March 1991 show that the
flaring site was characterized by a new rising `emerging flux region'
(EFR) near the highly sheared magnetic field configuration. The polarity
axis of the emerging flux was nearly perpendicular to the pre-existing
magnetic neutral line. The location of the EFR was the site of initial
brightening in Hα. The post-flare magnetograms show higher magnetic
shear at the flare location compared to the post-flare magnetograms,
which might indicate that the EFR was sheared at the time of its
emergence. As the new EFR coincided with the occurrence of the flare,
we suggest that it might have triggered the observed flare. Observations
from Big Bear Solar Observatory and Marshall Space Flight Center also
show that there was emergence of new flux at the same location prior
to two other X-class flares. We find that out of five observed X-class
flares in NOAA 6555, at least in three cases there are clear signatures
of flare-related flux emergence. Therefore, it is concluded that EFRs
might play an important role in destabilizing the observed sheared
magnetic structures leading to large X-class flares of NOAA 6555.
Title: A Tripler to 220 GHz Using a Back-To-Back Barrier N-N+
Varactor Diode
Authors: Choudhary, D.; Siegel, P. H.; Räisänen, A. V.; Martin,
S. C.; Smith, R. P.
Bibcode: 1994stt..conf..475C
Altcode:
No abstract at ADS
Title: Spectroscopic study of comets with special reference to
Halley's Comet
Authors: Choudhary, Debi Prasad
Bibcode: 1988PhDT.......187C
Altcode:
Abstract not available.