ambastha

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Author name code: ambastha
ADS astronomy entries on 2022-09-14
author:"Ambastha, Ashok"

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Title: Magnetic and Velocity Field Topology in Active Regions of
Descending Phase of Solar Cycle 23
Authors: Maurya, R. A.; Ambastha, A.
2020SoPh..295..106M Altcode: 2020arXiv200613602M
We analyze the topology of photospheric magnetic fields and
sub-photospheric flows of several active regions (ARs) that are
observed during the peak to descending phase of Solar Cycle 23. Our
analysis shows clear evidence of hemispheric preferences in all the
topological parameters such as the magnetic, current and kinetic
helicities, and the `curl-divergence'. We found that 68%(67%) ARs in
the northern (southern) hemisphere with negative (positive) magnetic
helicity. Same hemispheric preference sign is found for the current
helicity in 68%(68%) ARs. The hemispheric preferences are found to exist
statistically for all the time except in a few ARs observed during the
peak and the end phases of the solar cycle. This means that magnetic
fields are dominantly left(right)-helical in scales smaller than
individual ARs of northern(southern) hemisphere. We found that magnetic
and current helicity parameters show equatorward propagation similar
to the sunspot cycle. The kinetic helicity showed similar hemispheric
trend to that of magnetic and current helicity parameters. There are
65%(56%) ARs with negative (positive) kinetic helicity as well as
divergence-curl, at the depth of 2.4 Mm, in the northern (southern)
hemisphere. The hemispheric preference of the kinetic helicity
becomes more evident at larger depths, e.g., 69%(67%) at the depth
of 12.6 Mm. A similar hemispheric trend of kinetic helicity to that
of the current helicity supports the mean-field dynamo model. We also
found that the hemispheric preference of all the parameters increases
with the field strength of ARs. The topology of photospheric magnetic
fields and near-surface sub-photospheric flow fields did not show good
association but the correlation between them enhances with depths,
which could be indicating more aligned flows at deeper layers of ARs.

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Title: Solar activity and explosive transient eruptions
Authors: Ambastha, Ashok
2016AsJPh..25..267A Altcode:
We discuss active and explosive behavior of the Sun observable in a
wide range of wavelengths (or energies) and spatio-temporal scales
that are not possible for any other star. On the longer time scales,
the most notable form of solar activity is the well known so called
11-year solar activity cycle. On the other hand, at shorter time scales
of a few minutes to several hours, spectacular explosive transient
events, such as, solar flares, prominence eruptions, and coronal mass
ejections (CMEs) occur in the outer layers of solar atmosphere. These
solar activity cycle and explosive phenomena influence and disturb
the space between the Sun and planets. The state of the interplanetary
medium, including planetary and terrestrial surroundings, or "the space
weather", and its forecasting has important practical consequences. The
reliable forecasting of space weather lies in continuously observing
of the Sun. We present an account of the recent developments in our
understanding of these phenomena using both space-borne and ground-based
solar observations.

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Title: Activity-related variations of high-degree p-mode amplitude,
width, and energy in solar active regions
Authors: Maurya, R. A.; Ambastha, A.; Chae, J.
2014A&A...561A.123M Altcode: 2013arXiv1310.6458M
Context. Solar energetic transients such as flares and coronal mass
ejections occur mostly within active regions (ARs) and release large
amounts of energy, which is expected to excite acoustic waves by
transferring the mechanical impulse of the thermal expansion of the
flare on the photosphere. On the other hand, strong magnetic fields of
AR sunspots absorb the power of the photospheric oscillation modes. Aims: We study the properties of high-degree p-mode oscillations in
flaring and dormant ARs and compare them with those in corresponding
quiet regions (QRs) to find the association of the mode parameters with
magnetic- and flare-related activities. Methods: We computed the
mode parameters using the ring-diagram technique. The magnetic-activity
indices (MAIs) of ARs and QRs were determined from the line-of-sight
magnetograms. The flare indices (FIs) of ARs were obtained from the GOES
X-ray fluxes. Mode parameters were corrected for foreshortening, duty
cycle, and MAI using multiple non-linear regression. Results:
Our analysis of several flaring and dormant ARs observed during the
Carrington rotations 1980-2109 showed a strong association of the mode
amplitude, width, and energy with magnetic and flare activities,
although their changes are combined effects of foreshortening,
duty cycle, magnetic-activity, flare-activity, and measurement
uncertainties. We find that the largest reduction in mode amplitude
and background power of an AR are caused by the angular distance of
the AR from the solar disc centre. After correcting the mode parameters
for foreshortening and duty cycle, we find that the mode amplitudes of
flaring and dormant ARs are lower than in corresponding QRs reducing
with increasing MAI, suggesting a stronger mode power suppression in
ARs with larger magnetic fields. The mode widths in ARs are larger
than in corresponding QRs and increase with MAI, indicating shorter
lifetimes of modes in ARs than in QRs. The variations in mode amplitude
and width with MAI are not same in different frequency bands. The
largest amplification (reduction) in mode amplitude (mode width) of
dormant ARs is found in the five-minute frequency band. The average
mode energy of both the flaring and dormant ARs is smaller than in
their corresponding QRs, reducing with increasing MAI. But the average
mode energy reduction rate in flaring ARs is smaller than in dormant
ARs. Moreover, the increase in mode width rate in dormant (flaring)
ARs is followed by a decrease (increase) in the amplitude variation
rate. Furthermore, including the mode corrections for MAI shows that
mode amplitude and mode energy of flaring ARs escalate with FI, while
the mode width shows an opposite trend, suggesting excitations of modes
and growth in their lifetimes by flares. The increase (decrease) in mode
amplitude (width) is larger in the five-minute and higher-frequency
bands. The enhancement in width variation rate is followed by a rapid
decline in the amplitude variation rate.

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Title: Magnetic structure of solar active region NOAA 11158
Authors: Vemareddy, P.; Ambastha, A.; Wiegelmann, T.
2013BASI...41..183V Altcode: 2013arXiv1310.6895V
Magnetic fields in the solar corona are responsible for a wide range
of phenomena. However, any direct measurements of the coronal magnetic
fields are very difficult due to lack of suitable spectral lines, weak
magnetic fields, and high temperatures. Therefore, one extrapolates
photospheric field measurements into the corona. Owing to low coronal
plasma β, we can apply a force-free model in lowest order to study the
slow evolution of active region (AR) magnetic fields. On applying these
models to AR 11158 and compared with coronal plasma tracers, we found
that (1) the approximation of potential field to coronal structures
over large length scales is a reasonable one, 2) linear force-free
(LFF) assumption to AR coronal fields may not be applicable model
as it assumes uniform twist over the entire AR, and 3) for modeling
fields at sheared, stressed locations where energy release in the
form of flares are usually observed, non-linear force free fields
(NLFFF) seem to provide a good approximation. The maximum available
free-energy profile shows step-wise decrease that is sufficient to
power an M-class flare as observed.

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Title: Helicity injection by flux motions and its role in flare
and CMEs
Authors: Panditi, Vemareddy; Ambastha, A.; Maurya, R.; Chae, J.;
Ambstha, A.; Maurya, R. A.; Chae, J.
2013SPD....4430003P Altcode:
An investigation of helicity injection by photospheric shear motions
is carried out for two active regions (ARs), NOAA 11158 and 11166,
using line-of-sight magnetic field observations obtained from
the Helioseismic and Magnetic Imager on board the Solar Dynamics
Observatory. We derived the horizontal velocities in the ARs from the
differential affine velocity estimator (DAVE) technique. Persistent
strong shear motions at maximum velocities in the range of 0.6-0.9
km s-1 along the magnetic polarity inversion line and outward flows
from the peripheral regions of the sunspots were observed in the two
ARs. The helicities injected in NOAA 11158 and 11166 during their
six-day evolution period were estimated as 14.16x10$^42$ Mx$^2$
and 9.5x10$^42$ Mx$^2$, respectively. The estimated injection rates
decreased up to 13% by increasing the time interval between the
magnetograms from 12 minutes to 36 minutes, and increased up to 9%
by decreasing the DAVE window size from 21x18 to 9x6 pixel$^2$,
resulting in 10% variation in the accumulated helicity. In both
ARs, the flare-prone regions (R2) had inhomogeneous helicity flux
distribution with mixed helicities of both signs and coronal mass
ejection (CME) prone regions had almost homogeneous distribution of
helicity flux dominated by a single sign. The temporal profiles of
helicity injection showed impulsive variations during some flares/CMEs
due to negative helicity injection into the dominant region of positive
helicity flux. A quantitative analysis reveals a marginally significant
association of helicity flux with CMEs but not flares in AR 11158,
while for the AR 11166, we find a marginally significant association
of helicity flux with flares but not CMEs, providing evidence of the
role of helicity injection at localized sites of the events. These
short-term variations of helicity flux are further discussed in
view of possible flare-related effects. This study suggests that flux
motions and spatial distribution of helicity injection are important to
understanding the complex nature of the magnetic flux system of the AR,
and how it can lead to conditions favorable for eruptive events.

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Title: On the Role of Rotating Sunspots in the Activity of Solar
Active Region NOAA 11158
Authors: Panditi, Vemareddy; Ambastha, A.; Maurya, R.
2013SPD....44..120P Altcode:
We study the role of rotating sunspots in relation to the evolution
of various physical parameters characterizing the non-potentiality
of the active region (AR) NOAA 11158 and its eruptive events using
the magnetic field data from the Helioseismic and Magnetic Imager
(HMI) and multi-wavelength observations from the Atmospheric Imaging
Assembly (AIA) on board the Solar Dynamics Observatory. From the
evolutionary study of HMI intensity and AIA channels, it is observed
that the AR consists of two major rotating sunspots, one connected
to a flare-prone region and another with coronal mass ejection
(CME). The constructed space-time intensity maps reveal that the
sunspots exhibited peak rotation rates coinciding with the occurrence of
major eruptive events. Further, temporal profiles of twist parameters,
namely, average shear angle, αav, αbest, derived from HMI vector
magnetograms, and the rate of helicity injection, obtained from the
horizontal flux motions of HMI line-of-sight magnetograms, correspond
well with the rotational profile of the sunspot in the CME-prone
region, giving predominant evidence of rotational motion causig
magnetic non-potentiality. Moreover, the mean value of free energy
from the virial theorem calculated at the photospheric level shows
a clear step-down decrease at the onset time of the flares revealing
unambiguous evidence of energy release intermittently that is stored
by flux emergence and/or motions in pre-flare phases. Additionally,
distribution of helicity injection is homogeneous in the CME-prone
region while in the flare-prone region it is not and often changes
sign. This study provides a clear picture that both proper and
rotational motions of the observed fluxes played significant roles
in enhancing the magnetic non-potentiality of the AR by injecting
helicity, twisting the magnetic fields and thereby increasing the free
energy, leading to favorable conditions for the observed transient
activity.Abstract (2,250 Maximum Characters): We study the role of
rotating sunspots in relation to the evolution of various physical
parameters characterizing the non-potentiality of the active region
(AR) NOAA 11158 and its eruptive events using the magnetic field data
from the Helioseismic and Magnetic Imager (HMI) and multi-wavelength
observations from the Atmospheric Imaging Assembly (AIA) on board
the Solar Dynamics Observatory. From the evolutionary study of HMI
intensity and AIA channels, it is observed that the AR consists of
two major rotating sunspots, one connected to a flare-prone region and
another with coronal mass ejection (CME). The constructed space-time
intensity maps reveal that the sunspots exhibited peak rotation rates
coinciding with the occurrence of major eruptive events. Further,
temporal profiles of twist parameters, namely, average shear angle,
αav, αbest, derived from HMI vector magnetograms, and the rate of
helicity injection, obtained from the horizontal flux motions of HMI
line-of-sight magnetograms, correspond well with the rotational profile
of the sunspot in the CME-prone region, giving predominant evidence
of rotational motion causig magnetic non-potentiality. Moreover,
the mean value of free energy from the virial theorem calculated at
the photospheric level shows a clear step-down decrease at the onset
time of the flares revealing unambiguous evidence of energy release
intermittently that is stored by flux emergence and/or motions in
pre-flare phases. Additionally, distribution of helicity injection is
homogeneous in the CME-prone region while in the flare-prone region
it is not and often changes sign. This study provides a clear picture
that both proper and rotational motions of the observed fluxes played
significant roles in enhancing the magnetic non-potentiality of the
AR by injecting helicity, twisting the magnetic fields and thereby
increasing the free energy, leading to favorable conditions for the
observed transient activity.

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Title: Rotating sunspots and their role in the activity of solar
active region NOAA 11158
Authors: Vemareddy, P.; Ambastha, A.; Maurya, R. A.
2013enss.confE...6V Altcode:
We study the role of rotating sunspots in relation to the evolution
of various physical parameters characterizing the non-potentiality of
the active region (AR) NOAA 11158 and its eruptive events using the
magnetic field data from the Helioseismic and Magnetic Imager (HMI)
and multi-wavelength observations from the Atmospheric Imaging Assembly
(AIA) on board the Solar Dynamics Observatory. From the evolutionary
study of HMI intensity and AIA channels, it is observed that the AR
consists of two major rotating sunspots, one connected to a flare-prone
region and another with coronal mass ejection (CME). The constructed
space-time intensity maps reveal that the sunspots exhibited peak
rotation rates coinciding with the occurrence of major eruptive
events. Further, temporal profiles of twist parameters, namely,
average shear angle, α_{av}, α_{best}, derived from HMI vector
magnetograms, and the rate of helicity injection, obtained from the
horizontal flux motions of HMI line-of-sight magnetograms, correspond
well with the rotational profile of the sunspot in the CME-prone
region, giving predominant evidence of rotational motion causing
magnetic non-potentiality. Moreover, the mean value of free energy
from the virial theorem calculated at the photospheric level shows
a clear step-down decrease at the onset time of the flares revealing
unambiguous evidence of energy release intermittently that is stored
by flux emergence and/or motions in pre-flare phases. Additionally,
distribution of helicity injection is homogeneous in the CME-prone
region while in the flare-prone region it is not and often changes
sign. This study provides a clear picture that both proper and
rotational motions of the observed fluxes played significant roles in
enhancing the magnetic non-potentiality of the AR by injecting helicity,
twisting the magnetic fields and thereby increasing the free energy,
leading to favorable conditions for the observed transient activity.

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Title: Helicity Injection by the Shearing Motion of Fluxes in Relation
to Flares and Coronal Mass Ejections
Authors: Vemareddy, P.; Ambastha, A.; Maurya, R. A.; Chae, J.
2013enss.confE...8V Altcode:
An investigation of helicity injection by photospheric shear motions is
carried out for two active regions (ARs), NOAA 11158 and 11166, using
line-of-sight magnetic field observations obtained from the Helioseismic
and Magnetic Imager on board the Solar Dynamics Observatory. We derived
the horizontal velocities in the ARs from the differential affine
velocity estimator (DAVE) technique. Persistent strong shear motions at
maximum velocities in the range of 0.6-0.9 km s^{-1} along the magnetic
polarity inversion line and outward flows from the peripheral regions
of the sunspots were observed in the two ARs. The helicities injected
in NOAA 11158 and 11166 during their six-day evolution period were
estimated as 14.16x10^{42} Mx^2 and 9.5×10^{42} Mx^2, respectively. The
estimated injection rates decreased up to 13% by increasing the time
interval between the magnetograms from 12 minutes to 36 minutes, and
increased up to 9% by decreasing the DAVE window size from 21×18 to
9×6 pixel2, resulting in 10% variation in the accumulated helicity. In
both ARs, the flare-prone regions (R2) had inhomogeneous helicity
flux distribution with mixed helicities of both signs and coronal
mass ejection (CME) prone regions had almost homogeneous distribution
of helicity flux dominated by a single sign. The temporal profiles of
helicity injection showed impulsive variations during some flares/CMEs
due to negative helicity injection into the dominant region of positive
helicity flux. A quantitative analysis reveals a marginally significant
association of helicity flux with CMEs but not flares in AR 11158,
while for the AR 11166, we find a marginally significant association
of helicity flux with flares but not CMEs, providing evidence of the
role of helicity injection at localized sites of the events. These
short-term variations of helicity flux are further discussed in
view of possible flare-related effects. This study suggests that flux
motions and spatial distribution of helicity injection are important to
understanding the complex nature of the magnetic flux system of the AR,
and how it can lead to conditions favorable for eruptive events.

---------------------------------------------------------
Title: On the Role of Rotating Sunspots in the Activity of Solar
Active Region NOAA 11158
Authors: Vemareddy, P.; Ambastha, A.; Maurya, R. A.
2012ApJ...761...60V Altcode: 2012arXiv1210.3912V
We study the role of rotating sunspots in relation to the evolution
of various physical parameters characterizing the non-potentiality
of the active region (AR) NOAA 11158 and its eruptive events using
the magnetic field data from the Helioseismic and Magnetic Imager
(HMI) and multi-wavelength observations from the Atmospheric Imaging
Assembly (AIA) on board the Solar Dynamics Observatory. From the
evolutionary study of HMI intensity and AIA channels, it is observed
that the AR consists of two major rotating sunspots, one connected to
a flare-prone region and another with coronal mass ejection (CME). The
constructed space-time intensity maps reveal that the sunspots exhibited
peak rotation rates coinciding with the occurrence of major eruptive
events. Further, temporal profiles of twist parameters, namely, average
shear angle, αav, αbest, derived from HMI vector
magnetograms, and the rate of helicity injection, obtained from the
horizontal flux motions of HMI line-of-sight magnetograms, correspond
well with the rotational profile of the sunspot in the CME-prone
region, giving predominant evidence of rotational motion causing
magnetic non-potentiality. Moreover, the mean value of free energy
from the virial theorem calculated at the photospheric level shows
a clear step-down decrease at the onset time of the flares revealing
unambiguous evidence of energy release intermittently that is stored
by flux emergence and/or motions in pre-flare phases. Additionally,
distribution of helicity injection is homogeneous in the CME-prone
region while in the flare-prone region it is not and often changes
sign. This study provides a clear picture that both proper and
rotational motions of the observed fluxes played significant roles in
enhancing the magnetic non-potentiality of the AR by injecting helicity,
twisting the magnetic fields and thereby increasing the free energy,
leading to favorable conditions for the observed transient activity.

---------------------------------------------------------
Title: On the Injection of Helicity by the Shearing Motion of Fluxes
in Relation to Flares and Coronal Mass Ejections
Authors: Vemareddy, P.; Ambastha, A.; Maurya, R. A.; Chae, J.
2012ApJ...761...86V Altcode: 2012arXiv1202.5195V
An investigation of helicity injection by photospheric shear motions is
carried out for two active regions (ARs), NOAA 11158 and 11166, using
line-of-sight magnetic field observations obtained from the Helioseismic
and Magnetic Imager on board the Solar Dynamics Observatory. We derived
the horizontal velocities in the ARs from the differential affine
velocity estimator (DAVE) technique. Persistent strong shear motions at
maximum velocities in the range of 0.6-0.9 km s-1 along the
magnetic polarity inversion line and outward flows from the peripheral
regions of the sunspots were observed in the two ARs. The helicities
injected in NOAA 11158 and 11166 during their six-day evolution
period were estimated as 14.16 × 1042 Mx2 and
9.5 × 1042 Mx2, respectively. The estimated
injection rates decreased up to 13% by increasing the time interval
between the magnetograms from 12 minutes to 36 minutes, and increased
up to 9% by decreasing the DAVE window size from 21 × 18 to 9 ×
6 pixel2, resulting in 10% variation in the accumulated
helicity. In both ARs, the flare-prone regions (R2) had inhomogeneous
helicity flux distribution with mixed helicities of both signs and
coronal mass ejection (CME) prone regions had almost homogeneous
distribution of helicity flux dominated by a single sign. The temporal
profiles of helicity injection showed impulsive variations during
some flares/CMEs due to negative helicity injection into the dominant
region of positive helicity flux. A quantitative analysis reveals
a marginally significant association of helicity flux with CMEs but
not flares in AR 11158, while for the AR 11166, we find a marginally
significant association of helicity flux with flares but not CMEs,
providing evidence of the role of helicity injection at localized
sites of the events. These short-term variations of helicity flux
are further discussed in view of possible flare-related effects. This
study suggests that flux motions and spatial distribution of helicity
injection are important to understanding the complex nature of the
magnetic flux system of the AR, and how it can lead to conditions
favorable for eruptive events.

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Title: The Venus Twilight Experiment: Probing The Mesosphere In 2004
And 2012
Authors: Tanga, Paolo; Widemann, T.; Ambastha, A.; Babcock, B. A.;
Berthier, J.; Bouley, S.; Braga-Ribas, F.; Brasch, K.; Burke, W.;
Colas, F.; Fukuhara, T.; Fulham, L.; Imai, M.; Lu, M.; Machado, P.;
Maquet, L.; Pasachoff, J. M.; Roberts, J.; Schneider, G.; Sheehan,
W.; Sigismondi, C.; Thouvenin, N.; Vachier, F.; Veillet, C.; Wang, X.
2012DPS....4450807T Altcode:
During the Venus transit in 2004 several observers collected data
useful to the characterization of the mesosphere of the planet, by
observing the solar light refracted at the corresponding altitude
range. The "aureole" thus formed, is observable during the ingress
and egress phases of the transit, when Venus is crossing the solar
limb. For the 2012 opportunity we prepared a set of coronagraphs
to obtain multi-wavelength, space- and time-resolved photometry of
the aureole, in collaboration with other space- and ground-based
campaigns. The coronagraphs were distributed in the visibility area
around the Pacific, over eight sites where local logistic support and
scientific expertise were present. Several sites obtained useful data
at frame rates of several images/sec. We will give an account of the
campaign presenting first results obtained at 450, 535, 607 and 760 nm
(FWHM 10 nm). A comparison with data collected at the 2004 transit shows
that variations in the aspects of the aureole are present. These can
be linked to variations in the vertical distribution of the absorbers
(aerosols and cloud-top level). A common feature in both the recent
transits is the presence of a brightness peak at high latitude, which
was imaged several minutes before and after first and last contact,
respectively. The historical record of the aureole contains hints of
varying features and a constant polar-spot presence, which can now be
interpreted in the light of the measurements obtained in 2004 and 2012.

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Title: On the Injection of Helicity by Shearing Motion of Fluxes in
Relation to Flares and CMEs
Authors: Panditi, Vemareddy; Ambastha, Ashok; Maurya, Ram Ajor
2012cosp...39.1440P Altcode: 2012cosp.meet.1440P
An investigation of helicity injection by photospheric shear motions
is presented for two active regions (ARs) NOAA 11158 and 11166,
using line-of-sight magnetic field observations obtained from
the Helioseismic and magnetic Imager (HMI) on-board Solar Dynamics
Observatory (SDO). We derived the horizontal flux velocities in the ARs
from Differential Affine Velocity Estimator (DAVE) technique. During
the six day evolution period of the ARs, we found persistent strong
shear motions at a maximum velocity in the range of 0.5-0.7 km-s-1
along the magnetic polarity inversion line (PIL) and outward flows
from the peripheral regions of the sunspots. The helicities injected
in AR 11158 and AR 11166 during the six days' period were estimated as
13.30×10^42 Mx2 and 9.5×10^42 Mx2, respectively. Temporal profiles
of helicity injection showed impulsive variations at the onset times
of flares/CMEs due to the negative helicity injection in the dominant
region of positive helicity density. The spatial examination of helicity
density maps showed that these variations resulted mainly due to the
negative helicity injection in the regions of opposite helicity that
were co-spatial with flaring sites. These co-spatial and co-temporal
variations of helicity injection with flares are interpreted to be due
to the relaxation from the state of high shear by the observed motions
to a lower or shear-free state releasing energy in the form of eruptive
events. This agrees with the simulations by Kusano et al. (2004)
for the triggering mechanism of flares. However, for the flares of
smaller magnitude no clear evidence of such changes was available. We
expect that this difficulty could be addressed by careful calculations
for the emergence term of helicity injection using the vector magnetic
field data. Our study suggests that the existence of opposite helicity
fluxes can trigger eruptive events, viz., flares and CMEs, and promises
to be useful in forecasting the transient activity of ARs.

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Title: Photospheric Transients and Spectral Line Changes Associated
with a Large X2.2 Flare
Authors: Ambastha, Ashok; Maurya, Ram Ajor; Panditi, Vemareddy
2012cosp...39...42A Altcode: 2012cosp.meet...42A
Solar energetic transients occurring in solar atmosphere are associated
with catastrophic release of energy in the solar corona. These
transients inject a part of their energy by various physical processes
to the deeper, denser photospheric layer at which velocity and magnetic
fields are measured using suitable spectral lines. Some signatures of
these processes have been observed during the first X-class flare of
the current solar cycle 24 in Active Region NOAA 11158 of 2011 February
15 using the data obtained from the Helioseismic and Magnetic Imager
(HMI) on board Solar Dynamics Observatory (SDO). In particular, we
detected short-lived magnetic and Doppler velocity transients together
with abnormal polarity reversals during the impulsive phase of this
large energetic flare. The observed photospheric changes associated
with energetic transients have been an issue of serious debates as
the magnetic (and Doppler) measurements are expected to be affected by
flare-induced line profile changes. We explain the observed transient
phenomena during the flare's impulsive phase using the SDO-HMI spectral
data obtained before, during and after the flare. The corresponding
physical processes are discussed in the light of recent flare models.

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Title: Filament Eruption in NOAA 11093 Leading to a Two-Ribbon M1.0
Class Flare and CME
Authors: Vemareddy, P.; Maurya, R. A.; Ambastha, A.
2012SoPh..277..337V Altcode: 2011SoPh..tmp..413V; 2011arXiv1103.3168V; 2011SoPh..tmp..416V;
2011arXiv1103.3168R
We present a multi-wavelength analysis of an eruption event that
occurred in active region NOAA 11093 on 7 August 2010, using
data obtained from SDO, STEREO, RHESSI, and the GONG Hα network
telescope. From these observations, we inferred that an upward
slow rising motion of an inverse S-shaped filament lying along the
polarity inversion line resulted in a CME subsequent to a two-ribbon
flare. Interaction of overlying field lines across the filament with
the side-lobe field lines, associated EUV brightening, and flux
emergence/cancelation around the filament were the observational
signatures of the processes leading to its destabilization and the
onset of eruption. Moreover, the time profile of the rising motion of
the filament/flux rope corresponded well with flare characteristics,
viz., the reconnection rate and hard X-ray emission profiles. The flux
rope was accelerated to the maximum velocity as a CME at the peak phase
of the flare, followed by deceleration to an average velocity of 590 km
s−1. We suggest that the observed emergence/cancelation
of magnetic fluxes near the filament caused it to rise, resulting
in the tethers to cut and reconnection to take place beneath the
filament; in agreement with the tether-cutting model. The corresponding
increase/decrease in positive/negative photospheric fluxes found in
the post-peak phase of the eruption provides unambiguous evidence of
reconnection as a consequence of tether cutting.

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Title: High Energy Emissions from Young Stellar Objects
Authors: Das, A. C.; Ambastha, Ashok
2012JApA...33....1D Altcode: 2012JApA..tmp...40D
X-ray emissions from Young Stellar Objects (YSO) are detected by
many X-ray missions that are providing important information about
their properties. However, their emission processes are not fully
understood. In this research note, we propose a model for the generation
of emissions from a YSO on the basis of a simple interaction between
the YSO and its surrounding circumstellar accretion disc containing
neutral gas and charged dust. It is assumed that the YSO has a weak
dipole type magnetic field and its field lines are threaded into the
circumstellar disc. Considering the motion of ions and charged dust
particles in the presence of neutral gas, we show that the sheared
dust-neutral gas velocities can lead to a current along the direction
of ambient magnetic field. Magnitude of this current can become large
and is capable of generating an electric field along the magnetic
field lines. It is shown how the particles can gain energy up to MeV
range and above, which can produce high-energy radiations from the YSO.

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Title: Velocity and Magnetic Transients Driven by the X2.2 White-light
Flare of 2011 February 15 in NOAA 11158
Authors: Maurya, R. A.; Vemareddy, P.; Ambastha, A.
2012ApJ...747..134M Altcode: 2011arXiv1106.4166M
The first X-class flare of the current solar cycle 24 occurred in
Active Region NOAA 11158 during its central meridian passage on 2011
February 15. This two-ribbon white-light flare was observed by the
Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics
Observatory. During the peak phase of the flare, we detected magnetic
and Doppler velocity (DV) transients appearing near the umbral boundary
of the main sunspot. These transients persisted for a few minutes and
showed spatial and temporal correspondence with the flare kernels. The
observed magnetic polarity at the transients' locations underwent a sign
reversal, together with a large enhancement in DVs. We explain this
observational phenomenon using the HMI spectral data obtained before,
during, and after the flare. These changes were reflected in the maps
of the active region in all the Stokes parameters. Association of the
transient features with various signatures of the flare and the cause
and effects of their appearance are also presented on the basis of
present theoretical models.

---------------------------------------------------------
Title: Spectral line profile changes associated with energetic
solar transients
Authors: Ambastha, Ashok; Maurya, Ram A.
2012ASInC...6..197A Altcode: 2012arXiv1203.3326A
Solar energetic transients occurring in solar atmosphere are associated
with catastrophic release of energy in the solar corona. These
transients inject a part of their energy by various physical processes
to the deeper, denser photospheric layer at which velocity and
magnetic fields are measured using suitable spectral lines. Serious
questions have been raised about the nature of the observed magnetic
(and velocity) field changes associated with energetic transients as
their measurements are expected to be affected by flare-induced line
profile changes. In this paper, we shall discuss some recent progress on
our understanding of the physical processes associated with such events.

---------------------------------------------------------
Title: Kinetic and magnetic helicities in solar active regions
Authors: Maurya, Ram Ajor; Ambastha, Ashok; Reddy, Vema
2011JPhCS.271a2003M Altcode:
We have studied the kinetic and magnetic helicities in sub-photospheric
flows and photospheric magnetic fields, respectively, of a sample of
91 ARs of solar cycle 23. Hemispheric trend is investigated in the
kinetic helicity of sub-photospheric flows averaged in the depth range
of 2.5-12 Mms. Magnetic helicity parameters for the ARs are derived
using photospheric vector magnetograms to examine their correlation
with the corresponding kinetic helicities. We found no significant
association between the two helicity parameters.

---------------------------------------------------------
Title: Variations in p-mode parameters and sub-surface flows of
active regions with flare activity
Authors: Maurya, R. A.; Ambastha, A.
2011ASInC...2..189M Altcode: 2011arXiv1106.4180M
We examine the characteristic properties of photospheric p-modes and
sub-photospheric flows of active regions (ARs) observed during the
period of 26-31 October 2003. Using ring diagram analysis of Doppler
velocity data obtained from the Global Oscillations Network Group
(GONG), we have found that p-mode parameters evolve with ARs and show a
strong association with flare activity. Sub-photospheric flows, derived
using inversions of p-modes, show strong twist at the locations of ARs,
and large variation with flare activity.

---------------------------------------------------------
Title: Sub-surface Meridional Flow, Vorticity, and the Lifetime of
Solar Active Regions
Authors: Maurya, R. A.; Ambastha, A.
2010ApJ...714L.196M Altcode: 2010arXiv1003.5273M
Solar sub-surface fluid topology provides an indirect approach to
examine the internal characteristics of active regions (ARs). Earlier
studies have revealed the prevalence of strong flows in the interior
of ARs having complex magnetic fields. Using the Doppler data obtained
by the Global Oscillation Network Group project for a sample of 74
ARs, we have discovered the presence of steep gradients in meridional
velocity at depths ranging from 1.5 to 5 Mm in flare productive ARs. The
sample of these ARs is taken from the Carrington rotations 1980-2052
covering the period 2001 August-2007 January. The gradients showed
an interesting hemispheric trend of negative (positive) signs in the
northern (southern) hemisphere, i.e., directed toward the equator. We
have discovered three sheared layers in the depth range of 0-10 Mm,
providing evidence of complex flow structures in several ARs. An
important inference derived from our analysis is that the location of
the deepest zero vertical vorticity is correlated with the remaining
lifetime of ARs. This new finding may be employed as a tool for
predicting the life expectancy of an AR.

---------------------------------------------------------
Title: A Technique for Automated Determination of Flare Ribbon
Separation and Energy Release
Authors: Maurya, R. A.; Ambastha, A.
2010SoPh..262..337M Altcode: 2009arXiv0910.4245M; 2010SoPh..tmp...21M
We present a technique for automatic determination of flare ribbon
separation and the energy released during the course of two-ribbon
flares. We have used chromospheric Hα filtergrams and photospheric
line-of-sight magnetograms to analyse flare ribbon separation and
magnetic field structures, respectively. Flare ribbons were first
enhanced and then extracted by the technique of "region growing", i.e.,
a morphological operator to help resolve the flare ribbons. Separation
of flare ribbons was then estimated from the magnetic-polarity reversal
line using an automatic technique implemented into an Interactive Data
Language (IDLTM) platform. Finally, the rate of flare-energy
release was calculated using photospheric magnetic field data and the
corresponding separation of the chromospheric Hα flare ribbons. This
method could be applied to measure the motion of any feature of interest
(e.g., intensity, magnetic, Doppler) from a given point of reference.

---------------------------------------------------------
Title: Magnetic and Velocity Field Changes Related to the Solar
Flares of 28 and 29 October 2003
Authors: Maurya, R. A.; Ambastha, A.
2010ASSP...19..517M Altcode: 2010mcia.conf..517M; 2009arXiv0906.3965M
Magnetic and velocity field measurements of solar active regions
suffer from ambiguities caused by the change in spectral line profiles
that occur during the impulsive phase of a major flare. This leads
to difficulties in correct interpretation of any flare-related
changes. Using magnetic and Doppler movies taken with GONG and MDI,
we have detected transient, "moving" features around the peak phases
of the X17.2/4B flare observed on 28 October 2003 and the X10/2B flare
observed on 29 October 2003 in super-active region NOAA 10486. These
features were located near the compact acoustic sources reported earlier
by Donea and Lindsey (2005) and the seismic sources reported by Zharkova
and Zharkov (2007).We find a moving feature, spatially and temporally
associated with the flare ribbons, that separates away at speeds
ranging from 30 to 50 km s-1 as observed in photospheric
white light and in temperature-minimum (1600 Å), chromospheric (Hα),
and transition-region (284Å ) intensities.We suggest that such moving
features arise from the line-profile changes attributed to downward
electron jets associated with the flare, and do not reflect real changes
in the photospheric magnetic and velocity fields. However, abrupt and
persistent changes in the pre- and post-flare phases were also found,
which do not seem to be affected by line-profile changes. The detailed
results have been appeared in Maurya and Ambastha (2009).

---------------------------------------------------------
Title: Flows in Flaring and Dormant Active Regions
Authors: Maurya, R. A.; Ambastha, A.
2010ASSP...19..516M Altcode: 2010mcia.conf..516M
During cycle 23, some active regions (ARs) produced extremely energetic
flares and coronal mass ejections. These ARs are expected to be distinct
from dormant ARs and quiet regions (QRs). It is of interest to identify
whether the internal structure and dynamics of ARs is related to their
outburst activity. For this investigation, we have obtained subsurface
velocity flows in several ARs and QRs using ring diagram analysis, and
derived the corresponding vorticities and kinetic helicity densities.

---------------------------------------------------------
Title: Heliophysical Processes
Authors: Gopalswamy, Natchimuthuk; Hasan, S.; Ambastha, Ashok
2010ASSP...18.....G Altcode: 2010hepr.book.....G
No abstract at ADS

---------------------------------------------------------
Title: The Association of Energetic Events with p-Mode Energy
Authors: Maurya, Ram Ajor; Ambastha, Ashok
2010cosp...38.3025M Altcode: 2010cosp...38.3025A; 2010cosp.meet.3025M
The energetic events, e.g., flares, CMEs, etc. release large amount
of energy, which may be able to excite acoustic waves (p-mode) by
exerting mechanical impulse of the thermal expansion of the flare on
the photosphere. This implied that during an energetic flare, energy
of p-modes must be weighted by the energy of excited modes. To get
the inferences of flare related enhances of p-mode energy, we have
derived the magnetic energy released during a flare from magnetic
field observations and associated p-mode energy from ring-diagram
analysis. For the statistical studied of their association, we took
samples of several high energy flares from Carrington rota-tions
1980-2052 covering the period August 2001-January 2007. We find
significant relationship between them. A detailed description of our
findings will be presented in the paper.

---------------------------------------------------------
Title: Solar Interior
Authors: Ambastha, Ashok
2010ASSP...18...15A Altcode: 2010hepr.book...15A
The solar interior is not visible by direct means, and until a
few decades back its understanding was based only on the surface
observations pertaining to its global properties (e.g., temperature,
luminosity, radius, etc.). Solar neutrinos and global solar oscillations
have provided more "direct" probes of the Sun's internal structure and
dynamics. In these lectures we discuss some aspects of standard solar
models, the neutrino problem and recent developments in helioseismology.

---------------------------------------------------------
Title: Variations in p-Mode Parameters with Changing Onset Time of
a Large Flare
Authors: Maurya, R. A.; Ambastha, A.; Tripathy, S. C.
2009ApJ...706L.235M Altcode: 2009arXiv0910.4247M
It is expected that energetic solar flares releasing a large amount
of energy at the photosphere may be able to excite the acoustic
(p-) modes of oscillations. We have determined the characteristic
properties of mode parameters by applying the ring diagram technique
to three-dimensional power spectra obtained for solar active region
NOAA 10486 during the long-duration energetic X17.2/4B flare of 2003
October 28. Strong evidence of substantial increase in mode amplitude
and systematic variations in sub-surface flows, i.e., meridional
and zonal components of velocity, kinetic helicity, and vorticity,
is found from comparison of the pre- to the post-flare phases.

---------------------------------------------------------
Title: Transient Magnetic and Doppler Features Related to the
White-Light Flares in NOAA 10486
Authors: Maurya, R. A.; Ambastha, A.
2009SoPh..258...31M Altcode: 2009arXiv0903.2138M
Rapidly moving transient features have been detected in magnetic
and Doppler images of super-active region NOAA 10486 during the
X17/4B flare of 28 October 2003 and the X10/2B flare of 29 October
2003. Both these flares were extremely energetic white-light events. The
transient features appeared during impulsive phases of the flares and
moved with speeds ranging from 30 to 50 km s−1. These
features were located near the previously reported compact acoustic
(Donea and Lindsey, Astrophys. J.630, 1168, 2005) and seismic sources
(Zharkova and Zharkov, Astrophys. J.664, 573, 2007). We examine the
origin of these features and their relationship with various aspects
of the flares, viz., hard X-ray emission sources and flare kernels
observed at different layers: i) photosphere (white-light continuum),
ii) chromosphere (Hα 6563 Å), iii) temperature minimum region (UV
1600 Å), and iv) transition region (UV 284 Å).

---------------------------------------------------------
Title: Magnetic and velocity field variations in the active regions
NOAA 10486 and NOAA 10488
Authors: Maurya, Ram Ajor; Ambastha, Ashok
2008JApA...29..103M Altcode:
We study the magnetic and velocity field evolution in the two
magnetically complex active regions NOAA 10486 and NOAA 10488 observed
during October-November 2003. We have used the available data to
examine net flux and Doppler velocity time profiles to identify changes
associated with evolutionary and transient phenomena. In particular,
we report detection of rapid moving features observed in NOAA 10486
during the maximum phase of the X17.2/4B superflare of October 28,
2003. The velocity of this moving feature is estimated around 40 km/s,
i.e., much greater than the usual Hα flare-ribbons' separation speed
of 3-10 km/s, but similar to the velocity of seismic waves, i.e.,
∼45 km/s reported earlier by Kosovichev &amp; Zharkova (1998).

---------------------------------------------------------
Title: Hα intensity oscillations in large flares
Authors: Maurya, Ram Ajor; Ambastha, Ashok
2008JApA...29..249M Altcode:
We reinvestigate the problem of HÎ± intensity oscillations in large
flares, particularly those classified as X-class flares. We have used
high spatial and temporal resolution digital observations obtained
from Udaipur Solar Observatory during the period 1998-2006 and selected
several events. Normalized Lomb-Scargle periodogram method for spectral
analysis was used to study the oscillatory power in quiet and active
chromospheric locations, including the flare ribbons.

---------------------------------------------------------
Title: Helioseismic effects of energetic transients
Authors: Ambastha, Ashok
2008JApA...29...93A Altcode:
Photospheric and chromospheric signatures related to large, energetic
transients such as flares and CMEs, have been extensively reported
during the last several years. In addition, energetic solar transients
are expected to cause helioseismic effects. Some of the recent
results are reviewed here; in particular, the helioseismic effects of
the powerful flares in superactive region, NOAA 10486, including the
4B/X17 superflare of October 28, 2003. We also examine the temporal
variations of power in low-l modes during the period May 1995-October
2005, and compare with daily, disk-integrated flare- and CME-indices
to infer the effect of transients on the scale of whole solar disk.

---------------------------------------------------------
Title: Excitation of solar p-modes by energetic transients
Authors: Ambastha, Ashok
2008BASIP..25...29A Altcode: 2008BASI...25S..29A
Several recent studies have reported helioseismic effects of large
flares on solar p-mode characteristics. Using 3-D power spectra of
p-mode oscillations, we found statistically significant p-mode power
enhancement corresponding to large flares at the scale of individual
active regions. An important result is the presence of steep gradient
in the meridional velocity in the sub-photospheric regions below a depth
of 5 Mm in flare productive active regions. In order to explore whether
energetic transients have a role in p-mode excitation on the global
scale, we have analysed low-l mode power time-series over the period
of May 1995-October 2005; now available from GONG network. It shows a
poor correlation between the running means of disk-integrated flare-
(and CME-) indices and the mode power. This behaviour is also reflected
in the distribution of mode power. The variations in the running mean
mode power corresponding to l=0 modes with different radial orders
are generally stochastic in nature as predicted theoretically.

---------------------------------------------------------
Title: Signatures of large flares on photospheric magnetic and
velocity fields
Authors: Ambastha, A.
2007BASI...35..419A Altcode:
We have analysed the spatial and temporal evolution of photospheric
magnetic and doppler velocities in active regions, particularly in
the superactive region NOAA 10486, to detect pre- and post-flare
changes. These findings have been compared with recent reports
by other workers, and significance of these results has been
discussed. Helioseismic response of large flares, and the role of
sub-photospheric flows in flare-productive as compared to that in less
flare-productive active regions are presented.

---------------------------------------------------------
Title: Evolution of Magnetic and Velocity Fields in Super-active
Region NOAA10486 and the Large 4B/X17.2 Flare of October 28, 2003
Authors: Ambastha, A.
2007SunGe...2...13A Altcode:
We have used high cadence GONG + photospheric magnetograms, dopplergrams
and Udaipur Solar Observatory (USO) chromospheric Hα-filtergrams to
study the spatial and temporal evolution of the active region NOAA
10486 in relation to the X17.2/4B flare of October 28, 2003. New flux
emergences, large proper motions and development of steady velocity
flows have been identified around the flare site. In addition, filament
activation and eruption leading to fast CMEs were noticed. During the
flare, NOAA 10486 was located near the disk-center; well suited for the
ring diagram analysis. Therefore, we have obtained the 3-D power spectra
to search for helioseismic response of the large flare on the amplitude,
frequency and width of the p-modes. Power enhancement was found during
the post-flare phase, and NOAA 10486 possessed steep gradient in the
meridional velocity as compared to the less flare-productive active
regions.

---------------------------------------------------------
Title: Solar Activity
Authors: Ambastha, Ashok
2007AIPC..919..173A Altcode:
Understanding the solar activity is a fundamental problem which has
essentially led to create modern solar physics. The Sun's magnetic
field and differential rotation give rise to much complexity, in
particular, to solar activity over a large range in both spatial
and temporal scales. Explosive transient events occur in the solar
atmosphere in shorter time-scales of minutes to hours, such as,
solar flares and coronal mass ejections (CMEs). At the longer scales,
the most notable is the solar activity cycle of 11 years, or magnetic
cycle of 22 years. Understanding of the solar activity is important
as it affects the space weather, i.e., the interplanetary medium
and geo-magnetic environment. We present an account of the recent
developments in our understanding of these phenomena, using both
space-borne and ground-based observations.

---------------------------------------------------------
Title: Signatures of Large Flares on Photospheric Magnetic and
Velocity Fields
Authors: Ambastha, A.
2006ihy..workE..26A Altcode:
a_ambastha@rediffmail.com We have studied spatial and temporal evolution
of some flare productive active regions using high cadence photospheric
magnetograms and Dopplergrams. In addition, chromospheric H-alpha
filtergrams have been used to identify flux emergences, large proper
motions and development of velocity flows in relation to the flare
sites. Magnetic flux and velocity changes have been found at these
sites before and after large flares. The 3-D power spectra of p-mode
oscillations have been obtained using ring diagram technique. These
spectra are then used to look for helioseismic response of the flares
on the amplitude, frequency and width of the p-modes. In the flaring
active region, p-mode power enhancement and a steep gradient in the
meridional velocity are found as compared to the quiet regions. A
comparison of flaring active regions has been carried out with less
productive active regions.

---------------------------------------------------------
Title: Possibility of Excitation of Low-ℓ P-Modes by Energetic
Solar Transients
Authors: Ambastha, Ashok; Antia, H. M.
2006SoPh..238..219A Altcode: 2006SoPh..tmp...63A
We examine the temporal variation of power in low-ℓ modes using GONG
data for the period of May 1995-October 2005 and compare this with
disk-integrated flare and CME indices. A poor correlation between
the running means of Flare Index and mode power is found. A similar
result is found for CME Index also. Variations in the running mean mode
power corresponding to ℓ = 0 modes with different radial orders are
generally stochastic in nature. This behaviour is also reflected in
the distribution of mode power.

---------------------------------------------------------
Title: Photospheric, Chromospheric and Helioseismic Signatures of
a Large Flare in Super-active Region NOAA 10486
Authors: Ambastha, Ashok
2006JApA...27..255A Altcode:
NOAA 10486 produced several powerful flares, including the 4B/X17.2
superflare of October 28, 2003/11:10 UT. This flare was extensively
covered by the Hα and GONG instruments operated at the
Udaipur Solar Observatory (USO). The central location of the active
region on October 28, 2003 was well-suited for the ring diagram analysis
to obtain the 3-D power spectra and search for helioseismic response
of this large flare on the amplitude, frequency and width of the
p-modes. Further, using USO observations, we have identified the sites
of new flux emergences, large proper motions and line-of-sight velocity
flows in the active region and their relationship with the flare.

---------------------------------------------------------
Title: Hα Observations of 8 June, 2004 Venus Transit
Authors: Ambastha, Ashok; Ravindra, B.; Gosain, Sanjay
2006SoPh..233..171A Altcode:
The cosmic event of Venus transit across the solar disk occurred on 8
June, 2004. The previous such event was witnessed about 122 years ago
on 6 December, 1882. We observed this rare transit in Hα
6563 Å line-center from Udaipur Solar Observatory (USO) using both
the full-disk and small field-of-view solar telescopes. In the earlier
historical transits, a "black-drop" effect was observed in white light
images, during the contact phases. The transit of 8 June, 2004 provided
a unique opportunity to observe this effect, for the first time, in
Hα. We report that the "black-drop" effect is present in
Hα also, as in the white light observations made by the
ground-based Global Oscillation Network Group (GONG) instrument and the
space-borne Transition Region and Coronal Explorer (TRACE) satellite. We
did not observe any noticeable "aureole" (atmospheric glow) around Venus
during the ingress or egress phases. We have compared the Hα
images with the multi-wavelength data obtained from the TRACE satellite.

---------------------------------------------------------
Title: Spectral Characterization of Solar Active Region NOAA 8242
in Quiet and Sunspot Locations
Authors: Janve, Vaibhav A.; Ambastha, Ashok
2005BASI...33..360J Altcode:
We present results from the analysis of simultaneous Stokes I profile
measurements in Fe I twin line in 6302 Å region and Ca II K 3934 Å
line over NOAA 8242 on 13th June 1998. Advanced Stokes Polarimeter
(ASP) instrument of the Vacuum Tower Telescope (VTT) of Sac Peak,
NSO, U.S.A was used for obtaining these observations. Along with
the spectral data simultaneous filtergrams in G-band, Hα 6562.8 Å,
and 6118.7 Å continuum were also obtained using the UBF filter. The
slope corrected spectral data was used to construct spectroheliograms,
and the maps of spectral asymmetry, velocity, FWHM, Equivalent width
by a Gaussian fitting of spectral line profiles for both the quiet and
sunspot regions. The implications for strong and weak magnetic field
locations have been discussed.

---------------------------------------------------------
Title: Magnetic Evolution of Super-Active Region NOAA AR 10486 and
the Large 4B/X17.2 Class Flare Observed During Octbober 28, 2003
Authors: Ambastha, Ashok
2005BASI...33..364A Altcode:
Extensive flare activity was observed in super-active region NOAA10486
during its disk passage of October 22-November 04, 2003. An extremely
energetic 4B/X17.2 flare on October 28, 2003/11:10 UT was observed
from USO when the active region was located at S16E08, i.e., close to
the disk-centre. This flare was rated the third largest X-ray flare
recorded by GOES satellite, and the largest in the optical class
(4B) observed so far from USO. Chromospheric H-alpha filtergrams
were obtained before, during and in the decay phase of the two-ribbon
flare at a cadence of 3-4 seconds. The temporal and spatial structure
evolution was analyzed with the help of a movie constructed using more
than 4000 images. Magnetograms from NASA-MSFC showed large magnetic
shear around the flare site which was delineated by a large active
filament. The filament erupted as the flare progressed. In the decay
phase of the flare, a system of post-flare loops developed at the site
of the erupted filament. Observation from TRACE also exhibited these
loop structures. Associated with this flare, a fast Earthward moving
halo CME was also detected by SOHO, which initiated a major geomagnetic
storm on October 29, 2003 at 06:13 UT, i.e., within a record time of
19 hours after the flare. This large flare was followed by another
2B/X11 event on October 29, 2003/20:49 UT, not observed from USO as it
occurred in our night-time. We have used white light full disk images
and line-of-sight magnetograms obtained from SOHO-MDI for determination
of proper motion of the main sunspots and corresponding magnetic fluxes
in order to understand rapid magnetic energy build-up in the active
region, giving rise to the two large flares within such a short time.

---------------------------------------------------------
Title: Preliminary Results of Venus Transit of June 8, 2004 Observed
in Hα 6563 Å
Authors: Ravindra, B.; Ambastha, Ashok; Gosain, Sanjay
2005BASI...33..366R Altcode:
The rare cosmic event of Venus transit across the solar disk occurred
on 8th June 2004, i.e. ~ 122 years after the last such event which
occurred in observed on 6 December 1882. The event was observed
at Udaipur Solar Observatory using full-disk, as well as, small
field-of-view high resolution solar telescopes, and recorded in the
nearly monochromatic light of Hα 6563 Å. High resolution solar images
were taken at a cadence of 3 seconds during the period 05:10-05:30 UT
which covered the Ist and IInd contacts of Venus. This was repeated
during the period 11:10-11:30 UT covering the IIIrd and IVth contacts,
while the images were obtained at a lower cadence between the period of
the IInd and IIIrd contacts. Altogether, around 4000 filtergrams were
obtained. We have compared our observations with the multi-wavelength
data obtained from TRACE satellite. We studied the optical effects
that cause the "black-drop" and the "atmospheric-glow" around Venus at
the time of its I-II, and III-IV contacts. We have also compared the
difference in contact timings observed in different wavelength bands.

---------------------------------------------------------
Title: Variations in LOW-l Solar P-Modes with Flare Activity
Authors: Ambastha, A.; Antia, H. M.
2004ESASP.559..289A Altcode: 2004soho...14..289A
No abstract at ADS

---------------------------------------------------------
Title: Solar P-Mode Characteristics Associated with
Superactive-Regions Observed during OCT NOV 2003
Authors: Ambastha, A.; Basu, S.; Antia, H. M.; Bogart, R. S.
2004ESASP.559..293A Altcode: 2004soho...14..293A
No abstract at ADS

---------------------------------------------------------
Title: Flare-Induced Excitation of Solar p modes
Authors: Ambastha, Ashok; Basu, Sarbani; Antia, H. M.
2003SoPh..218..151A Altcode:
Solar flares release large amounts of energy at different layers
of the solar atmosphere, including at the photosphere in the case of
exceptionally major events. Therefore, it is expected that large flares
would be able to excite acoustic waves on the solar surface, thereby
affecting the p-mode oscillation characteristics. We have applied
the ring-diagram analysis technique to 3-D power spectra obtained
for different flare regions in order to study how flares affect the
amplitude, frequency and width of the acoustic modes. Data from the
Michelson Doppler Imager (MDI) on board the Solar and Heliospheric
Observatory (SOHO) has been used. We have used data obtained for
several active regions of the current solar cycle that have produced
flares. In most cases, during the period of high flare activity, power
in p modes appears to be larger when compared to that in non-flaring
regions of similar magnetic field strength.

---------------------------------------------------------
Title: White Light and Emission Line Polarization of Solar Corona
during TSE of June 21, 2001
Authors: Ambastha, Ashok; Gosain, Sanjay
2003BASI...31..295A Altcode:
Coronal intensity and polarization maps have been obtained for the total
solar eclipse (TSE) o o of June 21, 2001, observed from Lusaka, Zambia
(location: 28 17.5E 15 24.5S, Alt. 1300 mtr) at broadband H 6563Å
(FWHM 80Å), as well as, around the coronal emission lines 5303Å
(FWHM 12Å), and 6374Å (FWHM 12Å). The results are discussed.

---------------------------------------------------------
Title: Excitation of Solar p-mode Oscillations by Flares
Authors: Ambastha, Ashok; Basu, Sarbani; Antia, H. M.
2003BASI...31..319A Altcode:
Solar flares release large amounts of energy at different layers of
the solar atmosphere. It is, therefore, expected that major flares
would be able to excite waves, thereby affecting the p-mode oscillation
characteristics. From the analysis of MDI data, we find that power in
p-modes appears to increase for some flares, beyond the normal values
expected from the influence of magnetic field.

---------------------------------------------------------
Title: Performance Evaluation of Adaptive Optics Systems
Authors: Sridharan, Rengaswamy; Raja Bayanna, A.; Srivastava, Nandita;
Kumar, Brajesh; Ravindra, B.; Gupta, S. K.; Jain, Naresh; Ambastha,
A.; Venkatakrishnan, P.
2003BASI...31..455S Altcode:
Adaptive Optics (AO) systems improve the resolution of ground based
telescopes and allow for long exposure images. Their performance
depends on the seeing conditions at the time of observations. In this
paper, we evaluate the performance of an AO system under various seeing
conditions through simulations. Then we present the wave-front sensing
and correction schemes that would be used in the first phase of the
AO system to be developed at the Udaipur Solar Observatory.

---------------------------------------------------------
Title: Effects of flares on solar oscillation characteristics
Authors: Ambastha, Ashok; Basu, Sarbani; Antia, H. M.
2003ESASP.517..219A Altcode: 2003soho...12..219A
We use ring diagram analysis to study the effects of solar flares
on p-mode oscillation characteristics. We study the changes in the
amplitude, frequency and width of acoustic modes using data before,
during and after a few of the major flares during the current solar
cycle. Mode power is found to be enhanced during and after some flares,
though the enhancement is not seen in all flares.

---------------------------------------------------------
Title: The Active and Explosive Sun
Authors: Ambastha, A.
2003LNP...619..127A Altcode: 2003lsp..conf..127A
The Sun's magnetic field and differential rotation give rise to
much complexity in its structure and activity over a large range
in both spatial and temporal scales. The most notable among these
is the solar activity cycle of 11 years, or magnetic cycle of 22
years. On shorter time scales of a few seconds to several hours,
spectacular explosive events occur in the solar atmosphere, such
as, solar flares, prominence eruptions, and coronal mass ejections
(CMEs). The explosive energy release takes place in the form of
accelerated particles, bulk mass motion, and enhancement of radiation
over the entire electromagnetic spectrum ranging from γ-rays to radio
wavelengths. These solar transients are essentially the source of
disturbance in the interplanetary medium, and also cause geomagnetic
effects upon their encounter with the Earth. We present an account of
the recent developments in our understanding of these phenomena using
both space-borne, and ground-based observations.

---------------------------------------------------------
Title: Effect of flares on solar oscillations characteristics
Authors: Ambastha, Ashok; Basu, Sarbani; Antia, H. M.
2002ESASP.508...43A Altcode: 2002soho...11...43A
We use ring diagram analysis to study the effect of solar flares on
oscillation mode characteristics, using data from GONG+ and MDI. The
data taken around the flares of June 6-7, 2000; March 29, 2001 and
April 10-11, 2001 are studied. We find that during some flares, the
power in acoustic modes increases beyond the normal values expected
from the influence of magnetic field.

---------------------------------------------------------
Title: Digital Imaging Techniques for Solar Magnetic and Velocity
Fields
Authors: Ambastha, Ashok
2002adaa.conf..177A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: A catalogue of Solar Filament Analysis: Year 2000
Authors: Ambastha, A.; Agrawal, Reetu
2001csfa.rept.....A Altcode:
This volume is an observational catalog of solar filament/prominence
data obtained during the year 2000 at the Udaipur Solar
Observatory, Udaipur, India. Physical ephemeris, listing of observed
filament/prominences along with their characteristics, daily full disk
H-alpha and VMG (GONG) images are compiled. Some selected events of
filament eruptions are also presented.

---------------------------------------------------------
Title: A Comparison of Flux Emergence, Cancellation, and Motions
in Flaring and Non-Flaring Sites of NOAA 8038 Observed by USO
Magnetograph
Authors: Ambastha, A.; Mathew, S. K.
2001ASPC..236..313A Altcode: 2001aspt.conf..313A
No abstract at ADS

---------------------------------------------------------
Title: Magnetic field gradient and flare: study of a small flare in
NOAA 8038
Authors: Mathew, Shibu K.; Ambastha, Ashok
2000SoPh..197...75M Altcode:
Active region NOAA 8038 was observed from 10 to 13 May, 1997 using the
USO solar video magnetograph. During this period, the active region
was mostly inactive, and gave rise to only a single notable flare of
1N/C1.3 class on May 12, 1997/04:45 UT. The flare occurred in a weak
field location, but new emerging fluxes were observed prior to the
flare onset. Horizontal motions of the network photospheric magnetic
fluxes were inferred using USO and SOHO magnetograms, and velocities in
the range 300-800 m s−1 were estimated. The initial flare
brightening was observed at the flux cancellation site where magnetic
field gradients were found to increase. Detailed analyses of flux
motions, cancellation and their relation with the flare are presented.

---------------------------------------------------------
Title: A Rapidly Evolving Active Region NOAA 8032 Observed on April
15th 1997
Authors: Mathew, Shibu K.; Ambastha, Ashok
2000JApA...21..233M Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Relationship of Non-potentially and Flaring: Intercomparison
for an Mclass Flare
Authors: Ambastha, Ashok; Mathew, Shibu K.
2000JApA...21..271A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: A catalogue of Solar Filament Analysis: Year 1999
Authors: Ambastha, A.; Pathak, Kumud; Agrawal, Reetu
2000csfa.rept.....A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Dynamical properties of quiescent prominence in He D3 5876
&amp;Aring line emission
Authors: Prasad, C. Debi; Ambastha, Ashok; Mathew, Shibu K.
1999BASI...27..411P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Dynamical properties of quiescent prominence in He D3 5876
Å line emission.
Authors: Debi Prasad, C.; Ambastha, A.; Mathew, S. K.
1999BASI...27..411D Altcode:
The authors have observed a quiescent prominence with the Coudé
spectrograph of Udaipur Solar Observatory during May 24 - 25,
1995. Analysis of two dimensional spectroscopic data was carried
out in order to obtain the line shifts and line widths of He D3 5876
Å emission lines. These quantities are used to study the dynamical
properties of the observed prominence.

---------------------------------------------------------
Title: Complex H α Loop Activity in a Long Duration Flare
Authors: Prasad, D.; Gary, G.; Ambastha, A.
1999ASPC..183..523P Altcode: 1999hrsp.conf..523P
No abstract at ADS

---------------------------------------------------------
Title: Circular polarization measurement using a tunable lithium
niobate Fabry-Perot filter
Authors: Mathew, S. K.; Bhatnagar, A.; Debi Prasad, C.; Ambastha, A.
1999ASSL..243..321M Altcode: 1999sopo.conf..321M
No abstract at ADS

---------------------------------------------------------
Title: High Resolution Longitudinal Magnetic Field Measurements
Using a Fabry-Perot Lithium Niobate Filter Based Video Magnetograph
Authors: Mathew, S.; Bhatnagar, A.; Debi Prasad, C.; Ambastha, A.
1999ASPC..183..256M Altcode: 1999hrsp.conf..256M
No abstract at ADS

---------------------------------------------------------
Title: Fabry-Perot filter based solar video magnetograph
Authors: Mathew, S. K.; Bhatnagar, A.; Prasad, C. D.; Ambastha, A.
1998A&AS..133..285M Altcode:
A tunable Lithium Niobate (LiNbO_3) Fabry-Perot filter (FP) (passband
165 m Angstroms/ at 6122 Angstroms) based video magnetograph has
been designed and fabricated. This instrument is capable of providing
near simultaneous observations of photospheric longitudinal magnetic
field, chromospheric Hα , and photospheric CaI pictures using the same
telescope and back-end set-up. The magnetic field measurements are made
by using the polarization properties of the Zeeman components of the
photospheric CaI line at 6122 Angstroms/ (Landé g factor of 1.75). The
CaI line has been chosen due to its low temperature sensitivity and no
blend with other solar or atmospheric lines. A variable electro-optic
quarter wave retarder, KD(*) P (Potassium di-Deuterium Phosphate)
along with a linear polarizer is used for analyzing the circular
polarization of the Zeeman components. The filter tuned at 140 m
Angstroms/ away from the line center in the blue wing is found to give
the best linear response for the field strength up to 1500 Gauss. A
field of view (FOV) of ~ 4 x 3 arcmin on the solar disk is imaged
using a 699 x 288 pixel Cohu CCD camera in synchronous with the KD(*)
P modulation. The &lt;~mbda/4 modulation is achieved by applying +/-2100
volts to the KD(*) P to obtain alternate frames of oppositely circular
polarized images. These images are stored in separate frame buffers of
an image acquisition system. To achieve high signal to noise ratio,
a large number of images (maximum 256) are added in the respective
frame buffers and then the difference between the left and the right
circularly polarized images is obtained. This difference is related to
the magnetic field strength. On comparing the video magnetograms (VMG)
obtained at Udaipur Solar Observatory (USO) on 09 April 1997 at 09:32
UT with those taken by SOHO/MDI at 09:41 UT, it was found that all
the magnetic features matched very well in both the magnetograms. In
this paper we present the details of the instrument and examples
of observations.

---------------------------------------------------------
Title: Solar physics in India during the next solar maximum and
beyond. Proceedings. Physical Research Laboratory (Ahmedabad) Golden
Jubilee Workshop, Udaipur (India), 7 - 10 Oct 1996.
Authors: Ambastha, A.
1998BASI...26.....A Altcode:
The workshop covered widely varied topics on solar physics such as
instrumentation, magnetic fields, flares and transients, solar wind
and the interplanetary medium, solar eclipses, helioseismology and
international projects, e.g. GONG, YOHKOH, and SOHO/SUMER.

---------------------------------------------------------
Title: Emerging Flux and X-class Flares in NOAA 6555
Authors: Choudhary, Debi Prasad; Ambastha, Ashok; Ai, G.
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: Probing the Solar Interior: Hearing the Heartbeats of 1he Sun
Authors: Ambastha, A.
1998Reson...3...18A Altcode:
Recent developments in solar seismology have enabled us to observe
and analyse the vibrations of the sun, and help to probe its hidden
interior. Understanding the sun's internal structure and dynamics
promises to test and expand our knowledge of physics, cosmology, and
astrophysics. Seismic sounding of the sun has begun to shed light on
its hidden internal anatomy.

---------------------------------------------------------
Title: Dynamics of Helically Twisted Prominence of January 22, 1979
Authors: Srivastava, Nandita; Ambastha, Ashok
1998Ap&SS.262...29S Altcode: 1999Ap&SS.262...29S
We have studied the dynamics of a macroscopically twisted helical
prominence observed in Hα line on January 22, 1979 from Udaipur Solar
Observatory. The analysis carried out is similar to that of March 11,
1979 event (Srivastava et al., 1991) wherein we had studied the role
of twisted force-free magnetic fields in the prominence system. In
the present study, it is found that of the two helically braided
prominence tubes, one was dynamically more active. We have examined
the temporal evolution of force-free parameter alpha, and the axial
currents associated with the prominence system that decreased with
time. We find that the magnitude of the electric currents and also the
rate of energy release during the untwisting of the prominence was of
comparatively higher order ~ 10^30 ergs s^-1 than that of March 11, 1979
event, in agreement with the physical dimensions of the two prominences.

---------------------------------------------------------
Title: The Role of Magnetic Shear and New Emerging Fluxes in Producing
Large Solar Flares
Authors: Ambastha, Ashok
1998ASPC..140..113A Altcode: 1998ssp..conf..113A
No abstract at ADS

---------------------------------------------------------
Title: Solar Photospheric and Chromospheric Observations using a
Lithium Niobate Fabry-Perot ETALON
Authors: Debi Prasad, C.; Mathew, Shibu K.; Bhatnagar, Arvind;
Ambastha, Ashok
1998ExA.....8..125D Altcode:
We have made a narrow band tunable filter for solar observations using
a Lithium Niobate Fabry-Perot etalon. The 60 mm aperture etalon with
a free spectral range of 4.22 Å and finesse of 26 at lambda = 6122
Å has been procured from CSIRO, Australia. The wavelength tuning is
achieved by applying high voltage to the etalon substrate at the rate
of 0.45 Å per 1000 Volts. The filter is being used for imaging the
sun in Hα line and obtaining Doppler- and Magnetogram in CaI 6122 Å
line. In this paper, we present some initial observations carried out
with this filter at Udaipur Solar Observatory.

---------------------------------------------------------
Title: Chromospheric Evolution and the Flare Activity of Super-Active
Region NOAA 6555
Authors: Debi Prasad, C.; Ambastha, Ashok; Srivastava, Nandita;
Tripathy, Sushanta C.; Hagyard, Mona J.
1997JApA...18...39D Altcode:
Super-active region NOAA 6555 was highly flare productive during the
period March 21st-27th, 1991 of its disk passage. We have st udied its
chromospheric activity using high spatial resolution Hα filtergrams
taken at Udaipur along with MSFC vector magnetograms. A possible
relationship of flare productivity and the variation in shear has
been explored. Flares were generally seen in those subareas of the
active region which possessed closed magnetic field configuration,
whereas only minor flares and/or surges occurred in subareas showing
open magnetic field configuration. Physical mechanisms responsible
for the observed surges are also discussed.

---------------------------------------------------------
Title: Seismic investigation of the solar structure using GONG
frequencies
Authors: Tripathy, S. C.; Antia, H. M.; Hill, F.; Ambastha, A.
1997astro.ph..3179T Altcode:
Using the recently obtained GONG frequencies, we investigate the
properties of the solar interior by constructing solar models with
various input physics like opacities, equation of state, nuclear
reaction rates etc. The differential asymptotic inversion technique is
then used to infer the relative difference in sound speed between the
Sun and solar models. Here we apply these results to test equation of
state and different formulation for calculating the convective flux.

---------------------------------------------------------
Title: No signature of circum-solar dust ring up to 5 Rsun
from optical polarization and near IR observations of 24 October
1995 total solar eclipse.
Authors: Prasad C., D.; Ambastha, A. K.
1997KodOB..13...17P Altcode:
A circum-solar dust ring, consisting of interplanetary particles, is
believed to be situated at 4 Rsun, and temporarily variable
in anti-phase with the solar activity cycle. The signatures of such
a ring should be observable through: (1) a highly polarized scattered
solar radiation, and (2) its thermal component in near IR. The observed
coronal polarization and surface brightness were found to be consistent
with the overall expected in the corona at the solar minimum phase. The
authors report that no ring signature was observed by their experiment.

---------------------------------------------------------
Title: The Role of Magnetic Shear and New Emerging Fluxes in Producing
Large Solar Flares in NOAA AR 6555
Authors: Ambastha, Ashok; Prasad, C. Debi
1997IAUJD..19E..11A Altcode:
The evolution of a flare-productive active region NOAA AR 6555 has
been extensively studied in response to the changes taking place at
the photospheric level, such as, sunspot proper motions and emerging
fluxes. Photospheric vector magnetic fields have been used in deriving
such information as flux distributions, sites of abnormal polarities
(inverted bipoles, delta-structures), steep magnetic field gradients,
magnetic shear etc. A particular site in the active region produced
several X-class flares, while other areas were relatively dormant,
notwithstanding the existence of strong shear. It is thus inferred
that large magnetic shear alone is not sufficient to explain flare
productivity, or flare occurrence of an active region. Using high
resolution H alpha filtergrams from USO, vector magnetograms from
Huairou and MSFC, and Doppler-grams from Huairou, we find that many
other parameters contributed to the X-class flares in NOAA 6555. These
were: rapid evolution in overall magnetic properties, large sunspot
motions, new flux emergence, development of magnetic null point over
the flaring site, orthogonality of magnetic and velocity (Doppler)
neutral lines.

---------------------------------------------------------
Title: Electric Currents and Magnetic Shear Variations during Some
Flares of M- and X-Class
Authors: Ambastha, A.; Fontenla, J. M.; Hagyard, M. J.
1996mpsa.conf..533A Altcode: 1996IAUCo.153..533A
We study the magnetic field evolution during flares of M and
X-class using overlays of cotemporal halpha filtergrams and
magnetograms.Significant decrease in the area-averaged magnetic
shear was found in some cases around the flare onset-time, increasing
subsequently as the flare progressed. However, the changes were less
pronounced for relatively smaller M-class flares. Strong Lorentz forces
were found to exist at the photosphere, acting against the observed
motions of sunspots. Extrapolated potential field indicated presence
of a magnetic null at the upper chromosphere/lower coronal height,
directly above a large Halpha flare.

---------------------------------------------------------
Title: A large flare in an area of weak magnetic field and low shear:
a counter - example!
Authors: Ambastha, Ashok; Mona, Hagyard J.; West, E. A.
1995BASI...23..428A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Chromospheric, photospheric, magnetic field evolution and
flare activity of the super active region NOAA 6555
Authors: Debi Prasad, C.; Srivastava, Nandita; Tripathy, Sushantha C.;
Ambastha, Ashok
1995BASI...23..427D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Magnetic Evolution of AR 6555 Which LED to Two Impulsive,
Relatively Compact, X-Type Flares
Authors: Fontenla, J. M.; Ambastha, A.; Kalman, B.; Csepura, Gy.
1995ApJ...440..894F Altcode:
We study the evolution of the vector magnetic field and the sunspot
motions observed in AR 6555 during 1991 March 23-26. This region
displays two locations of large magnetic shear that were also sites
of flare activity. The first location produced two large (X-class)
flares during the period covered by our observations. The second
location had larger magnetic shear than the first but produced only
small (M- and C-class) flares during our observations. We study the
evolution of the photospheric magnetic field in relation to the large
flares in the first location. These flares occurred around the same
included polarity and have very similar characteristics (soft X-ray
light curves, energies, etc.). However, the whole active region has
changed substantially in the period between them. We found several
characteristics of the region that appear related to the occurrence of
these flares: (1) The flares occurred near regions of large magnetic
"shear," but not at the locations of maximum shear or maximum field. (2)
Potential field extrapolations of the observed field suggest that the
topology changed, prior to the first of the two flares, in such a way
that a null appeared in the coarse magnetic field. (3) This null was
located close to both X-class flares and remained in that location for
a few days while the two flares were observed. (4) The flaring region
has a pattern of vector field and sunspot motions in which material
is "squeezed" along the polarity inversion line. This pattern is very
different from that usually associated with shearing arcades, but it is
similar to that suggested previously by Fontenla and Davis. The vertical
electric currents, inferred from the transverse field, are consistent
with this pattern. (5) A major reconfiguration of the longitudinal
field and the vertical electric currents occurred just prior to the
first of the two flares. Both changes imply substantial variations of
the magnetic structure of the region. On the basis of the available
data we suggest that these changes made the flaring possible, and we
develop a scenario that can explain the origin of the magnetic free-
energy that was released in these flares.

---------------------------------------------------------
Title: Evolutionary and flare-associated magnetic shear variations
observed in a complex, flare-productive active region
Authors: Ambastha, Ashok; Hagyard, Mona J.; West, E. A.
1993SoPh..148..277A Altcode:
Evolution of nonpotential structures in a complex, flare-productive
active region NOAA AR 6555 has been studied during the period March
23-26, 1991, using a quantitative description of the degree of magnetic
shear at both local and regional scales. Distinct shear evolution in
four subareas of AR 6555 is inferred from daily variation of their
shear indices. Subareas which showed significant evolution in their
overall magnetic structure are found to be most active. Hα flare
ribbons mostly formed bordering, andnot within, areas of large shear
and expanded over areas of lower shear.

---------------------------------------------------------
Title: Evolution and Flare-Associated Magnetic Shear Variation in
Solar Active Regions
Authors: Ambastha, A.; Hagyard, M. J.; West, E. A.
1993BAAS...25R1189A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: A Study of Photospheric Magnetic Fields Associated with the
June 1991 Active Region
Authors: Hagyard, M. J.; West, E. A.; Smith, J. E.; Ambastha, A.;
Kenny, E. G.
1993BAAS...25Q1190H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Stability of Self-Gravitating Finite Disks
Authors: Ambastha, A.
1993icpc.proc..151A Altcode: 1993IAUCo.132..151A
No abstract at ADS

---------------------------------------------------------
Title: Filament Eruptions Flaring Arches and Eruptive Flares
Authors: Bhatnagar, A.; Ambastha, A.; Srivastava, N.
1992LNP...399...59B Altcode: 1992esf..coll...59B; 1992IAUCo.133...59B
Several cases of erupting filaments showing distinctly their feet
have been studied. Role of the feet and their anchorage with the
photosphere in maintaining filament stability is established; apart
from the footpoint separation and height criteria. Further, a homologous
series of more energetic events, namely, the flaring arches and eruptive
flares of March 5-7, 1991, suggest a repetitive restoration of magnetic
field conditions and energy build-up within a day. High resolution
H-alpha observations of these events indicate that large amount of
ejected material was siphoned out from the chromosphere through,
the top. of a low-lying compact emission loop within the active region.

---------------------------------------------------------
Title: The large two-ribbon flare of 1990 March 28.
Authors: Ambastha, A.
1991BASI...19..208A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Helically twisted prominence eruption event of 1979 March 11.
Authors: Srivastava, N.; Ambastha, A.; Bhatnagar, A.
1991BASI...19..208S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Results of GONG site survey program at the Udaipur Solar
Observatory.
Authors: Ambastha, A.; Bhatnagar, A.; Srivastava, N.; Jain, R. M.;
Gupta, S. K.; Sharma, R.; Agrawal, G.
1991BASI...19..211A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: GONG site evaluation program at Udaipur Solar Observatory
Authors: Ambastha, Ashok; Bhatnagar, Arvind; Jain, Rajmal; Srivastava,
Nandita; Gupta, Sudhir; Sharma, Richa; Agrawal, Gopal; Kumawat,
Vishnu; Hill, Frank; Fischer, George
1991BASI...19..215A Altcode:
The Global Oscillations Network Group (GONG) project to study the
internal structure and dynamic of the sun is discussed. The GONG project
will measure waves that penetrate throughout the sun's core. Power
spectra of the monthly average transparency for each calendar month were
obtained and analyzed. The resulting clear and dark time distributions
and monthly average extinction coefficients and transparency power
spectra are shown and discussed. The various possible networks from
the candidate sites participating in the GONG project are compared.

---------------------------------------------------------
Title: Evolution of helically twisted prominence structures of March
11, 1979
Authors: Srivastava, N.; Ambastha, A.; Bhatnagar, A.
1991SoPh..133..339S Altcode:
Helical structures are generally associated with many eruptive solar
prominences. Thus, study of their evolution in the solar atmosphere
assumes importance. We present a study of a flare-associated
erupting prominence of March 11, 1979, with conspicuous helically
twisted structure, observed in Hα line center. We have attempted to
understand the role played by twisted force-free magnetic fields in this
event. In the analysis, we have assumed that the helical structures
visible in Hα outline the field lines in which prominence tubes are
embedded. Untwisting of observed prominence tubes and later, formation
of open prominence structures provide evidence of restructuring of the
magnetic field configuration over the active region during the course
of prominence eruption. Temporal evolution of the force-free parameter
α is obtained for two main prominence tubes observed to be intertwined
in a rope-like structure. Axial electric currents associated with the
prominence tubes are estimated to be of the order of 1011
A which decreased with time. Correspondingly, it is estimated that the
rate of energy release was ≈ 1028 erg s−1
during the prominence eruption.

---------------------------------------------------------
Title: On impulsive and gradual optical solar flares.
Authors: Bhatnagar, A.; Ambastha, A.; Jain, R. M.; Srivastava, N.
1989sasf.confP.207B Altcode: 1988sasf.conf..207B; 1989IAUCo.104P.207B
A comparative study of H-alpha intensity and area development of several
flares is presented in an attempt to investigate characteristic features
of various types of optical solar flares.

---------------------------------------------------------
Title: The GONG site survey.
Authors: Hill, F.; Ambastha, A.; Ball, W.; Duhalde, O.; Farris,
D.; Fischer, G.; Hieda, L.; Zhen, Huang; Ingram, B.; Jackson, P.;
Jones, H.; Jones, W.; Kennewell, J.; Kunkel, W.; Kupke, R.; Labonte,
B.; Leibacher, J.; Libbrecht, K.; Lu, W.; Morrison, L.; Odell, C.;
Pallé, P.; Saá, O.; Sousa, E.; Stebbins, T.; Xiao, Suming; GONG
Site Survey Team
1988ESASP.286..209H Altcode:
The Global Oscillation Network Group (GONG) project is planning to
place six observing stations around the world to observe the solar
oscillations as continuously as possible. This paper describes the
procedures that are being used to select the six sites. The latest
results of measurements of cloud cover obtained by networks of 6
(out of 10) radiometers show a duty cycle of over 93%, with the first
diurnal sidelobe in the window power spectrum suppressed by a factor
of 400. The results are in good agreement with the predictions of a
computer model of the expected cloud cover at individual sites.

---------------------------------------------------------
Title: Proceedings of the Indo-US Workshop on Interplanetary
Scintillations and Solar Activity
Authors: Bhonsle, R. V.; Bhatnagar, A.; Dryer, M.; Alurkar, S. K.;
Ambastha, A.; Vats, H. O.
1988issa.conf.....B Altcode:
Under the Indo-US sub-commission on science and technology, the
Department of Science and technology had approved two projects for
the study of solar activity and interplanetary medium in 1987. The
objectives of these projects include the study of solar activity using
optical technique and solar wind using IPS technique. These proceedings
are an attempt to capture some of the highlights of the deliberations of
the workshop organised in Udaipur and Ahmedabad during January-February
1988 to have mutual discussions by Indian and American sides on these
inter-related projects.

---------------------------------------------------------
Title: Sunspot proper motions in active region NOAA 2372 and its
flare activity during SMY period of 1980 April 4-13
Authors: Ambastha, Ashok; Bhatnagar, Arvind
1988JApA....9..137A Altcode:
Solar active region NOAA 2372 was observed extensively by the Solar
Maximum Mission (SMM) satellite and several ground-based observatories
during 1980 April 4 - 13 in the Solar Maximum Year. The authors have
studied photospheric and chromospheric observations of this active
region together with Marshall Space Flight Center magnetograms and
X-ray data from HXIS aboard the SMM satellite. In particular, they
discuss the relationship of the flare-productivity with sunspot proper
motions and emergence of new regions of magnetic flux in the active
region from its birth to its disappearance at the W-limb.

---------------------------------------------------------
Title: Umbral solar flares.
Authors: Ambastha, A.
1988BASI...16...84A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Reduction of photoheliograms by IMB-PC controlled digitizer.
Authors: Ambastha, Ashok
1988KodOB...9..201A Altcode:
A method is described here to determine heliographic coordinates
of sunspots on a photoheliogram using an IBM-PC controlled
digitizer. Sunspot proper motions and build up of magnetic energy in
solar active regions may be studied using this method. We discuss also
other applications of this technique.

---------------------------------------------------------
Title: Sunspot Motion in NOAA:2372 and Associated Flare Activity
during Solar Maximum Year Period 1980APR4-13
Authors: Ambastha, A.; Bhatnagar, A.
1987BASI...15...19A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Photographic atlas of the solar chromosphere
Authors: Ambastha, Ashok; Bhatnagar, A.
1985pasc.book.....A Altcode: 1985QB528.A45......
In this atlas, we present sequences of chromospheric photographs
of some typical solar phenomena observed during 1976-84, through a
15-cm aperture refractor in conjunction with a Halle narrow passband
H-alpha filter mounted on a 4.5 m solar spar at the Udaipur Solar
Observatory, Udaipur, India. The solar photographs are grouped to
present characteristics of solar flares, mass ejections, filament
activations, etc. The purpose of this photographic atlas is to
apprise new entrants, students, and researchers in solar physics,
both observational and theoretical, with the variety of intricate
active phenomena that occur on the Sun's chromosphere.

---------------------------------------------------------
Title: Stability of a finite disc under the influence of a spherical
halo
Authors: Ambastha, A.
1984JApA....5..285A Altcode:
The stability of finite gaseous disks against large-scale perturbations
under the influence of spherical, massive haloes have been studied. A
surface-density distribution consistent with the observed spiral-tracer
profiles in disk galaxies is considered for the disk. It is found that
growing eigenmodes with both 'trailing' and 'leading' spirals exist
in 'cold' disks for a wide range of values of the halo mass and its
radius. The amplification rates of the unstable modes reduce as the
ratio of the mass of the halo to the mass of the disk is increased. A
uniform halo is not very effective towards stabilizing the disk against
these modes. The results from the present study are considered vis-a-vis
previous studies on the global modes of self-gravitating disks.

---------------------------------------------------------
Title: On the global density waves in self-gravitating flat disks
Authors: Ambastha, A.; Varma, R. K.
1983ApJ...264..413A Altcode:
The problem of global stability and structure of a class of disk
models, both cold and 'warm', against axisymmetric and nonaxisymmetric
perturbations has been reconsidered in the form of an eigenvalue problem
involving a triply infinite matrix. Some large wavelength (or open),
nonaxisymmetric modes are found to stabilize in centrally concentrated
disks. Unstable 'leading' modes of cold disks become, gradually,
'trailing' as the thermal energy of the disk is increased. The growth
rates of short wavelength modes are lowered significantly by pressure
effects in warm disks.

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Title: Global stability of disk-bulge systems - Spiral structure of
disk galaxies
Authors: Ambastha, A.; Varma, R. K.
1982JApA....3..125A Altcode:
The spiral arms of disk galaxies are very sensitive to various
morphological properties, such as, the gas content, the disk-to-bulge
ratio, etc. Here, the stability of self-gravitating annular disks
surrounding the central rigid bulge component has been studied in
order to explain the transition from the tight spiral arms in Sa
galaxies to rather open patterns in Sc galaxies as the central amorphous
component diminishes. Smooth spiral patterns are found associated with
the dominant (or the fastest growing) modes of the system. When the
disk-to-bulge mass ratio is small, a tight pattern results restricted to
the inner regions of the disk. This pattern opens up and occupies larger
disk areas as the disk component becomes comparable to the bulge. It
is found here that the 'explosive' instabilities of the global density
waves do not occur in the presence of a massive bulge. The growth-rates
of the eigenmodes decrease as the disk-to-bulge mass ratio decreases. It
is also found that unstable modes of the annular disk can be suppressed
by increasing the thermal pressure sufficiently.

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Title: Global spiral waves in disk-bulge systems.
Authors: Ambastha, A.; Varma, R. K.
1982BASI...10...39A Altcode:
No abstract at ADS

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Title: Global density waves in self gravitating flat disks.
Authors: Ambastha, A.; Varma, R. K.
1982BASI...10...40A Altcode:
No abstract at ADS

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Title: Black Holes and Astrophysics: Compilation of Vikram Sarabhai
Memorial Lectures of Prof. S. Chandrasekhar
Authors: Ambastha, A.; Mohan, M.
1982bhac.book.....A Altcode:
No abstract at ADS

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Title: A Study on the Dynamics of Gravitational Systems
Authors: Ambastha, Ashok
1981PhDT........95A Altcode:
The origin and maintenance of spiral structures, one of the most
striking morphological properties of many galaxies, has been studied in
this thesis. While the version of the density wave theory as proposed
by Lin and his coworkers has explained many observational facts,
it appears to be incomplete and unsatisfactory on many counts. In
order to maintain the spiral density waves against various damping
mechanisms it is necessary to find methods which would contribute to
the excitation of the density waves in the galactic disks. ....A more
complete global theory of spiral patterns as the allowed, eigenfunctions
of a self-gravitating disk would require a self-consistent solution
as an eigenvalue problem with proper boundary conditions. We have
carried out such studies for various disk models with solid body as
well as differential rotation. It is found that spiral patters arise
naturally in differentially rotating disks.

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Title: Astrophysical Problems-A dynamical Approach: Compilation of
Lectures of Prof. I.P. Williams
Authors: Ambastha, A.; Anandarao, B. G.; Chandrasekhar, T.
1981apda.conf.....A Altcode:
This material is a compilation of the notes of lectures delivered
by Prof. I.P. Williams, Queen Mary College, University of London,
in March-April 1981 at the Physical Research Laboratory, Ahmedabad.

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Title: Global spiral density waves in disk galaxies.
Authors: Ambastha, A.; Verma, R. K.
1981BASI....9...80A Altcode:
No abstract at ADS

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Title: Plasma Physics and Astrophysics: Compilation of Lectures of
Prof. D. ter Haar
Authors: Mohan, M.; Ambastha, A.
1978ppa..conf.....M Altcode:
The following notes are based on the lectures delivered by Professor
D. ter Haar during his stay at the Physical Research Laboratory,
Ahmedabad, India, on the occassion of the Plasma Physics Symposium
held in November 1978.

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Title: Accretion-Induced Overstability of Density Waves in a
Self-Gravitating Disk
Authors: Ambastha, A.; Varma, R. K.
1978Ap&SS..55..459A Altcode:
A two-component differentially rotating disk of self-gravitating
particles is considered in the hydrodynamical framework. This system
is shown to sustain two pairs of density waves, corresponding to
the familiar Jeans modes and an acoustic type of modes (similar to
ion-acoustic modes in plasmas). As a result of mass and momentum
transfer from the gaseous to the stellar component (an accretion
process), the acoustic modes suffer a strong damping, whereas the Jeans
modes which were oscillatory, now become overstable provided the thermal
velocity of stars is larger than that of gaseous component. The waves
with frequencies near the corotation have a rather large growth
rate. This amplification can explain the maintenance of spiral
structure and a ‘selective’ amplification could even determine
the wave-frequency (or pattern velocity).

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Title: Gravitational Kinetic Theory : Compilation of Lectures of
Prof. George Severne)
Authors: Ambastha, A.
1977gkt..rept.....A Altcode:
No abstract at ADS