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Author name code: venkatakrishnan
ADS astronomy entries on 2022-09-14
author:"Venkatakrishnan, Paramewswaran"
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Title: Fizeau Mask Interferometry of Solar Features Using the
Multi-application Solar Telescope at the Udaipur Solar Observatory
Authors: Raja Bayanna, A.; Venkatakrishnan, P.; Rengaswamy, Sridharan;
Mathew, Shibu K.
2020SoPh..295...30R Altcode:
Efforts are made to demonstrate high-resolution observations of the
solar atmosphere using spatial interferometry. Covering the telescope
pupil with a Fizeau mask consisting of two small circular apertures
separated by a vector distance known as the baseline is the first step
towards interferometric imaging. A mask with two circular holes of
diameter 7 cm each and separated by a distance of 19 cm is placed in the
pupil plane of the Multi-application solar telescope at Udaipur solar
observatory. The fringe pattern observed in the image plane signifies
the presence of solar structures with sizes smaller than the fringe
period. The study is extended with baselines of 29 cm and 38 cm. It
is observed that an increase in the baseline causes a reduction in
the fringe period and the fringe contrast. Observations are carried
out in two spectral lines/bands, centered at 656.3 nm and 861.0 nm
using filters of bandwidth 1 nm and 330 nm, respectively. The effect
of bandwidth on the fringe visibility is also discussed based on the
bandwidth decorrelation function.
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Title: The Multi Application Solar Telescope
Authors: Venkatakrishnan, P.; Mathew, Shibu K.; Srivastava, Nandita;
Bayanna, A. R.; Kumar, Brajesh; Ramya, Bireddy; Jain, Naresh;
Saradava, Mukesh
2017CSci..113..686V Altcode:
No abstract at ADS
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Title: Imaging Spectropolarimeter for the Multi-Application
Solar Telescope at Udaipur Solar Observatory: Characterization of
Polarimeter and Preliminary Observations
Authors: Tiwary, Alok Ranjan; Mathew, Shibu K.; Bayanna, A. Raja;
Venkatakrishnan, P.; Yadav, Rahul
2017SoPh..292...49T Altcode: 2017arXiv170107019R
The Multi-Application Solar Telescope (MAST) is a 50 cm off-axis
Gregorian telescope that has recently become operational at the
Udaipur Solar Observatory (USO). An imaging spectropolarimeter is
being developed as one of the back-end instruments of MAST to gain a
better understanding of the evolution and dynamics of solar magnetic
and velocity fields. This system consists of a narrow-band filter and
a polarimeter. The polarimeter includes a linear polarizer and two sets
of liquid crystal variable retarders (LCVRs). The instrument is intended
for simultaneous observations in the spectral lines 6173 Å and 8542 Å,
which are formed in the photosphere and chromosphere, respectively. In
this article, we present results from the characterization of the
LCVRs for the spectral lines of interest and the response matrix of
the polarimeter. We also present preliminary observations of an active
region obtained using the spectropolarimeter. For verification purposes,
we compare the Stokes observations of the active region obtained from
the Helioseismic Magnetic Imager (HMI) onboard the Solar Dynamics
Observatory (SDO) with that of MAST observations in the spectral
line 6173 Å. We find good agreement between the two observations,
considering the fact that MAST observations are limited by seeing.
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Title: Inferring the Magnetic Structure of a Sunspot from
Decomposition of Photospheric Vertical Current Density into Twist
and Shear Components
Authors: Venkatakrishnan, P.
2015AGUFMSH13D2455V Altcode:
The electric current density, derived from the curl of the vector
magnetic field, can be decomposed into the so-called twist and
shear components. We examined this decomposition for 2 highly flare
productive active regions undergoing magnetic flux emergence, viz. AR
10930 observed by SOT aboard Hinode and AR 11158 observed by HMI aboard
SDO. We found the following common evolutionary behaviour in both the
cases: 1. The percentage of variation of the spatially averaged value
of the ratio of the shear current density to the twist current density
is much smaller than the percentage of variation of the twist current
throughout the evolution. 2. The average ratio was negative in sign,
viz. the shear current density was opposite in sign to the twist current
density in most of the pixels. 3. The magnitude of the average ratio
increased with increase of pixel size. The above 3 results were examined
for two extreme kinds of magnetic structure: a) a monolithic continuous
didtribution of magnetic flux and b) an intermittent cluster of magnetic
fibrils separated by field free regions. We arrive at the conclusion
that the above mentioned observations tend to favor the monolithic
model of sunspot magnetic field over the fibril model, although a
fibril structure at deeper layers cannot be ruled out. Observations of
vector magnetic fields of sunspots using the infrared FeI line around
1560 nm could perhaps resolve this issue.
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Title: Division II: Commission 12: Solar Radiation and Structure
Authors: Kosovichev, Alexander; Cauzzi, Gianna; Martinez Pillet,
Valentin; Asplund, Martin; Brandenburg, Axel; Chou, Dean-Yi;
Christensen-Dalsgaard, Jorgen; Gan, Weiqun; Kuznetsov, Vladimir D.;
Rovira, Marta G.; Shchukina, Nataliya; Venkatakrishnan, P.
2015IAUTB..28..109K Altcode:
The President of C12, Alexander Kosovichev, presented the status of
the Commission and its working Group(s). Primary activities included
organization of international meetings (IAU Symposia, Special Sessions
and Joint Discussion); review and support of proposals for IAU sponsored
meetings; organization of working groups on the Commission topics
to promote the international cooperation; preparation of triennial
report on the organizational and science activities of Commission
members. Commission 12 broadly encompasses topics of solar research
which include studies of the Sun's internal structure, composition,
dynamics and magnetism (through helioseismology and other techniques),
studies of the quiet photosphere, chromosphere and corona, and also
research of the mechanisms of solar radiation, and its variability on
various time scales. Some overlap with topics covered by Commission
10 Solar Activity is unavoidable, and many activities are sponsored
jointly by these two commissions. The Commission website can be found
at http://sun.stanford.edu/IAU-Com12/, with information about related
IAU Symposiums and activities, and links to appropriate web sites.
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Title: Near-Sun speed of CMEs and the magnetic nonpotentiality of
their source active regions
Authors: Tiwari, Sanjiv K.; Falconer, David A.; Moore, Ronald L.;
Venkatakrishnan, P.; Winebarger, Amy R.; Khazanov, Igor G.
2015GeoRL..42.5702T Altcode: 2015arXiv150801532T
We show that the speed of the fastest coronal mass ejections (CMEs)
that an active region (AR) can produce can be predicted from a
vector magnetogram of the AR. This is shown by logarithmic plots of
CME speed (from the SOHO Large Angle and Spectrometric Coronagraph
CME catalog) versus each of ten AR-integrated magnetic parameters
(AR magnetic flux, three different AR magnetic-twist parameters,
and six AR free-magnetic-energy proxies) measured from the vertical
and horizontal field components of vector magnetograms (from the
Solar Dynamics Observatory's Helioseismic and Magnetic Imager)
of the source ARs of 189 CMEs. These plots show the following: (1)
the speed of the fastest CMEs that an AR can produce increases with
each of these whole-AR magnetic parameters and (2) that one of the AR
magnetic-twist parameters and the corresponding free-magnetic-energy
proxy each determine the CME-speed upper limit line somewhat better
than any of the other eight whole-AR magnetic parameters.
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Title: Membrane-based deformable mirror: intrinsic aberrations and
alignment issues
Authors: Raja Bayanna, A.; Louis, Rohan E.; Chatterjee, S.; Mathew,
Shibu K.; Venkatakrishnan, P.
2015ApOpt..54.1727R Altcode: 2015arXiv150204424R
A Deformable Mirror (DM) is an important component of an Adaptive
Optics system. It is known that an on-axis spherical/parabolic optical
component, placed at an angle to the incident beam introduces defocus
as well as astigmatism in the image plane. Although the former can be
compensated by changing the focal plane position, the latter cannot be
removed by mere optical re-alignment. Since the DM is to be used to
compensate a turbulence-induced curvature term in addition to other
aberrations, it is necessary to determine the aberrations induced by
such (curved DM surface) an optical element when placed at an angle
(other than 0 degree) of incidence in the optical path. To this effect,
we estimate to a first order, the aberrations introduced by a DM as a
function of the incidence angle and deformation of the DM surface. We
record images using a simple setup in which the incident beam is
reflected by a 37 channel Micro-machined Membrane Deformable Mirror
for various angles of incidence. It is observed that astigmatism is a
dominant aberration which was determined by measuring the difference
between the tangential and sagital focal planes. We justify our results
on the basis of theoretical simulations and discuss the feasibility
of using such a system for adaptive optics considering a trade-off
between wavefront correction and astigmatism due to deformation.
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Title: Speed of CMEs and the magnetic non-potentiality of their
source active regions
Authors: Tiwari, S. K.; Falconer, D. A.; Moore, R. L.; Venkatakrishnan,
P.
2014AGUFMSH21C4134T Altcode:
Most fast coronal mass ejections (CMEs) originate from solar active
regions (ARs). Non-potentiality of ARs is expected to determine the
speed and size of CMEs in the outer corona. Several other unexplored
parameters might be important as well. To find out the correlation
between the initial speed of CMEs and the non-potentiality of source
ARs, we associated over a hundred of CMEs with source ARs via their
co-produced flares. The speed of the CMEs are collected from the SOHO
LASCO CME catalog. We have used vector magnetograms obtained mainly
with HMI/SDO, also with Hinode (SOT/SP) when available within an hour
of a CME occurence, to evaluate various magnetic non-potentiality
parameters, e.g. magnetic free-energy proxies, computed magnetic
free energy, twist, shear angle, signed shear angle etc. We have
also included several other parameters e.g. total unsigned flux, net
current, magnetic area of ARs, area of sunspots, to investigate their
correlation, if any, with the initial speeds of CMEs. Our preliminary
results show that the ARs with larger non-potentiality and area mostly
produce fast CMEs but they can also produce slower ones. The ARs with
lesser non-potentiality and area generally produce only slower CMEs,
however, there are a few exceptions. The total unsigned flux correlate
with the non-potentiality parameters and area of ARs but some ARs with
large unsigned flux are also found to be least non-potential. A more
detailed analysis is underway. SKT is supported by an appointment to
the NASA Postdoctoral Program at the NASA Marshall Space Flight Center,
administered by Oak Ridge Associated Universities through a contract
with NASA. RLM is supported by funding from the Living With a Star
Targeted Research and Technology Program of the Heliophysics Division
of NASA's Science Mission Directorate. Support for MAG4 development
comes from NASA's Game Changing Development Program, and Johnson Space
Center's Space Radiation Analysis Group (SRAG).
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Title: A study of the relation between intensity oscillations and
magnetic field parameters in a sunspot: Hinode observations
Authors: Raja Bayanna, A.; Mathew, Shibu K.; Kumar, Brajesh; Louis,
Rohan E.; Venkatakrishnan, P.
2014RAA....14.1458R Altcode: 2014arXiv1407.7363R
We present properties of intensity oscillations of a sunspot in the
photosphere and chromosphere using G band and Ca II H filtergrams
from Hinode. Intensity power maps as function of magnetic field
strength and frequency reveal reduction of power in the G band
with an increase in photospheric magnetic field strength at all
frequencies. In Ca II H, however, stronger fields exhibit more power
at high frequencies, particularly in the 4.5-8.0 mHz band. Power
distributions in different locations of the active region show that
the oscillations in Ca II H exhibit more power compared to that of
the G band. We also relate the power in intensity oscillations with
different components of the photospheric vector magnetic field using
near simultaneous spectro-polarimetric observations of the sunspot
from the Hinode spectropolarimeter. The photospheric umbral power
is strongly anti-correlated with the magnetic field strength and
its line-of-sight component but there is a good correlation with
the transverse component. A reversal of this trend is observed in
the chromosphere except at low frequencies (ν <= 1.5 mHz). The
power in sunspot penumbrae is anti-correlated with the magnetic field
parameters at all frequencies (1.0 <= ν <= 8.0 mHz) in both
the photosphere and chromosphere, except that the chromospheric power
shows a strong correlation in the frequency range 3-3.5 mHz.
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Title: Narrow-Band Imaging System for the Multi-application Solar
Telescope at Udaipur Solar Observatory: Characterization of Lithium
Niobate Etalons
Authors: Raja Bayanna, A.; Mathew, Shibu K.; Venkatakrishnan, P.;
Srivastava, N.
2014SoPh..289.4007R Altcode: 2014arXiv1407.7627R; 2014SoPh..tmp...96R
The Multi-application Solar Telescope is a 50 cm off-axis Gregorian
telescope that has been installed at the lake site of Udaipur Solar
Observatory. For quasi-simultaneous photospheric and chromospheric
observations, a narrow-band imager has been developed as one of the
back-end instruments for this telescope. Narrow-band imaging is achieved
using two lithium niobate Fabry-Perot etalons working in tandem as a
filter. This filter can be tuned to different wavelengths by changing
either voltage, tilt, or temperature of the etalons. To characterize
the etalons, a Littrow spectrograph was set up in conjunction with a
15 cm Carl Zeiss Coudé solar telescope. The etalons were calibrated
for the solar spectral lines Fe I 6173 Å, and Ca II 8542 Å. In this
work, we discuss the characterization of the Fabry-Perot etalons,
specifically, the temperature and voltage tuning of the system for the
spectral lines proposed for observations. We present the details of
the calibration set-up and various tuning parameters. We also present
solar images obtained using the system.
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Title: Anomalous flows in a sunspot penumbra
Authors: Louis, Rohan E.; Beck, Christian; Mathew, Shibu K.;
Venkatakrishnan, P.
2014A&A...570A..92L Altcode: 2014arXiv1408.6690L
Context. The photospheric Evershed flow is a distinct characteristic of
penumbrae that is closely associated with the photometric and magnetic
structure of sunspots. <BR /> Aims: We analyse the properties of an
anomalous flow in the photosphere in a sunspot penumbra and compare it
with those of the regular Evershed flow. <BR /> Methods: High-resolution
spectropolarimetric observations of active region NOAA 11271 were
obtained with the spectro-polarimeter (SP) on board Hinode. We used the
magnetic field properties derived by a Milne-Eddington inversion. In
addition, we used Ca II H images obtained by the broad-band filter
instrument to study the low chromospheric response to the photospheric
structure and Dopplergrams from the Helioseismic and Magnetic Imager
to follow the evolution of the photospheric flows. <BR /> Results:
We detect a blueshifted feature that appeared on the limb-side
penumbra of a sunspot and that was present intermittently during
the next two hours. It exhibited a maximum blueshift of 1.6 km
s<SUP>-1</SUP>, an area of 5.2 arcsec<SUP>2</SUP>, and a maximum
uninterrupted lifetime of 1 h. The blueshifted feature, when present,
lies parallel to redshifts. Both blue- and redshifts flank a highly
inclined or horizontal magnetic structure that is radially oriented
in the penumbra. The low-cadence SP maps reveal changes in size,
radial position in the penumbra, and line-of-sight (LOS) velocity
of the blueshifted feature, from one scan to the next. There was an
increase of nearly 500 G in the field strength with the onset of the
blueshifts, particularly when the LOS velocity in the feature exceeded
1.5 km s<SUP>-1</SUP>. There was only a marginal reduction in the
field inclination of about 10° with the increase in blueshifts. In the
chromosphere, intense, arc-shaped brightenings were observed close to
the location of the photospheric blueshifts, which extend from the edge
of the umbral core to the penumbra-quiet Sun boundary. The intensity
of these brightenings exceeds the background intensity by 30% to 65%
with the strongest and largest brightenings observed about 30 min after
the strongest blueshifts were detected at the photosphere. The close
spatial proximity of the two phenomenon strongly suggests a causal
relationship. <BR /> Conclusions: The blueshifted feature represents
plasma motion that could be related to a magnetic structure that rises
in the solar atmosphere and subsequently reconnects with the ambient
chromospheric magnetic field of the sunspot or an inverse Evershed flow,
which would be unique in the photosphere. This transient phenomena is
presumably related to the dynamic stability of the sunspot because the
corresponding umbral core separated two days later at the location of
the blueshifts and fragmented subsequently.
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Title: The Evolution of the Net Twist Current and the Net Shear
Current in Active Region NOAA 10930
Authors: Suthar, Yogita; Venkatakrishnan, P.; Ravindra, B.; Jaaffrey,
S. N. A.
2014SoPh..289.2459S Altcode: 2014SoPh..tmp...30S
The electric current exists because of the non-potential magnetic
field in solar active regions. We present the evolution of net current
in the solar active region NOAA 10930 as the sum of shear current and
twist current by using 27 high-resolution vector magnetograms obtained
with Hinode/SOT-SP during 9 - 15 December 2006. This active region
was highly eruptive and produced a large number of flares ranging
from B to X class. We derived local distribution of shear and twist
current densities in this active region and studied the evolution of
net shear current (NSC) and net twist current (NTC) in the N-polarity
and S-polarity regions separately. We found the following: i) The
twist current density was dominant in the umbrae. ii) The footpoint
of the emerging flux rope showed a dominant twist current. iii) The
shear current density and twist current density appeared in alternate
bands around the umbrae. iv) On the scale of the active region, NTC
was always larger than NSC. v) Both NTC and NSC decreased after the
onset of an X3.4 class flare that occurred on 13 December 2006.
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Title: A New Technique for Solar Imaging Spectro-polarimetry using
Shack-Hartmann and Fabry-Pérot
Authors: Gosain, S.; Sankarasubramanian, K.; Venkatakrishnan, P.;
Raja Bayanna, A.
2012ASPC..463..301G Altcode:
A new technique for solar imaging spectro-polarimetry is
presented. Using the combination of a Shack-Hartmann (SH) and
a Fabry-Pérot (FP) interferometer, high-cadence spectroscopic
observations can be obtained at discrete wavelength positions
simultaneously, thereby avoiding errors due to non-simultaneity of the
wavelength scans. A SH mask is used to generate multiple images of the
same field-of-view (FOV). These multiple images when passed through the
FP in a collimated-beam arrangement are shifted in wavelength due to
the angular dependence of the FP filter transmission profile. Thus,
by re-imaging one obtains multiple images of the FOV which are
tuned to different wavelength points across the spectral line, in
a single exposure. The schematic of the setup and the laboratory
simulation of such a configuration is presented. The technique has
an advantage of simultaneity over conventional wavelength scanning
filtergraphs and has potential for observing highly-dynamic phenomena
like solar flares. Also, one can exploit the method to perform
snapshot spectropolarimetry by designing a special polarization
modulator. The limitation of this technique is that it downgrades the
spatial resolution due to the downsampling of the pupil into smaller
sub-apertures. However, for large aperture telescopes like 4 meter
class telescopes (ATST) this is not a major issue and one can still
work at sub-arcsec resolution, though not at the diffraction limit of
the full aperture.
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Title: Observations of Supersonic Downflows in a Sunspot Light Bridge
as Revealed by Hinode
Authors: Louis, R. E.; Rubio, L. R. B.; Mathew, S. K.; Venkatakrishnan,
P.
2012ASPC..454..205L Altcode: 2012arXiv1206.4347L
Recent high resolution spectropolarimetric observations from Hinode
detected the presence of supersonic downflows in a sunspot light bridge
(Louis et al. 2009). These downflows occurred in localized patches,
close to regions where the field azimuth changed by a large value. This
apparent discontinuity in the field azimuth was seen along a thin
ridge running along the western edge of the light bridge. Some, but not
all, of these downflowing patches were co-spatial with chromospheric
brightness enhancements seen in Ca II H filtergrams. The presence
of magnetic inhomogeneities at scales of 0.″3 could facilitate the
reconnection of field lines in the lower chromosphere whose signatures
might be the supersonic downflows and the brightness enhancements that
have been observed.
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Title: Coherent Lateral Motion of Penumbral Filaments during the
X-class Flare of 13 December 2006
Authors: Gosain, S.; Venkatakrishnan, P.; Tiwari, S. K.
2012ASPC..454..273G Altcode:
The high-resolution pictures of the solar photosphere from space
based 50 cm Solar Optical Telescope (SOT) onboard Hinode spacecraft,
are now routinely observed. Such images of a δ-sunspot in NOAA 10930
were obtained by Hinode during 13 December 2006 while a X-class flare
occurred in this active region. Two bright ribbons were visible even
in white light and G-band images apart from chromospheric Ca II H
images. We register the sunspot globally using cross-correlation
technique and analyse local effects during flare interval. We find
that during flare the penumbral filaments show lateral motion. Also,
we locate two patches, one in either polarity, which show converging
motion towards the polarity inversion line (PIL). In Ca II H images
we find kernel with pre-flare brightening which lie along the PIL.
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Title: Analysis of a Fragmenting Sunspot Using Hinode Observations
Authors: Louis, Rohan E.; Ravindra, B.; Mathew, Shibu K.; Bellot Rubio,
Luis R.; Raja Bayanna, A.; Venkatakrishnan, P.
2012ApJ...755...16L Altcode: 2012arXiv1205.6669L
We employ high-resolution filtergrams and polarimetric measurements
from Hinode to follow the evolution of a sunspot for eight days
starting on 2007 June 28. The imaging data were corrected for intensity
gradients, projection effects, and instrumental stray light prior to
the analysis. The observations show the formation of a light bridge
at one corner of the sunspot by a slow intrusion of neighboring
penumbral filaments. This divided the umbra into two individual
umbral cores. During the light bridge formation, there was a steep
increase in its intensity from 0.28 to 0.7 I <SUB>QS</SUB> in nearly
4 hr, followed by a gradual increase to quiet-Sun (QS) values in 13
hr. This increase in intensity was accompanied by a large reduction
in the field strength from 1800 G to 300 G. The smaller umbral core
gradually broke away from the parent sunspot nearly two days after
the formation of the light bridge, rendering the parent spot without a
penumbra at the location of fragmentation. The penumbra in the fragment
disappeared first within 34 hr, followed by the fragment whose area
decayed exponentially with a time constant of 22 hr. In comparison, the
parent sunspot area followed a linear decay rate of 0.94 Mm<SUP>2</SUP>
hr<SUP>-1</SUP>. The depleted penumbra in the parent sunspot regenerated
when the inclination of the magnetic field at the penumbra-QS boundary
became within 40° from being completely horizontal and this occurred
near the end of the fragment's lifetime. After the disappearance of the
fragment, another light bridge formed in the parent which had similar
properties as the fragmenting one, but did not divide the sunspot. The
significant weakening in field strength in the light bridge along with
the presence of granulation is suggestive of strong convection in the
sunspot, which might have triggered the expulsion and fragmentation of
the smaller spot. Although the presence of QS photospheric conditions
in sunspot umbrae could be a necessary condition for fragmentation,
it is not a sufficient one.
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Title: Observations of Supersonic Downflows near the Umbra-Penumbra
Boundary of Sunspots as Revealed by Hinode
Authors: Louis, R. E.; Bellot Rubio, L. R.; Mathew, S. K.;
Venkatakrishnan, P.
2012ASPC..455...75L Altcode: 2011arXiv1101.0751L
High resolution spectropolarimetric observations by Hinode have revealed
the existence of supersonic downflows at the umbra-penumbra boundary
of 3 sunspots (Louis et al. 2011). These downflows are observed to be
co-spatial with bright penumbral filaments and occupy an area greater
than 1.6 arcsec<SUP>2</SUP>. They are located at the center-side
penumbra and have the same polarity as the sunspot which suggests
that they are not associated with the Evershed flow. In this paper
we describe the supersonic velocities observed in NOAA AR 10923 and
discuss the photospheric as well as chromospheric brightenings that
lie close to the downflowing areas. Our observations suggest that this
phenomenon is driven by dynamic and energetic physical processes in the
inner penumbra which affect the chromosphere, providing new constraints
to numerical models of sunspots.
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Title: Commission 12: Solar Radiation and Structure
Authors: Kosovichev, Alexander; Cauzzi, Gianna; Pillet, Valentin
Martinez; Asplund, Martin; Brandenburg, Axel; Chou, Dean-Yi;
Christensen-Dalsgaard, Jorgen; Gan, Weiqun; Kuznetsov, Vladimir D.;
Rovira, Marta G.; Shchukina, Nataliya; Venkatakrishnan, P.
2012IAUTA..28...81K Altcode: 2012IAUTB..28...81K
Commission 12 of the International Astronomical Union encompasses
investigations of the internal structure and dynamics of the Sun, mostly
accessible through the techniques of local and global helioseismology,
the quiet solar atmosphere, solar radiation and its variability, and
the nature of relatively stable magnetic structures like sunspots,
faculae and the magnetic network. The Commission sees participation
of over 350 scientists worldwide.
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Title: On the Flare-induced Seismicity in the Active Region NOAA
10930 and Related Enhancement of Global Waves in the Sun
Authors: Kumar, Brajesh; Venkatakrishnan, P.; Mathur, Savita; Tiwari,
Sanjiv Kumar; García, R. A.
2011ApJ...743...29K Altcode: 2011arXiv1110.6309K
A major flare (of class X3.4) occurred on 2006 December 13 in the active
region NOAA 10930. This flare event has remained interesting to solar
researchers for studies related to particle acceleration during the
flare process and the reconfiguration of magnetic fields as well as
fine-scale features in the active region. The energy released during
flares is also known to induce acoustic oscillations in the Sun. Here,
we analyze the line-of-sight velocity patterns in this active region
during the X3.4 flare using the Dopplergrams obtained by the Global
Oscillation Network Group (GONG) instrument. We have also analyzed the
disk-integrated velocity observations of the Sun obtained by the Global
Oscillation at Low Frequency (GOLF) instrument on board the Solar and
Heliospheric Observatory spacecraft as well as full-disk collapsed
velocity signals from GONG observations during this flare to study
any possible connection between the flare-related changes seen in the
local and global velocity oscillations in the Sun. We apply wavelet
transform to the time series of the localized velocity oscillations
as well as the global velocity oscillations in the Sun spanning the
flare event. The line-of-sight velocity shows significant enhancement
in some localized regions of the penumbra of this active region during
the flare. The affected region is seen to be away from the locations of
the flare ribbons and the hard X-ray footpoints. The sudden enhancement
of this velocity seems to be caused by the Lorentz force driven by
the "magnetic jerk" in the localized penumbral region. Application of
wavelet analysis to these flare-induced localized seismic signals shows
significant enhancement in the high-frequency domain (5 <ν < 8
mHz) and a feeble enhancement in the p-mode oscillations (2 <ν <
5 mHz) during the flare. On the other hand, the wavelet analysis of GOLF
velocity data and the full-disk collapsed GONG velocity data spanning
the flare event indicates significant post-flare enhancements in the
high-frequency global velocity oscillations in the Sun, as evident
from the wavelet power spectrum and the corresponding scale-average
variance. The present observations of the flare-induced seismic signals
in the active region in context of the driving force are different as
compared to previous reports on such cases. We also find indications
of a connection between flare-induced localized seismic signals and
the excitation of global high-frequency oscillations in the Sun.
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Title: Evolution of Currents of Opposite Signs in the Flare-productive
Solar Active Region NOAA 10930
Authors: Ravindra, B.; Venkatakrishnan, P.; Tiwari, Sanjiv Kumar;
Bhattacharyya, R.
2011ApJ...740...19R Altcode: 2011arXiv1108.5818R
Analysis of a time series of high spatial resolution vector magnetograms
of the active region NOAA 10930 available from the Solar Optical
Telescope SpectroPolarimeter on board Hinode revealed that there is
a mixture of upward and downward currents in the two footpoints of
an emerging flux rope. The flux emergence rate is almost the same
in both the polarities. We observe that along with an increase in
magnetic flux, the net current in each polarity increases initially
for about three days after which it decreases. This net current is
characterized by having exactly opposite signs in each polarity while
its magnitude remains almost the same most of the time. The decrease
of the net current in both the polarities is due to the increase of
current having a sign opposite to that of the net current. The dominant
current, with the same sign as the net current, is seen to increase
first and then decreases during the major X-class flares. Evolution
of non-dominant current appears to be a necessary condition for flare
initiation. The above observations can be plausibly explained in terms
of the superposition of two different force-free states resulting in a
non-zero Lorentz force in the corona. This Lorentz force then pushes the
coronal plasma and might facilitate the magnetic reconnection required
for flares. Also, the evolution of the net current is found to follow
the evolution of magnetic shear at the polarity inversion line.
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Title: Evolution of twist-shear and dip-shear in flaring active
region NOAA 10930
Authors: Gosain, Sanjay; Venkatakrishnan, P.
2011IAUS..273..212G Altcode: 2010arXiv1010.0532G
We study the evolution of magnetic shear angle in a flare productive
active region NOAA 10930. The magnetic shear angle is defined as the
deviation in the orientation of the observed magnetic field vector with
respect to the potential field vector. The shear angle is measured in
horizontal as well as vertical plane. The former is computed by taking
the difference between the azimuth angles of the observed and potential
field and is called the twist-shear, while the latter is computed by
taking the difference between the inclination angles of the observed
and potential field and is called the dip-shear. The evolution of the
two shear angles is then tracked over a small region located over the
sheared penumbra of the delta sunspot in NOAA 10930. We find that,
while the twist-shear shows an increasing trend after the flare the
dip-shear shows a significant drop after the flare.
---------------------------------------------------------
Title: Evolution of Magnetic Field Twist and Tilt in Active Region
NOAA 10930
Authors: Ravindra, B.; Venkatakrishnan, P.; Tiwari, Sanjiv Kumar
2011aogs...27..153R Altcode: 2010arXiv1012.0120R
Magnetic twist of the active region has been measured over a decade
using photospheric vector field data, chromospheric H<SUB>α</SUB>
data, and coronal loop data. The twist and tilt of the active regions
have been measured at the photospheric level with the vector magnetic
field measurements. The active region NOAA 10930 is a highly twisted
emerging region. The same active region produced several flares and has
been extensively observed by Hinode. In this paper, we will show the
evolution of twist and tilt in this active region leading up to the two
X-class flares. We find that the twist initially increases with time
for a few days with a simultaneous decrease in the tilt until before
the X3.4 class flare on December 13, 2006. The total twist acquired
by the active region is larger than one complete winding before the
X3.4 class flare and it decreases in later part of observations. The
injected helicity into the corona is negative and it is in excess of
10<SUP>43</SUP> Mx<SUP>2</SUP> before the flares.
---------------------------------------------------------
Title: Actors of the main activity in large complex centres during
the 23 solar cycle maximum
Authors: Schmieder, B.; Démoulin, P.; Pariat, E.; Török, T.;
Molodij, G.; Mandrini, C. H.; Dasso, S.; Chandra, R.; Uddin, W.;
Kumar, P.; Manoharan, P. K.; Venkatakrishnan, P.; Srivastava, N.
2011AdSpR..47.2081S Altcode:
During the maximum of Solar Cycle 23, large active regions had a long
life, spanning several solar rotations, and produced large numbers of
X-class flares and CMEs, some of them associated to magnetic clouds
(MCs). This is the case for the Halloween active regions in 2003. The
most geoeffective MC of the cycle (Dst = -457) had its source during
the disk passage of one of these active regions (NOAA 10501) on
18 November 2003. Such an activity was presumably due to continuous
emerging magnetic flux that was observed during this passage. Moreover,
the region exhibited a complex topology with multiple domains of
different magnetic helicities. The complexity was observed to reach
such unprecedented levels that a detailed multi-wavelength analysis
is necessary to precisely identify the solar sources of CMEs and
MCs. Magnetic clouds are identified using in situ measurements and
interplanetary scintillation (IPS) data. Results from these two
different sets of data are also compared.
---------------------------------------------------------
Title: Issues with external occultation of a coronagraph
Authors: Bayanna, A. Raja; Mathew, Shibu K.; Sankarasubramanian, K.;
Venkatakrishnan, P.; Singh, J.; Prasad, B. R.
2011ExA....29..145B Altcode: 2011ExA...tmp....1B
This paper addresses some of the issues related to externally occulted
solar coronagraph; vignetting and achievable resolution due to an
external occulter. The analytical expression by Evans (J Opt Soc Am
38:1083-1085, 1948) is used to perform the initial calculations. An
expression for the vignetting for a given external occulter and field
angle is derived. The values obtained with the derived expression are
verified with those obtained by ZEMAX an Optical design software. The
degradation in angular resolution of the system due to vignetting is
also presented and an empirical relation to calculate the normalized
resolution for a given amount of vignetting is obtained.
---------------------------------------------------------
Title: Acoustic Power Absorption and its Relation to Vector Magnetic
Field of a Sunspot
Authors: Gosain, S.; Mathew, S. K.; Venkatakrishnan, P.
2011SoPh..268..335G Altcode: 2010arXiv1008.1456G; 2010SoPh..tmp..163G
The distribution of acoustic power over sunspots shows an enhanced
absorption near the umbra - penumbra boundary. Previous studies revealed
that the region of enhanced absorption coincides with the region of
strongest transverse potential field. The aim of this paper is to i)
utilize the high-resolution vector magnetograms derived using Hinode
SOT/SP observations and study the relationship between the vector
magnetic field and power absorption and ii) study the variation of
power absorption in sunspot penumbrae due to the presence of spine-like
radial structures.
---------------------------------------------------------
Title: Proposed visible emission line space solar coronagraph
Authors: Singh, Jagdev; Prasad, B. Raghavendra; Venkatakrishnan, P.;
Sankarasubramanian, K.; Banerjee, Dipankar; Bayanna, Raja; Mathew,
Shibu; Murthy, Jayant; Subramanian, Prasad; Ramesh, R.; Kathiravan,
S.; Nagabhushana, S.; Mahesh, P. K.; Manoharan, P. K.; Uddin,
Wahab; Sriram, S.; Kumar, Amir; Srivastava, N.; Rao, Koteswara;
Nagendra, C. L.; Chakraborthy, P.; Sriram, K. V.; Venkateswaran,
R.; Krishnamurthy, T.; Sreekumar, P.; Sarma, K. S.; Murthy, Raghava;
Navalgund, K. HJ.; Samudraiah, D. R. M.; Babu, P. Narayan; Patra, Asit
2011CSci..100..167S Altcode:
The outer atmosphere of the sun - called the corona - has been observed
during total solar eclipse for short periods (typically <6 min),
from as early as the eighteenth century. In the recent past, space-based
instruments have permitted us to study the corona uninterruptedly. In
spite of these developments, the dynamic corona and its high temperature
(1-2 million K) are yet to be fully understood. It is conjectured
that their dynamic nature and associated energetic events are possible
reasons behind the high temperature. In order to study these in detail,
a visible emission line space solar coronagraph is being proposed as a
payload under the small-satellite programme of the Indian Space Research
Organisation. The satellite is named as Aditya-1 and the scientific
objectives of this payload are to study: (i) the existence of intensity
oscillations for the study of wave-driven coronal heating; (ii) the
dynamics and formation of coronal loops and temperature structure of the
coronal features; (iii) the origin, cause and acceleration of coronal
mass ejections (CMEs) and other solar active features, and (iv) coronal
magnetic field topology and three-dimensional structures of CMEs using
polarization information. The uniqueness of this payload compared to
previously flown space instruments is as follows: (a) observations in
the visible wavelength closer to the disk (down to 1.05 solar radii);
(b) high time cadence capability (better than two-images per second),
and (c) simultaneous observations of at least two spectral windows
all the time and three spectral windows for short durations.
---------------------------------------------------------
Title: Supersonic Downflows at the Umbra-Penumbra Boundary of Sunspots
Authors: Louis, Rohan E.; Bellot Rubio, Luis R.; Mathew, Shibu K.;
Venkatakrishnan, P.
2011ApJ...727...49L Altcode: 2010arXiv1010.0519L
High-resolution spectropolarimetric observations of three sunspots
taken with Hinode demonstrate the existence of supersonic downflows
at or close to the umbra-penumbra boundary which have not been
reported before. These downflows are confined to large patches,
usually encompassing bright penumbral filaments, and have lifetimes of
more than 14 hr. The presence of strong downflows in the center-side
penumbra near the umbra rules out an association with the Evershed
flow. Chromospheric filtergrams acquired close to the time of the
spectropolarimetric measurements show large, strong, and long-lived
brightenings in the neighborhood of the downflows. The photospheric
intensity also exhibits persistent brightenings comparable to the
quiet Sun. Interestingly, the orientation of the penumbral filaments
at the site of the downflows is similar to that resulting from the
reconnection process described by Ryutova et al. The existence of such
downflows in the inner penumbra represents a challenge for numerical
models of sunspots because they have to explain them in terms of
physical processes likely affecting the chromosphere.
---------------------------------------------------------
Title: Analysis of peculiar penumbral flows observed in the active
region NOAA 10930 during a major solar flare
Authors: Kumar, Brajesh; Venkatakrishnan, P.; Mathur, Savita; Tiwari,
Sanjiv Kumar; García, R. A.
2011JPhCS.271a2020K Altcode:
It is believed that the high energetic particles and tremendous amount
of energy released during the flares can induce velocity oscillations in
the Sun. Using the Dopplergrams obtained by Global Oscillation Network
Group (GONG) telescope, we analyze the velocity flows in the active
region NOAA 10930 during a major flare (of class X3.4) that occurred on
13 December 2006. We observe peculiar evolution of velocity flows in
some localized portions of the penumbra of this active region during
the flare. Application of Wavelet transform to these velocity flows
reveals that there is major enhancement of velocity oscillations in the
high-frequency regime (5-8 mHz), while there is feeble enhancement in
the p mode oscillations (2-5 mHz) in the aforementioned location. It
has been recently shown that flares can induce high-frequency global
oscillations in the Sun. Therefore, it appears that during the flare
process there might be a common origin for the excitation of local
and global high-frequency oscillations in the Sun.
---------------------------------------------------------
Title: The Evolution of the Twist Shear and Dip Shear During X-class
Flare of 2006 December 13: Hinode Observations
Authors: Gosain, Sanjay; Venkatakrishnan, P.
2010ApJ...720L.137G Altcode: 2010arXiv1007.2702G
The non-potentiality of solar magnetic fields is traditionally measured
in terms of a magnetic shear angle, i.e., the angle between the observed
and potential field azimuths. Here, we introduce another measure of the
shear that has not been previously studied in solar active regions,
i.e., the one that is associated with the inclination angle of the
magnetic field. This form of the shear, which we call "dip shear,"
can be calculated by taking the difference between the observed and the
potential field inclination. In this Letter, we study the evolution of
the dip shear as well as the conventional twist shear in a δ-sunspot
using high-resolution vector magnetograms from the Hinode space
mission. We monitor these shears in a penumbral region located close
to a flaring site during 2006 December 12 and 13. It is found that (1)
the penumbral area close to the flaring site shows a high value of the
twist shear and dip shear as compared with other parts of the penumbra,
(2) after the flare, the value of the dip shear drops in this region
while the twist shear tends to increase, (3) the dip shear and twist
shear are correlated such that pixels with a large twist shear also
tend to exhibit a large dip shear, and (4) the correlation between
the twist shear and dip shear is tighter after the flare. The present
study suggests that monitoring the twist shear alone during the flare is
not sufficient, but we need to monitor it together with the dip shear.
---------------------------------------------------------
Title: Magnetic Non-potentiality of Solar Active Regions and Peak
X-ray Flux of the Associated Flares
Authors: Tiwari, Sanjiv Kumar; Venkatakrishnan, P.; Gosain, Sanjay
2010ApJ...721..622T Altcode: 2010arXiv1007.4876T
Predicting the severity of solar eruptive phenomena such as flares and
coronal mass ejections remains a great challenge despite concerted
efforts to do so over the past several decades. However, the advent
of high-quality vector magnetograms obtained from Hinode (SOT/SP) has
increased the possibility of meeting this challenge. In particular,
the spatially averaged signed shear angle (SASSA) seems to be a
unique parameter for quantifying the non-potentiality of active
regions. We demonstrate the usefulness of the SASSA for predicting
flare severity. For this purpose, we present case studies of the
evolution of magnetic non-potentiality using 115 vector magnetograms of
four active regions, namely, ARs NOAA 10930, 10960, 10961, and 10963
during 2006 December 8-15, 2007 June 3-10, 2007 June 28-July 5, and
2007 July 10-17, respectively. The NOAA ARs 10930 and 10960 were very
active and produced X and M class flares, respectively, along with many
smaller X-ray flares. On the other hand, the NOAA ARs 10961 and 10963
were relatively less active and produced only very small (mostly A-
and B-class) flares. For this study, we have used a large number of
high-resolution vector magnetograms obtained from Hinode (SOT/SP). Our
analysis shows that the peak X-ray flux of the most intense solar
flare emanating from the active regions depends on the magnitude of
the SASSA at the time of the flare. This finding of the existence of
a lower limit of the SASSA for a given class of X-ray flares will be
very useful for space weather forecasting. We have also studied another
non-potentiality parameter called the mean weighted shear angle (MWSA)
of the vector magnetograms along with the SASSA. We find that the MWSA
does not show such distinction as the SASSA for upper limits of the
GOES X-ray flux of solar flares; however, both the quantities show
similar trends during the evolution of all active regions studied.
---------------------------------------------------------
Title: On the Estimate of Magnetic Non-potentiality of Sunspots
Derived Using Hinode SOT/SP Observations: Effect of Polarimetric Noise
Authors: Gosain, Sanjay; Tiwari, Sanjiv Kumar; Venkatakrishnan, P.
2010ApJ...720.1281G Altcode: 2010arXiv1007.2505G
The accuracy of Milne-Eddington (ME) inversions, used to retrieve the
magnetic field vector, depends upon the signal-to-noise ratio (S/N)
of the spectro-polarimetric observations. The S/N in real observations
varies from pixel to pixel; therefore the accuracy of the field vector
also varies over the map. The aim of this work is to study the effect
of polarimetric noise on the inference of the magnetic field vector
and the magnetic non-potentiality of a real sunspot. To this end,
we use the Hinode SOT/SP vector magnetogram of a real sunspot NOAA
10933 as an input to generate synthetic Stokes profiles under ME model
assumptions. We then add normally distributed polarimetric noise of
the level 0.5% of continuum intensity to these synthetic profiles and
invert them again using the ME code. This process is repeated 100 times
with different realizations of noise. It is found that within most of
the sunspot areas (>90% area) the spread in the (1) field strength
is less than 8 G, (2) field inclination is less than 1°, and (3)
field azimuth is less than 5°. Further, we determine the uncertainty
in the magnetic non-potentiality of a sunspot as determined by the
force-free parameter α<SUB> g </SUB> and spatially averaged signed
shear angle (SASSA). It is found that for the sunspot studied here
these parameters are α<SUB> g </SUB> = -3.5 ± 0.37(×10<SUP>-9</SUP>
m<SUP>-1</SUP>) and SASSA = -1.68 ± 0fdg014. This suggests that the
SASSA is a less dispersed non-potentiality parameter as compared to
α<SUB> g </SUB>. Further, we examine the effect of increasing noise
levels, viz. 0.01%, 0.1%, 0.5%, and 1% of continuum intensity, and
find that SASSA is less vulnerable to noise as compared to the α<SUB>
g </SUB> parameter.
---------------------------------------------------------
Title: Magnetic tension of sunspot fine structures
Authors: Venkatakrishnan, P.; Tiwari, Sanjiv Kumar
2010A&A...516L...5V Altcode: 2010arXiv1005.3899V
Context. The equilibrium structure of sunspots depends critically on
its magnetic topology and is dominated by magnetic forces. Tension
force is one component of the Lorentz force, which balances the
gradient of magnetic pressure in force-free configurations. <BR />
Aims: We employ the tension term of the Lorentz force to clarify
the structure of sunspot features like penumbral filaments, umbral
light bridges, and outer penumbral fine structures. <BR /> Methods:
We computed the vertical component of the tension term of Lorentz force
over two active regions, NOAA AR 10933 and NOAA AR 10930 observed on 5
January 2007 and 12 December 2006, respectively. The former is a simple
active region while the latter is a complex one with highly sheared
polarity inversion line (PIL). We obtained the vector magnetograms from
Hinode(SOT/SP). <BR /> Results: We find an inhomogeneous distribution of
tension with both positive and negative signs in various features of the
sunspots. The existence of positive tension at locations of lower field
strength and higher inclination is compatible with the uncombed model
of the penumbral structure. Positive tension is also seen in umbral
light bridges, which could be indication of uncombed structure of the
light bridge. Likewise, the upwardly directed tension associated with
bipolar regions in the penumbra could be a direct confirmation of the
sea serpent model of penumbral structures. Upwardly directed tension
at the PIL of AR 10930 seems to be related to flux emergence. The
magnitude of the tension force is greater than the force of gravity
in some places, implying a nearly force-free configuration for these
sunspot features. <BR /> Conclusions: From our study, magnetic tension
emerges as a useful diagnostic of the local equilibrium of the sunspot
fine structures. <P />Figures A.1-A.3 are only available in electronic
form at <A href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: On the Flare Induced High-Frequency Global Waves in the Sun
Authors: Kumar, Brajesh; Mathur, Savita; García, R. A.;
Venkatakrishnan, P.
2010ApJ...711L..12K Altcode: 2010arXiv1001.4331K
Recently, Karoff & Kjeldsen presented evidence of strong
correlation between the energy in the high-frequency part (5.3 <
ν < 8.3 mHz) of the acoustic spectrum of the Sun and the solar
X-ray flux. They have used disk-integrated intensity observations of
the Sun obtained from the Variability of solar IRradiance and Gravity
Oscillations instrument on board Solar and Heliospheric Observatory
(SOHO) spacecraft. Similar signature of flares in velocity observations
has not been confirmed till now. The study of low-degree high-frequency
waves in the Sun is important for our understanding of the dynamics of
the deeper solar layers. In this Letter, we present the analysis of
the velocity observations of the Sun obtained from the Michelson and
Doppler Imager (MDI) and the Global Oscillations at Low Frequencies
(GOLF) instruments on board SOHO for some major flare events of the
solar cycle 23. Application of wavelet techniques to the time series
of disk-integrated velocity signals from the solar surface using
the full-disk Dopplergrams obtained from the MDI clearly indicates
that there is enhancement of high-frequency global waves in the Sun
during the flares. This signature of flares is also visible in the
Fourier Power Spectrum of these velocity oscillations. On the other
hand, the analysis of disk-integrated velocity observations obtained
from the GOLF shows only marginal evidence of effects of flares on
high-frequency oscillations.
---------------------------------------------------------
Title: Can major solar flares excite high-frequency global waves in
the Sun ?
Authors: Kumar, Brajesh; Mathur, Savita; Garcia, R. A.;
Venkatakrishnan, P.
2010arXiv1003.2263K Altcode:
The study of low-degree high-frequency waves in the Sun can provide
new insight into the dynamics of the deeper layers of the Sun. Here, we
present the analysis of the velocity observations of the Sun obtained
from the Michelson and Doppler Imager (MDI) and Global Oscillations
at Low Frequency (GOLF) instruments on board Solar and Heliospheric
Observatory (SOHO) spacecraft for the major flare event of 2003 October
28 during the solar cycle 23. We have applied wavelet transform to the
time series of disk-integrated velocity signals from the solar surface
using the full-disk Dopplergrams obtained from MDI. The wavelet power
spectrum computed from MDI velocity series clearly shows that there
is enhancement of high-frequency global waves in the Sun during the
flare. We do observe this signature of flare in the Fourier Power
Spectrum of these velocity oscillations. However, the analysis of
disk-integrated velocity observations obtained from GOLF shows only
feeble effect of flare on high-frequency oscillations.
---------------------------------------------------------
Title: Coherent Lateral Motion of Penumbral Filaments during
X-class Flare
Authors: Gosain, S.; Venkatakrishnan, P.; Tiwari, Sanjiv Kumar
2010arXiv1002.0397G Altcode:
The high-resolution pictures of the solar photosphere from space
based 50 cm Solar Optical Telescope (SOT) on-board Hinode spacecraft,
are now routinely observed. Such images of a delta-sunspot in NOAA
10930 were obtained by Hinode during 13 December 2006 while a X-class
flare occurred in this active region. Two bright ribbons were visible
even in white light and G-band images apart from chromospheric Ca II
H images. We register the sunspot globally using cross-correlation
technique and analyse local effects during flare interval. We find
that during flare the penumbral filaments show lateral motion. Also,
we locate two patches, one in either polarity, which show converging
motion towards the polarity inversion line (PIL). In Ca II H images
we find kernel with pre-flare brightening which lie along the PIL.
---------------------------------------------------------
Title: Helicity at Photospheric and Chromospheric Heights
Authors: Tiwari, S. K.; Venkatakrishnan, P.; Sankarasubramanian, K.
2010ASSP...19..443T Altcode: 2009arXiv0904.4353T; 2010mcia.conf..443T
In the solar atmosphere, the twist parameter α has the same sign as
magnetic helicity. It has been observed using photospheric vector
magnetograms that negative/positive helicity is dominant in the
northern/southern hemisphere of the Sun. Chromospheric features show
dextral/sinistral dominance in the northern/ southern hemisphere
and sigmoids observed in X-rays also have a dominant sense of
reverse-S/forward-S in the northern/southern hemisphere. It
is of interest whether individual features have one-to-one
correspondence in terms of helicity at different atmospheric
heights. We use UBF Hα images from the Dunn Solar Telescope (DST)
and other Hα data from Udaipur Solar Observatory and Big Bear Solar
Observatory. Near-simultaneous vector magnetograms from the DST are
used to establish one-to-one correspondence of helicity at photospheric
and chromospheric heights. We plan to extend this investigation with
more data including coronal intensities.
---------------------------------------------------------
Title: Phase III of the USO Solar Vector Magnetograph
Authors: Gosain, S.; Venkatakrishnan, P.
2010ASSP...19..395G Altcode: 2009arXiv0907.5267G; 2010mcia.conf..395G
The solar vector magnetograph (SVM) is a modern imaging
spectropolarimeter installed at Udaipur Solar Observatory (USO). Earlier
phases saw the development of the instrument using off-the-shelf
components with in-house software development. Subsequently,
improvements were done in the opto-mechanical design of the sub-systems
and the telescope tracking system. The third phase of the instrument
development saw three major improvements: (1) installation of a
web-camera-based telescope guiding system, developed in-house,
(2) high-cadence spectropolarimetry using liquid-crystal variable
retarders and a fast CCD camera, and (3) inclusion of the Na I D1
line for chromospheric observations, in addition to the regularly used
photospheric Fe I 6302 Å line.
---------------------------------------------------------
Title: Strong, Localized Downflows in a Sunspot Light Bridge
Authors: Louis, R. E.; Rubio, L. R. Bellot; Mathew, S. K.;
Venkatakrishnan, P.
2010ASSP...19..509L Altcode: 2010mcia.conf..509L
We investigate spectropolarimetric observations of a sunspot light
bridge in NOAA AR 10953 taken on 1 May 2007 with Hinode using the Fe
I line pair at 630 nm. The SIR code (Ruiz Cobo and del Toro Iniesta
1992, ApJ, 398, 375) was used to invert the observed Stokes profiles,
obtaining temperature stratifications and height-independent values
of the magnetic field and Doppler velocity. The maps of the physical
parameters show that the light bridge is a penumbral penetration into
the umbra and has a relatively weak, inclined magnetic field. The
highlight of our inversions is the presence of strong downflow patches
in the light bridge, with line-of-sight velocities exceeding 4 km
s<SUP>-1</SUP>. The field azimuth also shows large rotation along a
thin ridge close to one edge of the light bridge, essentially seen
as a discontinuity in azimuth. Some of the downflow patches are also
co-spatial with brightness enhancements in the Ca IIH chromospheric
filtergrams (Louis et al. 2008, Solar Phys., 252, 43). Inspection of
the Stokes profiles for the downflow patches indicates doubly red-lobed
Stokes-V signals. These profiles were also inverted with SIR using a
two-component atmosphere with varying degrees of complexity. All such
inversions indicate that the downflow patches consist of supersonic
flows of about 10 km s<SUP>-1</SUP>. Interestingly, the linear
polarization also appears to be anomalous at the ridge demarcating the
field azimuth change.We believe that the anomalous Q and U profiles
result from mixing of the light bridge and the umbral magnetic field,
which through reconnection may result in supersonic downflows in
the photosphere and brightness enhancements in the chromosphere. The
light bridge represents a strong inhomogeneity within a fairly regular
sunspot. Our observations serve as useful inputs to future numerical
models of light bridges.
---------------------------------------------------------
Title: Flare-Driven Acoustic Modes in the Sun
Authors: Kumar, B.; Venkatakrishnan, P.
2010ASSP...19..405K Altcode: 2010mcia.conf..405K
We have analyzed disk-integrated velocity signals at the solar
surface obtained from full-disk Doppler images as observed with MDI
(Michelson and Doppler Imager) on board SoHO (Solar and Heliospheric
Observatory) to study the effect of solar flares on acoustic velocity
oscillations. It is seen that during a flare these oscillations
are enhanced significantly in the higher frequency band, beyond the
acoustic cut-off-frequency in the solar photosphere of about 5mHz,
while there is feeble to no enhancement of these oscillations in the
5-min (3.3mHz) band. Enhancement in the high-frequency component of
the acoustic spectrum of the Sun during major flares has also been
reported recently in disk-integrated intensity observations of the Sun.
---------------------------------------------------------
Title: Wave Heating of Coronal Loops with Steady Flows
Authors: Pandey, V. S.; Venkatakrishnan, P.; Narayanan, A. S.
2010ASSP...19..520P Altcode: 2010mcia.conf..520P
We derive a general dispersion relation for hydromagnetic waves in
inhomogeneous coronal loops with steady plasma flows. We model static
loops by assuming the equilibrium magnetic and plasma configuration
of solar coronal loops as slab geometry, with the z-axis of the
slab along the uniform background magnetic field. We assume a density
stratification perpendicular to the direction of the background magnetic
field. Here we evaluate the extent to which hydromagnetic waves can
be collisionally dissipated, that is, their dissipation by viscosity
and heat conduction in the coronal loops when invoking steady flow,
and we compare the results with the minimum that is required to satisfy
the optically thin radiative cooling rate.
---------------------------------------------------------
Title: A 3D view of eruptive filaments by STEREO
Authors: Gosain, Sanjay; Schmieder, Brigitte; Venkatakrishnan, P.;
Chandra, Ramesh; Artzner, Guy
2010cosp...38.2916G Altcode: 2010cosp.meet.2916G
STEREO/SECHI/EUVI A and B observe different views of the eruption of a
quiescent filament. We will concentrate on two events: (i) May 20 to 22,
2008 event (A and B separated by 52.4 degrees from each other), and (ii)
September 25 to 26, 2009 event (A and B more than 100 degrees from each
other. After using different techniques of reconstruction we obtained
a 3 dimensional view of untwisted flux ropes in He II 304 Angstrom,
with fine structures. The entire disappearance phase lasted more than
ten hours. The filament evolved very slowly ( 5 km/s) from a dense
structure with a thick spine into fine threads. Individual threads are
seen to be oscillating and rising to an altitude of about 150 Mm with
velocities of about 100 km/s. The plasma disappears by diffusion in
the corona. Weak CME events are recorded by LASCO at the beginning of
the disappearance. In this paper we shall present the dynamics of the
filament eruptions as viewed in 3D by STEREO using different methods. We
shall explore the causes and consequences of the filament disappearance.
---------------------------------------------------------
Title: Actors of the main activity of large complex centres during
the 23 Solar Cycle maximum
Authors: Schmieder, Brigitte; Chandra, Ramesh; Demoulin, Pascal;
Mandrini, Cristina H.; Venkatakrishnan, P.; Manoharan, P. K.; Uddin,
Wahab; Pariat, Etienne; Toeroek, Tibor; Molodij, Guillaume; Kumar, P.
2010cosp...38.1861S Altcode: 2010cosp.meet.1861S
During the maximum of the last Solar Cycle solar cycle 23, large
active regions had a long life spanning several solar rotations and
produced a large number of X-ray class flares, CMEs and Magnetic
clouds (MC). This was the case for the Halloween active regions in
2003. The most geoeffective magnetic cloud of the cycle (Dst=-457)
has its source in one passage of the active region (NOAA 10501) on
November 18, 2003. Such an activity is presumably due to continuous
emerging magnetic flux that was observed during this passage. Moreover,
the region exhibited a complex topology with multiple domains of
distinct magnetic helicities. The complexity is observed to reach
such unprecedented levels that a detailed multi wavelength analysis
is necessary to precisely identify the sources of CMEs and MCs.
---------------------------------------------------------
Title: Evidence of Enhanced High Frequency Velocity Oscillations in
the Sun Observed During Solar Flare Using Disk Integrated Doppler
Signals
Authors: Kumar, B.; Venkatakrishnan, P.
2009ASPC..416..333K Altcode:
Disk integrated velocity signals at the solar surface obtained from
Global Oscillation Network Group (GONG) full-disk Dopplergrams have
been used to study the flare-induced velocity oscillations during
the major solar flare (X17.6/4B) of 28 October 2003. We observe that
these velocity oscillations are enhanced significantly during the
flare in the higher frequency band (5-6.5 mHz) while there is feeble
or no enhancement of these oscillations in lower frequency band (2-4
mHz). We also observe the presence of high frequency power in these
velocity oscillations of the Sun comparable to granulation power even
in non-flaring condition.
---------------------------------------------------------
Title: HINODE Observations of Coherent Lateral Motion of Penumbral
Filaments During an X-Class Flare
Authors: Gosain, S.; Venkatakrishnan, P.; Tiwari, Sanjiv Kumar
2009ApJ...706L.240G Altcode: 2009arXiv0910.5336G
The X-3.4 class flare of 2006 December 13 was observed with a high
cadence of 2 minutes at 0.2 arcsec resolution by HINODE/SOT FG
instrument. The flare ribbons could be seen in G-band images also. A
careful analysis of these observations after proper registration
of images shows flare-related changes in penumbral filaments of the
associated sunspot for the first time. The observations of sunspot
deformation, decay of penumbral area, and changes in magnetic flux
during large flares have been reported earlier in the literature. In
this Letter, we report lateral motion of the penumbral filaments in
a sheared region of the δ-sunspot during the X-class flare. Such
shifts have not been seen earlier. The lateral motion occurs in two
phases: (1) motion before the flare ribbons move across the penumbral
filaments and (2) motion afterward. The former motion is directed away
from expanding flare ribbons and lasts for about 4 minutes. The latter
motion is directed in the opposite direction and lasts for more than
40 minutes. Further, we locate a patch in adjacent opposite polarity
spot moving in opposite direction to the penumbral filaments. Together
these patches represent conjugate footpoints on either side of the
polarity inversion line, moving toward each other. This converging
motion could be interpreted as shrinkage of field lines.
---------------------------------------------------------
Title: Evolution of Fine Structures in an Eruptive Active Region:
Hinode (SOT/SP) Observations
Authors: Tiwari, S. K.; Venkatakrishnan, P.
2009AGUFMSH51A1268T Altcode:
We study the evolution of an active region NOAA 10930 by using a
large number of high resolution vector magnetograms obtained from
Hinode (SOT/SP). A X3.4 class solar flare was observed from the active
region NOAA 10930 (S06W35), which started at 02:14 UT on 13th December
2006. We have used HINODE (SOT/SP) data for 12, 13 and 14 December
2006 for studying spatial and temporal changes in pre and post eruption
cases. The evolution of twist (computed from signed shear angle (SSA))
in the vector magnetograms is studied. It is known that the magnetic
tension is reduced in highly sheared magnetic field regions e.g.,
polarity inversion lines. We study the evolution of magnetic tension
near the polarity inversion line to check if the loss of magnetic
tension was the possible cause of its eruption. We also study the
evolution of AR NOAA 10961 as a non-erupting case. The difference
between the evolution of fine structures in erupting and non-erupting
active regions is the main motivation of this study.
---------------------------------------------------------
Title: On the Absence of Photospheric Net Currents in Vector
Magnetograms of Sunspots Obtained from Hinode (Solar Optical
Telescope/Spectro-Polarimeter)
Authors: Venkatakrishnan, P.; Tiwari, Sanjiv Kumar
2009ApJ...706L.114V Altcode: 2009arXiv0910.3751V
Various theoretical and observational results have been reported
regarding the presence/absence of net electric currents in the
sunspots. The limited spatial resolution of the earlier observations
perhaps obscured the conclusions. We have analyzed 12 sunspots observed
from Hinode (Solar Optical Telescope/Spectro-polarimeter) to clarify
the issue. The azimuthal and radial components of magnetic fields and
currents have been derived. The azimuthal component of the magnetic
field of sunspots is found to vary in sign with azimuth. The radial
component of the field also varies in magnitude with azimuth. While the
latter pattern is a confirmation of the interlocking combed structure
of penumbral filaments, the former pattern shows that the penumbra is
made up of a "curly interlocking combed" magnetic field. The azimuthally
averaged azimuthal component is seen to decline much faster than 1/piv
in the penumbra, after an initial increase in the umbra, for all the
spots studied. This confirms the confinement of magnetic fields and
absence of a net current for sunspots as postulated by Parker. The
existence of a global twist for a sunspot even in the absence of a net
current is consistent with a fibril-bundle structure of the sunspot
magnetic fields.
---------------------------------------------------------
Title: 3D Evolution of a Filament Disappearance Event Observed
by STEREO
Authors: Gosain, S.; Schmieder, B.; Venkatakrishnan, P.; Chandra,
R.; Artzner, G.
2009SoPh..259...13G Altcode: 2009arXiv0910.0786G
A filament disappearance event was observed on 22 May 2008 during
our recent campaign JOP 178. The filament, situated in the Southern
Hemisphere, showed sinistral chirality consistent with the hemispheric
rule. The event was well observed by several observatories,
in particular by THEMIS. One day, before the disappearance, Hα
observations showed up- and down-flows in adjacent locations along the
filament, which suggest plasma motions along twisted flux rope. THEMIS
and GONG observations show shearing photospheric motions leading
to magnetic flux canceling around barbs. STEREO A, B spacecraft
with separation angle 52.4°, showed quite different views of this
untwisting flux rope in He II 304 Å images. Here, we reconstruct the
three-dimensional geometry of the filament during its eruption phase
using STEREO EUV He II 304 Å images and find that the filament was
highly inclined to the solar normal. The He II 304 Å movies show
individual threads, which oscillate and rise to an altitude of about
120 Mm with apparent velocities of about 100 km s<SUP>−1</SUP>
during the rapid evolution phase. Finally, as the flux rope expands
into the corona, the filament disappears by becoming optically thin to
undetectable levels. No CME was detected by STEREO, only a faint CME
was recorded by LASCO at the beginning of the disappearance phase at
02:00 UT, which could be due to partial filament eruption. Further,
STEREO Fe XII 195 Å images showed bright loops beneath the filament
prior to the disappearance phase, suggesting magnetic reconnection
below the flux rope.
---------------------------------------------------------
Title: Supersonic Downflows in a Sunspot Light Bridge
Authors: Louis, Rohan E.; Bellot Rubio, Luis R.; Mathew, Shibu K.;
Venkatakrishnan, P.
2009ApJ...704L..29L Altcode: 2009arXiv0908.3465L
We report the discovery of supersonic downflows in a sunspot light
bridge using measurements taken with the spectropolarimeter onboard
the Hinode satellite. The downflows occur in small patches close to
regions where the vector magnetic field changes orientation rapidly,
and are associated with anomalous circular polarization profiles. An
inversion of the observed Stokes spectra reveals velocities of up to
10 km s<SUP>-1</SUP>, making them the strongest photospheric flows
ever measured in light bridges. Some (but not all) of the downflowing
patches are cospatial and cotemporal with brightness enhancements in
chromospheric Ca II H filtergrams. We suggest that these flows are due
to magnetic reconnection in the upper photosphere/lower chromosphere,
although other mechanisms cannot be ruled out.
---------------------------------------------------------
Title: Global Twist of Sunspot Magnetic Fields Obtained from
High-Resolution Vector Magnetograms
Authors: Tiwari, Sanjiv Kumar; Venkatakrishnan, P.; Sankarasubramanian,
K.
2009ApJ...702L.133T Altcode: 2009arXiv0907.5064T
The presence of fine structures in sunspot vector magnetic fields has
been confirmed from Hinode as well as other earlier observations. We
studied 43 sunspots based on the data sets taken from ASP/DLSP, Hinode
(SOT/SP), and SVM (USO). In this Letter, (1) we introduce the concept
of signed shear angle (SSA) for sunspots and establish its importance
for non-force-free fields. (2) We find that the sign of global α
(force-free parameter) is well correlated with that of the global SSA
and the photospheric chirality of sunspots. (3) Local α patches of
opposite signs are present in the umbra of each sunspot. The amplitude
of the spatial variation of local α in the umbra is typically of the
order of the global α of the sunspot. (4) We find that the local α
is distributed as alternately positive and negative filaments in the
penumbra. The amplitude of azimuthal variation of the local α in the
penumbra is approximately an order of magnitude larger than that in the
umbra. The contributions of the local positive and negative currents
and α in the penumbra cancel each other giving almost no contribution
for their global values for the whole sunspot. (5) Arc-like structures
(partial rings) with a sign opposite to that of the dominant sign of
α of the umbral region are seen at the umbral-penumbral boundaries
of some sunspots. (6) Most of the sunspots studied belong to the
minimum epoch of the 23rd solar cycle and do not follow the so-called
hemispheric helicity rule.
---------------------------------------------------------
Title: Effect of Polarimetric Noise on the Estimation of Twist and
Magnetic Energy of Force-Free Fields
Authors: Tiwari, Sanjiv Kumar; Venkatakrishnan, P.; Gosain, Sanjay;
Joshi, Jayant
2009ApJ...700..199T Altcode: 2009arXiv0904.4594T
The force-free parameter α, also known as helicity parameter or twist
parameter, bears the same sign as the magnetic helicity under some
restrictive conditions. The single global value of α for a whole active
region gives the degree of twist per unit axial length. We investigate
the effect of polarimetric noise on the calculation of global α value
and magnetic energy of an analytical bipole. The analytical bipole
has been generated using the force-free field approximation with a
known value of constant α and magnetic energy. The magnetic parameters
obtained from the analytical bipole are used to generate Stokes profiles
from the Unno-Rachkovsky solutions for polarized radiative transfer
equations. Then we add random noise of the order of 10<SUP>-3</SUP>
of the continuum intensity (I <SUB> c </SUB>) in these profiles to
simulate the real profiles obtained by modern spectropolarimeters such
as Hinode (SOT/SP), SVM (USO), ASP, DLSP, POLIS, and SOLIS etc. These
noisy profiles are then inverted using a Milne-Eddington inversion
code to retrieve the magnetic parameters. Hundred realizations of this
process of adding random noise and polarimetric inversion is repeated
to study the distribution of error in global α and magnetic energy
values. The results show that (1) the sign of α is not influenced
by polarimetric noise and very accurate values of global twist can
be calculated, and (2) accurate estimation of magnetic energy with
uncertainty as low as 0.5% is possible under the force-free condition.
---------------------------------------------------------
Title: A 2-dimensional Scanning Solar Vector Magnetograph at Udaipur
Solar Observatory
Authors: Gosain, S.; Venkatakrishnan, P.
2009ASPC..405..467G Altcode:
We describe a newly developed instrument used for performing filter
based spectro-polarimetry of solar active regions. The instrument
consists of a tunable Fabry-Perot etalon kept in collimated arrangement
for spectroscopy in the wavelength range 550 to 700 nm. The polarimeter
consists of two quarter wave-plates and a dual-beam calcite analyzer
(Savart plate). We present the instrument design and the interactive
tools for data analysis and visualization. These tools facilitate
Milne-Eddington inversion, visualization, heliographic vector
transformations and 180 degree ambiguity resolution. It is planned
to upgrade the instrument to observe in chromospheric Na D2 and H-α
lines together with photospheric Fe I 630.2 nm line pair, in near
simultaneous mode.
---------------------------------------------------------
Title: Commission 12: Solar Radiation and Structure
Authors: Martínez Pillet, Valentin; Kosovichev, Alexander;
Mariska, John T.; Bogdan, Thomas J.; Asplund, Martin; Cauzzi, Gianna;
Christensen-Dalsgaard, Jørgen; Cram, Lawrence E.; Gan, Weiqun; Gizon,
Laurent; Heinzl, Petr; Rovira, Marta G.; Venkatakrishnan, P.
2009IAUTA..27..104M Altcode:
Commission 12 encompasses investigations on the internal structure
and dynamics of the Sun, mostly accessible through the techniques of
local and global helioseismology, the quiet solar atmosphere, solar
radiation and its variability, and the nature of relatively stable
magnetic structures like sunspots, faculae and the magnetic network. A
revision of the progress made in these fields is presented. For some
specific topics, the review has counted with the help of experts
outside the Commission Organizing Committee that are leading and/or
have recently presented relevant works in the respective fields. In
this cases the contributor's name is given in parenthesis.
---------------------------------------------------------
Title: Dynamics of Sunspot Light Bridges as Revealed by
High-Resolution Images from Hinode
Authors: Louis, Rohan E.; Bayanna, A. Raja; Mathew, Shibu K.;
Venkatakrishnan, P.
2008SoPh..252...43L Altcode: 2008SoPh..tmp..147L
We present G-band and Ca II H filtergrams of two sunspot light
bridges in NOAA AR 10953 taken from the 50-cm Solar Optical Telescope
onboard the Japanese space satellite Hinode on 1 May 2007. The
two light bridges differ in structure, with one of them resembling
the filamentary penumbra and the other possessing a dark central
lane running along the axis of the bridge having a width of 170 km,
which is close to the diffraction limit of the telescope. Velocity
measurements of the light bridges using proper motion displacements of
inhomogeneities, averaged over the entire time series, show a nonuniform
flow with velocities peaking at 250 and 180 m s<SUP>−1</SUP> for
the two bridges, respectively. We report observations of an archlike
structure over one of the light bridges in the Ca images. Brightness
enhancements are seen traveling along this arch as well as along the
light bridge. Observations suggest that these enhancements over light
bridges could possibly be a signature of lower chromospheric heating.
---------------------------------------------------------
Title: Chapter 4: Solar Magnetism
Authors: Venkatakrishnan, P.; Gosain, Sanjay
2008psa..book...39V Altcode:
This chapter is basically divided into 2 parts. In the first part, the
important properties of the solar magnetic field are summarized. The
discussion begins with a simple introduction to solar magneto
hydrodynamics. This introduction will be sufficient to understand
the current status of the solar dynamo theory that follows. Some very
curious and interesting results on force free fields are then presented
in very basic terms. Finally, the application of this theoretical
framework to the problems of coronal heating, solar flares and coronal
mass ejections are developed in a simple unified scheme, based on a
hierarchy of physical conditions. The second part consists of a tutorial
on magnetographs. It begins with a description of polarization of light
from very fundamental notions of coherence of light. This is followed
by simple but comprehensive explanations of the Zeeman and Hanle effects
along with the necessary basic ideas of quantum physics of scattering of
light. Then the working of a few important magnetographs is outlined,
with special emphasis on a solar vector magnetograph developed for
USO, to provide a ”hands on" perspective. The article concludes with
a few brief remarks on the possible future directions for research in
the domain of solar magnetism...
---------------------------------------------------------
Title: MHD Waves at a Tangential Discontinuity with Inclined Magnetic
Fields and Flows
Authors: Satya Narayanan, A.; Pandey, V.; Venkatakrishnan, P.
2008ESPM...12.3.64S Altcode:
Propagation of magnetohydrodynamic (MHD) waves in inhomogenous magnetic
plasmas is interesting from space and astrophysical point of view. In
this study, the combined effect of non-parallel propagation, steady
flow and inclined magnetic fields on either side of the tangential
discontinuity is examined, with change in the field strength of the
magnetic field, though uniform in each layer. The density is also
assumed to be different on both sides of the interfacial layer. It is
assumed that the fluid is perfectly conducting, infinite in extent
with an interface that supports both body waves as well as surface
waves. This model will also support fast, Alfven modes depending on
the parametric values of the system. Some special cases are discussed
briefly. These modes have interesting applications in the solar corona
and solar wind
---------------------------------------------------------
Title: Co-spatial evolution of photospheric Doppler enhancements and
Hα flare ribbons observed during the solar flare of 2003 October 28
Authors: Venkatakrishnan, P.; Kumar, Brajesh; Uddin, Wahab
2008MNRAS.387L..69V Altcode:
The active region NOAA (National Oceanic and Atmospheric Administration)
AR 10486 which appeared on the solar disc in 2003 October produced
a lot of space weather related activity. Here, we report on the
co-spatial evolution of the photospheric Doppler enhancements and the
chromospheric Hα flare ribbons observed during the 4B/X17.2 class
solar flare of 2003 October 28 in this active region. These velocity
enhancements exactly match the Hα brightness enhancements in space,
and are delayed by approximately 1min in time. Hα brightness attains
a maximum nearly at the same time as the peak seen in light curves in
high-energy emission observed by KORONAS (Kuznetsov et al. 2006).
---------------------------------------------------------
Title: Software for interactively visualizing solar vector
magnetograms of udaipur solar observatory
Authors: Gosain, Sanjay; Tiwari, Sanjiv; Joshi, Jayant;
Venkatakrishnan, P.
2008JApA...29..107G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Development of a low-order Adaptive Optics system at Udaipur
Solar Observatory
Authors: Bayanna, A. R.; Kumar, B.; Louis, R. E.; Venkatakrishnan,
P.; Mathew, S. K.
2008JApA...29..353B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Developmental Aspects of a Multi-Slit Spectro-Polarimeter
Authors: George, K.; Sankarasubramanian, R.; Bayanna, R.; Lin, H.;
Venkatakrishnan, P.
2008eic..work..515G Altcode:
We report the development aspects of an integral field unit,
multi-slit spectro-polarimeter (MSSP) optimized for optical to near
infrared regime, which can be used to derive simultaneous spectral
and vector magnetic field information at high spatial, spectral and
temporal resolution of any extended astronomical object like the Sun,
with limited spectral coverage of few Angstrom. The instrument will be
first developed and tested in laboratory which in a later stage will
be used as a focal plane instrument for the Multi Application Solar
Telescope (MAST). The major technological challenges involved in setting
up and calibration of the instrument are discussed. The scientific
motivation for the system is highlighted, with special emphasis on
science limitations imposed by similar existing instruments elsewhere.
---------------------------------------------------------
Title: Evolution of Magnetic Helicity in NOAA 10923 Over Three
Consecutive Solar Rotations
Authors: Tiwari, Sanjiv Kumar; Joshi, Jayant; Gosain, Sanjay;
Venkatakrishnan, P.
2008ASSP...12..329T Altcode: 2009arXiv0904.4024T; 2008tdad.conf..329T
We have studied the evolution of magnetic helicity and chirality
in an active region over three consecutive solar rotations. The
region where it first appeared was named NOAA10923 and in subsequent
rotations it was numbered NOAA 10930, 10935 and 10941. We compare the
chirality of these regions at photospheric, chromospheric and coronal
heights. The observations used for photospheric and chromospheric
heights are taken from Solar Vector Magnetograph (SVM) and H-α imaging
telescope of Udaipur Solar Observatory (USO), respectively. We discuss
the chirality of the sunspots and associated H-α filaments in these
regions. We find that the twistedness of superpenumbral filaments is
maintained in the photospheric transverse field vectors also. We also
compare the chirality at photospheric and chromospheric heights with
the chirality of the associated coronal loops, as observed from the
HINODE X-Ray Telescope.
---------------------------------------------------------
Title: An image stabilization system for solar observations
Authors: Sridharan, R.; Raja Bayanna, A.; Louis, Rohan Eugene; Kumar,
Brajesh; Mathew, Shibu K.; Venkatakrishnan, P.
2007SPIE.6689E..0ZS Altcode: 2007SPIE.6689E..29S
An image stabilization system has been developed and demonstrated
for solar observations in the visible wave-length at Udaipur Solar
Observatory (USO) with a 15 cm Coudé-refractor. The softwa4re and
hardware components of the system are similar to that of the low cost
solar adaptive optics system developed for the 1.5 m McMath-Pierce
solar telescope at Kitt Peak observatory for solar observations in the
infrared. The first results presented. The system has a closed loop
correction bandwidth in the range of 70 to 100 Hz. The root mean by a
factor of 10 to 20. The software developes and key issues concerning
optimum system performance have been addressed.
---------------------------------------------------------
Title: High resolution imaging system for Udaipur Solar Observatory
Authors: Bayanna, A. Raja; Louis, Rohan Eugene; Kumar, Brajesh;
Mathew, Shibu K.; Venkatakrishnan, P.
2007SPIE.6689E..0WB Altcode: 2007SPIE.6689E..26B
A Multi-Application Solar Telescope (MAST) is proposed to be installed
at the Udaipur Solar Observatory (USO) in India to monitor the Sun in
optical and near infra-red wavelengths. The median value of the Fried's
parameter at this site is 4 cm. USO is in the process of building
an Adaptive optics (AO) system in order to have diffraction limited
performance of the MAST under this moderate seeing condition. AO helps
in achieving high-resolution imaging by compensating the atmospheric
turbulence in real-time. We have performed simulations to evaluate
the performance of AO for various seeing conditions. It was concluded
that with the present availability of AO system components, a 55
cm aperture telescope would yield optimum performance with AO, in
combination with post-processing techniques like speckle imaging and
phase diversity. At present, we are developing a proto-type AO system
at USO to demonstrate its performance with a 15 cm Coudé refracting
telescope as a preparation for the main AO system to be deployed on
the MAST. The prototype AO system is being realized in two phases. In
the first phase, we have developed an image stabilization system
to compensate the global tilt of the wave-front. The second phase
consists of sensing and correcting the local tilts of the wave-front
by integrating a micro-machined membrane deformable mirror with
the image stabilization system and is currently in progress. Here,
we present the details of our proto-type AO system. We also present
preliminary results obtained from simulations using Phase Diversity
as a post processing technique.
---------------------------------------------------------
Title: Site Characterization Using Solar Hα Images
Authors: Kumar, Brajesh; Venkatakrishnan, P.; Bayanna, A. Raja;
Venugopalan, K.
2007SoPh..241..427K Altcode:
A Multi-Application Solar Telescope (MAST) is proposed to be installed
at the lake site (Lake Fatehsagar) of Udaipur Solar Observatory (USO)
in India. The lake site Observatory of USO is located on a small island
in the middle of the lake. To determine the optimum size of the MAST
(for use with an adaptive optics system), it was decided to quantify
the seeing conditions prevailing at the lake site during the different
months of the year. For this purpose, we have used short-exposure (3 ms)
high-resolution Hα (6563 Å) images (spatial scale of ∼0.55 arc sec
per pixel) of the Sun taken in burst mode with the 15-cm refractor Spar
telescope located at the lake site of USO. Spectral ratio technique
as reported by von der Lühe (1984, J. Opt. Soc. Am.A1, 510) has been
used to estimate the Fried's parameter (r<SUB>0</SUB>) at this site,
which gives the quantitative measure of astronomical seeing. This
study has been carried out daily on an hourly basis during 4:30 -
10:30 UT over the months January - June of the years 2005 and 2006
to understand the diurnal and seasonal variations in r<SUB>0</SUB>
at this site. It is noteworthy that the lake was almost dry during the
observing period in 2005, while it overflowed during our observations
in 2006 because of abundant monsoon rains. The seeing in the presence
of water shows improvement in r<SUB>0</SUB> by about 1.0 cm with
respect to the previous year's dry condition and mean r<SUB>0</SUB>
varies between 4.0 and 4.5 cm as evident from the data obtained between
January and June, 2006.
---------------------------------------------------------
Title: Commission 12: Solar Radiation & Structure
Authors: Bogdan, Thomas. J.; Martínez Pillet, Valentin; Asplund,
M.; Christensen-Dalsgaard, J.; Cauzzi, G.; Cram, L. E.; Dravins, D.;
Gan, W.; Henzl, P.; Kosovichev, A.; Mariska, J. T.; Rovira, M. G.;
Venkatakrishnan, P.
2007IAUTA..26...89B Altcode:
Commission 12 covers research on the internal structure and dynamics
of the Sun, the "quiet" solar atmosphere, solar radiation and its
variability, and the nature of relatively stable magnetic structures
like sunspots, faculae and the magnetic network. There is considerable
productive overlap with the other Commissions of Division II as
investigations move progressively toward the fertile intellectual
boundaries between traditional research disciplines. In large part,
the solar magnetic field provides the linkage that connects these
diverse themes. The same magnetic field that produces the more subtle
variations of solar structure and radiative output over the 11 yr
activity cycle is also implicated in rapid and often violent phenomena
such as flares, coronal mass ejections, prominence eruptions, and
episodes of sporadic magnetic reconnection.The last three years have
again brought significant progress in nearly all the research endeavors
touched upon by the interests of Commission 12. The underlying causes
for this success remain the same: sustained advances in computing
capabilities coupled with diverse observations with increasing levels
of spatial, temporal and spectral resolution. It is all but impossible
to deal with these many advances here in anything except a cursory and
selective fashion. Thankfully, the Living Reviews in Solar Physics; has
published several extensive reviews over the last two years that deal
explicitly with issues relevant to the purview of Commission 12. The
reader who is eager for a deeper and more complete understanding of
some of these advances is directed to http://www.livingreviews.org
for access to these articles.
---------------------------------------------------------
Title: Multi Application Solar Telescope (MAST): A Versatile Tool
for Studying the Physics of Solar Eruptions
Authors: Venkatakrishnan, P.
2006ihy..workE..33V Altcode:
pvk@prl.res.in Contemporary solar research is progressing along several
fronts. Solar magnetism and its role in powering solar eruptions is one
basic theme. Quantitative evaluation of the different manifestations of
the free energy available for eruption is one major task. This requires
vector magnetograms of a large number of active regions monitored
closely in time with high polarimetric accuracy. The second task is
to obtain greater clarity about various triggering mechanisms for
the eruptions. This requires observations of line-of-sight magnetic
fields and velocity fields with high spatial resolution. Both tasks
need mutually exclusive requirements leading to the concept of the
multi application telescope. In this talk, I outline the various steps,
like site characterization, optical design, adaptive optics development
and schemes for back-end instrumentation that culminated in the present
concept of MAST. I will emphasize the constraints posed by availability
of and access to technology which played a significant role in deciding
the concept. I conclude by highlighting certain unique features of
MAST which can provide special insights into a few scientific problems.
---------------------------------------------------------
Title: Acoustic power and magnetic field orientation in a large
sunspot
Authors: Gosain, S.; Venkatakrishnan, P.; Venugopalan, K.
2006ESASP.624E..59G Altcode: 2006soho...18E..59G
No abstract at ADS
---------------------------------------------------------
Title: On the enhanced velocity oscillations observed during solar
flares
Authors: Kumar, B.; Venkatakrishnan, P.; Venugopalan, K.
2006ESASP.624E..65K Altcode: 2006soho...18E..65K
No abstract at ADS
---------------------------------------------------------
Title: Development of Solar Scintillometer
Authors: Gupta, Sudhir Kumar; Mathew, Shibu K.; Venkatakrishnan, P.
2006JApA...27..315G Altcode:
The index of scintillation measurement is a good parameter to compare
different sites for image quality or `seeing'.We have developed
a scintillometer, which is deployed on the high resolution SPAR
telescope in the island site of Udaipur Solar Observatory, for the site
characterization to specify the proposed MAST (Multi Application Solar
Telescope). The scintillometer consists of a miniature telescope, termed
as micro telescope (4mm aperture, 15mm focal length) mounted on a drive
which tracks the Sun continuously, associated amplifiers and a data
acquisition system. A photodiode is used as the detector. The telescope
along with detector was obtained from National Solar Observatory
(NSO), and is similar to the one used for Advanced Technology Solar
Telescope (ATST) site survey. At USO we developed the amplifier and
data acquisition system for the scintillometer. A 24-bit analog to
digital converter based system was designed, assembled, tested and
used as the data acquisition system (DAS). In this paper, we discuss
the instrumentation and present the initial results.
---------------------------------------------------------
Title: Phase Diversity Technique for High Resolution Solar Imaging
Authors: Bayanna, A. Raja; Sridharan, R.; Venkatakrishnan, P.
2005BASI...33..413B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Experiments and Design Activities for the Multiapplication
Solar Telescope
Authors: Venkatakrishnan, P.
2005BASI...33..353V Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Imaging with insolated mirrors
Authors: Venkatakrishnan, P.; Sridharan, R.; Gupta, S. K.
2005BASI...33..265V Altcode:
Modern solar telescope designs are different from the conven- tional
concept of vacuum telescopes. These new designs are "open" telescopes
which try to minimize the temperature difference between various parts
of the telescope and the ambient air. In this paper, we address a
few issues related to the thermal response and image quality of such
insolated mirrors. We estimate the distortion produced by thermal
and material inhomogeneities and present limiting values of allowable
temperature differences and percentage change of expansion coefficients
for different aperture diameters, for typical materials under best
possible seeing conditions. We predict the evolution of surface
temperature of an insolated mirror using a simplified theoretical
approach and show that it is compatible with the experimental values
to a large extent. The results indicate the possibility of avoiding
active cooling of the mirror surfaces, at least for primary mirrors
with aperture diameter less than or equal to 50 cm.
---------------------------------------------------------
Title: Imaging with Insolated Mirrors
Authors: Venkatakrishnan, P.; Sridharan, R.; Gupta, S. K.
2005BASI...33..415V Altcode:
No abstract at ADS
---------------------------------------------------------
Title: An Image Stabilization System for Solar Observations
Authors: Sridharan, R.; Bayanna, A. Raja; Kumar, Brajesh;
Venkatakrishnan, P.
2005BASI...33..414S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Preliminary Results on the Calibration and Control of an
Adaptive Optics System
Authors: Kumar, Brajesh; Sridharan, R.; Bayanna, A. Raja;
Venkatakrishnan, P.
2005BASI...33..413K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Simulations of Solar AO Systems
Authors: Sridharan, R.; Bayanna, A. Raja; Venkatakrishnan, P.
2005sao..conf..132S Altcode:
In this paper, first we compare the two kinds of algorithms that are
being used in solar AO systems to sense a distorted wave-front through
simulations. Then, we comment on the various issues related to solar
AO systems and describe solar features that can be studied using AO
as a tool. Then we briefly describe the laboratory model of AO that
is being built at the Udaipur Solar Observatory (USO), India.
---------------------------------------------------------
Title: The Pattern of Moving Magnetic Inhomogeneities in and Around
Sunspots
Authors: Ravindra, B.; Venkatakrishnan, P.; Kumar, Brajesh
2004SoPh..225...47R Altcode:
High-resolution MDI magnetograms are used to study the pattern of
moving magnetic inhomogeneities in sunspots. We examine the inward and
outward moving features in sunspots. The velocity of these features
is small in the umbra while it is about 0.5 km s<SUP>−1</SUP> in
the outer penumbra. The inward and outward moving features may be
the possible origin for the long-term fluctuations of magnetic field
strength in sunspots.
---------------------------------------------------------
Title: Solar and interplanetary sources of major geomagnetic storms
during 1996-2002
Authors: Srivastava, Nandita; Venkatakrishnan, P.
2004JGRA..10910103S Altcode:
During the 7-year period of the current solar cycle, 64 geoeffective
coronal mass ejections (CMEs) were found to produce major geomagnetic
storms (D<SUB>ST</SUB> < -100 nT) at the Earth. In this paper we
examine solar and interplanetary properties of these geoeffective
coronal mass ejections (CMEs). The observations reveal that full-halo
CMEs are potential sources of intense geomagnetic activity at the
Earth. However, not all full-halo CMEs give rise to major geomagnetic
storms, which complicates the task of space weather forecasting. We
examine solar origins of the geoeffective CMEs and their interplanetary
effects, namely, solar wind speed, interplanetary shocks, and the
southward component of the interplanetary magnetic field, in order
to investigate the relationship between the solar and interplanetary
parameters. In particular, the present study aims at ascertaining solar
parameters that govern important interplanetary parameters responsible
for producing major geomagnetic storms. Our investigation shows that
fast full-halo CMEs associated with strong flares and originating from
a favorable location, i.e., close to the central meridian and low and
middle latitudes, are the most potential candidates for producing
strong ram pressure at the Earth's magnetosphere and hence intense
geomagnetic storms. The results also show that the intensity of
geomagnetic storms depends most strongly on the southward component
of the interplanetary magnetic field, followed by the initial speed
of the CME and the ram pressure.
---------------------------------------------------------
Title: Estimation of Fried's Parameter from Long-Exposure Solar Images
Authors: Sridharan, R.; Dashora, Nirvikar; Venkatakrishnan, P.
2004SoPh..222...35S Altcode:
We propose a criterion for extending the parameter search method
(Krishnakumar and Venkatakrishnan, 1997) of estimating the point spread
function to solar data. In the parameter search method, the number of
pixels with negative intensity values in the restored object is used
as an estimator for determining the unknown parameters of the point
spread function. As a solar image has a high background, the restored
object does not contain negative values, thereby making the method
unsuitable for solar data. We propose to use the intrinsic contrast of
solar features as a criterion for identifying the unknown parameter. We
validate our method through simulations. This method can not be used
for image restoration but can be used for monitoring daytime seeing.
---------------------------------------------------------
Title: Design of Instrument Control Software for Solar Vector
Magnetograph at Udaipur Solar Observatory
Authors: Gosain, Sanjay; Venkatakrishnan, P.; Venugopalan, K.
2004ExA....18...31G Altcode:
A magnetograph is an instrument which makes measurement of solar
magnetic field by measuring Zeeman induced polarization in solar
spectral lines. In a typical filter based magnetograph there are three
main modules namely, polarimeter, narrow-band spectrometer (filter),
and imager(CCD camera). For a successful operation of magnetograph
it is essential that these modules work in synchronization with each
other. Here, we describe the design of instrument control system
implemented for the Solar Vector Magnetograph under development at
Udaipur Solar Observatory. The control software is written in Visual
Basic and exploits the Component Object Model (COM) components for
a fast and flexible application development. The user can interact
with the instrument modules through a Graphical User Interface
(GUI) and can program the sequence of magnetograph operations. The
integration of Interactive Data Language (IDL) ActiveX components
in the interface provides a powerful tool for online visualization,
analysis and processing of images.
---------------------------------------------------------
Title: Relationship between CME velocity and active region magnetic
energy
Authors: Venkatakrishnan, P.; Ravindra, B.
2003GeoRL..30.2181V Altcode: 2003GeoRL..30wSSC2V
We find an empirical relationship between the initial speed of
Coronal Mass Ejection (CME) and the potential magnetic field energy
of the associated active region (AR) that closely resembles the Sedov
relation between the speed of a blast wave and the blast energy. We
conclude that it is the magnetic energy of an AR that drives the
CME. The restructuring of the AR field lines in the corona which
can push material with Alfven speed and thus inject energy into the
plasma on a time scale shorter than the dynamical time of the corona,
is a likely process that can drive the CME. The empirical relationship
allows the prediction of the maximum speed of a CME that can result
from an AR of a given magnetic energy.
---------------------------------------------------------
Title: Structure and Evolution of the Transition Region Network
Observed in he ii λ304
Authors: Ravindra, B.; Venkatakrishnan, P.
2003SoPh..215..239R Altcode:
The length scale and life time of the transition region network cells
were studied using He ii λ304 filtergrams. The temporal structure
function was calculated from spatially aligned He ii λ304 images. The
estimated life time of the network cell was about 27 hr. We compared
this life time with the life time of photospheric magnetic network and
of the extrapolated magnetic network. The spatial structure function
was calculated from the He ii λ304 filtergrams. The calculated spatial
structure function saturates at ∼ 25 000 km. The transition region
network elements are bigger in size than the photospheric magnetic
network element. The magnetic network element equals the size of the
He ii λ304 network element when the photospheric magnetic field is
extrapolated to a height of 3000 km above the photosphere where the
magnetic fluxes are deployed. The derived value of the diffusion speed
of the network elements was 0.098 km s<SUP>−1</SUP>.
---------------------------------------------------------
Title: On the Correlation Between the he ii λ304 Network Brightening
and the Photospheric Magnetic Field
Authors: Ravindra, B.; Venkatakrishnan, P.
2003SoPh..214..267R Altcode:
Near simultaneous coronal EUV images were used to show that the He ii
λ304 network brightening is independent of coronal EUV radiation. We
studied the quantitative relation between the intensity of He ii
λ304 network brightening with the associated magnetic elements. An
almost linear relationship was found between the He ii λ304 network
brightening and the magnetic field for a field strength higher than 10
G with exceptions at neutral lines and in the intra-network. We also
calculated the most probable formation height of He ii λ304 network
elements using the potential extrapolation of the photospheric magnetic
field. The results show that He ii λ304 network elements form at or
around 3000 km above the height of the layer at which the sources of
magnetic flux are deployed.
---------------------------------------------------------
Title: On the Rapid Variations of Solar Magnetic Fields
Authors: Venkatakrishnan, P.; Kumar, Brajesh; Ravindra, B.
2003BASI...31..299V Altcode:
We report on the rapid variations of solar magnetic fields that
appear to be enhanced significantly above the background variability,
at a few locations within the solar active regions, as observed with
the Michelson Doppler Imager (MDI) on board the SOHO spacecraft. The
pressure fluctuations estimated to arise from this variability far
exceed the general level of acoustic pressure fluctuations. The
equivalent mechanical flux that could be generated from these rapid
magnetic variations is more than adequate for the heating of the active
region chromosphere and corona.
---------------------------------------------------------
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: Solar Magnetic Fields
Authors: Venkatakrishnan, P.
2003LNP...619..202V Altcode: 2003lsp..conf..202V
The proximity of the Sun allows us to make detailed measurements
on the properties of solar magnetic fields. The long term systematic
changes in the solar magnetic field pattern indicate a global origin. A
global dynamo can be sustained by the interaction of solar convection
with solar rotation. The rudiments of such a dynamo mechanism are
discussed. Some recent issues arising out of new theoretical and
observational developments are mentioned. The importance of magnetic
topology for various solar phenomena is highlighted. Finally, a few
methods of measuring solar magnetic fields are described.
---------------------------------------------------------
Title: Propagation Characteristics of Geo-Effective CMES
Authors: Srivastava, Nandita; Venkatakrishnan, P.
2003IAUJD...7E..19S Altcode:
The expansion speeds of halo CMEs observed by LASCO aboard SoHO
duirng 1996-2002 which were geo-effective (Dst <-100 nT) have been
measured. The radial propagation profiles of these CMEs have been
inferred from the measured expansion speeds. We also investigate
if the propagation profiles of these geoeffective CMEs is of blast
wave nature. It is found that the profiles of geoeffective CMEs
associated with the flares and eruptive prominences are distinctly
different. This provides a crucial clue to the initial trigger mechanism
of geo-effective halo CMEs and their nature of propagation which in
turn has important implication on forecasting of the space weather.
---------------------------------------------------------
Title: Estimation of Fried's Parameter From Specklegrams of Solar
Features
Authors: Sridharan, R.; Venkatakrishnan, P.; Verma, V. K.
2002SoPh..211..395S Altcode:
A few methods of estimating Fried's parameter (r<SUB>0</SUB>) from
specklegrams of solar features are described. Some of these methods
were used to estimate r<SUB>0</SUB> for the speckle data obtained from
Kodaikanal Observatory (KO), Uttar Pradesh State Observatory (UPSO)
and Udaipur Solar Observatory (USO). The average value of r<SUB>0</SUB>
was found to be ∼ 3 cm at USO and UPSO during our observations. At
KO, values of r<SUB>0</SUB> ranging from 6 to 10 cm were estimated.
---------------------------------------------------------
Title: Variation of Acoustic Power with Magnetic Field as Seen in
Gong+ Data
Authors: Venkatakrishnan, P.; Kumar, Brajesh; Tripathy, S. C.
2002SoPh..211...77V Altcode: 2002astro.ph..7524V
The acoustic spectra in sunspots are known to be richer in higher
frequency power. We have attempted a generalized study of the effect
of magnetic fields on the shape of the acoustic spectrum using GONG+
bread-board data (spatial scale of ∼ 2 arc sec per pixel) of 11
May 2000 and 12 June 2000. The mean power spectra of the velocity
oscillations were obtained by averaging over several spectra for
different values of the magnetic field. With increasing magnetic field,
the acoustic power increases at higher frequencies and decreases at
lower frequencies with a transition at ≃ 5 mHz. This behavior is
slightly different from earlier results obtained from SOHO/MDI data.
---------------------------------------------------------
Title: On Magnetic Flux Imbalance in Solar Active Regions
Authors: Choudhary, Debi Prasad; Venkatakrishnan, P.; Gosain, Sanjay
2002ApJ...573..851C Altcode:
The magnetic flux imbalance of active regions has been studied using the
longitudinal magnetograms obtained from the National Solar Observatory
at Kitt Peak. The maximum and the median value of the flux imbalance in
137 active regions situated near the disk center is found to be about
62% and 9.5%, respectively. The detailed analysis of a few selected
active regions shows that the local flux asymmetry is compensated on
global scales. For example, the NOAA Active Region 7978, which appeared
during the solar activity minimum period and evolved during five solar
rotations (1996 July-October), shows a flux imbalance of about 9.5%,
with an excess of following negative flux. However, on a global scale,
the positive and negative flux in the entire solar disk was found to be
nearly balanced during the same period. The global flux imbalance of
the Sun during a full magnetic cycle is estimated from the Carrington
maps. These maps are made by merging the solar images obtained during
a complete solar rotation (about 27 days), hence each represents the
entire surface of the Sun. The 10°-40° active latitudinal zone in the
individual hemispheres during the solar maximum shows a flux imbalance
of more than 20%. This is reduced to below 10% when the entire Sun
is considered. The present study indicates that a fraction of the
magnetic field from the localized active regions connects with far
away locations.
---------------------------------------------------------
Title: Relationship between CME Speed and Geomagnetic Storm Intensity
Authors: Srivastava, Nandita; Venkatakrishnan, P.
2002GeoRL..29.1287S Altcode: 2002GeoRL..29i...1S
In this paper, we discuss the solar origin and interplanetary
consequences of the coronal mass ejection of March 29, 2001 that was
responsible for the most intense geomagnetic storm (D<SUB>ST</SUB> ~
-377 nT) of the current solar cycle to date. A comparison of the CME
of March 29, 2001, with a set of geo-effective halo CMEs associated
with X-class flares showed that the strength of the geomagnetic storm
at the earth is well correlated with the speed of the halo. Our study
shows that the fast ejection is responsible for building up the ram
pressure at the earth's magnetosphere. This may serve as a useful tool
in the forecasting of intense geomagnetic storms.
---------------------------------------------------------
Title: Useful Aspects of Chromospheric Magnetic Field Data
Authors: Sakurai, T.; Choudhary, D. P.; Venkatakrishnan, P.
2002stma.conf...37S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Chromospheric Magnetic Field of Solar Active Regions
Authors: Choudhary, Debi Prasad; Sakurai, Takashi; Venkatakrishnan, P.
2001ApJ...560..439C Altcode:
The three-dimensional magnetic field structure of 137 solar active
regions is studied by comparing the observed and computed chromospheric
magnetograms. The model chromospheric field is obtained by extrapolating
the observed photospheric field into the chromosphere with a potential
(current-free) magnetic-field model in Cartesian geometry. The
best correlation between the observed and the model chromospheric
magnetograms is found at the height of 800 km, which also corresponds
to the height of the line formation for Ca II 854.2 nm. In the weak
field range, within +/-300 G, most of the observed field is close to
the potential field. However, departures of about 50 G are observed
in a few active regions. For field values greater than +/-500 G, the
observed field does not always match the model. Whereas a part of this
could be due to the magnetogram calibration, it might also originate
from the “nonpotentiality” of the chromospheric field. In the case
of long-lived active regions, which make multiple disk passages, the
strong-field nonpotentiality is observed during their initial phase,
and converges to a potential field configuration later.
---------------------------------------------------------
Title: Multi-aperture solar telescope
Authors: Venkatakrishnan, P.
2001BASI...29..467V Altcode:
A proposal for a new ground based solar telescope of very high angular
resolution is presented. The science goals that drive the need for this
facility are enumerated. The proposed strategy, of combining several
smaller aperture telescopes to provide the desired angular resolution,
is out-lined.
---------------------------------------------------------
Title: Search for Spatial Variability in the Solar Acoustic Spectrum
Authors: Venkatakrishnan, P.; Kumar, Brajesh; Tripathy, S. C.
2001SoPh..202..229V Altcode: 2001astro.ph..5377V
Motivated by the various examples of spatial variability in the power
of the acoustic spectrum, we attempted to look for spatial variability
in the peak frequency of the spectrum. However, the determination of
this peak frequency on a spatial scale of a single pixel (8 arc sec
for the GONG data) is limited by the stochastic variations in the power
spectrum presumably caused by the stochastic nature of the excitation
process. Averaging over a large number of spectra (100 spectra from a
10 × 10 pixel area) produced stabler spectra. The peak frequencies of
130 such locations were found to be distributed with a FWHM of about
130 μHz. A map of the spatial variation of this peak frequency did
not show any strong feature with statistically significant deviation
from the mean of the distribution. Likewise, the scatter in the peak
frequencies masked the detection of magnetic-field-induced changes in
the peak frequency. On a much larger scale, the N latitudes showed
a slightly lower value of the peak frequency as compared to the S
latitudes, although the difference (25 μHz) is barely larger than
the r.m.s. spread (20 μHz).
---------------------------------------------------------
Title: Vector Polarimetry using the Kodaikanal Tower Telescope
Authors: Sankarasubramanian, K.; Venkatakrishnan, P.
2001ASPC..236..297S Altcode: 2001aspt.conf..297S
No abstract at ADS
---------------------------------------------------------
Title: Is a Sunspot in Static or Dynamic Equilibrium?
Authors: Venkatakrishnan, P.
2000JApA...21..171V Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Stokes Polarimetry at the Kodiakanal Tower Tunnel Telescope
Authors: Sankarasubramanian, K.; Srinivasulu, G.; Ananth, A. V.;
Venkatakrishnan, P.
2000JApA...21..241S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Morphology of Ca II K bright points and their link to G band
bright points
Authors: Krishnakumar, V.; Venkatakrishnan, P.; Srikanth, R.
2000BASI...28..123K Altcode:
We present the results of a preliminary analysis of a time series of
a sequence of Ca II K bright points and G band bright points observed
co-spatially and co-temporally. Inspection of these images shows that
the larger Ca II K bright points occur between two G band bright points
suggesting a loop structure with the top of the loop housing the Ca II
K bright point. The velocities of the center of mass of G band pair and
that of the corresponding K line bright point seem to be correlated
even though these features appear at different heights in the solar
atmosphere. The velocity of the K line bright point is estimated to lie
between 0.5 km/sec to 5.0km/ sec, while that of G band bright points
between 3.0 km/sec and 8.0 km/sec. The diffuse K line brightenings
are found to trace out the boundary of photospheric granulation. We
estimate the sizes of the cells enclosed by Ca II K brightenings.
---------------------------------------------------------
Title: Science from "Solar X-ray Spectrometer (SOXS)" - Proposed
payload onboard Indian satellite
Authors: Jain, Rajmal; Rao, A. R.; Deshpande, M. R.; Dwivedi,
B. N.; Manoharan, P. K.; Seetha, S.; Vahia, M. N.; Vats, Hari Om;
Venkatakrishnan, P.
2000BASI...28..117J Altcode:
It is proposed to fly a high spectral and temporal resolution "Solar
X-ray Spectrometer (SOXS)" onboard Indian satellite to understand
the mechanisms of energy release and particle acceleration in solar
flares. The SOXS will provide the disk integrated flux in the energy
range 2 keV-10 MeV. The proposed SOXS will consist of two detector
modules - SOXS Low Energy Detector (SLD) and SOXS High Energy Detector
(SHD). The proposed instrument will enable us to measure precisely the
low energy cut-off below 60 keV to estimate the total energy release in
the flare. It is proposed that high spectral and temporal resolution
efficiencies of our detectors will reveal, perhaps for the first
time, the observed break below 60 keV in the characteristic double
power-law shape of hard X-ray spectrum. Whether electrons and protons
are accelerated simultaneously may be also answered by correlating
high temporal spectra of SLD and SHD. The high temporal and sub-keV
resolution spectra from SLD will be capable to investigate the nature
of micro/nano flares considered responsible to heat the chromosphere
and corona. It is proposed to use the observations from this space
borne instrument, along with extensive simultaneous ground based high
spatial and time resolution observations in optical and radio wavebands
for better understanding of the flare phenomena.
---------------------------------------------------------
Title: Characteristics of Flare and Surge Locations in the Super
Active Regions of Cycle 22
Authors: Venkatakrishnan, P.; Prasad Choudhary, Debi
2000IAUJD...7E...3V Altcode:
The superactive regions NOAA 6555 and NOAA 6659 were highly flare
productive during their disk passage in cycle 22. Many of these
activities caused the near earth space weather disturbances. Here,
we study the properties of the location of surges and flares. From
the longitudinal magnetogram movies, it is clearly evident that the
emergence of magnetic flux is the prime cause for both the type of
activities. Flares were generally seen in those sub-areas of active
region which possessed closed magnetic field configuration, whereas
only minor flares and surges occurred in subareas showing open magnetic
field configuration. The active region NOAA 6555 had several locations
of highly sheared magnetic field structure, yet only one of them was
the site of all the X-class flares during its disk passage. There are
clear evidence of pre-flare flux emergence for all these events. In this
paper, we present the detailed photospheric and chromospheric morphology
of these active regions. We also present the three dimensional magnetic
topology derived from the daily evolution of the active regions.
---------------------------------------------------------
Title: A Speckle Experiment during the Partial Eclipse
Authors: Saha, S. K.; Nagabhushana, B. S.; Ananth, A. V.;
Venkatakrishnan, P.
1999astro.ph.10304S Altcode:
An experiment for the speckle reconstruction of solar features was
developed for observing the partial eclipse of the sun as viewed from
Bangalore on October 24, 1995. No data could be obtained because of
cloudy sky but the experimental details are described.
---------------------------------------------------------
Title: Blind iterative deconvolution of binary star images
Authors: Saha, S. K.; Venkatakrishnan, P.
1999astro.ph.10381S Altcode:
The technique of Blind Iterative De-convolution (BID) was used to
remove the atmospherically induced point spread function (PSF) from
short exposure images of two binary stars, HR 5138 and HR 5747 obtained
at the cassegrain focus of the 2.34 meter Vainu Bappu Telescope(VBT),
situated at Vainu Bappu Observatory (VBO), Kavalur. The position angles
and separations of the binary components were seen to be consistent
with results of the auto-correlation technique, while the Fourier
phases of the reconstructed images were consistent with published
observations of the binary orbits.
---------------------------------------------------------
Title: Comparative morphology of EIT/SOHO images and He II excitation
Authors: Venkatakrishnan, P.
1999SoPh..187...23V Altcode:
Morphological differences between coronal images on the one hand, and
a He i image on the other, are used to demonstrate the independence of
He ii excitation from coronal radiation. The distribution of magnetic
flux is found to be more important for He ii excitation. Collisional
excitation by non-thermal electrons produced in nano-flare events is
proposed as the mechanism for He ii excitation.
---------------------------------------------------------
Title: Transverse motions and wave heating of the solar atmosphere
Authors: Krishnakumar, V.; Venkatakrishnan, P.
1999SoPh..186...43K Altcode:
Periodic shaking or buffeting of magnetic flux tubes could generate
magnetohydrodynamic waves which propagate along the flux tubes and
dissipate energy in the chromosphere and/or corona. If we make an
assumption that the G-band bright points represent flux tubes, then
there should exist a relationship between the transverse motions and
the brightening of these bright points. We tracked a total of 56 bright
points, obtained their velocity and intensity power spectrum. We also
estimated the r.m.s. velocity, average velocity, r.m.s. intensity
and average intensity of these bright points. We do not see any clear
evidence for a relationship between these estimated quantities.
---------------------------------------------------------
Title: Measurement of instrumental polarisation of the Kodaikanal
tunnel tower telescope
Authors: Sankarasubramanian, K.; Samson, J. P. A.; Venkatakrishnan, P.
1999ASSL..243..313S Altcode: 1999sopo.conf..313S
No abstract at ADS
---------------------------------------------------------
Title: A Search for Vector Magnetic Field Variations Associated with
the M-Class Flares of 10 June 1991 IN AR 6659
Authors: Hagyard, M. J.; Stark, B. A.; Venkatakrishnan, P.
1999SoPh..184..133H Altcode:
A careful analysis of a 6-hour time sequence of vector magnetograms of
AR 6659, observed on 10 June 1991 with the MSFC vector magnetograph,
has revealed only minor changes in the vector magnetic field azimuths
in the vicinity of two M-class flares, and the association of these
changes with the flares is not unambiguous. In this paper we present our
analysis of the data which includes comparison of vector magnetograms
prior to and during the flares, calculation of distributions of the
r.m.s. variation of the azimuth at each pixel in the field of view of
the active region, and examination of the variation with time of the
azimuths along the flaring neutral lines and at every pixel covered by
the main flare emissions as observed with the Hα telescope coaligned
with the vector magnetograph.
---------------------------------------------------------
Title: Erratum: "Determination of the atmospheric point spread
function by a parameter search" [Astron. Astrophys., Suppl. Ser.,
Vol. 126, No. 1, p. 177 - 181 (Nov 1997)].
Authors: Krishnakumar, V.; Venkatakrishnan, P.
1998A&AS..131..195K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: High Resolution Imaging of the Sun and Other Extended Sources
Authors: Krishnakumar, V.; Venkatakrishnan, P.
1998ASPC..154.2003K Altcode: 1998csss...10.2003K
The functional form of the long exposure OTF P(u,v) is known (Fried
1966; Kirshnakumar & Venkatakrishnan 1997a) and i(x,y) is what
we record. The knowledge of P(u,v) is complete once r<SUB>o</SUB>
is estimated. Our aim is to recover o<SUB>e</SUB>(x,y) which is
close to o(x,y). We present here the results of estimation of Fried's
parameter r<SUB>o</SUB> using a parameter search method (Kirshnakumar
& Venkatakrishnan 1997b) and an modified Wiener filter (U filter)
(Roddier 1981) for image restoration. The estimation of r<SUB>o</SUB>
and restoration using the U filter on simulations and images of an
extended stellar source, globular cluster NGC 1409, is presented.
---------------------------------------------------------
Title: On the correlation between line width and line depth of the
solar HeI 1083 NM line
Authors: Venkatakrishnan, P.; Sakurai, T.; Suematsu, Y.; Ichimoto, K.
1997BASI...25..527V Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Determination of the atmospheric point spread function by a
parameter search
Authors: Krishnakumar, V.; Venkatakrishnan, P.
1997A&AS..126..177K Altcode:
The result of blind deconvolution is a reconstructed image that has
non positive intensities. The number of these non positive pixels
has been used as an estimator for the departure from a perfect
reconstruction. Simulations of reconstruction of objects convolved
with a kernel having one or two parameters are shown to demonstrate the
efficiency of the estimator. We thus present a technique of determining
the unknown parameters of the point spread function by searching for
the point in parameter space with the lowest number of nonpositive
pixels. It is also shown that the parameters of the convolving kernel
can be obtained even in the presence of noise. This method was validated
using a long exposure image of NGC 1409.
---------------------------------------------------------
Title: Blind iterative deconvolution of binary star images
Authors: Saha, S. K.; Venkatakrishnan, P.
1997BASI...25..329S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Narrow band photometry in emission lines during the solar
eclipse of October 24, 1995.
Authors: Singh, J.; Cowsik, R.; Venkatakrishnan, P.; Srinivasan, R.;
Chinnapan, V.; Raju, K. P.; Srikanth, R.
1997KodOB..13...37S Altcode:
To investigate the spatial variation of temperature and density within
coronal structures, narrow band photometry in three coronal emission
lines was done during this eclipse. The emission lines 6374 (Fe X),
7892 (Fe XI) and 5303 Å (Fe XIV) represented temperatures in the Å
range of 1 - 2 million degrees. Peltier cooled CCD camera was used to
record the coronal images through 5 Å passband interference filters
with pixel resolution of 11 arcsec.
---------------------------------------------------------
Title: Reconstruction of solar features from an image of partially
eclipsed Sun.
Authors: Krishnakumar, V.; Venkatakrishnan, P.
1997KodOB..13...95K Altcode:
The photoheliograms obtained at Kodaikanal during the partial
eclipse of the Sun on October 24, 1995 were digitised on a PDS
microdensitometer. The lunar limb was used as a standard source for
the digital reconstruction of a sunspot image. The details on the
reconstruction are provided along with an example of a reconstructed
image.
---------------------------------------------------------
Title: A speckle experiment during the partial eclipse.
Authors: Saha, S. K.; Nagabhushana, B. S.; Ananth, A. V.;
Venkatakrishnan, P.
1997KodOB..13...91S Altcode:
An experiment for the speckle reconstruction of solar features was
developed for observing the partial eclipse of the sun as viewed from
Bangalore on October 24, 1995. No data could be obtained because of
cloudy sky but the experimental details are described.
---------------------------------------------------------
Title: Ellipsometry of Coelostat Coatings Using a Babinet Compensator:
Simulation of the Experimental Accuracy
Authors: Sankarasubramanian, K.; Venkatakrishnan, P.
1996SoPh..167....1S Altcode:
The precise measurement of solar magnetic fields requires an accurate
measurement of the Muller matrix of the optical components in the
path of the light beam, which again requires a careful measurement
of the optical constants of the reflecting surfaces in the case of
a 3-mirror coelostat system. Here we present a method to measure the
optical constants (the real and imaginary part of the refractive index)
to an accuracy of the order of 1% for bulk aluminium. This work is
directed towards the measurement of instrumental polarisation at the
Kodaikanal solar tower telescope, although it can be used for any
metallic coated optics.
---------------------------------------------------------
Title: Onset of Hydrodynamic Non-Equilibrium in Helmet Streamers
Authors: Venkatakrishnan, P.
1996SoPh..166..195V Altcode:
The steady-state solar wind solution is examined for different
geometries of the flow tube that mimics a helmet streamer. Onset of
non-equilibrium is seen whenever the spatial variation of the flow
geometry crosses critical values. It is suggested that the dynamical
response of the flow to the onset of non-equilibrium can manifest as
a coronal mass ejection.
---------------------------------------------------------
Title: Observability of Coronal Heating Processes
Authors: Venkatakrishnan, P.
1996Ap&SS.243...43V Altcode: 1996IAUCo.154...43V
The mechanisms that could possibly heat the corona are briefly reviewed
with emphasis on their observability. Observing enhanced wave flux at
footpoints of active regions would confirm wave heating. Observation
of nonthermal electrons in tiny coronal events (nanoflares) would
confirm dissipation of current sheets. Presence of large scale
flows in coronal arcades would underline the importance of turbulent
resistivity for coronal heating. A comparison of HeI absorption in
quiet and active regions demonstrates the difficulty of interpreting
data that connect chromospheric dynamics with coronal heating. Finally,
the implications of the search for observations of coronal heating
processes are mentioned.
---------------------------------------------------------
Title: Enhanced He{I} Absorption at the Feet of Solar X-Ray Loops
Authors: Venkatakrishnan, P.; Sakurai, Takashi; Suematsu, Yoshinori;
Ichimoto, Kiyoshi
1996PASJ...48L...1V Altcode:
A comparison of He{I} spectroheliograms and Yohkoh soft X-ray images
of active regions indicates that He{I} absorption is enhanced at
the feet of hot X-ray loops. It is suggested that the conduction of
heat from the loops into the transition region at their feet would
produce enhanced transition-region emission around 50 eV that would
in turn cause enhanced excitation of He{I} leading to the excess He{I}
absorption that is observed.
---------------------------------------------------------
Title: The Evolutton Of The Magnetic Structure of the Solar Corona
With The Solar Cycle
Authors: Dikpati, Mausumi; Choudhuri, Arnab Rai; Venkatakrishnan, P.
1996ASPC...95..309D Altcode: 1996sdit.conf..309D
No abstract at ADS
---------------------------------------------------------
Title: A Search for Vector Magnetic Field Variations Associated with
the M-Class Flares of 1991 June 10 in AR 6659
Authors: Hagyard, M. J.; West, E. A.; Smith, J. E.; Venkatakrishnan, P.
1995SPD....26..304H Altcode: 1995BAAS...27..953H
No abstract at ADS
---------------------------------------------------------
Title: Observable signals of coronal heating processes
Authors: Venkatakrishnan, P.
1995HiA....10..305V Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Image Restoration by Blind Iterative Deconvolution - Results
Obtained from VBT
Authors: Saha, S. K.; Venkatakrishnan, P.
1995JApAS..16Q.446S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Personal Computer-Based Imaging Stokes Polarimeter for
Solar Observations
Authors: Ananth, A. V.; Venkatakrishnan, P.; Narayanan, R. S.;
Bhattacharyya, J. C.
1994SoPh..151..231A Altcode:
For measurements of vector magnetic field over solar active regions,
a Stokes polarimeter for studying the polarisation profiles on
selected spectral lines is described. This paper gives details of
the relevant CCD imaging system and the personal computer (PC)-based
acquisition, together with the image analysis techniques necessary
for the task. Field trials and tests of the system are also described.
---------------------------------------------------------
Title: Time variability of the He i 10830 Å line profile
Authors: Singh, Jagdev; Jain, S. K.; Venkatakrishnan, P.
1994SoPh..150...49S Altcode:
We have studied the time-dependent behaviour of the HeI 10830 å
line. These studies show that (i) the fluctuations of the line width
are uncorrelated with the equivalent-width fluctuations and (ii)
the autocorrelation curves for the equivalent-width fluctuations
are broader than those for line-width fluctuations. These results
could be interpreted as the signatures of the eruption of density
inhomogeneities, like spicules, into the altitudes of formation of
the HeI 10830 å line.
---------------------------------------------------------
Title: Can Coronal Magnetic Structures be Quasi-Static?
Authors: Venkatakrishnan, P.
1994ASPC...68..409V Altcode: 1994sare.conf..409V
No abstract at ADS
---------------------------------------------------------
Title: Coronal Heating and Chromospheric Energy Density - an
Observational Association
Authors: Venkatakrishnan, P.
1993SoPh..148..233V Altcode:
The time-averaged equivalent width of the HeI 10830 å line is seen
to be correlated with the time-averaged line width. This correlation
is interpreted as evidence for the association of the chromospheric
energy density with the heating of the overlying corona.
---------------------------------------------------------
Title: Correlation Between Magnetic Shear and Magnetic Tension in
a Solar Active Region
Authors: Venkatakrishnan, P.; Narayanan, R. S.; Prasad, N. D. N.
1993SoPh..144..315V Altcode:
The difference between the magnetic tension and magnetic shear
was calculated for four vector magnetograms of NOAA AR 4474. It
was seen that this difference between the two independent angular
measures of magnetic stress is less than 18° for more than 50% of the
pixels. Magnetic tension is thus found to be fairly well correlated
with magnetic shear for AR 4474.
---------------------------------------------------------
Title: On the evaluation of magnetic shear from H-alpha pictures
Authors: Venkatakrishnan, P.
1993SoPh..143..385V Altcode:
The recently reported pre-flare activity in H-alpha filaments
(Sivaraman, K. R., Rausaria, R. R., and Aleem, S. M.: 1992,Solar
Phys.138,353) is shown to be unrelated to changes in magnetic shear. An
alternative interpretation for these observations is suggested.
---------------------------------------------------------
Title: Spatio-temporal fluctuations in HeI 10830 Å line parameters:
Evidence for spicule formation
Authors: Venkatakrishnan, P.; Jain, S. K.; Singh, Jagdev; Recely,
F.; Livingston, W. C.
1992SoPh..138..107V Altcode:
The equivalent width, line depth, line width, and Doppler shift
of the He I 10830 Å line were extracted from two time series of
spectra. Scatter plots of time-averaged line depth, line width, and
Doppler shifts, as well as the root mean square temporal fluctuation of
these quantities against the time-averaged equivalent width at a few
hundred spatial locations were obtained. The statistical behaviour of
these line parameters and their fluctuations was used to infer plausible
reasons for the fluctuations. Examination of these results showed
that the line parameter fluctuations could be caused by fluctuations
in the coronal UV radiation (which could drive the spicules) or by
the appearance of density inhomogeneities such as spicules within the
line forming domain. In either case, the data can be interpreted as
representing the initial phases of spicules.
---------------------------------------------------------
Title: The Kodaikanal solar vector magnetograph - Laboratory
evaluation of the polarimeter
Authors: Venkatakrishnan, P.; Narayanan, R. S.
1991BASI...19..243V Altcode:
Laboratory experiments performance to evaluate the sensitivity of the
polarimeter of the Kodaikanal vector magnetograph. The polarimeter
does not produce significant cross talk and has a sensitivity of about
1 percent polarization.
---------------------------------------------------------
Title: The pyhsics of "seeing".
Authors: Venkatakrishnan, P.
1991nlt..work..129V Altcode:
The properties of images formed through turbulence are reviewed. The
causes of turbulence are also listed and some suggestions are made
for site selection based on this physical reasoning.
---------------------------------------------------------
Title: Loss of Magnetic Tension in Pre-Flare Magnetic Configurations
Authors: Venkatakrishnan, P.
1990SoPh..128..371V Altcode: 1990IAUCo.121P.371V
We demonstrate that magnetic tension vanishes at regions of large
magnetic `shear' on the polarity inversion line. The characteristics
of these tension-free fields depend on the density of the medium and,
therefore, change as a consequence of instabilities which modify the
density. These instabilities may possibly evolve into solar flares. We
suggest this as a possible explanation for the observed occurrence
of flares at locations of large magnetic shear along the polarity
inversion line.
---------------------------------------------------------
Title: Nonpotential Magnetic Fields at Sites of Gamma-Ray Flares
Authors: Hagyard, M. J.; Venkatakrishnan, P.; Smith, J. B., Jr.
1990ApJS...73..159H Altcode:
The relation between the degree of nonpotentiality of photospheric
magnetic fields and the occurrence of gama-ray flares is examined to
determine whether there are special signatures of the stressed fields
for this type of flare. Observations of the flares in the active
region of April 1984 (AR 4474) are analyzed, showing that the big
flare initiated at the location on the magnetic neutral line where
the field deviated the most from a potential field. The nonpotential
signatures of AR 4474 are compared with those of four other regions. The
results suggest that gamma-ray flares are associated with strongly
nonpotential fields that extend over relatively larger lengths of the
magnetic neutral line that the fields associated with flares that do
not produce gamma-ray events.
---------------------------------------------------------
Title: Implications of Tension-Free Equilibria for Pre-Flare Energy
Build UP
Authors: Venkatakrishnan, P.
1990IAUS..142..323V Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Evaluation of Magnetic Shear in Off-Disk Center Active Regions
Authors: Venkatakrishnan, P.; Hagyard, M. J.; Hathaway, D. H.
1989SoPh..122..215V Altcode:
We analyze the changes that projection effects produce in the evaluation
of magnetic shear in off-disk center active regions by comparing angular
shear calculated in image plane and heliographic coordinates. We
describe the procedure for properly evaluating magnetic shear by
transforming the observed vector magnetic field into the heliographic
system and then apply this procedure to evaluate magnetic shear along
the magnetic neutral line in an active region that was observed on 1984
April 24 at a longitude offset of -45°. In particular, we show that
the number of `critically sheared' pixels along an east-west directed
segment of the neutral line in the leader sunspot group changes from 16
in the image plane magnetogram to 14 in the heliographic magnetogram. We
also show that the critical shear as calculated in the image plane
served as a good predictor for the location of flaring activity since
the flare ribbons of the great flare of April 24 bracketed the inversion
line where the critical shear was located. These results indicate that
for this particular region, projection effects did not significantly
affect the evaluation of critical shear.
---------------------------------------------------------
Title: Off Disk Center Potential Field Calculations Using Vector
Magnetograms
Authors: Venkatakrishnan, P.; Gary, G. Allen
1989SoPh..120..235V Altcode:
We investigate a potential field calculation for off disk-center
vector magnetograms that uses all the three components of the measured
field. There is neither any need for interpolation of grid points
between the image plane and the heliographic plane nor for an extension
or a truncation to a heliographic rectangle. Hence, the method provides
the maximum information content from the photospheric field as well
as the most consistent potential field independent of the viewing
angle. The introduction of polarimetric noise produces a less tolerant
extrapolation procedure than using the line-of-sight extrapolation,
but the resultant standard deviation is still small enough for the
practical utility of this method.
---------------------------------------------------------
Title: Evaluation of Magnetic Shear in Off-Disk Center Active Regions
Authors: Hagyard, M. J.; Hathaway, D. H.; Venkatakrishnan, P.
1989BAAS...21..838H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Book-Review - Solar and Stellar Physics / 5TH European Solar
Meeting / Titisee / Schwarzwald Germany - 1987APR
Authors: Schroter, E. H.; Schussler, M.; Venkatakrishnan, P.
1988BASI...16..248S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Book-Review - Cool Stars Stellar Systems and the Sun / 5TH
Cambridge Workshop / Boulder, Colorado 1987JUL
Authors: Linsky, J. L.; Stencel, R. E.; Venkatakrishnan, P.
1988BASI...16..248L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Vector Magnetic Configurations of AR 4474 Before the
Flares of April 25 and April 28, 1984: A Comparison
Authors: Venkatakrishnan, P.
1988BAAS...20..711V Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Elimination of Projection Effects from Vector Magnetograms -
the Pre-Flare Configuration of Active Region AR:4474
Authors: Venkatakrishnan, P.; Hagyard, M. J.; Hathaway, D. H.
1988SoPh..115..125V Altcode:
We demonstrate a simple method of transforming vector magnetograms
to heliographic coordinates. The merits of this transformation are
illustrated using a vector magnetogram obtained with the MSFC vector
magnetograph 80 minutes prior to a white light flare in active region AR
4474 on 25 April, 1984. The original magnetogram shows strong magnetic
shear along the neutral line at both the flare site and a non-flaring
site. The transformation of the magnetogram to heliographic coordinates
shows that the elimination of projection effects results in a much
shorter length of the sheared region at the non-flaring site than
what is inferred from the image plane vector magnetogram. The length
of the sheared region at the flare site is relatively less affected
by the transformation.
---------------------------------------------------------
Title: Nonpotential magnetic fields at sites of gamma ray flares
Authors: Hagyard, M. J.; Venkatakrishnan, P.; Smith, J. B., Jr.
1988STIN...9026785H Altcode:
The relation between the degree of nonpotentiality of photospheric
magnetic fields and the occurrence of gamma ray flares is examined. The
parameter delta phi (magnetic shear) and the strength of the magnetic
field intensity are used as measures of the degree of nonpotentiality,
where delta phi is defined as the angular difference between the
observed direction of the transverse component of the photospheric field
and the direction of the potential field prescribed by the distribution
of measured photospheric flux. An analysis of the great flare of April
24 to 25, 1984 is presented as an example of this technique to quantify
the nonpotential characteristics of the pre-flare magnetic field. For
this flare, which produced a large gamma ray event, strong shear and
high field strengths prevailed over an extended length of the magnetic
neutral line where the flare occurred. Moreover, the flare began near
the area of strongest measured shear (89 to 90 deg). Four other flaring
regions were analyzed; one of these produced a moderate gamma ray event
while the other three did not produce detectable gamma rays. For all
four regions the flares were located in the area where the field was
not nonpotential, regardless of the class of flare. The fields of the
gamma ray flares were compared with those associated with the flares
without gamma rays, and little distinction was found in the degree
of magnetic shear. The major difference is seen in the extent of the
sheared field: for gamma ray events, the field is sheared over a longer
length of the neutral line.
---------------------------------------------------------
Title: On the saturation of the refractive index structure
function. II - Influence of the correlation length on astronomical
'seeing'
Authors: Venkatakrishnan, P.
1987MNRAS.229..379V Altcode:
A physical length scale in the wavefront corresponding to the parameter
(r<SUB>0)</SUB> characterizing the loss in detail in a long exposure
image is identified, and the influence of the correlation scale of
turbulence as r<SUB>0</SUB> approaches this scale is shown. Allowing
for the effect of 2-point correlations in the fluctuations of the
refractive index, Venkatakrishnan and Chatterjee (1987) proposed a
modified law for the phase structure function. It is suggested that
the departure of the phase structure function from the 5/3 power law
for length scales in the wavefront approaching the correlation scale
of turbulence may lead to better 'seeing' at longer wavelengths.
---------------------------------------------------------
Title: Thermal overstability of hydromagnetic surface waves.
Authors: Joarder, P. S.; Gokhale, M. H.; Venkatakrishnan, P.
1987SoPh..110..255J Altcode:
We investigate the effects of radiative heat losses and thermal
conductivity on the hydromagnetic surface waves along a magnetic
discontinuity in a plasma of infinite electrical conductivity. We show
that the effects of radiative heat losses on such surface waves are
appreciable only when values of the plasma pressure on the two sides
of the discontinuity are substantially different. Overstability of
a surface wave requires that the medium in which it gives larger
first-order compression should satisfy the criterion of Field
(1965). Possible applications of the study to magnetic discontinuities
in solar corona are briefly discussed.
---------------------------------------------------------
Title: Inhibition of convective collapse of solar magnetic flux
tubes by radiative diffusion
Authors: Venkatakrishnan, P.
1986Natur.322..156V Altcode:
Interaction of convection with a magnetic field leads to an intermittent
distribution of magnetic flux<SUP>1</SUP>. Such a process operating
on the solar surface can lead to `equipartition' fields of 700 G
(ref. 2). These fields are further prone to a convective instability
and eventually collapse to kilogauss intensity<SUP>3-5</SUP>. I show
here that radiative diffusion can inhibit this collapse to a varying
degree, depending on the field strength and the thickness of the flux
elements. As a consequence, one would expect the field strength of the
photospheric magnetic flux elements to depend on their sizes. It is
shown that at one end of such a distribution there would be kilogauss
tubes with small dispersion in field strength and large dispersion in
size. At the other extreme of the spectrum would be thin tubes of fairly
constant size but with a wide range in field strength, from kilogauss
intensities to the equipartition values of 700 G. High-resolution
observations from space-borne telescopes should reveal the existence
of the latter variety of tubes.
---------------------------------------------------------
Title: Nonlinear Response of Slender Magnetic Flux Tubes to External
Pressure Fluctuations
Authors: Venkatakrishnan, P.
1986SoPh..104..347V Altcode:
The effect of applying external pressure fluctuations on slender flux
tubes is studied as a nonlinear initial value problem. Large amplitude
velocity oscillations are seen to be produced when the frequency
of the imposed fluctuations matches the natural frequency of the
tube. Radiative cooling does not significantly damp these resonantly
built-up oscillations. The absence of observational evidence for such
a resonant response of the tubes is used to put a constraint on the
length of tubes.
---------------------------------------------------------
Title: Supercritical winds from cool `Canonical' stars caused by
evolution on the Main Sequence
Authors: Venkatakrishnan, P.
1986Ap&SS.119...51V Altcode:
Even the very slow expansion of a star's radius due to evolution on
the Main Sequence is shown to be supercritical for cool stars without
coronae. Since steady sphericaily-symmetric supercitical solutions
are theoretically impossible, unsteady supercritical solutions are
studied. It is seen that smooth sonic transitions are possible in the
unsteady case, but are accompanied by enhancement of pressure over
the critical values.
---------------------------------------------------------
Title: Measurement of Vector Magnetic Fields - Part One - Theoretical
Approach to the Instrumental Polarization of the Kodaikanal Solar
Tower
Authors: Balasubramaniam, K. S.; Venkatakrishnan, P.; Bhattacharyya,
J. C.
1985SoPh...99..333B Altcode:
The observations of Stokes line profiles require an accurate knowledge
of the instrumental polarisation caused by optical components in the
path of the light beam. In this context we present a theoretical
approach to the instrumental polarisation caused by the 3-mirror
coelostat system of the Kodaikanal Solar Tower.
---------------------------------------------------------
Title: Physical Limits to the Sizes of Magnetic Flux Tubes
Authors: Venkatakrishnan, P.
1985tphr.conf..200V Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Nonlinear development of convective instability within slender
flux tubes. II - The effect of radiative heat transport
Authors: Venkatakrishnan, P.
1985JApA....6...21V Altcode:
Inclusion of radiative heat transport in the energy equation for a
slender flux tube leads to oscillations of the tube. The amplitude
of the oscillations depends on the radius of the tube when lateral
heat exchange alone is considered. Longitudinal heat transport has
a greater influence on the evolution of the instability than lateral
heat exchange for the particular value of tube radius considered in
the calculation. Heat transport is seen to reduce the efficiency of
concentration of magnetic fields by convective collapse in the case
of polytropic tubes.
---------------------------------------------------------
Title: Physical limits to the sizes of magnetic flux tubes.
Authors: Venkatakrishnan, P.
1985MPARp.212..200V Altcode:
Inclusion of radiative exchange of heat by the flux tubes with their
surroundings leads to a transition from instability to overstability at
a critical value of tube radius. The implications of this are (1) the
existence of weak as well as strong field tubes and (2) the existence
of limits to the sizes of these tubes.
---------------------------------------------------------
Title: Notes and News
Authors: Gurm, H. S.; Raju, P. K.; Venkatakrishnan, P.
1984BASI...12...86G Altcode:
Report on the IAU Symposium No. 105, held at Geneva 1983 September 12
- 16.
---------------------------------------------------------
Title: Colloquium on magnetic field-plasma interaction on the sun,
Kodaikanal, 1984 January 23.
Authors: Raju, P. K.; Venkatakrishnan, P.
1984BASI...12...92R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Convection and the Phenomenon of Kilogausss Magnetic Fields
on the Sun
Authors: Venkatakrishnan, P.
1984KodOB...4...19V Altcode:
The role played by convection in the formation of slender magnetic
flux tubes and in the dynamics of the gas within the tube is discussed
in the case of simplified models. Convection instability cannot drive
systematic downflows whereas convective buffetting of the tube can. The
inclusion of heat transport reduces the efficiency of convective
collapse for the formation of strong fields. The implications of these
two results for the solar magnetic flux tubes is pointed out.
---------------------------------------------------------
Title: Nonlinear development of convective instability within slender
flux tubes. I - Adiabatic flow
Authors: Venkatakrishnan, P.
1983JApA....4..135V Altcode:
The method of characteristics is used to investigate the nonlinear
development of convective instability within slender flux tubes. It is
found that the initial magnetic field affects the development of the
instability. The boundary conditions dictate the asymptotic state of
the unstable tube. Flux tubes subjected to 'open' boundary conditions
exhibit better evidence for field amplification than those subjected to
'closed' boundary conditions. In either case, convective instability
gives rise to the generation of significant gas flow within slender
flux tubes. If a constant pressure is maintained at both ends of the
tube, then the final state depends on the initial perturbations as
well as on the boundary conditions. An initial updraft results in a
more intense tube with steady upflow.
---------------------------------------------------------
Title: On the absorption of He I 10830 Å line by spicules.
Authors: Venkatakrishnan, P.; Jain, S. K.
1983BASI...11..369V Altcode:
The possible applications of time-dependent measurements of the
equivalent width of the He I 10830 Å line for a better understanding
of the growth of spicules and the evolution of coronal holes and X-ray
bright points on the sun are discussed.
---------------------------------------------------------
Title: Influence of solar wind variability on the recurrence of
droughts.
Authors: Venkatakrishnan, P.
1982SoPh...81..193V Altcode:
The observed effects of solar flares and interplanetary sector
crossings seem to indicate that particle precipitation in the Earth's
upper atmosphere decreases cyclonic activity in the troposphere. As
an extrapolation to longer term effects, it is suggested that the
recurrence of prolonged periods of enhanced solar wind particle
precipitation in the upper atmosphere during alternate solar minima
could cause the recurrence of extreme droughts.
---------------------------------------------------------
Title: Transient Response of the Solar Wind to Changes in Flow
Geometry - Flows in Coronal Holes
Authors: Hasan, S. S.; Venkatakrishnan, P.
1982SoPh...80..385H Altcode:
The transient response of the solar wind to changes in geometry is
examined. An initial stationary flow in a configuration that diverges
as r<SUP>2</SUP> is assumed. This state corresponds to the usual solar
wind solution. The effect on the flow through a tube whose area A(r,
t) diverges faster than r<SUP>2</SUP>, with the degree of divergence
increasing in time, is considered. The asymptotic form of A(r, t)
is chosen to mimic the form inferred in coronal holes. A detailed
parameter study relating the form of A(r, t) to the pattern of flow in
the tube is presented. It is observed that in the limit of large time
(large compared to τ, the time constant for change in geometry of a
flow tube) the solutions obtained from a time-dependent analysis can
depend upon τ. For sufficiently large τ, the asymptotic solution is
the same as the steady state solution obeying the correct boundary
conditions and possessing a smooth sonic transition. However, if
the geometry changes rapidly enough, solutions exhibiting shock-like
discontinuities can also exist. This is essentially a new feature that
emerges from the present investigation. Finally, it is suggested that
this study may be useful in describing flows in evolving coronal holes.
---------------------------------------------------------
Title: A magnetogasdynamical analysis of the shock transition model
of the solar cycle.
Authors: Gokhale, M. H.; Venkatakrishnan, P.
1982BASI...10...35G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Comment on the paper `a new resonance in the solar atmosphere'
by Joseph V. Hollweg
Authors: Venkatakrishnan, P.; Hasan, S. S.
1982SoPh...75...79V Altcode:
In the absence of genuine forcing terms, there is no resonance between
linear fast mhd and gravito-acoustic waves.
---------------------------------------------------------
Title: Synchronization in Binary Stars
Authors: Rajamohan, R.; Venkatakrishnan, P.
1981BASI....9..309R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Time Dependent Model for Spicule Flow
Authors: Hasan, S. S.; Venkatakrishnan, P.
1981SoPh...73...45H Altcode:
A time dependent model for the flow of gas in a spicule is studied. In
this model, the flow occurs in a magnetic flux sheath. Starting from
hydrostatic equilibrium, the flux sheath is allowed to collapse normal
to itself. The collapse induces a flow of gas along the magnetic field
and this flow is identified as a spicule. A variety of sheath geometries
and velocity patterns for the normal flow have been studied. It is
observed that a large curvature in the field geometry and a large
initial value for the normal flow are necessary to achieve spicule-like
velocities. The duration for which a large velocity of normal flow is
required is much shorter than the average lifetime of a spicule. It is
proposed that the initial rapid collapse occurs during an `impulsive
spicule' phase and it is the subsequent gradual relaxation of the flow
which is observed as a spicule.
---------------------------------------------------------
Title: Wave Propagation in Solar Magnetic Tubes
Authors: Venkatakrishnan, P.
1981BASI....9..214V Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Time-dependent interaction of granules with magnetic flux tubes
Authors: Venkatakrishnan, P.; Hasan, S. S.
1981JApA....2..133V Altcode:
The time-dependent interaction of the granulation velocity field with
a magnetic flux tube is investigated here. It is seen that when a
magnetic field line is displaced normal to itself so as to simulate
the buffeting action of granules, a flow of gas is initiated along the
field. By choosing a lateral velocity field which is consistent with
observations of granules, it is found that the resulting gas motion is
a downward flow with a velocity compatible with the observed downflow
in isolated photospheric flux tubes. It is therefore proposed that the
observed photospheric downflow is a manifestation of the interaction
of granules with flux tubes.
---------------------------------------------------------
Title: A time dependent model for spicule flow.
Authors: Hasan, S. S.; Venkatakrishnan, P.
1981BASI....9...74H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Rotation in Close Binaries
Authors: Rajamohan, R.; Venkatakrishnan, P.
1980IAUS...88...27R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Flow of Gas Along a Magnetic Field with Time Dependent Geometry
Authors: Hasan, S. S.; Venkatakrishnan, P.
1980KodOB...3....6H Altcode:
The flow of gas along a magnetic field with time dependent geometry
has been studied. It is seen that the velocity of the flow in the
direction of the magnetic field depends both on the magnitude of the
velocity of flow perpendicular to the field as well as on its spatial
variation. Further, the nature of the flow is not very sensitive to
the choice of base temperature and polytropic index. The application
of this study to magnetofluid dynamic flow on the Sun is discussed.
---------------------------------------------------------
Title: Frequency response of magnetic flux sheaths.
Authors: Venkatakrishnan, P.
1979SoPh...63..135V Altcode:
When a sound wave is incident on a magnetic flux sheath, it causes
fluctuations in the mean magnetic field of the sheath. We have
calculated the space-average of the longitudinal component of these
fluctuations and plotted this against the frequency of the incident
sound wave. The main result is the presence of local maxima and minima
in the response curve. If such maxima and minima could be detected
in any actual observation then these would provide an estimate of the
thickness of these magnetic structures.
---------------------------------------------------------
Title: Intensity changes in sunspots and starspots
Authors: Venkatakrishnan, P.
1978BASI....6..100V Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A phenomenological, kinematical model of the coronal magnetic
fields in terms of thin flux tubes rising from the photosphere.
Authors: Gokhale, M. H.; Venkatakrishnan, P.
1978Prama..11..547G Altcode:
A kinematical model is necessary for understanding the gross structure
of the coronal magnetic field and its slow evolution in consistency with
the small-scale structure of the photospheric fields. A preliminary
phenomenological model is developed in terms of flux tubes of flux
amounting to approximately 10 to the 17 - 10 to the 18.5 Mx rising
across the inner corona in the form of arches and opening out in the
outer corona. In contrast to Parker's (1975) estimate, this model
is consistent with the observed spans of the chromospheric fibrils
and X-ray arches. It is also consistent with the number of flux tubes
present above the photosphere as estimated from the observed abundance
of spicules.
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Title: The Orbits of Five Minor Planets and Corrections to the FK4
Equator and Equinox.
Authors: Venkatakrishnan, P.; Gokhale, M. H.
1977SoPh...54..371V Altcode:
The presence of finite current sheets at the boundaries of a magnetic
flux sheath will lead to a somewhat reduced transmission of the energy
of an incident acoustic wave.
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Title: Transmission of energy across a magnetic flux sheath.
Authors: Gokhale, M. H.; Venkatakrishnan, P.
1976BASI....4Q..78G Altcode:
No abstract at ADS
