Author name code: venkatakrishnan ADS astronomy entries on 2022-09-14 author:"Venkatakrishnan, Paramewswaran" ------------------------------------------------------------------------ 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. Bibcode: 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. 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 Bibcode: 2017CSci..113..686V Altcode: No abstract at ADS 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 Bibcode: 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. Title: Inferring the Magnetic Structure of a Sunspot from Decomposition of Photospheric Vertical Current Density into Twist and Shear Components Authors: Venkatakrishnan, P. Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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. Title: Membrane-based deformable mirror: intrinsic aberrations and alignment issues Authors: Raja Bayanna, A.; Louis, Rohan E.; Chatterjee, S.; Mathew, Shibu K.; Venkatakrishnan, P. Bibcode: 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. 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. Bibcode: 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). 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. Bibcode: 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. 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. Bibcode: 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. Title: Anomalous flows in a sunspot penumbra Authors: Louis, Rohan E.; Beck, Christian; Mathew, Shibu K.; Venkatakrishnan, P. Bibcode: 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.
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.
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.
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-1, an area of 5.2 arcsec2, 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-1. 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.
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. 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. Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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. Title: Coherent Lateral Motion of Penumbral Filaments during the X-class Flare of 13 December 2006 Authors: Gosain, S.; Venkatakrishnan, P.; Tiwari, S. K. Bibcode: 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. 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. Bibcode: 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 QS 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 Mm2 hr-1. 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. 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. Bibcode: 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 arcsec2. 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. 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. Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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. Title: Evolution of twist-shear and dip-shear in flaring active region NOAA 10930 Authors: Gosain, Sanjay; Venkatakrishnan, P. Bibcode: 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 Bibcode: 2011aogs...27..153R Altcode: 2010arXiv1012.0120R Magnetic twist of the active region has been measured over a decade using photospheric vector field data, chromospheric Hα 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 1043 Mx2 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 g and spatially averaged signed shear angle (SASSA). It is found that for the sunspot studied here these parameters are α g = -3.5 ± 0.37(×10-9 m-1) and SASSA = -1.68 ± 0fdg014. This suggests that the SASSA is a less dispersed non-potentiality parameter as compared to α g . 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 g parameter. Title: Magnetic tension of sunspot fine structures Authors: Venkatakrishnan, P.; Tiwari, Sanjiv Kumar Bibcode: 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.
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.
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).
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.
Conclusions: From our study, magnetic tension emerges as a useful diagnostic of the local equilibrium of the sunspot fine structures.

Figures A.1-A.3 are only available in electronic form at http://www.aanda.org Title: On the Flare Induced High-Frequency Global Waves in the Sun Authors: Kumar, Brajesh; Mathur, Savita; García, R. A.; Venkatakrishnan, P. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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-1. 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-1. 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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−1 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. Bibcode: 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-1, 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. Bibcode: 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 Bibcode: 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 10-3 of the continuum intensity (I c ) 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. Bibcode: 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. Bibcode: 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. Bibcode: 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−1 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 Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 (r0) 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 r0 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 r0 by about 1.0 cm with respect to the previous year's dry condition and mean r0 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 2005BASI...33..413B Altcode: No abstract at ADS Title: Experiments and Design Activities for the Multiapplication Solar Telescope Authors: Venkatakrishnan, P. Bibcode: 2005BASI...33..353V Altcode: No abstract at ADS Title: Imaging with insolated mirrors Authors: Venkatakrishnan, P.; Sridharan, R.; Gupta, S. K. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 2005BASI...33..413K Altcode: No abstract at ADS Title: Simulations of Solar AO Systems Authors: Sridharan, R.; Bayanna, A. Raja; Venkatakrishnan, P. Bibcode: 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 Bibcode: 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−1 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. Bibcode: 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 (DST < -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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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−1. Title: On the Correlation Between the he ii λ304 Network Brightening and the Photospheric Magnetic Field Authors: Ravindra, B.; Venkatakrishnan, P. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 2002SoPh..211..395S Altcode: A few methods of estimating Fried's parameter (r0) from specklegrams of solar features are described. Some of these methods were used to estimate r0 for the speckle data obtained from Kodaikanal Observatory (KO), Uttar Pradesh State Observatory (UPSO) and Udaipur Solar Observatory (USO). The average value of r0 was found to be ∼ 3 cm at USO and UPSO during our observations. At KO, values of r0 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. Bibcode: 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 Bibcode: 2002ApJ...573..851C Altcode: The magnetic flux imbalance of active regions has been studied using the longitudinal magnetograms obtained from the National Solar Observatory at Kitt Peak. The maximum and the median value of the flux imbalance in 137 active regions situated near the disk center is found to be about 62% and 9.5%, respectively. The detailed analysis of a few selected active regions shows that the local flux asymmetry is compensated on global scales. For example, the NOAA Active Region 7978, which appeared during the solar activity minimum period and evolved during five solar rotations (1996 July-October), shows a flux imbalance of about 9.5%, with an excess of following negative flux. However, on a global scale, the positive and negative flux in the entire solar disk was found to be nearly balanced during the same period. The global flux imbalance of the Sun during a full magnetic cycle is estimated from the Carrington maps. These maps are made by merging the solar images obtained during a complete solar rotation (about 27 days), hence each represents the entire surface of the Sun. The 10°-40° active latitudinal zone in the individual hemispheres during the solar maximum shows a flux imbalance of more than 20%. This is reduced to below 10% when the entire Sun is considered. The present study indicates that a fraction of the magnetic field from the localized active regions connects with far away locations. Title: Relationship between CME Speed and Geomagnetic Storm Intensity Authors: Srivastava, Nandita; Venkatakrishnan, P. Bibcode: 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 (DST ~ -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. Bibcode: 2002stma.conf...37S Altcode: No abstract at ADS Title: Chromospheric Magnetic Field of Solar Active Regions Authors: Choudhary, Debi Prasad; Sakurai, Takashi; Venkatakrishnan, P. Bibcode: 2001ApJ...560..439C Altcode: The three-dimensional magnetic field structure of 137 solar active regions is studied by comparing the observed and computed chromospheric magnetograms. The model chromospheric field is obtained by extrapolating the observed photospheric field into the chromosphere with a potential (current-free) magnetic-field model in Cartesian geometry. The best correlation between the observed and the model chromospheric magnetograms is found at the height of 800 km, which also corresponds to the height of the line formation for Ca II 854.2 nm. In the weak field range, within +/-300 G, most of the observed field is close to the potential field. However, departures of about 50 G are observed in a few active regions. For field values greater than +/-500 G, the observed field does not always match the model. Whereas a part of this could be due to the magnetogram calibration, it might also originate from the ``nonpotentiality'' of the chromospheric field. In the case of long-lived active regions, which make multiple disk passages, the strong-field nonpotentiality is observed during their initial phase, and converges to a potential field configuration later. Title: Multi-aperture solar telescope Authors: Venkatakrishnan, P. Bibcode: 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. Bibcode: 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. Bibcode: 2001ASPC..236..297S Altcode: 2001aspt.conf..297S No abstract at ADS Title: Is a Sunspot in Static or Dynamic Equilibrium? Authors: Venkatakrishnan, P. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 ro is estimated. Our aim is to recover oe(x,y) which is close to o(x,y). We present here the results of estimation of Fried's parameter ro using a parameter search method (Kirshnakumar & Venkatakrishnan 1997b) and an modified Wiener filter (U filter) (Roddier 1981) for image restoration. The estimation of ro 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 1995SPD....26..304H Altcode: 1995BAAS...27..953H No abstract at ADS Title: Observable signals of coronal heating processes Authors: Venkatakrishnan, P. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1994ASPC...68..409V Altcode: 1994sare.conf..409V No abstract at ADS Title: Coronal Heating and Chromospheric Energy Density - an Observational Association Authors: Venkatakrishnan, P. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1987MNRAS.229..379V Altcode: A physical length scale in the wavefront corresponding to the parameter (r0) characterizing the loss in detail in a long exposure image is identified, and the influence of the correlation scale of turbulence as r0 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. Bibcode: 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. Bibcode: 1986Natur.322..156V Altcode: Interaction of convection with a magnetic field leads to an intermittent distribution of magnetic flux1. 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 intensity3-5. 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1984BASI...12...92R Altcode: No abstract at ADS Title: Convection and the Phenomenon of Kilogausss Magnetic Fields on the Sun Authors: Venkatakrishnan, P. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 r2 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 r2, 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. Bibcode: 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. Bibcode: 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. Bibcode: 1981BASI....9..309R Altcode: No abstract at ADS Title: A Time Dependent Model for Spicule Flow Authors: Hasan, S. S.; Venkatakrishnan, P. Bibcode: 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. Bibcode: 1981BASI....9..214V Altcode: No abstract at ADS Title: Time-dependent interaction of granules with magnetic flux tubes Authors: Venkatakrishnan, P.; Hasan, S. S. Bibcode: 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. Bibcode: 1981BASI....9...74H Altcode: No abstract at ADS Title: Rotation in Close Binaries Authors: Rajamohan, R.; Venkatakrishnan, P. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Title: The Orbits of Five Minor Planets and Corrections to the FK4 Equator and Equinox. Authors: Venkatakrishnan, P.; Gokhale, M. H. Bibcode: 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. Title: Transmission of energy across a magnetic flux sheath. Authors: Gokhale, M. H.; Venkatakrishnan, P. Bibcode: 1976BASI....4Q..78G Altcode: No abstract at ADS