Author name code: ambastha ADS astronomy entries on 2022-09-14 author:"Ambastha, Ashok" ------------------------------------------------------------------------ Title: Magnetic and Velocity Field Topology in Active Regions of Descending Phase of Solar Cycle 23 Authors: Maurya, R. A.; Ambastha, A. Bibcode: 2020SoPh..295..106M Altcode: 2020arXiv200613602M We analyze the topology of photospheric magnetic fields and sub-photospheric flows of several active regions (ARs) that are observed during the peak to descending phase of Solar Cycle 23. Our analysis shows clear evidence of hemispheric preferences in all the topological parameters such as the magnetic, current and kinetic helicities, and the `curl-divergence'. We found that 68%(67%) ARs in the northern (southern) hemisphere with negative (positive) magnetic helicity. Same hemispheric preference sign is found for the current helicity in 68%(68%) ARs. The hemispheric preferences are found to exist statistically for all the time except in a few ARs observed during the peak and the end phases of the solar cycle. This means that magnetic fields are dominantly left(right)-helical in scales smaller than individual ARs of northern(southern) hemisphere. We found that magnetic and current helicity parameters show equatorward propagation similar to the sunspot cycle. The kinetic helicity showed similar hemispheric trend to that of magnetic and current helicity parameters. There are 65%(56%) ARs with negative (positive) kinetic helicity as well as divergence-curl, at the depth of 2.4 Mm, in the northern (southern) hemisphere. The hemispheric preference of the kinetic helicity becomes more evident at larger depths, e.g., 69%(67%) at the depth of 12.6 Mm. A similar hemispheric trend of kinetic helicity to that of the current helicity supports the mean-field dynamo model. We also found that the hemispheric preference of all the parameters increases with the field strength of ARs. The topology of photospheric magnetic fields and near-surface sub-photospheric flow fields did not show good association but the correlation between them enhances with depths, which could be indicating more aligned flows at deeper layers of ARs. Title: Solar activity and explosive transient eruptions Authors: Ambastha, Ashok Bibcode: 2016AsJPh..25..267A Altcode: We discuss active and explosive behavior of the Sun observable in a wide range of wavelengths (or energies) and spatio-temporal scales that are not possible for any other star. On the longer time scales, the most notable form of solar activity is the well known so called 11-year solar activity cycle. On the other hand, at shorter time scales of a few minutes to several hours, spectacular explosive transient events, such as, solar flares, prominence eruptions, and coronal mass ejections (CMEs) occur in the outer layers of solar atmosphere. These solar activity cycle and explosive phenomena influence and disturb the space between the Sun and planets. The state of the interplanetary medium, including planetary and terrestrial surroundings, or "the space weather", and its forecasting has important practical consequences. The reliable forecasting of space weather lies in continuously observing of the Sun. We present an account of the recent developments in our understanding of these phenomena using both space-borne and ground-based solar observations. Title: Activity-related variations of high-degree p-mode amplitude, width, and energy in solar active regions Authors: Maurya, R. A.; Ambastha, A.; Chae, J. Bibcode: 2014A&A...561A.123M Altcode: 2013arXiv1310.6458M Context. Solar energetic transients such as flares and coronal mass ejections occur mostly within active regions (ARs) and release large amounts of energy, which is expected to excite acoustic waves by transferring the mechanical impulse of the thermal expansion of the flare on the photosphere. On the other hand, strong magnetic fields of AR sunspots absorb the power of the photospheric oscillation modes.
Aims: We study the properties of high-degree p-mode oscillations in flaring and dormant ARs and compare them with those in corresponding quiet regions (QRs) to find the association of the mode parameters with magnetic- and flare-related activities.
Methods: We computed the mode parameters using the ring-diagram technique. The magnetic-activity indices (MAIs) of ARs and QRs were determined from the line-of-sight magnetograms. The flare indices (FIs) of ARs were obtained from the GOES X-ray fluxes. Mode parameters were corrected for foreshortening, duty cycle, and MAI using multiple non-linear regression.
Results: Our analysis of several flaring and dormant ARs observed during the Carrington rotations 1980-2109 showed a strong association of the mode amplitude, width, and energy with magnetic and flare activities, although their changes are combined effects of foreshortening, duty cycle, magnetic-activity, flare-activity, and measurement uncertainties. We find that the largest reduction in mode amplitude and background power of an AR are caused by the angular distance of the AR from the solar disc centre. After correcting the mode parameters for foreshortening and duty cycle, we find that the mode amplitudes of flaring and dormant ARs are lower than in corresponding QRs reducing with increasing MAI, suggesting a stronger mode power suppression in ARs with larger magnetic fields. The mode widths in ARs are larger than in corresponding QRs and increase with MAI, indicating shorter lifetimes of modes in ARs than in QRs. The variations in mode amplitude and width with MAI are not same in different frequency bands. The largest amplification (reduction) in mode amplitude (mode width) of dormant ARs is found in the five-minute frequency band. The average mode energy of both the flaring and dormant ARs is smaller than in their corresponding QRs, reducing with increasing MAI. But the average mode energy reduction rate in flaring ARs is smaller than in dormant ARs. Moreover, the increase in mode width rate in dormant (flaring) ARs is followed by a decrease (increase) in the amplitude variation rate. Furthermore, including the mode corrections for MAI shows that mode amplitude and mode energy of flaring ARs escalate with FI, while the mode width shows an opposite trend, suggesting excitations of modes and growth in their lifetimes by flares. The increase (decrease) in mode amplitude (width) is larger in the five-minute and higher-frequency bands. The enhancement in width variation rate is followed by a rapid decline in the amplitude variation rate. Title: Magnetic structure of solar active region NOAA 11158 Authors: Vemareddy, P.; Ambastha, A.; Wiegelmann, T. Bibcode: 2013BASI...41..183V Altcode: 2013arXiv1310.6895V Magnetic fields in the solar corona are responsible for a wide range of phenomena. However, any direct measurements of the coronal magnetic fields are very difficult due to lack of suitable spectral lines, weak magnetic fields, and high temperatures. Therefore, one extrapolates photospheric field measurements into the corona. Owing to low coronal plasma β, we can apply a force-free model in lowest order to study the slow evolution of active region (AR) magnetic fields. On applying these models to AR 11158 and compared with coronal plasma tracers, we found that (1) the approximation of potential field to coronal structures over large length scales is a reasonable one, 2) linear force-free (LFF) assumption to AR coronal fields may not be applicable model as it assumes uniform twist over the entire AR, and 3) for modeling fields at sheared, stressed locations where energy release in the form of flares are usually observed, non-linear force free fields (NLFFF) seem to provide a good approximation. The maximum available free-energy profile shows step-wise decrease that is sufficient to power an M-class flare as observed. Title: Helicity injection by flux motions and its role in flare and CMEs Authors: Panditi, Vemareddy; Ambastha, A.; Maurya, R.; Chae, J.; Ambstha, A.; Maurya, R. A.; Chae, J. Bibcode: 2013SPD....4430003P Altcode: An investigation of helicity injection by photospheric shear motions is carried out for two active regions (ARs), NOAA 11158 and 11166, using line-of-sight magnetic field observations obtained from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. We derived the horizontal velocities in the ARs from the differential affine velocity estimator (DAVE) technique. Persistent strong shear motions at maximum velocities in the range of 0.6-0.9 km s-1 along the magnetic polarity inversion line and outward flows from the peripheral regions of the sunspots were observed in the two ARs. The helicities injected in NOAA 11158 and 11166 during their six-day evolution period were estimated as 14.16x10$^42$ Mx$^2$ and 9.5x10$^42$ Mx$^2$, respectively. The estimated injection rates decreased up to 13% by increasing the time interval between the magnetograms from 12 minutes to 36 minutes, and increased up to 9% by decreasing the DAVE window size from 21x18 to 9x6 pixel$^2$, resulting in 10% variation in the accumulated helicity. In both ARs, the flare-prone regions (R2) had inhomogeneous helicity flux distribution with mixed helicities of both signs and coronal mass ejection (CME) prone regions had almost homogeneous distribution of helicity flux dominated by a single sign. The temporal profiles of helicity injection showed impulsive variations during some flares/CMEs due to negative helicity injection into the dominant region of positive helicity flux. A quantitative analysis reveals a marginally significant association of helicity flux with CMEs but not flares in AR 11158, while for the AR 11166, we find a marginally significant association of helicity flux with flares but not CMEs, providing evidence of the role of helicity injection at localized sites of the events. These short-term variations of helicity flux are further discussed in view of possible flare-related effects. This study suggests that flux motions and spatial distribution of helicity injection are important to understanding the complex nature of the magnetic flux system of the AR, and how it can lead to conditions favorable for eruptive events. Title: On the Role of Rotating Sunspots in the Activity of Solar Active Region NOAA 11158 Authors: Panditi, Vemareddy; Ambastha, A.; Maurya, R. Bibcode: 2013SPD....44..120P Altcode: We study the role of rotating sunspots in relation to the evolution of various physical parameters characterizing the non-potentiality of the active region (AR) NOAA 11158 and its eruptive events using the magnetic field data from the Helioseismic and Magnetic Imager (HMI) and multi-wavelength observations from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory. From the evolutionary study of HMI intensity and AIA channels, it is observed that the AR consists of two major rotating sunspots, one connected to a flare-prone region and another with coronal mass ejection (CME). The constructed space-time intensity maps reveal that the sunspots exhibited peak rotation rates coinciding with the occurrence of major eruptive events. Further, temporal profiles of twist parameters, namely, average shear angle, αav, αbest, derived from HMI vector magnetograms, and the rate of helicity injection, obtained from the horizontal flux motions of HMI line-of-sight magnetograms, correspond well with the rotational profile of the sunspot in the CME-prone region, giving predominant evidence of rotational motion causig magnetic non-potentiality. Moreover, the mean value of free energy from the virial theorem calculated at the photospheric level shows a clear step-down decrease at the onset time of the flares revealing unambiguous evidence of energy release intermittently that is stored by flux emergence and/or motions in pre-flare phases. Additionally, distribution of helicity injection is homogeneous in the CME-prone region while in the flare-prone region it is not and often changes sign. This study provides a clear picture that both proper and rotational motions of the observed fluxes played significant roles in enhancing the magnetic non-potentiality of the AR by injecting helicity, twisting the magnetic fields and thereby increasing the free energy, leading to favorable conditions for the observed transient activity.Abstract (2,250 Maximum Characters): We study the role of rotating sunspots in relation to the evolution of various physical parameters characterizing the non-potentiality of the active region (AR) NOAA 11158 and its eruptive events using the magnetic field data from the Helioseismic and Magnetic Imager (HMI) and multi-wavelength observations from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory. From the evolutionary study of HMI intensity and AIA channels, it is observed that the AR consists of two major rotating sunspots, one connected to a flare-prone region and another with coronal mass ejection (CME). The constructed space-time intensity maps reveal that the sunspots exhibited peak rotation rates coinciding with the occurrence of major eruptive events. Further, temporal profiles of twist parameters, namely, average shear angle, αav, αbest, derived from HMI vector magnetograms, and the rate of helicity injection, obtained from the horizontal flux motions of HMI line-of-sight magnetograms, correspond well with the rotational profile of the sunspot in the CME-prone region, giving predominant evidence of rotational motion causig magnetic non-potentiality. Moreover, the mean value of free energy from the virial theorem calculated at the photospheric level shows a clear step-down decrease at the onset time of the flares revealing unambiguous evidence of energy release intermittently that is stored by flux emergence and/or motions in pre-flare phases. Additionally, distribution of helicity injection is homogeneous in the CME-prone region while in the flare-prone region it is not and often changes sign. This study provides a clear picture that both proper and rotational motions of the observed fluxes played significant roles in enhancing the magnetic non-potentiality of the AR by injecting helicity, twisting the magnetic fields and thereby increasing the free energy, leading to favorable conditions for the observed transient activity. Title: Rotating sunspots and their role in the activity of solar active region NOAA 11158 Authors: Vemareddy, P.; Ambastha, A.; Maurya, R. A. Bibcode: 2013enss.confE...6V Altcode: We study the role of rotating sunspots in relation to the evolution of various physical parameters characterizing the non-potentiality of the active region (AR) NOAA 11158 and its eruptive events using the magnetic field data from the Helioseismic and Magnetic Imager (HMI) and multi-wavelength observations from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory. From the evolutionary study of HMI intensity and AIA channels, it is observed that the AR consists of two major rotating sunspots, one connected to a flare-prone region and another with coronal mass ejection (CME). The constructed space-time intensity maps reveal that the sunspots exhibited peak rotation rates coinciding with the occurrence of major eruptive events. Further, temporal profiles of twist parameters, namely, average shear angle, α_{av}, α_{best}, derived from HMI vector magnetograms, and the rate of helicity injection, obtained from the horizontal flux motions of HMI line-of-sight magnetograms, correspond well with the rotational profile of the sunspot in the CME-prone region, giving predominant evidence of rotational motion causing magnetic non-potentiality. Moreover, the mean value of free energy from the virial theorem calculated at the photospheric level shows a clear step-down decrease at the onset time of the flares revealing unambiguous evidence of energy release intermittently that is stored by flux emergence and/or motions in pre-flare phases. Additionally, distribution of helicity injection is homogeneous in the CME-prone region while in the flare-prone region it is not and often changes sign. This study provides a clear picture that both proper and rotational motions of the observed fluxes played significant roles in enhancing the magnetic non-potentiality of the AR by injecting helicity, twisting the magnetic fields and thereby increasing the free energy, leading to favorable conditions for the observed transient activity. Title: Helicity Injection by the Shearing Motion of Fluxes in Relation to Flares and Coronal Mass Ejections Authors: Vemareddy, P.; Ambastha, A.; Maurya, R. A.; Chae, J. Bibcode: 2013enss.confE...8V Altcode: An investigation of helicity injection by photospheric shear motions is carried out for two active regions (ARs), NOAA 11158 and 11166, using line-of-sight magnetic field observations obtained from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. We derived the horizontal velocities in the ARs from the differential affine velocity estimator (DAVE) technique. Persistent strong shear motions at maximum velocities in the range of 0.6-0.9 km s^{-1} along the magnetic polarity inversion line and outward flows from the peripheral regions of the sunspots were observed in the two ARs. The helicities injected in NOAA 11158 and 11166 during their six-day evolution period were estimated as 14.16x10^{42} Mx^2 and 9.5×10^{42} Mx^2, respectively. The estimated injection rates decreased up to 13% by increasing the time interval between the magnetograms from 12 minutes to 36 minutes, and increased up to 9% by decreasing the DAVE window size from 21×18 to 9×6 pixel2, resulting in 10% variation in the accumulated helicity. In both ARs, the flare-prone regions (R2) had inhomogeneous helicity flux distribution with mixed helicities of both signs and coronal mass ejection (CME) prone regions had almost homogeneous distribution of helicity flux dominated by a single sign. The temporal profiles of helicity injection showed impulsive variations during some flares/CMEs due to negative helicity injection into the dominant region of positive helicity flux. A quantitative analysis reveals a marginally significant association of helicity flux with CMEs but not flares in AR 11158, while for the AR 11166, we find a marginally significant association of helicity flux with flares but not CMEs, providing evidence of the role of helicity injection at localized sites of the events. These short-term variations of helicity flux are further discussed in view of possible flare-related effects. This study suggests that flux motions and spatial distribution of helicity injection are important to understanding the complex nature of the magnetic flux system of the AR, and how it can lead to conditions favorable for eruptive events. Title: On the Role of Rotating Sunspots in the Activity of Solar Active Region NOAA 11158 Authors: Vemareddy, P.; Ambastha, A.; Maurya, R. A. Bibcode: 2012ApJ...761...60V Altcode: 2012arXiv1210.3912V We study the role of rotating sunspots in relation to the evolution of various physical parameters characterizing the non-potentiality of the active region (AR) NOAA 11158 and its eruptive events using the magnetic field data from the Helioseismic and Magnetic Imager (HMI) and multi-wavelength observations from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory. From the evolutionary study of HMI intensity and AIA channels, it is observed that the AR consists of two major rotating sunspots, one connected to a flare-prone region and another with coronal mass ejection (CME). The constructed space-time intensity maps reveal that the sunspots exhibited peak rotation rates coinciding with the occurrence of major eruptive events. Further, temporal profiles of twist parameters, namely, average shear angle, αav, αbest, derived from HMI vector magnetograms, and the rate of helicity injection, obtained from the horizontal flux motions of HMI line-of-sight magnetograms, correspond well with the rotational profile of the sunspot in the CME-prone region, giving predominant evidence of rotational motion causing magnetic non-potentiality. Moreover, the mean value of free energy from the virial theorem calculated at the photospheric level shows a clear step-down decrease at the onset time of the flares revealing unambiguous evidence of energy release intermittently that is stored by flux emergence and/or motions in pre-flare phases. Additionally, distribution of helicity injection is homogeneous in the CME-prone region while in the flare-prone region it is not and often changes sign. This study provides a clear picture that both proper and rotational motions of the observed fluxes played significant roles in enhancing the magnetic non-potentiality of the AR by injecting helicity, twisting the magnetic fields and thereby increasing the free energy, leading to favorable conditions for the observed transient activity. Title: On the Injection of Helicity by the Shearing Motion of Fluxes in Relation to Flares and Coronal Mass Ejections Authors: Vemareddy, P.; Ambastha, A.; Maurya, R. A.; Chae, J. Bibcode: 2012ApJ...761...86V Altcode: 2012arXiv1202.5195V An investigation of helicity injection by photospheric shear motions is carried out for two active regions (ARs), NOAA 11158 and 11166, using line-of-sight magnetic field observations obtained from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. We derived the horizontal velocities in the ARs from the differential affine velocity estimator (DAVE) technique. Persistent strong shear motions at maximum velocities in the range of 0.6-0.9 km s-1 along the magnetic polarity inversion line and outward flows from the peripheral regions of the sunspots were observed in the two ARs. The helicities injected in NOAA 11158 and 11166 during their six-day evolution period were estimated as 14.16 × 1042 Mx2 and 9.5 × 1042 Mx2, respectively. The estimated injection rates decreased up to 13% by increasing the time interval between the magnetograms from 12 minutes to 36 minutes, and increased up to 9% by decreasing the DAVE window size from 21 × 18 to 9 × 6 pixel2, resulting in 10% variation in the accumulated helicity. In both ARs, the flare-prone regions (R2) had inhomogeneous helicity flux distribution with mixed helicities of both signs and coronal mass ejection (CME) prone regions had almost homogeneous distribution of helicity flux dominated by a single sign. The temporal profiles of helicity injection showed impulsive variations during some flares/CMEs due to negative helicity injection into the dominant region of positive helicity flux. A quantitative analysis reveals a marginally significant association of helicity flux with CMEs but not flares in AR 11158, while for the AR 11166, we find a marginally significant association of helicity flux with flares but not CMEs, providing evidence of the role of helicity injection at localized sites of the events. These short-term variations of helicity flux are further discussed in view of possible flare-related effects. This study suggests that flux motions and spatial distribution of helicity injection are important to understanding the complex nature of the magnetic flux system of the AR, and how it can lead to conditions favorable for eruptive events. Title: The Venus Twilight Experiment: Probing The Mesosphere In 2004 And 2012 Authors: Tanga, Paolo; Widemann, T.; Ambastha, A.; Babcock, B. A.; Berthier, J.; Bouley, S.; Braga-Ribas, F.; Brasch, K.; Burke, W.; Colas, F.; Fukuhara, T.; Fulham, L.; Imai, M.; Lu, M.; Machado, P.; Maquet, L.; Pasachoff, J. M.; Roberts, J.; Schneider, G.; Sheehan, W.; Sigismondi, C.; Thouvenin, N.; Vachier, F.; Veillet, C.; Wang, X. Bibcode: 2012DPS....4450807T Altcode: During the Venus transit in 2004 several observers collected data useful to the characterization of the mesosphere of the planet, by observing the solar light refracted at the corresponding altitude range. The "aureole" thus formed, is observable during the ingress and egress phases of the transit, when Venus is crossing the solar limb. For the 2012 opportunity we prepared a set of coronagraphs to obtain multi-wavelength, space- and time-resolved photometry of the aureole, in collaboration with other space- and ground-based campaigns. The coronagraphs were distributed in the visibility area around the Pacific, over eight sites where local logistic support and scientific expertise were present. Several sites obtained useful data at frame rates of several images/sec. We will give an account of the campaign presenting first results obtained at 450, 535, 607 and 760 nm (FWHM 10 nm). A comparison with data collected at the 2004 transit shows that variations in the aspects of the aureole are present. These can be linked to variations in the vertical distribution of the absorbers (aerosols and cloud-top level). A common feature in both the recent transits is the presence of a brightness peak at high latitude, which was imaged several minutes before and after first and last contact, respectively. The historical record of the aureole contains hints of varying features and a constant polar-spot presence, which can now be interpreted in the light of the measurements obtained in 2004 and 2012. Title: On the Injection of Helicity by Shearing Motion of Fluxes in Relation to Flares and CMEs Authors: Panditi, Vemareddy; Ambastha, Ashok; Maurya, Ram Ajor Bibcode: 2012cosp...39.1440P Altcode: 2012cosp.meet.1440P An investigation of helicity injection by photospheric shear motions is presented for two active regions (ARs) NOAA 11158 and 11166, using line-of-sight magnetic field observations obtained from the Helioseismic and magnetic Imager (HMI) on-board Solar Dynamics Observatory (SDO). We derived the horizontal flux velocities in the ARs from Differential Affine Velocity Estimator (DAVE) technique. During the six day evolution period of the ARs, we found persistent strong shear motions at a maximum velocity in the range of 0.5-0.7 km-s-1 along the magnetic polarity inversion line (PIL) and outward flows from the peripheral regions of the sunspots. The helicities injected in AR 11158 and AR 11166 during the six days' period were estimated as 13.30×10^42 Mx2 and 9.5×10^42 Mx2, respectively. Temporal profiles of helicity injection showed impulsive variations at the onset times of flares/CMEs due to the negative helicity injection in the dominant region of positive helicity density. The spatial examination of helicity density maps showed that these variations resulted mainly due to the negative helicity injection in the regions of opposite helicity that were co-spatial with flaring sites. These co-spatial and co-temporal variations of helicity injection with flares are interpreted to be due to the relaxation from the state of high shear by the observed motions to a lower or shear-free state releasing energy in the form of eruptive events. This agrees with the simulations by Kusano et al. (2004) for the triggering mechanism of flares. However, for the flares of smaller magnitude no clear evidence of such changes was available. We expect that this difficulty could be addressed by careful calculations for the emergence term of helicity injection using the vector magnetic field data. Our study suggests that the existence of opposite helicity fluxes can trigger eruptive events, viz., flares and CMEs, and promises to be useful in forecasting the transient activity of ARs. Title: Photospheric Transients and Spectral Line Changes Associated with a Large X2.2 Flare Authors: Ambastha, Ashok; Maurya, Ram Ajor; Panditi, Vemareddy Bibcode: 2012cosp...39...42A Altcode: 2012cosp.meet...42A Solar energetic transients occurring in solar atmosphere are associated with catastrophic release of energy in the solar corona. These transients inject a part of their energy by various physical processes to the deeper, denser photospheric layer at which velocity and magnetic fields are measured using suitable spectral lines. Some signatures of these processes have been observed during the first X-class flare of the current solar cycle 24 in Active Region NOAA 11158 of 2011 February 15 using the data obtained from the Helioseismic and Magnetic Imager (HMI) on board Solar Dynamics Observatory (SDO). In particular, we detected short-lived magnetic and Doppler velocity transients together with abnormal polarity reversals during the impulsive phase of this large energetic flare. The observed photospheric changes associated with energetic transients have been an issue of serious debates as the magnetic (and Doppler) measurements are expected to be affected by flare-induced line profile changes. We explain the observed transient phenomena during the flare's impulsive phase using the SDO-HMI spectral data obtained before, during and after the flare. The corresponding physical processes are discussed in the light of recent flare models. Title: Filament Eruption in NOAA 11093 Leading to a Two-Ribbon M1.0 Class Flare and CME Authors: Vemareddy, P.; Maurya, R. A.; Ambastha, A. Bibcode: 2012SoPh..277..337V Altcode: 2011SoPh..tmp..413V; 2011arXiv1103.3168V; 2011SoPh..tmp..416V; 2011arXiv1103.3168R We present a multi-wavelength analysis of an eruption event that occurred in active region NOAA 11093 on 7 August 2010, using data obtained from SDO, STEREO, RHESSI, and the GONG Hα network telescope. From these observations, we inferred that an upward slow rising motion of an inverse S-shaped filament lying along the polarity inversion line resulted in a CME subsequent to a two-ribbon flare. Interaction of overlying field lines across the filament with the side-lobe field lines, associated EUV brightening, and flux emergence/cancelation around the filament were the observational signatures of the processes leading to its destabilization and the onset of eruption. Moreover, the time profile of the rising motion of the filament/flux rope corresponded well with flare characteristics, viz., the reconnection rate and hard X-ray emission profiles. The flux rope was accelerated to the maximum velocity as a CME at the peak phase of the flare, followed by deceleration to an average velocity of 590 km s−1. We suggest that the observed emergence/cancelation of magnetic fluxes near the filament caused it to rise, resulting in the tethers to cut and reconnection to take place beneath the filament; in agreement with the tether-cutting model. The corresponding increase/decrease in positive/negative photospheric fluxes found in the post-peak phase of the eruption provides unambiguous evidence of reconnection as a consequence of tether cutting. Title: High Energy Emissions from Young Stellar Objects Authors: Das, A. C.; Ambastha, Ashok Bibcode: 2012JApA...33....1D Altcode: 2012JApA..tmp...40D X-ray emissions from Young Stellar Objects (YSO) are detected by many X-ray missions that are providing important information about their properties. However, their emission processes are not fully understood. In this research note, we propose a model for the generation of emissions from a YSO on the basis of a simple interaction between the YSO and its surrounding circumstellar accretion disc containing neutral gas and charged dust. It is assumed that the YSO has a weak dipole type magnetic field and its field lines are threaded into the circumstellar disc. Considering the motion of ions and charged dust particles in the presence of neutral gas, we show that the sheared dust-neutral gas velocities can lead to a current along the direction of ambient magnetic field. Magnitude of this current can become large and is capable of generating an electric field along the magnetic field lines. It is shown how the particles can gain energy up to MeV range and above, which can produce high-energy radiations from the YSO. Title: Velocity and Magnetic Transients Driven by the X2.2 White-light Flare of 2011 February 15 in NOAA 11158 Authors: Maurya, R. A.; Vemareddy, P.; Ambastha, A. Bibcode: 2012ApJ...747..134M Altcode: 2011arXiv1106.4166M The first X-class flare of the current solar cycle 24 occurred in Active Region NOAA 11158 during its central meridian passage on 2011 February 15. This two-ribbon white-light flare was observed by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory. During the peak phase of the flare, we detected magnetic and Doppler velocity (DV) transients appearing near the umbral boundary of the main sunspot. These transients persisted for a few minutes and showed spatial and temporal correspondence with the flare kernels. The observed magnetic polarity at the transients' locations underwent a sign reversal, together with a large enhancement in DVs. We explain this observational phenomenon using the HMI spectral data obtained before, during, and after the flare. These changes were reflected in the maps of the active region in all the Stokes parameters. Association of the transient features with various signatures of the flare and the cause and effects of their appearance are also presented on the basis of present theoretical models. Title: Spectral line profile changes associated with energetic solar transients Authors: Ambastha, Ashok; Maurya, Ram A. Bibcode: 2012ASInC...6..197A Altcode: 2012arXiv1203.3326A Solar energetic transients occurring in solar atmosphere are associated with catastrophic release of energy in the solar corona. These transients inject a part of their energy by various physical processes to the deeper, denser photospheric layer at which velocity and magnetic fields are measured using suitable spectral lines. Serious questions have been raised about the nature of the observed magnetic (and velocity) field changes associated with energetic transients as their measurements are expected to be affected by flare-induced line profile changes. In this paper, we shall discuss some recent progress on our understanding of the physical processes associated with such events. Title: Kinetic and magnetic helicities in solar active regions Authors: Maurya, Ram Ajor; Ambastha, Ashok; Reddy, Vema Bibcode: 2011JPhCS.271a2003M Altcode: We have studied the kinetic and magnetic helicities in sub-photospheric flows and photospheric magnetic fields, respectively, of a sample of 91 ARs of solar cycle 23. Hemispheric trend is investigated in the kinetic helicity of sub-photospheric flows averaged in the depth range of 2.5-12 Mms. Magnetic helicity parameters for the ARs are derived using photospheric vector magnetograms to examine their correlation with the corresponding kinetic helicities. We found no significant association between the two helicity parameters. Title: Variations in p-mode parameters and sub-surface flows of active regions with flare activity Authors: Maurya, R. A.; Ambastha, A. Bibcode: 2011ASInC...2..189M Altcode: 2011arXiv1106.4180M We examine the characteristic properties of photospheric p-modes and sub-photospheric flows of active regions (ARs) observed during the period of 26-31 October 2003. Using ring diagram analysis of Doppler velocity data obtained from the Global Oscillations Network Group (GONG), we have found that p-mode parameters evolve with ARs and show a strong association with flare activity. Sub-photospheric flows, derived using inversions of p-modes, show strong twist at the locations of ARs, and large variation with flare activity. Title: Sub-surface Meridional Flow, Vorticity, and the Lifetime of Solar Active Regions Authors: Maurya, R. A.; Ambastha, A. Bibcode: 2010ApJ...714L.196M Altcode: 2010arXiv1003.5273M Solar sub-surface fluid topology provides an indirect approach to examine the internal characteristics of active regions (ARs). Earlier studies have revealed the prevalence of strong flows in the interior of ARs having complex magnetic fields. Using the Doppler data obtained by the Global Oscillation Network Group project for a sample of 74 ARs, we have discovered the presence of steep gradients in meridional velocity at depths ranging from 1.5 to 5 Mm in flare productive ARs. The sample of these ARs is taken from the Carrington rotations 1980-2052 covering the period 2001 August-2007 January. The gradients showed an interesting hemispheric trend of negative (positive) signs in the northern (southern) hemisphere, i.e., directed toward the equator. We have discovered three sheared layers in the depth range of 0-10 Mm, providing evidence of complex flow structures in several ARs. An important inference derived from our analysis is that the location of the deepest zero vertical vorticity is correlated with the remaining lifetime of ARs. This new finding may be employed as a tool for predicting the life expectancy of an AR. Title: A Technique for Automated Determination of Flare Ribbon Separation and Energy Release Authors: Maurya, R. A.; Ambastha, A. Bibcode: 2010SoPh..262..337M Altcode: 2009arXiv0910.4245M; 2010SoPh..tmp...21M We present a technique for automatic determination of flare ribbon separation and the energy released during the course of two-ribbon flares. We have used chromospheric Hα filtergrams and photospheric line-of-sight magnetograms to analyse flare ribbon separation and magnetic field structures, respectively. Flare ribbons were first enhanced and then extracted by the technique of "region growing", i.e., a morphological operator to help resolve the flare ribbons. Separation of flare ribbons was then estimated from the magnetic-polarity reversal line using an automatic technique implemented into an Interactive Data Language (IDLTM) platform. Finally, the rate of flare-energy release was calculated using photospheric magnetic field data and the corresponding separation of the chromospheric Hα flare ribbons. This method could be applied to measure the motion of any feature of interest (e.g., intensity, magnetic, Doppler) from a given point of reference. Title: Magnetic and Velocity Field Changes Related to the Solar Flares of 28 and 29 October 2003 Authors: Maurya, R. A.; Ambastha, A. Bibcode: 2010ASSP...19..517M Altcode: 2010mcia.conf..517M; 2009arXiv0906.3965M Magnetic and velocity field measurements of solar active regions suffer from ambiguities caused by the change in spectral line profiles that occur during the impulsive phase of a major flare. This leads to difficulties in correct interpretation of any flare-related changes. Using magnetic and Doppler movies taken with GONG and MDI, we have detected transient, "moving" features around the peak phases of the X17.2/4B flare observed on 28 October 2003 and the X10/2B flare observed on 29 October 2003 in super-active region NOAA 10486. These features were located near the compact acoustic sources reported earlier by Donea and Lindsey (2005) and the seismic sources reported by Zharkova and Zharkov (2007).We find a moving feature, spatially and temporally associated with the flare ribbons, that separates away at speeds ranging from 30 to 50 km s-1 as observed in photospheric white light and in temperature-minimum (1600 Å), chromospheric (Hα), and transition-region (284Å ) intensities.We suggest that such moving features arise from the line-profile changes attributed to downward electron jets associated with the flare, and do not reflect real changes in the photospheric magnetic and velocity fields. However, abrupt and persistent changes in the pre- and post-flare phases were also found, which do not seem to be affected by line-profile changes. The detailed results have been appeared in Maurya and Ambastha (2009). Title: Flows in Flaring and Dormant Active Regions Authors: Maurya, R. A.; Ambastha, A. Bibcode: 2010ASSP...19..516M Altcode: 2010mcia.conf..516M During cycle 23, some active regions (ARs) produced extremely energetic flares and coronal mass ejections. These ARs are expected to be distinct from dormant ARs and quiet regions (QRs). It is of interest to identify whether the internal structure and dynamics of ARs is related to their outburst activity. For this investigation, we have obtained subsurface velocity flows in several ARs and QRs using ring diagram analysis, and derived the corresponding vorticities and kinetic helicity densities. Title: Heliophysical Processes Authors: Gopalswamy, Natchimuthuk; Hasan, S.; Ambastha, Ashok Bibcode: 2010ASSP...18.....G Altcode: 2010hepr.book.....G No abstract at ADS Title: The Association of Energetic Events with p-Mode Energy Authors: Maurya, Ram Ajor; Ambastha, Ashok Bibcode: 2010cosp...38.3025M Altcode: 2010cosp...38.3025A; 2010cosp.meet.3025M The energetic events, e.g., flares, CMEs, etc. release large amount of energy, which may be able to excite acoustic waves (p-mode) by exerting mechanical impulse of the thermal expansion of the flare on the photosphere. This implied that during an energetic flare, energy of p-modes must be weighted by the energy of excited modes. To get the inferences of flare related enhances of p-mode energy, we have derived the magnetic energy released during a flare from magnetic field observations and associated p-mode energy from ring-diagram analysis. For the statistical studied of their association, we took samples of several high energy flares from Carrington rota-tions 1980-2052 covering the period August 2001-January 2007. We find significant relationship between them. A detailed description of our findings will be presented in the paper. Title: Solar Interior Authors: Ambastha, Ashok Bibcode: 2010ASSP...18...15A Altcode: 2010hepr.book...15A The solar interior is not visible by direct means, and until a few decades back its understanding was based only on the surface observations pertaining to its global properties (e.g., temperature, luminosity, radius, etc.). Solar neutrinos and global solar oscillations have provided more "direct" probes of the Sun's internal structure and dynamics. In these lectures we discuss some aspects of standard solar models, the neutrino problem and recent developments in helioseismology. Title: Variations in p-Mode Parameters with Changing Onset Time of a Large Flare Authors: Maurya, R. A.; Ambastha, A.; Tripathy, S. C. Bibcode: 2009ApJ...706L.235M Altcode: 2009arXiv0910.4247M It is expected that energetic solar flares releasing a large amount of energy at the photosphere may be able to excite the acoustic (p-) modes of oscillations. We have determined the characteristic properties of mode parameters by applying the ring diagram technique to three-dimensional power spectra obtained for solar active region NOAA 10486 during the long-duration energetic X17.2/4B flare of 2003 October 28. Strong evidence of substantial increase in mode amplitude and systematic variations in sub-surface flows, i.e., meridional and zonal components of velocity, kinetic helicity, and vorticity, is found from comparison of the pre- to the post-flare phases. Title: Transient Magnetic and Doppler Features Related to the White-Light Flares in NOAA 10486 Authors: Maurya, R. A.; Ambastha, A. Bibcode: 2009SoPh..258...31M Altcode: 2009arXiv0903.2138M Rapidly moving transient features have been detected in magnetic and Doppler images of super-active region NOAA 10486 during the X17/4B flare of 28 October 2003 and the X10/2B flare of 29 October 2003. Both these flares were extremely energetic white-light events. The transient features appeared during impulsive phases of the flares and moved with speeds ranging from 30 to 50 km s−1. These features were located near the previously reported compact acoustic (Donea and Lindsey, Astrophys. J.630, 1168, 2005) and seismic sources (Zharkova and Zharkov, Astrophys. J.664, 573, 2007). We examine the origin of these features and their relationship with various aspects of the flares, viz., hard X-ray emission sources and flare kernels observed at different layers: i) photosphere (white-light continuum), ii) chromosphere (Hα 6563 Å), iii) temperature minimum region (UV 1600 Å), and iv) transition region (UV 284 Å). Title: Magnetic and velocity field variations in the active regions NOAA 10486 and NOAA 10488 Authors: Maurya, Ram Ajor; Ambastha, Ashok Bibcode: 2008JApA...29..103M Altcode: We study the magnetic and velocity field evolution in the two magnetically complex active regions NOAA 10486 and NOAA 10488 observed during October-November 2003. We have used the available data to examine net flux and Doppler velocity time profiles to identify changes associated with evolutionary and transient phenomena. In particular, we report detection of rapid moving features observed in NOAA 10486 during the maximum phase of the X17.2/4B superflare of October 28, 2003. The velocity of this moving feature is estimated around 40 km/s, i.e., much greater than the usual Hα flare-ribbons' separation speed of 3-10 km/s, but similar to the velocity of seismic waves, i.e., ∼45 km/s reported earlier by Kosovichev & Zharkova (1998). Title: Hα intensity oscillations in large flares Authors: Maurya, Ram Ajor; Ambastha, Ashok Bibcode: 2008JApA...29..249M Altcode: We reinvestigate the problem of Hα intensity oscillations in large flares, particularly those classified as X-class flares. We have used high spatial and temporal resolution digital observations obtained from Udaipur Solar Observatory during the period 1998-2006 and selected several events. Normalized Lomb-Scargle periodogram method for spectral analysis was used to study the oscillatory power in quiet and active chromospheric locations, including the flare ribbons. Title: Helioseismic effects of energetic transients Authors: Ambastha, Ashok Bibcode: 2008JApA...29...93A Altcode: Photospheric and chromospheric signatures related to large, energetic transients such as flares and CMEs, have been extensively reported during the last several years. In addition, energetic solar transients are expected to cause helioseismic effects. Some of the recent results are reviewed here; in particular, the helioseismic effects of the powerful flares in superactive region, NOAA 10486, including the 4B/X17 superflare of October 28, 2003. We also examine the temporal variations of power in low-l modes during the period May 1995-October 2005, and compare with daily, disk-integrated flare- and CME-indices to infer the effect of transients on the scale of whole solar disk. Title: Excitation of solar p-modes by energetic transients Authors: Ambastha, Ashok Bibcode: 2008BASIP..25...29A Altcode: 2008BASI...25S..29A Several recent studies have reported helioseismic effects of large flares on solar p-mode characteristics. Using 3-D power spectra of p-mode oscillations, we found statistically significant p-mode power enhancement corresponding to large flares at the scale of individual active regions. An important result is the presence of steep gradient in the meridional velocity in the sub-photospheric regions below a depth of 5 Mm in flare productive active regions. In order to explore whether energetic transients have a role in p-mode excitation on the global scale, we have analysed low-l mode power time-series over the period of May 1995-October 2005; now available from GONG network. It shows a poor correlation between the running means of disk-integrated flare- (and CME-) indices and the mode power. This behaviour is also reflected in the distribution of mode power. The variations in the running mean mode power corresponding to l=0 modes with different radial orders are generally stochastic in nature as predicted theoretically. Title: Signatures of large flares on photospheric magnetic and velocity fields Authors: Ambastha, A. Bibcode: 2007BASI...35..419A Altcode: We have analysed the spatial and temporal evolution of photospheric magnetic and doppler velocities in active regions, particularly in the superactive region NOAA 10486, to detect pre- and post-flare changes. These findings have been compared with recent reports by other workers, and significance of these results has been discussed. Helioseismic response of large flares, and the role of sub-photospheric flows in flare-productive as compared to that in less flare-productive active regions are presented. Title: Evolution of Magnetic and Velocity Fields in Super-active Region NOAA10486 and the Large 4B/X17.2 Flare of October 28, 2003 Authors: Ambastha, A. Bibcode: 2007SunGe...2...13A Altcode: We have used high cadence GONG + photospheric magnetograms, dopplergrams and Udaipur Solar Observatory (USO) chromospheric Hα-filtergrams to study the spatial and temporal evolution of the active region NOAA 10486 in relation to the X17.2/4B flare of October 28, 2003. New flux emergences, large proper motions and development of steady velocity flows have been identified around the flare site. In addition, filament activation and eruption leading to fast CMEs were noticed. During the flare, NOAA 10486 was located near the disk-center; well suited for the ring diagram analysis. Therefore, we have obtained the 3-D power spectra to search for helioseismic response of the large flare on the amplitude, frequency and width of the p-modes. Power enhancement was found during the post-flare phase, and NOAA 10486 possessed steep gradient in the meridional velocity as compared to the less flare-productive active regions. Title: Solar Activity Authors: Ambastha, Ashok Bibcode: 2007AIPC..919..173A Altcode: Understanding the solar activity is a fundamental problem which has essentially led to create modern solar physics. The Sun's magnetic field and differential rotation give rise to much complexity, in particular, to solar activity over a large range in both spatial and temporal scales. Explosive transient events occur in the solar atmosphere in shorter time-scales of minutes to hours, such as, solar flares and coronal mass ejections (CMEs). At the longer scales, the most notable is the solar activity cycle of 11 years, or magnetic cycle of 22 years. Understanding of the solar activity is important as it affects the space weather, i.e., the interplanetary medium and geo-magnetic environment. We present an account of the recent developments in our understanding of these phenomena, using both space-borne and ground-based observations. Title: Signatures of Large Flares on Photospheric Magnetic and Velocity Fields Authors: Ambastha, A. Bibcode: 2006ihy..workE..26A Altcode: a_ambastha@rediffmail.com We have studied spatial and temporal evolution of some flare productive active regions using high cadence photospheric magnetograms and Dopplergrams. In addition, chromospheric H-alpha filtergrams have been used to identify flux emergences, large proper motions and development of velocity flows in relation to the flare sites. Magnetic flux and velocity changes have been found at these sites before and after large flares. The 3-D power spectra of p-mode oscillations have been obtained using ring diagram technique. These spectra are then used to look for helioseismic response of the flares on the amplitude, frequency and width of the p-modes. In the flaring active region, p-mode power enhancement and a steep gradient in the meridional velocity are found as compared to the quiet regions. A comparison of flaring active regions has been carried out with less productive active regions. Title: Possibility of Excitation of Low-ℓ P-Modes by Energetic Solar Transients Authors: Ambastha, Ashok; Antia, H. M. Bibcode: 2006SoPh..238..219A Altcode: 2006SoPh..tmp...63A We examine the temporal variation of power in low-ℓ modes using GONG data for the period of May 1995-October 2005 and compare this with disk-integrated flare and CME indices. A poor correlation between the running means of Flare Index and mode power is found. A similar result is found for CME Index also. Variations in the running mean mode power corresponding to ℓ = 0 modes with different radial orders are generally stochastic in nature. This behaviour is also reflected in the distribution of mode power. Title: Photospheric, Chromospheric and Helioseismic Signatures of a Large Flare in Super-active Region NOAA 10486 Authors: Ambastha, Ashok Bibcode: 2006JApA...27..255A Altcode: NOAA 10486 produced several powerful flares, including the 4B/X17.2 superflare of October 28, 2003/11:10 UT. This flare was extensively covered by the Hα and GONG instruments operated at the Udaipur Solar Observatory (USO). The central location of the active region on October 28, 2003 was well-suited for the ring diagram analysis to obtain the 3-D power spectra and search for helioseismic response of this large flare on the amplitude, frequency and width of the p-modes. Further, using USO observations, we have identified the sites of new flux emergences, large proper motions and line-of-sight velocity flows in the active region and their relationship with the flare. Title: Hα Observations of 8 June, 2004 Venus Transit Authors: Ambastha, Ashok; Ravindra, B.; Gosain, Sanjay Bibcode: 2006SoPh..233..171A Altcode: The cosmic event of Venus transit across the solar disk occurred on 8 June, 2004. The previous such event was witnessed about 122 years ago on 6 December, 1882. We observed this rare transit in Hα 6563 Å line-center from Udaipur Solar Observatory (USO) using both the full-disk and small field-of-view solar telescopes. In the earlier historical transits, a "black-drop" effect was observed in white light images, during the contact phases. The transit of 8 June, 2004 provided a unique opportunity to observe this effect, for the first time, in Hα. We report that the "black-drop" effect is present in Hα also, as in the white light observations made by the ground-based Global Oscillation Network Group (GONG) instrument and the space-borne Transition Region and Coronal Explorer (TRACE) satellite. We did not observe any noticeable "aureole" (atmospheric glow) around Venus during the ingress or egress phases. We have compared the Hα images with the multi-wavelength data obtained from the TRACE satellite. Title: Spectral Characterization of Solar Active Region NOAA 8242 in Quiet and Sunspot Locations Authors: Janve, Vaibhav A.; Ambastha, Ashok Bibcode: 2005BASI...33..360J Altcode: We present results from the analysis of simultaneous Stokes I profile measurements in Fe I twin line in 6302 Å region and Ca II K 3934 Å line over NOAA 8242 on 13th June 1998. Advanced Stokes Polarimeter (ASP) instrument of the Vacuum Tower Telescope (VTT) of Sac Peak, NSO, U.S.A was used for obtaining these observations. Along with the spectral data simultaneous filtergrams in G-band, Hα 6562.8 Å, and 6118.7 Å continuum were also obtained using the UBF filter. The slope corrected spectral data was used to construct spectroheliograms, and the maps of spectral asymmetry, velocity, FWHM, Equivalent width by a Gaussian fitting of spectral line profiles for both the quiet and sunspot regions. The implications for strong and weak magnetic field locations have been discussed. Title: Magnetic Evolution of Super-Active Region NOAA AR 10486 and the Large 4B/X17.2 Class Flare Observed During Octbober 28, 2003 Authors: Ambastha, Ashok Bibcode: 2005BASI...33..364A Altcode: Extensive flare activity was observed in super-active region NOAA10486 during its disk passage of October 22-November 04, 2003. An extremely energetic 4B/X17.2 flare on October 28, 2003/11:10 UT was observed from USO when the active region was located at S16E08, i.e., close to the disk-centre. This flare was rated the third largest X-ray flare recorded by GOES satellite, and the largest in the optical class (4B) observed so far from USO. Chromospheric H-alpha filtergrams were obtained before, during and in the decay phase of the two-ribbon flare at a cadence of 3-4 seconds. The temporal and spatial structure evolution was analyzed with the help of a movie constructed using more than 4000 images. Magnetograms from NASA-MSFC showed large magnetic shear around the flare site which was delineated by a large active filament. The filament erupted as the flare progressed. In the decay phase of the flare, a system of post-flare loops developed at the site of the erupted filament. Observation from TRACE also exhibited these loop structures. Associated with this flare, a fast Earthward moving halo CME was also detected by SOHO, which initiated a major geomagnetic storm on October 29, 2003 at 06:13 UT, i.e., within a record time of 19 hours after the flare. This large flare was followed by another 2B/X11 event on October 29, 2003/20:49 UT, not observed from USO as it occurred in our night-time. We have used white light full disk images and line-of-sight magnetograms obtained from SOHO-MDI for determination of proper motion of the main sunspots and corresponding magnetic fluxes in order to understand rapid magnetic energy build-up in the active region, giving rise to the two large flares within such a short time. Title: Preliminary Results of Venus Transit of June 8, 2004 Observed in Hα 6563 Å Authors: Ravindra, B.; Ambastha, Ashok; Gosain, Sanjay Bibcode: 2005BASI...33..366R Altcode: The rare cosmic event of Venus transit across the solar disk occurred on 8th June 2004, i.e. ~ 122 years after the last such event which occurred in observed on 6 December 1882. The event was observed at Udaipur Solar Observatory using full-disk, as well as, small field-of-view high resolution solar telescopes, and recorded in the nearly monochromatic light of Hα 6563 Å. High resolution solar images were taken at a cadence of 3 seconds during the period 05:10-05:30 UT which covered the Ist and IInd contacts of Venus. This was repeated during the period 11:10-11:30 UT covering the IIIrd and IVth contacts, while the images were obtained at a lower cadence between the period of the IInd and IIIrd contacts. Altogether, around 4000 filtergrams were obtained. We have compared our observations with the multi-wavelength data obtained from TRACE satellite. We studied the optical effects that cause the "black-drop" and the "atmospheric-glow" around Venus at the time of its I-II, and III-IV contacts. We have also compared the difference in contact timings observed in different wavelength bands. Title: Variations in LOW-l Solar P-Modes with Flare Activity Authors: Ambastha, A.; Antia, H. M. Bibcode: 2004ESASP.559..289A Altcode: 2004soho...14..289A No abstract at ADS Title: Solar P-Mode Characteristics Associated with Superactive-Regions Observed during OCT NOV 2003 Authors: Ambastha, A.; Basu, S.; Antia, H. M.; Bogart, R. S. Bibcode: 2004ESASP.559..293A Altcode: 2004soho...14..293A No abstract at ADS Title: Flare-Induced Excitation of Solar p modes Authors: Ambastha, Ashok; Basu, Sarbani; Antia, H. M. Bibcode: 2003SoPh..218..151A Altcode: Solar flares release large amounts of energy at different layers of the solar atmosphere, including at the photosphere in the case of exceptionally major events. Therefore, it is expected that large flares would be able to excite acoustic waves on the solar surface, thereby affecting the p-mode oscillation characteristics. We have applied the ring-diagram analysis technique to 3-D power spectra obtained for different flare regions in order to study how flares affect the amplitude, frequency and width of the acoustic modes. Data from the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO) has been used. We have used data obtained for several active regions of the current solar cycle that have produced flares. In most cases, during the period of high flare activity, power in p modes appears to be larger when compared to that in non-flaring regions of similar magnetic field strength. Title: White Light and Emission Line Polarization of Solar Corona during TSE of June 21, 2001 Authors: Ambastha, Ashok; Gosain, Sanjay Bibcode: 2003BASI...31..295A Altcode: Coronal intensity and polarization maps have been obtained for the total solar eclipse (TSE) o o of June 21, 2001, observed from Lusaka, Zambia (location: 28 17.5E 15 24.5S, Alt. 1300 mtr) at broadband H 6563Å (FWHM 80Å), as well as, around the coronal emission lines 5303Å (FWHM 12Å), and 6374Å (FWHM 12Å). The results are discussed. Title: Excitation of Solar p-mode Oscillations by Flares Authors: Ambastha, Ashok; Basu, Sarbani; Antia, H. M. Bibcode: 2003BASI...31..319A Altcode: Solar flares release large amounts of energy at different layers of the solar atmosphere. It is, therefore, expected that major flares would be able to excite waves, thereby affecting the p-mode oscillation characteristics. From the analysis of MDI data, we find that power in p-modes appears to increase for some flares, beyond the normal values expected from the influence of magnetic field. Title: Performance Evaluation of Adaptive Optics Systems Authors: Sridharan, Rengaswamy; Raja Bayanna, A.; Srivastava, Nandita; Kumar, Brajesh; Ravindra, B.; Gupta, S. K.; Jain, Naresh; Ambastha, A.; Venkatakrishnan, P. 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: Effects of flares on solar oscillation characteristics Authors: Ambastha, Ashok; Basu, Sarbani; Antia, H. M. Bibcode: 2003ESASP.517..219A Altcode: 2003soho...12..219A We use ring diagram analysis to study the effects of solar flares on p-mode oscillation characteristics. We study the changes in the amplitude, frequency and width of acoustic modes using data before, during and after a few of the major flares during the current solar cycle. Mode power is found to be enhanced during and after some flares, though the enhancement is not seen in all flares. Title: The Active and Explosive Sun Authors: Ambastha, A. Bibcode: 2003LNP...619..127A Altcode: 2003lsp..conf..127A The Sun's magnetic field and differential rotation give rise to much complexity in its structure and activity over a large range in both spatial and temporal scales. The most notable among these is the solar activity cycle of 11 years, or magnetic cycle of 22 years. On shorter time scales of a few seconds to several hours, spectacular explosive events occur in the solar atmosphere, such as, solar flares, prominence eruptions, and coronal mass ejections (CMEs). The explosive energy release takes place in the form of accelerated particles, bulk mass motion, and enhancement of radiation over the entire electromagnetic spectrum ranging from γ-rays to radio wavelengths. These solar transients are essentially the source of disturbance in the interplanetary medium, and also cause geomagnetic effects upon their encounter with the Earth. We present an account of the recent developments in our understanding of these phenomena using both space-borne, and ground-based observations. Title: Effect of flares on solar oscillations characteristics Authors: Ambastha, Ashok; Basu, Sarbani; Antia, H. M. Bibcode: 2002ESASP.508...43A Altcode: 2002soho...11...43A We use ring diagram analysis to study the effect of solar flares on oscillation mode characteristics, using data from GONG+ and MDI. The data taken around the flares of June 6-7, 2000; March 29, 2001 and April 10-11, 2001 are studied. We find that during some flares, the power in acoustic modes increases beyond the normal values expected from the influence of magnetic field. Title: Digital Imaging Techniques for Solar Magnetic and Velocity Fields Authors: Ambastha, Ashok Bibcode: 2002adaa.conf..177A Altcode: No abstract at ADS Title: A catalogue of Solar Filament Analysis: Year 2000 Authors: Ambastha, A.; Agrawal, Reetu Bibcode: 2001csfa.rept.....A Altcode: This volume is an observational catalog of solar filament/prominence data obtained during the year 2000 at the Udaipur Solar Observatory, Udaipur, India. Physical ephemeris, listing of observed filament/prominences along with their characteristics, daily full disk H-alpha and VMG (GONG) images are compiled. Some selected events of filament eruptions are also presented. Title: A Comparison of Flux Emergence, Cancellation, and Motions in Flaring and Non-Flaring Sites of NOAA 8038 Observed by USO Magnetograph Authors: Ambastha, A.; Mathew, S. K. Bibcode: 2001ASPC..236..313A Altcode: 2001aspt.conf..313A No abstract at ADS Title: Magnetic field gradient and flare: study of a small flare in NOAA 8038 Authors: Mathew, Shibu K.; Ambastha, Ashok Bibcode: 2000SoPh..197...75M Altcode: Active region NOAA 8038 was observed from 10 to 13 May, 1997 using the USO solar video magnetograph. During this period, the active region was mostly inactive, and gave rise to only a single notable flare of 1N/C1.3 class on May 12, 1997/04:45 UT. The flare occurred in a weak field location, but new emerging fluxes were observed prior to the flare onset. Horizontal motions of the network photospheric magnetic fluxes were inferred using USO and SOHO magnetograms, and velocities in the range 300-800 m s−1 were estimated. The initial flare brightening was observed at the flux cancellation site where magnetic field gradients were found to increase. Detailed analyses of flux motions, cancellation and their relation with the flare are presented. Title: A Rapidly Evolving Active Region NOAA 8032 Observed on April 15th 1997 Authors: Mathew, Shibu K.; Ambastha, Ashok Bibcode: 2000JApA...21..233M Altcode: No abstract at ADS Title: Relationship of Non-potentially and Flaring: Intercomparison for an Mclass Flare Authors: Ambastha, Ashok; Mathew, Shibu K. Bibcode: 2000JApA...21..271A Altcode: No abstract at ADS Title: A catalogue of Solar Filament Analysis: Year 1999 Authors: Ambastha, A.; Pathak, Kumud; Agrawal, Reetu Bibcode: 2000csfa.rept.....A Altcode: No abstract at ADS Title: Dynamical properties of quiescent prominence in He D3 5876 &Aring line emission Authors: Prasad, C. Debi; Ambastha, Ashok; Mathew, Shibu K. Bibcode: 1999BASI...27..411P Altcode: No abstract at ADS Title: Dynamical properties of quiescent prominence in He D3 5876 Å line emission. Authors: Debi Prasad, C.; Ambastha, A.; Mathew, S. K. Bibcode: 1999BASI...27..411D Altcode: The authors have observed a quiescent prominence with the Coudé spectrograph of Udaipur Solar Observatory during May 24 - 25, 1995. Analysis of two dimensional spectroscopic data was carried out in order to obtain the line shifts and line widths of He D3 5876 Å emission lines. These quantities are used to study the dynamical properties of the observed prominence. Title: Complex H α Loop Activity in a Long Duration Flare Authors: Prasad, D.; Gary, G.; Ambastha, A. Bibcode: 1999ASPC..183..523P Altcode: 1999hrsp.conf..523P No abstract at ADS Title: Circular polarization measurement using a tunable lithium niobate Fabry-Perot filter Authors: Mathew, S. K.; Bhatnagar, A.; Debi Prasad, C.; Ambastha, A. Bibcode: 1999ASSL..243..321M Altcode: 1999sopo.conf..321M No abstract at ADS Title: High Resolution Longitudinal Magnetic Field Measurements Using a Fabry-Perot Lithium Niobate Filter Based Video Magnetograph Authors: Mathew, S.; Bhatnagar, A.; Debi Prasad, C.; Ambastha, A. Bibcode: 1999ASPC..183..256M Altcode: 1999hrsp.conf..256M No abstract at ADS Title: Fabry-Perot filter based solar video magnetograph Authors: Mathew, S. K.; Bhatnagar, A.; Prasad, C. D.; Ambastha, A. Bibcode: 1998A&AS..133..285M Altcode: A tunable Lithium Niobate (LiNbO_3) Fabry-Perot filter (FP) (passband 165 m Angstroms/ at 6122 Angstroms) based video magnetograph has been designed and fabricated. This instrument is capable of providing near simultaneous observations of photospheric longitudinal magnetic field, chromospheric Hα , and photospheric CaI pictures using the same telescope and back-end set-up. The magnetic field measurements are made by using the polarization properties of the Zeeman components of the photospheric CaI line at 6122 Angstroms/ (Landé g factor of 1.75). The CaI line has been chosen due to its low temperature sensitivity and no blend with other solar or atmospheric lines. A variable electro-optic quarter wave retarder, KD(*) P (Potassium di-Deuterium Phosphate) along with a linear polarizer is used for analyzing the circular polarization of the Zeeman components. The filter tuned at 140 m Angstroms/ away from the line center in the blue wing is found to give the best linear response for the field strength up to 1500 Gauss. A field of view (FOV) of ~ 4 x 3 arcmin on the solar disk is imaged using a 699 x 288 pixel Cohu CCD camera in synchronous with the KD(*) P modulation. The <~mbda/4 modulation is achieved by applying +/-2100 volts to the KD(*) P to obtain alternate frames of oppositely circular polarized images. These images are stored in separate frame buffers of an image acquisition system. To achieve high signal to noise ratio, a large number of images (maximum 256) are added in the respective frame buffers and then the difference between the left and the right circularly polarized images is obtained. This difference is related to the magnetic field strength. On comparing the video magnetograms (VMG) obtained at Udaipur Solar Observatory (USO) on 09 April 1997 at 09:32 UT with those taken by SOHO/MDI at 09:41 UT, it was found that all the magnetic features matched very well in both the magnetograms. In this paper we present the details of the instrument and examples of observations. Title: Solar physics in India during the next solar maximum and beyond. Proceedings. Physical Research Laboratory (Ahmedabad) Golden Jubilee Workshop, Udaipur (India), 7 - 10 Oct 1996. Authors: Ambastha, A. Bibcode: 1998BASI...26.....A Altcode: The workshop covered widely varied topics on solar physics such as instrumentation, magnetic fields, flares and transients, solar wind and the interplanetary medium, solar eclipses, helioseismology and international projects, e.g. GONG, YOHKOH, and SOHO/SUMER. Title: Emerging Flux and X-class Flares in NOAA 6555 Authors: Choudhary, Debi Prasad; Ambastha, Ashok; Ai, G. Bibcode: 1998SoPh..179..133C Altcode: The active region NOAA 6555 had several locations of highly sheared magnetic field structure, yet, only one of them was the site for all the five X-class flares during its disk passage in March 1991. The pre-flare observations of high-resolution Hα filtergrams, vector magnetograms and Hβ Dopplergrams of the 2B/X5.3 flare on 25 March 1991 show that the flaring site was characterized by a new rising `emerging flux region' (EFR) near the highly sheared magnetic field configuration. The polarity axis of the emerging flux was nearly perpendicular to the pre-existing magnetic neutral line. The location of the EFR was the site of initial brightening in Hα. The post-flare magnetograms show higher magnetic shear at the flare location compared to the post-flare magnetograms, which might indicate that the EFR was sheared at the time of its emergence. As the new EFR coincided with the occurrence of the flare, we suggest that it might have triggered the observed flare. Observations from Big Bear Solar Observatory and Marshall Space Flight Center also show that there was emergence of new flux at the same location prior to two other X-class flares. We find that out of five observed X-class flares in NOAA 6555, at least in three cases there are clear signatures of flare-related flux emergence. Therefore, it is concluded that EFRs might play an important role in destabilizing the observed sheared magnetic structures leading to large X-class flares of NOAA 6555. Title: Probing the Solar Interior: Hearing the Heartbeats of 1he Sun Authors: Ambastha, A. Bibcode: 1998Reson...3...18A Altcode: Recent developments in solar seismology have enabled us to observe and analyse the vibrations of the sun, and help to probe its hidden interior. Understanding the sun's internal structure and dynamics promises to test and expand our knowledge of physics, cosmology, and astrophysics. Seismic sounding of the sun has begun to shed light on its hidden internal anatomy. Title: Dynamics of Helically Twisted Prominence of January 22, 1979 Authors: Srivastava, Nandita; Ambastha, Ashok Bibcode: 1998Ap&SS.262...29S Altcode: 1999Ap&SS.262...29S We have studied the dynamics of a macroscopically twisted helical prominence observed in Hα line on January 22, 1979 from Udaipur Solar Observatory. The analysis carried out is similar to that of March 11, 1979 event (Srivastava et al., 1991) wherein we had studied the role of twisted force-free magnetic fields in the prominence system. In the present study, it is found that of the two helically braided prominence tubes, one was dynamically more active. We have examined the temporal evolution of force-free parameter alpha, and the axial currents associated with the prominence system that decreased with time. We find that the magnitude of the electric currents and also the rate of energy release during the untwisting of the prominence was of comparatively higher order ~ 10^30 ergs s^-1 than that of March 11, 1979 event, in agreement with the physical dimensions of the two prominences. Title: The Role of Magnetic Shear and New Emerging Fluxes in Producing Large Solar Flares Authors: Ambastha, Ashok Bibcode: 1998ASPC..140..113A Altcode: 1998ssp..conf..113A No abstract at ADS Title: Solar Photospheric and Chromospheric Observations using a Lithium Niobate Fabry-Perot ETALON Authors: Debi Prasad, C.; Mathew, Shibu K.; Bhatnagar, Arvind; Ambastha, Ashok Bibcode: 1998ExA.....8..125D Altcode: We have made a narrow band tunable filter for solar observations using a Lithium Niobate Fabry-Perot etalon. The 60 mm aperture etalon with a free spectral range of 4.22 Å and finesse of 26 at lambda = 6122 Å has been procured from CSIRO, Australia. The wavelength tuning is achieved by applying high voltage to the etalon substrate at the rate of 0.45 Å per 1000 Volts. The filter is being used for imaging the sun in Hα line and obtaining Doppler- and Magnetogram in CaI 6122 Å line. In this paper, we present some initial observations carried out with this filter at Udaipur Solar Observatory. Title: Chromospheric Evolution and the Flare Activity of Super-Active Region NOAA 6555 Authors: Debi Prasad, C.; Ambastha, Ashok; Srivastava, Nandita; Tripathy, Sushanta C.; Hagyard, Mona J. Bibcode: 1997JApA...18...39D Altcode: Super-active region NOAA 6555 was highly flare productive during the period March 21st-27th, 1991 of its disk passage. We have st udied its chromospheric activity using high spatial resolution Hα filtergrams taken at Udaipur along with MSFC vector magnetograms. A possible relationship of flare productivity and the variation in shear has been explored. Flares were generally seen in those subareas of the active region which possessed closed magnetic field configuration, whereas only minor flares and/or surges occurred in subareas showing open magnetic field configuration. Physical mechanisms responsible for the observed surges are also discussed. Title: Seismic investigation of the solar structure using GONG frequencies Authors: Tripathy, S. C.; Antia, H. M.; Hill, F.; Ambastha, A. Bibcode: 1997astro.ph..3179T Altcode: Using the recently obtained GONG frequencies, we investigate the properties of the solar interior by constructing solar models with various input physics like opacities, equation of state, nuclear reaction rates etc. The differential asymptotic inversion technique is then used to infer the relative difference in sound speed between the Sun and solar models. Here we apply these results to test equation of state and different formulation for calculating the convective flux. Title: No signature of circum-solar dust ring up to 5 Rsun from optical polarization and near IR observations of 24 October 1995 total solar eclipse. Authors: Prasad C., D.; Ambastha, A. K. Bibcode: 1997KodOB..13...17P Altcode: A circum-solar dust ring, consisting of interplanetary particles, is believed to be situated at 4 Rsun, and temporarily variable in anti-phase with the solar activity cycle. The signatures of such a ring should be observable through: (1) a highly polarized scattered solar radiation, and (2) its thermal component in near IR. The observed coronal polarization and surface brightness were found to be consistent with the overall expected in the corona at the solar minimum phase. The authors report that no ring signature was observed by their experiment. Title: The Role of Magnetic Shear and New Emerging Fluxes in Producing Large Solar Flares in NOAA AR 6555 Authors: Ambastha, Ashok; Prasad, C. Debi Bibcode: 1997IAUJD..19E..11A Altcode: The evolution of a flare-productive active region NOAA AR 6555 has been extensively studied in response to the changes taking place at the photospheric level, such as, sunspot proper motions and emerging fluxes. Photospheric vector magnetic fields have been used in deriving such information as flux distributions, sites of abnormal polarities (inverted bipoles, delta-structures), steep magnetic field gradients, magnetic shear etc. A particular site in the active region produced several X-class flares, while other areas were relatively dormant, notwithstanding the existence of strong shear. It is thus inferred that large magnetic shear alone is not sufficient to explain flare productivity, or flare occurrence of an active region. Using high resolution H alpha filtergrams from USO, vector magnetograms from Huairou and MSFC, and Doppler-grams from Huairou, we find that many other parameters contributed to the X-class flares in NOAA 6555. These were: rapid evolution in overall magnetic properties, large sunspot motions, new flux emergence, development of magnetic null point over the flaring site, orthogonality of magnetic and velocity (Doppler) neutral lines. Title: Electric Currents and Magnetic Shear Variations during Some Flares of M- and X-Class Authors: Ambastha, A.; Fontenla, J. M.; Hagyard, M. J. Bibcode: 1996mpsa.conf..533A Altcode: 1996IAUCo.153..533A We study the magnetic field evolution during flares of M and X-class using overlays of cotemporal halpha filtergrams and magnetograms.Significant decrease in the area-averaged magnetic shear was found in some cases around the flare onset-time, increasing subsequently as the flare progressed. However, the changes were less pronounced for relatively smaller M-class flares. Strong Lorentz forces were found to exist at the photosphere, acting against the observed motions of sunspots. Extrapolated potential field indicated presence of a magnetic null at the upper chromosphere/lower coronal height, directly above a large Halpha flare. Title: A large flare in an area of weak magnetic field and low shear: a counter - example! Authors: Ambastha, Ashok; Mona, Hagyard J.; West, E. A. Bibcode: 1995BASI...23..428A Altcode: No abstract at ADS Title: Chromospheric, photospheric, magnetic field evolution and flare activity of the super active region NOAA 6555 Authors: Debi Prasad, C.; Srivastava, Nandita; Tripathy, Sushantha C.; Ambastha, Ashok Bibcode: 1995BASI...23..427D Altcode: No abstract at ADS Title: The Magnetic Evolution of AR 6555 Which LED to Two Impulsive, Relatively Compact, X-Type Flares Authors: Fontenla, J. M.; Ambastha, A.; Kalman, B.; Csepura, Gy. Bibcode: 1995ApJ...440..894F Altcode: We study the evolution of the vector magnetic field and the sunspot motions observed in AR 6555 during 1991 March 23-26. This region displays two locations of large magnetic shear that were also sites of flare activity. The first location produced two large (X-class) flares during the period covered by our observations. The second location had larger magnetic shear than the first but produced only small (M- and C-class) flares during our observations. We study the evolution of the photospheric magnetic field in relation to the large flares in the first location. These flares occurred around the same included polarity and have very similar characteristics (soft X-ray light curves, energies, etc.). However, the whole active region has changed substantially in the period between them. We found several characteristics of the region that appear related to the occurrence of these flares: (1) The flares occurred near regions of large magnetic "shear," but not at the locations of maximum shear or maximum field. (2) Potential field extrapolations of the observed field suggest that the topology changed, prior to the first of the two flares, in such a way that a null appeared in the coarse magnetic field. (3) This null was located close to both X-class flares and remained in that location for a few days while the two flares were observed. (4) The flaring region has a pattern of vector field and sunspot motions in which material is "squeezed" along the polarity inversion line. This pattern is very different from that usually associated with shearing arcades, but it is similar to that suggested previously by Fontenla and Davis. The vertical electric currents, inferred from the transverse field, are consistent with this pattern. (5) A major reconfiguration of the longitudinal field and the vertical electric currents occurred just prior to the first of the two flares. Both changes imply substantial variations of the magnetic structure of the region. On the basis of the available data we suggest that these changes made the flaring possible, and we develop a scenario that can explain the origin of the magnetic free- energy that was released in these flares. Title: Evolutionary and flare-associated magnetic shear variations observed in a complex, flare-productive active region Authors: Ambastha, Ashok; Hagyard, Mona J.; West, E. A. Bibcode: 1993SoPh..148..277A Altcode: Evolution of nonpotential structures in a complex, flare-productive active region NOAA AR 6555 has been studied during the period March 23-26, 1991, using a quantitative description of the degree of magnetic shear at both local and regional scales. Distinct shear evolution in four subareas of AR 6555 is inferred from daily variation of their shear indices. Subareas which showed significant evolution in their overall magnetic structure are found to be most active. Hα flare ribbons mostly formed bordering, andnot within, areas of large shear and expanded over areas of lower shear. Title: Evolution and Flare-Associated Magnetic Shear Variation in Solar Active Regions Authors: Ambastha, A.; Hagyard, M. J.; West, E. A. Bibcode: 1993BAAS...25R1189A Altcode: No abstract at ADS Title: A Study of Photospheric Magnetic Fields Associated with the June 1991 Active Region Authors: Hagyard, M. J.; West, E. A.; Smith, J. E.; Ambastha, A.; Kenny, E. G. Bibcode: 1993BAAS...25Q1190H Altcode: No abstract at ADS Title: Stability of Self-Gravitating Finite Disks Authors: Ambastha, A. Bibcode: 1993icpc.proc..151A Altcode: 1993IAUCo.132..151A No abstract at ADS Title: Filament Eruptions Flaring Arches and Eruptive Flares Authors: Bhatnagar, A.; Ambastha, A.; Srivastava, N. Bibcode: 1992LNP...399...59B Altcode: 1992esf..coll...59B; 1992IAUCo.133...59B Several cases of erupting filaments showing distinctly their feet have been studied. Role of the feet and their anchorage with the photosphere in maintaining filament stability is established; apart from the footpoint separation and height criteria. Further, a homologous series of more energetic events, namely, the flaring arches and eruptive flares of March 5-7, 1991, suggest a repetitive restoration of magnetic field conditions and energy build-up within a day. High resolution H-alpha observations of these events indicate that large amount of ejected material was siphoned out from the chromosphere through, the top. of a low-lying compact emission loop within the active region. Title: The large two-ribbon flare of 1990 March 28. Authors: Ambastha, A. Bibcode: 1991BASI...19..208A Altcode: No abstract at ADS Title: Helically twisted prominence eruption event of 1979 March 11. Authors: Srivastava, N.; Ambastha, A.; Bhatnagar, A. Bibcode: 1991BASI...19..208S Altcode: No abstract at ADS Title: Results of GONG site survey program at the Udaipur Solar Observatory. Authors: Ambastha, A.; Bhatnagar, A.; Srivastava, N.; Jain, R. M.; Gupta, S. K.; Sharma, R.; Agrawal, G. Bibcode: 1991BASI...19..211A Altcode: No abstract at ADS Title: GONG site evaluation program at Udaipur Solar Observatory Authors: Ambastha, Ashok; Bhatnagar, Arvind; Jain, Rajmal; Srivastava, Nandita; Gupta, Sudhir; Sharma, Richa; Agrawal, Gopal; Kumawat, Vishnu; Hill, Frank; Fischer, George Bibcode: 1991BASI...19..215A Altcode: The Global Oscillations Network Group (GONG) project to study the internal structure and dynamic of the sun is discussed. The GONG project will measure waves that penetrate throughout the sun's core. Power spectra of the monthly average transparency for each calendar month were obtained and analyzed. The resulting clear and dark time distributions and monthly average extinction coefficients and transparency power spectra are shown and discussed. The various possible networks from the candidate sites participating in the GONG project are compared. Title: Evolution of helically twisted prominence structures of March 11, 1979 Authors: Srivastava, N.; Ambastha, A.; Bhatnagar, A. Bibcode: 1991SoPh..133..339S Altcode: Helical structures are generally associated with many eruptive solar prominences. Thus, study of their evolution in the solar atmosphere assumes importance. We present a study of a flare-associated erupting prominence of March 11, 1979, with conspicuous helically twisted structure, observed in Hα line center. We have attempted to understand the role played by twisted force-free magnetic fields in this event. In the analysis, we have assumed that the helical structures visible in Hα outline the field lines in which prominence tubes are embedded. Untwisting of observed prominence tubes and later, formation of open prominence structures provide evidence of restructuring of the magnetic field configuration over the active region during the course of prominence eruption. Temporal evolution of the force-free parameter α is obtained for two main prominence tubes observed to be intertwined in a rope-like structure. Axial electric currents associated with the prominence tubes are estimated to be of the order of 1011 A which decreased with time. Correspondingly, it is estimated that the rate of energy release was ≈ 1028 erg s−1 during the prominence eruption. Title: On impulsive and gradual optical solar flares. Authors: Bhatnagar, A.; Ambastha, A.; Jain, R. M.; Srivastava, N. Bibcode: 1989sasf.confP.207B Altcode: 1988sasf.conf..207B; 1989IAUCo.104P.207B A comparative study of H-alpha intensity and area development of several flares is presented in an attempt to investigate characteristic features of various types of optical solar flares. Title: The GONG site survey. Authors: Hill, F.; Ambastha, A.; Ball, W.; Duhalde, O.; Farris, D.; Fischer, G.; Hieda, L.; Zhen, Huang; Ingram, B.; Jackson, P.; Jones, H.; Jones, W.; Kennewell, J.; Kunkel, W.; Kupke, R.; Labonte, B.; Leibacher, J.; Libbrecht, K.; Lu, W.; Morrison, L.; Odell, C.; Pallé, P.; Saá, O.; Sousa, E.; Stebbins, T.; Xiao, Suming; GONG Site Survey Team Bibcode: 1988ESASP.286..209H Altcode: The Global Oscillation Network Group (GONG) project is planning to place six observing stations around the world to observe the solar oscillations as continuously as possible. This paper describes the procedures that are being used to select the six sites. The latest results of measurements of cloud cover obtained by networks of 6 (out of 10) radiometers show a duty cycle of over 93%, with the first diurnal sidelobe in the window power spectrum suppressed by a factor of 400. The results are in good agreement with the predictions of a computer model of the expected cloud cover at individual sites. Title: Proceedings of the Indo-US Workshop on Interplanetary Scintillations and Solar Activity Authors: Bhonsle, R. V.; Bhatnagar, A.; Dryer, M.; Alurkar, S. K.; Ambastha, A.; Vats, H. O. Bibcode: 1988issa.conf.....B Altcode: Under the Indo-US sub-commission on science and technology, the Department of Science and technology had approved two projects for the study of solar activity and interplanetary medium in 1987. The objectives of these projects include the study of solar activity using optical technique and solar wind using IPS technique. These proceedings are an attempt to capture some of the highlights of the deliberations of the workshop organised in Udaipur and Ahmedabad during January-February 1988 to have mutual discussions by Indian and American sides on these inter-related projects. Title: Sunspot proper motions in active region NOAA 2372 and its flare activity during SMY period of 1980 April 4-13 Authors: Ambastha, Ashok; Bhatnagar, Arvind Bibcode: 1988JApA....9..137A Altcode: Solar active region NOAA 2372 was observed extensively by the Solar Maximum Mission (SMM) satellite and several ground-based observatories during 1980 April 4 - 13 in the Solar Maximum Year. The authors have studied photospheric and chromospheric observations of this active region together with Marshall Space Flight Center magnetograms and X-ray data from HXIS aboard the SMM satellite. In particular, they discuss the relationship of the flare-productivity with sunspot proper motions and emergence of new regions of magnetic flux in the active region from its birth to its disappearance at the W-limb. Title: Umbral solar flares. Authors: Ambastha, A. Bibcode: 1988BASI...16...84A Altcode: No abstract at ADS Title: Reduction of photoheliograms by IMB-PC controlled digitizer. Authors: Ambastha, Ashok Bibcode: 1988KodOB...9..201A Altcode: A method is described here to determine heliographic coordinates of sunspots on a photoheliogram using an IBM-PC controlled digitizer. Sunspot proper motions and build up of magnetic energy in solar active regions may be studied using this method. We discuss also other applications of this technique. Title: Sunspot Motion in NOAA:2372 and Associated Flare Activity during Solar Maximum Year Period 1980APR4-13 Authors: Ambastha, A.; Bhatnagar, A. Bibcode: 1987BASI...15...19A Altcode: No abstract at ADS Title: Photographic atlas of the solar chromosphere Authors: Ambastha, Ashok; Bhatnagar, A. Bibcode: 1985pasc.book.....A Altcode: 1985QB528.A45...... In this atlas, we present sequences of chromospheric photographs of some typical solar phenomena observed during 1976-84, through a 15-cm aperture refractor in conjunction with a Halle narrow passband H-alpha filter mounted on a 4.5 m solar spar at the Udaipur Solar Observatory, Udaipur, India. The solar photographs are grouped to present characteristics of solar flares, mass ejections, filament activations, etc. The purpose of this photographic atlas is to apprise new entrants, students, and researchers in solar physics, both observational and theoretical, with the variety of intricate active phenomena that occur on the Sun's chromosphere. Title: Stability of a finite disc under the influence of a spherical halo Authors: Ambastha, A. Bibcode: 1984JApA....5..285A Altcode: The stability of finite gaseous disks against large-scale perturbations under the influence of spherical, massive haloes have been studied. A surface-density distribution consistent with the observed spiral-tracer profiles in disk galaxies is considered for the disk. It is found that growing eigenmodes with both 'trailing' and 'leading' spirals exist in 'cold' disks for a wide range of values of the halo mass and its radius. The amplification rates of the unstable modes reduce as the ratio of the mass of the halo to the mass of the disk is increased. A uniform halo is not very effective towards stabilizing the disk against these modes. The results from the present study are considered vis-a-vis previous studies on the global modes of self-gravitating disks. Title: On the global density waves in self-gravitating flat disks Authors: Ambastha, A.; Varma, R. K. Bibcode: 1983ApJ...264..413A Altcode: The problem of global stability and structure of a class of disk models, both cold and 'warm', against axisymmetric and nonaxisymmetric perturbations has been reconsidered in the form of an eigenvalue problem involving a triply infinite matrix. Some large wavelength (or open), nonaxisymmetric modes are found to stabilize in centrally concentrated disks. Unstable 'leading' modes of cold disks become, gradually, 'trailing' as the thermal energy of the disk is increased. The growth rates of short wavelength modes are lowered significantly by pressure effects in warm disks. Title: Global stability of disk-bulge systems - Spiral structure of disk galaxies Authors: Ambastha, A.; Varma, R. K. Bibcode: 1982JApA....3..125A Altcode: The spiral arms of disk galaxies are very sensitive to various morphological properties, such as, the gas content, the disk-to-bulge ratio, etc. Here, the stability of self-gravitating annular disks surrounding the central rigid bulge component has been studied in order to explain the transition from the tight spiral arms in Sa galaxies to rather open patterns in Sc galaxies as the central amorphous component diminishes. Smooth spiral patterns are found associated with the dominant (or the fastest growing) modes of the system. When the disk-to-bulge mass ratio is small, a tight pattern results restricted to the inner regions of the disk. This pattern opens up and occupies larger disk areas as the disk component becomes comparable to the bulge. It is found here that the 'explosive' instabilities of the global density waves do not occur in the presence of a massive bulge. The growth-rates of the eigenmodes decrease as the disk-to-bulge mass ratio decreases. It is also found that unstable modes of the annular disk can be suppressed by increasing the thermal pressure sufficiently. Title: Global spiral waves in disk-bulge systems. Authors: Ambastha, A.; Varma, R. K. Bibcode: 1982BASI...10...39A Altcode: No abstract at ADS Title: Global density waves in self gravitating flat disks. Authors: Ambastha, A.; Varma, R. K. Bibcode: 1982BASI...10...40A Altcode: No abstract at ADS Title: Black Holes and Astrophysics: Compilation of Vikram Sarabhai Memorial Lectures of Prof. S. Chandrasekhar Authors: Ambastha, A.; Mohan, M. Bibcode: 1982bhac.book.....A Altcode: No abstract at ADS Title: A Study on the Dynamics of Gravitational Systems Authors: Ambastha, Ashok Bibcode: 1981PhDT........95A Altcode: The origin and maintenance of spiral structures, one of the most striking morphological properties of many galaxies, has been studied in this thesis. While the version of the density wave theory as proposed by Lin and his coworkers has explained many observational facts, it appears to be incomplete and unsatisfactory on many counts. In order to maintain the spiral density waves against various damping mechanisms it is necessary to find methods which would contribute to the excitation of the density waves in the galactic disks. ....A more complete global theory of spiral patterns as the allowed, eigenfunctions of a self-gravitating disk would require a self-consistent solution as an eigenvalue problem with proper boundary conditions. We have carried out such studies for various disk models with solid body as well as differential rotation. It is found that spiral patters arise naturally in differentially rotating disks. Title: Astrophysical Problems-A dynamical Approach: Compilation of Lectures of Prof. I.P. Williams Authors: Ambastha, A.; Anandarao, B. G.; Chandrasekhar, T. Bibcode: 1981apda.conf.....A Altcode: This material is a compilation of the notes of lectures delivered by Prof. I.P. Williams, Queen Mary College, University of London, in March-April 1981 at the Physical Research Laboratory, Ahmedabad. Title: Global spiral density waves in disk galaxies. Authors: Ambastha, A.; Verma, R. K. Bibcode: 1981BASI....9...80A Altcode: No abstract at ADS Title: Plasma Physics and Astrophysics: Compilation of Lectures of Prof. D. ter Haar Authors: Mohan, M.; Ambastha, A. Bibcode: 1978ppa..conf.....M Altcode: The following notes are based on the lectures delivered by Professor D. ter Haar during his stay at the Physical Research Laboratory, Ahmedabad, India, on the occassion of the Plasma Physics Symposium held in November 1978. Title: Accretion-Induced Overstability of Density Waves in a Self-Gravitating Disk Authors: Ambastha, A.; Varma, R. K. Bibcode: 1978Ap&SS..55..459A Altcode: A two-component differentially rotating disk of self-gravitating particles is considered in the hydrodynamical framework. This system is shown to sustain two pairs of density waves, corresponding to the familiar Jeans modes and an acoustic type of modes (similar to ion-acoustic modes in plasmas). As a result of mass and momentum transfer from the gaseous to the stellar component (an accretion process), the acoustic modes suffer a strong damping, whereas the Jeans modes which were oscillatory, now become overstable provided the thermal velocity of stars is larger than that of gaseous component. The waves with frequencies near the corotation have a rather large growth rate. This amplification can explain the maintenance of spiral structure and a ‘selective’ amplification could even determine the wave-frequency (or pattern velocity). Title: Gravitational Kinetic Theory : Compilation of Lectures of Prof. George Severne) Authors: Ambastha, A. Bibcode: 1977gkt..rept.....A Altcode: No abstract at ADS