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Combining visibilities from the Giant Meterwave Radio Telescope and the Nançay Radio Heliograph: High dynamic range snapshot images of the solar corona at 327 MHz  

Prasad Subramanian   Submitted: 2005-08-31 23:42

We report first results from an ongoing program of combining visibilities from the Giant Meterwave Radio Telescope (GMRT) and the Nançay Radio Heliograph (NRH) to produce composite snapshot images of the sun at meter wavelengths. We describe the data processing, including a specific multi-scale CLEAN algorithm. We present results of a) simulations for two models of the sun at 327 MHz, with differing complexity b) observations of a complex noise storm on the sun at 327 MHz on Aug 27 2002. Our results illustrate the capacity of this method to produce high dynamic range snapshot images when the solar corona has structures with scales ranging from the image resolution of 49'' to the size of the whole sun. We find that we cannot obtain reliable snapshot images for complex objects when the visibilities are sparsely sampled.

Authors: Claude Mercier (1), Prasad Subramanian (2), Alain Kerdraon (1), Monique Pick (1), S. Ananthakrishnan (3), P. Janardhan (4) ((1) Obs. Paris, Meudon, France, (2) IUCAA, Pune, India, (3) NCRA-TIFR, Pune, India, (4) PRL, Ahmedabad, India)
Projects: None

Publication Status: Accepted for publication in Astronomy & Astrophysics
Last Modified: 2005-08-31 23:42
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Energetics of Coronal Mass Ejections  

Prasad Subramanian   Submitted: 2005-02-07 05:51

We examine the energetics of the best examples of flux-rope CMEs observed by LASCO in 1996-2001. We find that 69% of the CMEs in our sample experience a driving power in the LASCO field of view. For these CMEs which are driven, we examine if they might be deriving most of their driving energy by coupling to the solar wind. We do not find conclusive evidence to support this hypothesis. We adopt two different methods to estimate the energy that can possibly be released by the internal magnetic fields of the CMEs. We find that the internal magnetic fields are a viable source of driving power for these CMEs.

Authors: Prasad Subramanian (IUCAA, Pune, India), Angelos Vourlidas (NRL, Washington, DC)
Projects: Soho-LASCO

Publication Status: To appear in proceedings of IAU Symposium 226, Coronal and Stellar Mass Ejections, Sept 13-17 2004, Beijing
Last Modified: 2005-02-07 05:51
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Noise storm continua: power estimates for electron acceleration  

Prasad Subramanian   Submitted: 2005-02-01 22:43

We use a generic stochastic acceleration formalism to examine the power L m in ({ m erg,s-1}) input to nonthermal electrons that cause noise storm continuum emission. The analytical approach includes the derivation of the Green's function for a general second-order Fermi process, and its application to obtain the particular solution for the nonthermal electron distribution resulting from the acceleration of a Maxwellian source in the corona. We compare L m in with the power L m out observed in noise storm radiation. Using typical values for the various parameters, we find that L m in sim 1023-26 { m erg,s-1}, yielding an efficiency estimate eta equiv L m out/L m in in the range 10-10 lsim eta lsim 10-6 for this nonthermal acceleration/radiation process. These results reflect the efficiency of the overall process, starting from electron acceleration and culminating in the observed noise storm emission.

Authors: Prasad Subramanian, Peter A. Becker
Projects: None

Publication Status: 2005, to appear in Solar Physics
Last Modified: 2005-02-01 22:43
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Giant Meterwave Radio Telescope observations of an M2.8 flare: insights into the initiation of a flare-coronal mass ejection event  

Prasad Subramanian   Submitted: 2004-08-26 10:08

We present the first observations of a solar flare with the GMRT. An M2.8 flare observed at 1060 MHz with the GMRT on Nov 17 2001 was associated with a prominence eruption observed at 17 GHz by the Nobeyama radioheliograph and the initiation of a fast partial halo CME observed with the LASCO C2 coronograph. Towards the start of the eruption, we find evidence for reconnection above the prominence. Subsequently, we find evidence for rapid growth of a vertical current sheet below the erupting arcade, which is accompanied by the flare and prominence eruption.

Authors: Prasad Subramanian, S. Ananthakrishnan, P. Janardhan, M. R. Kundu, S. M. White, V. I. Garaimov
Projects: None

Publication Status: Solar Physics, 2003, vol. 218, p. 247
Last Modified: 2004-08-26 10:08
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Noise storm continua: power estimates for electron acceleration  

Prasad Subramanian   Submitted: 2004-08-26 10:03

We use a generic stochastic acceleration formalism to examine the power L m in ({ m erg,s-1}) input to nonthermal electrons that cause noise storm continuum emission. The analytical approach includes the derivation of the Green's function for a general second-order Fermi process, and its application to obtain the particular solution for the nonthermal electron distribution resulting from the acceleration of a Maxwellian source in the corona. We compare L m in with the power L m out observed in noise storm radiation. Using typical values for the various parameters, we find that L m in sim 1023-26 { m erg,s-1}, yielding an efficiency estimate eta equiv L m out/L m in in the range 10-10 lsim eta lsim 10-6 for this nonthermal acceleration/radiation process. These results reflect the efficiency of the overall process, starting from electron acceleration and culminating in the observed noise storm emission.

Authors: Prasad Subramanian, Peter A. Becker
Projects: None

Publication Status: Accepted for publication in Solar Physics
Last Modified: 2004-08-26 10:03
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The Relationship of Coronal Mass Ejections to Streamers  

Prasad Subramanian   Submitted: 1999-12-03 14:02

We have examined images from the Large-Angle and Spectrometric Coronagraph (LASCO) to study the relationship of coronal mass ejections (CMEs) to coronal streamers. We wish to test the suggestion [{em Low}, 1996] that CMEs arise from flux ropes embedded in a streamer erupting and disrupting the streamer. The data span a period of 2 years near Sunspot minimum through a period of increased activity as Sunspot numbers increased. We have used LASCO data from the C2 coronagraph which records Thomson scattered white light from coronal electrons at heights between 1.5 and 6Rs. Maps of the coronal streamers have been constructed from LASCO C2 observations at a height of 2.5Rs at the east and west limbs. We have superposed the corresponding positions of CMEs observed with the C2 coronagraph onto the synoptic maps. We identified the different kinds of signatures CMEs leave on the streamer structure at this height (2.5Rs). We find four types of CMEs with respect to their effect on streamers: 1. CMEs that disrupt the streamer, 2. CMEs that have no effect on the streamer, even though they are related to it, 3. CMEs that create streamer-like structures and 4. CMEs that are latitudinally displaced from the streamer. CMEs in categories 3 and 4 are not related to the streamer structure. This is the most extensive observational study of the relation between CMEs and streamers to date. Previous studies using SMM data have made the general statement that CMEs are mostly associated with streamers and that they frequently disrupt it. However, we find that approximately 35% of the observed CMEs bear no relation to the preexisting streamer, while 46% have no effect on the observed streamer, even though they appear to be related to it. Our conclusions thus differ considerably from those of previous studies.

Authors: Subramanian, P., Dere, K. P., Rich, N. B., and Howard, R. A.
Projects:

Publication Status: 1999, Journal of Geophysical Research, vol. 104, pp. 22,321-22,330
Last Modified: 1999-12-03 14:02
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Further constraints on electron acceleration in solar noise storms
Combining visibilities from the Giant Meterwave Radio Telescope and the Nancay Radio Heliograph: High dynamic range snapshot images of the solar corona at 327 MHz
Energetics of Coronal Mass Ejections
Noise storm continua: power estimates for electron acceleration
Giant Meterwave Radio Telescope observations of an M2.8 flare: insights into the initiation of a flare-coronal mass ejection event
Noise storm continua: power estimates for electron acceleration
The Relationship of Coronal Mass Ejections to Streamers

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