Searcharxiv⌕ Search

arXiv subjects

Prasad Subramanian

Publications and source records attributed to Prasad Subramanian.

At least 55 records · Page 3Linked to original sources
arXiv2007

Electron acceleration in a post-flare decimetric continuum source

Aims: To calculate the power budget for electron acceleration and the efficiency of the plasma emission mechanism in a post-flare decimetric continuum source. Methods: We have imaged a high brightness temperature ($\sim 10^{9}$K) post-flare source at 1060 MHz with the Giant Metrewave Radio Telescope (GMRT). We use information from these images and the dynamic spectrum from the Hiraiso spectrograph together with the theoretical method described in Subramanian & Becker (2006) to calculate the power input to the electron acceleration process. The method assumes that the electrons are accelerated via a second-order Fermi acceleration mechanism. Results: We find that the power input to the nonthermal electrons is in the range $3\times 10^{25}$--$10^{26}$ erg/s. The efficiency of the overall plasma emission process starting from electron acceleration and culminating in the observed emission could range from $2.87\times 10^{-9}$ to $2.38 \times 10^{-8}$.

astro-ph↗
arXiv2007

Energetics of solar coronal mass ejections

Aims: We investigate whether solar coronal mass ejections are driven mainly by coupling to the ambient solar wind or through the release of internal magnetic energy. Methods: We examine the energetics of 39 flux-rope like coronal mass ejections (CMEs) from the Sun using data in the distance range $\sim$ 2--20 $R_{\odot}$ from the Large Angle Spectroscopic Coronograph (LASCO) aboard the Solar and Heliospheric Observatory (SOHO). This comprises a complete sample of the best examples of flux-rope CMEs observed by LASCO in 1996-2001. Results: We find that 69% of the CMEs in our sample experience a clearly identifiable driving power in the LASCO field of view. For the CMEs that are driven, we examine if they might be deriving most of their driving power by coupling to the solar wind. We do not find conclusive evidence in favor of this hypothesis. On the other hand, we find that their internal magnetic energy is a viable source of the required driving power. We have estimated upper and lower limits on the power that can possibly be provided by the internal magnetic field of a CME. We find that, on average, the lower limit to the available magnetic power is around 74% of what is required to drive the CMEs, while the upper limit can be as much as an order of magnitude larger.

astro-ph↗
arXiv2007

Electron acceleration in solar noise storms

We present an up-to-date review of the physics of electron acceleration in solar noi se storms. We describe the observed characteristics of noise storm emission, emphasi zing recent advances in imaging observations. We briefly describe the general method ology of treating particle acceleration problems and apply it to the specific proble m of electron acceleration in noise storms. We dwell on the issue of the efficiency of the overall noise storm emission process and outline open problems in this area.

astro-ph↗
arXiv2006

Further constraints on electron acceleration in solar noise storms

We reexamine the energetics of nonthermal electron acceleration in solar noise storms. A new result is obtained for the minimum nonthermal electron number density required to produce a Langmuir wave population of sufficient intensity to power the noise storm emission. We combine this constraint with the stochastic electron acceleration formalism developed by Subramanian & Becker (2005) to derive a rigorous estimate for the efficiency of the overall noise storm emission process, beginning with nonthermal electron acceleration and culminating in the observed radiation. We also calculate separate efficiencies for the electron acceleration -- Langmuir wave generation stage and the Langmuir wave -- noise storm production stage. In addition, we obtain a new theoretical estimate for the energy density of the Langmuir waves in noise storm continuum sources.

astro-ph↗
arXiv2005

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

We report first results from an ongoing program of combining visibilities from the Giant Meterwave Radio Telescope (GMRT) and the Nancay 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.

astro-ph↗
arXiv2005

Cosmic ray electron acceleration in the remnant of nova GK Persei?

Radio observations of the nova remnant of GK Per around 1985 (epoch 1) have shown it to have a spectral index of -0.4 (S propto nu^alpha) at frequencies <1.4 GHz and a spectral index of -0.7 at frequencies >1.4 GHz (Seaquist et al 1989, Biermann et al 1995). This spectrum is either due to secondary electrons from p-p interactions (Biermann et al 1995) or due to the presence of at least two electron populations which dominate the emission at different frequencies. Our radio observations (Anupama & Kantharia 2005) of the remnant of GK Persei using the Giant Metrewave Radio Telescope (GMRT) and archival Very Large Array (VLA) data show that the nova remnant has a low frequency (<1.3 GHz) spectral index alpha ~ -0.85 (in 2003; epoch 3) and a high frequency spectral index of alpha ~ -0.7 (1997; epoch 2). Moreover the higher frequencies show an annual secular decrease of ~ 2.1 % between epochs 1 and 2 whereas there is no change at 0.33 GHz. This suggests a significant evolution of the spectrum over 20 years. In this paper, we discuss a possible physical basis for this spectral evolution based on a comparison of the synchrotron loss and electron acceleration timescales.

astro-ph↗
arXiv2005

Using transport coefficients of cosmic rays in turbulent magnetic fields to determine hybrid viscosity in hot accretion disks around AGN

The nature of the viscosity operative in hot, two-temperature accretion disks around AGN has been a long-standing, unsolved problem. It has been previously suggested that protons, in conjunction with the turbulent magnetic field that is likely to exist in the accretion disk, might be crucial in providing this viscosity. Several authors have recently determined diffusion coefficients for charged particles (cosmic rays) propagating in turbulent magnetic fields by means of extensive Monte Carlo simulations. We use the diffusion coefficients for protons determined by these simulations to find the effective mean free path for protons in hot accretion disks. This in turn yields good estimates of the viscosity due to energetic protons embedded in the turbulent magnetic field of a hot, two-temperature accretion disk. We combine this with a simple two-temperature accretion disk model to determine the Shakura-Sunyaev $α$ viscosity parameter arising out of this mechanism. We find that protons diffusing in the turbulent magentic field embedded in a hot accretion d isk provide a physically reasonable source of viscosity in hot accretion disks around AGN.

astro-ph↗
arXiv2005

Energetics of Coronal Mass Ejections

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.

astro-ph↗
arXiv2004

Restrictions on the Physical Prescription for the Viscosity in Advection-Dominated Accretion Disks

It has recently been demonstrated that the Shakura-Sunyaev prescription for the kinematic viscosity in an advection-dominated accretion disk yields physically reasonable solutions for the structure of the inflow close to the event horizon. In particular, no violations of relativistic causality occur at the horizon. This is somewhat surprising considering the diffusive nature of the angular momentum transport in the Shakura-Sunyaev scenario, and it is therefore natural to ask whether one can also obtain acceptable solutions for the disk structure based on the various alternative models for the viscosity that have been proposed, including the "deterministic" forms. In this paper we perform a rigorous asymptotic analysis of the structure of an advection-dominated accretion disk close to the event horizon of a nonrotating black hole based on three of the alternative prescriptions for the viscosity that have been suggested in the literature. We constrain the physical disk model by stipulating that the stress must vanish at the horizon, which is the fundamental inner boundary condition imposed by general relativity. Surprisingly, we find that none of the three alternative viscosity prescriptions yield physically acceptable disk structures close to the horizon when the zero-torque condition is applied, whether the flow is in vertical hydrostatic equilibrium or free-fall. Hence we conclude that the original Shakura-Sunyaev prescription is the only one proposed so far that is physically consistent close to the event horizon.

astro-ph↗
arXiv2004

Angular momentum transport in accretion disks

We reexamine arguments advanced by Hayashi & Matsuda (2001), who claim that several simple, physically motivated derivations based on mean free path theory for calculating the viscous torque in a quasi-Keplerian accretion disk yield results that are inconsistent with the generally accepted model. If correct, the ideas proposed by Hayashi & Matsuda would radically alter our understanding of the nature of the angular momentum transport in the disk, which is a central feature of accretion disk theory. However, in this paper we point out several fallacies in their arguments and show that there indeed exists a simple derivation based on mean free path theory that yields an expression for the viscous torque that is proportional to the radial derivative of the angular velocity in the accretion disk, as expected. The derivation is based on the analysis of the epicyclic motion of gas parcels in adjacent eddies in the disk.

astro-ph↗
arXiv2004

Noise storm continua: power estimates for electron acceleration

We use a generic stochastic acceleration formalism to examine the power $L_{\rm in}$ (${\rm erg s^{-1}}$) 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 $L_{\rm in}$ with the power $L_{\rm out}$ observed in noise storm radiation. Using typical values for the various parameters, we find that $L_{\rm in} \sim 10^{23-26}$ ${\rm erg s^{-1}}$, yielding an efficiency estimate $η\equiv L_{\rm out}/L_{\rm in}$ in the range $10^{-10} \lsim η\lsim 10^{-6}$ 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.

astro-ph↗
arXiv2003

Giant Meterwave Radio Telescope observations of an M2.8 flare: insights into the initiation of a flare-coronal mass ejection event

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.

astro-ph↗
arXiv2001

Are the hotspots of radio galaxies the sites of in-situ acceleration of relativistic particles?

Using a large set of optically detected hotspots in powerful extragalactic double radio sources, we examine the basic question of whether the detection of optical synchrotron radiation requires in-situ acceleration of relativistic electrons within the hotspots/lobes. For this, we take into account the jet's bulk relativistic motion, as well as its likely misalignment from the plane of the sky. Together, both these factors can drastically reduce the apparent range of the ultra-relativistic electrons ejected from the nucleus in the form of a jet. The conventionally adopted parameter space for the fundamental variables, namely, the hotspot magnetic field, radio source orientation angle relative to the line-of-sight and the bulk speed of the jet plasma, is considered. We find that the observed optical/near-IR synchrotron emission of the hotspots can be explained even if the radiating relativistic electrons were accelerated exclusively within the nuclear region, provided the energy losses incurred by the electrons during their transport down the jet are dominated by inverse Compton upscatterings of the cosmic microwave background photons. Under this circumstance, in-situ acceleration of relativistic electrons inside the hotspots or lobes is not found to be mandated by their reported optical/near-infrared detections.

astro-ph↗
arXiv2001

Source Regions of Coronal Mass Ejections

Observations of the solar corona with the Large Angle Spectrometric Coronograph (LASCO) and Extreme ultraviolet Imaging Telescope (EIT) instruments on the Solar and Heliospheric Observatory (SOHO) provide an unprecedented opportunity to study coronal mass ejections (CMEs) from their initiation through their evolution out to 30 \rsun. The objective of this study is to gain an understanding of the source regions from which the CMEs emanate. To this end, we have developed a list of 32 CMEs whose source regions are located on the solar disk and are well observed in EIT 195 Å data during the period from so lar minimum in January 1996 through the rising part of the cycle in May 1998. We compare the EIT source regions with photospheric magnetograms from the Michelson Doppler Imager (MDI) instrument on SOHO and the NSO/Kitt Peak Observatory and also with H$α$ data from various sources. The overall results of our study show that 41% of the CME related transients observed are associated with active regions and have no prominence eruptions, 44% are associated with eruptions of prominences embedded in active regions and 15% are associated with eruptions of prominences outside active regions. Those CMEs that do not involve prominence eruptions originate in active regions both with and without prominences. We describe 6 especially well observed events. These case studies suggest that active region CMEs (without eruptive prominences) are associated with active regions with lifetimes between 11--80 days. They are also often associated with small scale emerging or cancelling flux over timescales of 6--7 hours. CMEs associated with active region prominence eruptions, on the other hand, are typically associated with old active regions with lifetimes $\sim$ 6-7 months.

astro-ph↗
arXiv2001

Relativistic Outflows from Advection-Dominated Accretion Disks around Black Holes

The Newtonian ADAF model has been generalized recently to obtain the ADIOS model that includes outflows of energy and angular momentum. The utilization of a Newtonian gravitational potential limits the ability of the ADIOS model to describe the inner region of the disk, where any relativistic outflows are likely to originate. In this paper we modify the ADIOS scenario to incorporate a pseudo-Newtonian potential, which approximates the effects of general relativity. The analysis yields a unique, self-similar solution for the structure of the coupled disk/wind system. Interesting features of the new solution include the relativistic character of the outflow in the vicinity of the radius of marginal stability, which may help to explain the formation of relativistic jets in active galaxies. The unique density variation obtained agrees with that implied by the dependence of the X-ray hard time lags on the Fourier frequency for a number of accreting galactic black hole candidates.

astro-ph↗
arXiv1999

LASCO Measurements of the Energetics of Coronal Mass Ejections

We examine the energetics of Coronal Mass Ejections (CMEs) with data from the LASCO coronagraphs on SOHO. The LASCO observations provide fairly direct measurements of the mass, velocity and dimensions of CMEs. Using these basic measurements, we determine the potential and kinetic energies and their evolution for several CMEs that exhibit a flux-rope morphology. Assuming flux conservation, we use observations of the magnetic flux in a variety of magnetic clouds near the Earth to determine the magnetic flux and magnetic energy in CMEs near the Sun. We find that the potential and kinetic energies increase at the expense of the magnetic energy as the CME moves out, keeping the total energy roughly constant. This demonstrates that flux rope CMEs are magnetically driven. Furthermore, since their total energy is constant, the flux rope parts of the CMEs can be considered to be a closed system above $\sim$ 2 $R_{\sun}$.

astro-ph↗
arXiv1999

Inner region accretion flows onto black holes

We examine here the inner region accretion flows onto black holes. A variety of models are presented. We also discuss viscosity mechanisms under a variety of circumstances, for standard accretion disks onto galactic black holes and supermassive black holes and hot accretion disks. Relevant work is presented here on unified aspects of disk accretion onto supermassive black holes and the possible coupling of thick disks to beams in the inner regions. We also explore other accretion flow scenarios. We conclude that a variety of scenarios yield high temperatures in the inner flows and that viscosity is likely not higher than alpha $\sim$ 0.01.

astro-ph↗