arXiv subjects

Rohit Sharma

Publications and source records attributed to Rohit Sharma.

46 records · Page 3Linked to original sources

arXiv2020

Study of the spatial association between an active region jet and a nonthermal type~${\rm III}$ radio burst

We aim to investigate the spatial location of the source of an active region (AR) jet and its relation with associated nonthermal type~III radio emission. An emission measure (EM) method was used to study the thermodynamic nature of the AR jet. The nonthermal type~{\rm III} radio burst observed at meterwavelength was studied using the Murchison Widefield Array (MWA) radio imaging and spectroscopic data. The local configuration of the magnetic field and the connectivity of the source region of the jet with open magnetic field structures was studied using a nonlinear force-free field (NLFFF) extrapolation and potential field source surface (PFSS) extrapolation respectively. The plane-of-sky velocity of the AR jet was found to be $\sim$136~km/s. The EM analysis confirmed the presence of low temperature 2~MK plasma for the spire, whereas hot plasma, between 5-8 MK, was present at the footpoint region which also showed the presence of Fe~{\sc xviii} emission. A lower limit on the electron number density was found to be 1.4$\times$10$^{8}$ cm$^{-3}$ for the spire and 2.2$\times$10$^{8}$~cm$^{-3}$ for the footpoint. A temporal and spatial correlation between the AR jet and nonthermal type III burst confirmed the presence of open magnetic fields. An NLFFF extrapolation showed that the photospheric footpoints of the null point were anchored at the location of the source brightening of the jet. The spatial location of the radio sources suggests an association with the extrapolated closed and open magnetic fields although strong propagation effects are also present. The multi-scale analysis of the field at local, AR, and solar scales confirms the interlink between different flux bundles involved in the generation of the type III radio signal with flux transferred from a small coronal hole to the periphery of the sunspot via null point reconnection with an emerging structure.

astro-ph.SR↗

arXiv2020

Radio and X-ray Observations of Short-lived Episodes of Electron Acceleration in a Solar Microflare

Solar flares are sudden energy release events in the solar corona, resulting from magnetic reconnection, that accelerates particles and heats the ambient plasma. During a flare, there are often multiple, temporally and spatially separated individual energy release episodes that can be difficult to resolve depending on the observing instrument. We present multi-wavelength imaging and spectroscopy observations of multiple electron acceleration episodes during a GOES B1.7-class two-ribbon flare on 2012 February 25, observed simultaneously with the Karl G. Jansky Very Large Array (VLA) at 1--2 GHz, the Reuven Ramatay High Energy Solar Spectroscopic Imager (RHESSI) in X-rays, and the Solar Dynamics Observatory in extreme ultraviolet (EUV). During the initial phase of the flare, five radio bursts were observed. A nonthermal X-ray source was seen co-temporal, but not co-spatial, with the first three radio bursts. Their radio spectra are interpreted as optically thick gyrosynchrotron emission. By fitting the radio spectra with a gyrosynchrotron model, we derive the magnetic field strength and nonthermal electron spectral parameters in each acceleration episode. Notably, the nonthermal parameters derived from X-rays differ considerably from the nonthermal parameters inferred from the radio. The observations are indicative of multiple, co-temporal acceleration episodes during the impulsive phase of a solar microflare. The X-ray and radio burst sources likely originate from separate electron distributions in different magnetic loops.

astro-ph.SR↗

arXiv2019

Possible glass-like random singlet magnetic state in 1T-TaS2

Two-dimensional layered transition-metal-dichalcogenide compound 1T-TaS2 shows the rare coexistence of charge density wave (CDW) and electron correlation driven Mott transition. In addition, atomic-cluster spins on the triangular lattice of the CDW state of 1T-TaS2 give rise to the possibility of the exotic spin-singlet state in which quantum fluctuations of spins are strong enough to prevent any long range magnetic ordering down to absolute zero ( 0 K). We present here the evidences of a glass-like random singlet magnetic state in 1T-TaS2 at low temperatures through a study of temperature and time dependence of magnetization. Comparing the experimental results with a representative canonical spin-glass system Au(1.8%Mn), we show that this glass-like state is distinctly different from the well established canonical spin-glass state.

cond-mat.str-el↗

arXiv2019

BioGen: Automated Biography Generation

A biography of a person is the detailed description of several life events including his education, work, relationships, and death. Wikipedia, the free web-based encyclopedia, consists of millions of manually curated biographies of eminent politicians, film and sports personalities, etc. However, manual curation efforts, even though efficient, suffers from significant delays. In this work, we propose an automatic biography generation framework BioGen. BioGen generates a short collection of biographical sentences clustered into multiple events of life. Evaluation results show that biographies generated by BioGen are significantly closer to manually written biographies in Wikipedia. A working model of this framework is available at nlpbiogen.herokuapp.com/home/.

cs.DL↗

arXiv2017

Opportunistic Self Organizing Migrating Algorithm for Real-Time Dynamic Traveling Salesman Problem

Self Organizing Migrating Algorithm (SOMA) is a meta-heuristic algorithm based on the self-organizing behavior of individuals in a simulated social environment. SOMA performs iterative computations on a population of potential solutions in the given search space to obtain an optimal solution. In this paper, an Opportunistic Self Organizing Migrating Algorithm (OSOMA) has been proposed that introduces a novel strategy to generate perturbations effectively. This strategy allows the individual to span across more possible solutions and thus, is able to produce better solutions. A comprehensive analysis of OSOMA on multi-dimensional unconstrained benchmark test functions is performed. OSOMA is then applied to solve real-time Dynamic Traveling Salesman Problem (DTSP). The problem of real-time DTSP has been stipulated and simulated using real-time data from Google Maps with a varying cost-metric between any two cities. Although DTSP is a very common and intuitive model in the real world, its presence in literature is still very limited. OSOMA performs exceptionally well on the problems mentioned above. To substantiate this claim, the performance of OSOMA is compared with SOMA, Differential Evolution and Particle Swarm Optimization.

cs.NE↗

arXiv2017

Quantifying weak non-thermal solar radio emission at low radio frequencies

The recent availability of fine grained high sensitivity data from the new generation low radio frequency instruments such as the Murchison Widefield Array (MWA) have opened up opportunities for using novel techniques for characterizing the nature of solar emission at these frequencies. Here we use this opportunity to look for evidence for the presence of weak non-thermal emissions in the 100-240 MHz band, at levels weaker than have usually been probed. The presence of such features is believed to be a necessary consequence of nanoflare-based coronal and chromospheric heating theories. We separate the calibrated MWA solar dynamic spectra into a slowly varying and an impulsive, and hence non-thermal, component. We demonstrate that Gaussian mixtures modeling can be used to robustly model the latter and we estimate the flux density distribution as well as the prevalence of impulsive non-thermal emission in the frequency-time plane. Evidence for presence of non-thermal emission at levels down to $\sim$0.2 SFU (1 SFU = 10$^4$ Jy) is reported, making them the weakest reported emissions of this nature. Our work shows the fractional occupancy of the non-thermal impulsive emission to lie in the 17--45\% range during a period of medium solar activity. We also find that the flux density radiated in the impulsive non-thermal emission is very similar in strength to that of the slowly varying component, dominated by the thermal bremsstrahlung. Such significant prevalence and strength of the weak impulsive non-thermal emission has not been appreciated earlier.

astro-ph.SR↗

arXiv2017

On the energisation of charged particles by fast magnetic reconnection

We study the role of turbulence in magnetic reconnection, within the framework of magneto-hydrodynamics, using three-dimensional direct numerical simulations. For small turbulent intensity we find that the reconnection rate obeys Sweet-Parker scaling. For large enough turbulent intensity reconnection rate departs significantly from Sweet-Parker behaviour, becomes almost a constant as a function of the Lundquist number. We further study energisation of test-particles in the same setup. We find that the speed of the energised particles obeys a Maxwellian distribution, whose variance also obeys Sweet-Parker scaling for small turbulent intensity but depends weakly on the Lundquist number for large turbulent intensity. Furthermore, the variance is found to increase with the strength of the reconnecting magnetic field.

physics.plasm-ph↗

arXiv2017

Wavelet-Based Characterization of Small-Scale Solar Emission Features at Low Radio Frequencies

Low radio frequency solar observations using the Murchison Widefield Array have recently revealed the presence of numerous weak, short-lived and narrow-band emission features, even during moderately quiet solar conditions. These non-thermal features occur at rates of many thousands per hour in the 30.72 MHz observing bandwidth, and hence, necessarily require an automated approach for their detection and characterization. Here, we employ continuous wavelet transform using a mother Ricker wavelet for feature detection from the dynamic spectrum. We establish the efficacy of this approach and present the first statistically robust characterization of the properties of these features. In particular, we examine distributions of their peak flux densities, spectral spans, temporal spans and peak frequencies. We can reliably detect features weaker than 1 SFU, making them, to the best of our knowledge, the weakest bursts reported in literature. The distribution of their peak flux densities follows a power law with an index of -2.23 in the 12-155 SFU range, implying that they can provide an energetically significant contribution to coronal and chromospheric heating. These features typically last for 1-2 seconds and possess bandwidths of about 4-5 MHz. Their occurrence rate remains fairly flat in the 140-210 MHz frequency range. At the time resolution of the data, they appear as stationary bursts, exhibiting no perceptible frequency drift. These features also appear to ride on a broadband background continuum, hinting at the likelihood of them being weak type-I bursts.

astro-ph.SR↗

arXiv2017

Estimating solar flux density at low radio frequencies using a sky brightness model

Sky models have been used in the past to calibrate individual low radio frequency telescopes. Here we generalize this approach from a single antenna to a two element interferometer and formulate the problem in a manner to allow us to estimate the flux density of the Sun using the normalized cross-correlations (visibilities) measured on a low resolution interferometric baseline. For wide field-of-view instruments, typically the case at low radio frequencies, this approach can provide robust absolute solar flux calibration for well characterized antennas and receiver systems. It can provide a reliable and computationally lean method for extracting parameters of physical interest using a small fraction of the voluminous interferometric data, which can be prohibitingly compute intensive to calibrate and image using conventional approaches. We demonstrate this technique by applying it to data from the Murchison Widefield Array and assess its reliability.

astro-ph.IM↗

arXiv2016

On the Bright Loop Top Emission in Post Eruption Arcades

The observations of post eruption arcades (PEAs) in X-rays and EUV reveal strong localised brightenings at the loop top regions. The origin of these brightenings and their dynamics is not well understood to date. Here, we study the dynamics of PEAs using one-dimensional hydrodynamic modelling with the focus on the the understanding of the formation of localised brightening.Our findings suggest that these brightenings are the result of collisions between the counter-streaming chromospheric evaporation from both the foot points. We further perform forward modelling of the emission observed in simulated results in various spectral lines observed by the Extreme-Ultraviolet Imaging Telescope aboard Hinode. The forward modelled intensities in various spectral lines are in close agreement with a flare observed in December 17, 2006 by EIS.

astro-ph.SR↗