arXiv subjects

Aveek Sarkar

Publications and source records attributed to Aveek Sarkar.

26 records · Page 2Linked to original sources

arXiv2018

Observation and Modeling of Chromospheric Evaporation in a Coronal Loop Related to Active Region Transient Brightening

Using the observations recorded by Atmospheric Imaging Assembly (AIA) on-board the Solar Dynamics Observatory (SDO), the Interface Region Imaging Spectrograph (IRIS) and the Extreme-ultraviolet Imaging Spectrometer (EIS) and X-Ray Telescope (XRT) both on-board Hinode, we present the evidence of chromospheric evaporation in a coronal loop after the occurrence of two active region transient brightenings (ARTBs) at the two footpoints. The chromospheric evaporation started nearly simultaneously in all the three hot channels of AIA such as 131~Å, 94~Å and 335~Å, which was observed to be temperature dependent, being fastest in the highest temperature channel. The whole loop became fully brightened following the ARTBs after $\approx25$~s in 131~Å, $\approx 40$~s in 94~Å, and $\approx 6.5$~min in 335~Å. The DEM measurements at the two footpoints (i.e., of two ARTBs) and the loop-top suggest that the plasma attained a maximum temperature of $\sim$10~MK at all these locations. The spectroscopic observations from IRIS revealed the presence of redshifted emission of $\sim$20~km~s$^{-1}$ in cooler lines like \ion{C}{2} and \ion{Si}{4} during the ARTBs that was co-temporal with the evaporation flow at the footpoint of the loop. During the ARTBs, the line width of \ion{C}{2} and \ion{Si}{4} increased nearly by a factor of two during the peak emission. Moreover, enhancement in the line width preceded that in the Doppler shift which again preceded enhancement in the intensity. The observed results were qualitatively reproduced by 1-D hydrodynamic simulations where energy was deposited at both the footpoints of a monolithic coronal loop that mimicked the ARTBs identified in the observations.

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arXiv2017

Simulating coronal loop implosion and compressible wave modes in a flare hit active region

There is considerable observational evidence of implosion of magnetic loop systems inside solar coronal active regions following high energy events like solar flares. In this work, we propose that such collapse can be modeled in three dimensions quite accurately within the framework of ideal magnetohydrodynamics. We furthermore argue that the dynamics of loop implosion is only sensitive to the transmitted disturbance of one or more of the system variables, e.g. velocity generated at the event site. This indicates that to understand loop implosion, it is sensible to leave the event site out of the simulated active region. Towards our goal, a velocity pulse is introduced to model the transmitted disturbance generated at the event site. Magnetic field lines inside our simulated active region are traced in real time, and it is demonstrated that the subsequent dynamics of the simulated loops closely resemble observed imploding loops. Our work highlights the role of plasma $β$ in regards to the rigidity of the loop systems and how that might affect the imploding loops' dynamics. Compressible magnetohydrodynamic modes such as kink and sausage are also shown to be generated during such processes, in accordance with observations.

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arXiv2015

Modeling solar coronal bright point oscillations with multiple nanoflare heated loops

Intensity oscillations of coronal bright points (BPs) have been studied for past several years. It has been known for a while that these BPs are closed magnetic loop like structures. However, initiation of such intensity oscillations is still an enigma. There have been many suggestions to explain these oscillations, but modeling of such BPs have not been explored so far. Using a multithreaded nanoflare heated loop model we study the behavior of such BPs in this work. We compute typical loop lengths of BPs using potential field line extrapolation of available data (Chandrashekhar et al. 2013), and set this as the length of our simulated loops. We produce intensity like observables through forward modeling and analyze the intensity time series using wavelet analysis, as was done by previous observers. The result reveals similar intensity oscillation periods reported in past observations. It is suggested these oscillations are actually shock wave propagations along the loop. We also show that if one considers different background subtractions, one can extract adiabatic standing modes from the intensity time series data as well, both from the observed and simulated data.

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arXiv2011

A pre-outburst signal in the long term optical lightcurve of the recurrent nova RS Ophiuchi

Recurrent novae are binary stars in which a white dwarf accretes matter from a less evolved companion, either a red giant or a main-sequence star. They have dramatic optical brightenings of around 5-6 mag in V in less than a day, several times a century. These occur at variable and unpredictable intervals, and are followed by an optical decline over several weeks, and activity from the X-ray to the radio. The unpredictability of recurrent novae and related stellar types can hamper systematic study of their outbursts. Here we analyse the long-term lightcurve of RS Ophiuchi, a recurrent nova with six confirmed outbursts, most recently in 2006 February. We confirm the previously suspected 1945 outburst, largely obscured in a seasonal gap. We also find a signal via wavelet analysis that can be used to predict an incipient outburst up to a few hundred days before hand. This has never before been possible. In addition this may suggest that the preferred thermonuclear runaway mechanism for the outbursts will have to be modified, as no pre-outburst signal is anticipated in that case. If our result indeed points to gaps in our understanding of how outbursts are driven, we will need to study such objects carefully to determine if the white dwarf is growing in mass, an essential factor if these systems are to become Type Ia Supernovae. Determining the likelihood of recurrent novae being an important source population will have implications for stellar and galaxy evolution.

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arXiv2009

EUV Observational consequences of the spatial localisation of nanoflare heating within a multi-stranded atmospheric loop

Determining the preferred spatial location of the energy input to solar coronal loops would be an important step forward towards a more complete understanding of the coronal heating problem. Following on from Sarkar & Walsh (2008) this paper presents a short 10e9 cm "global loop" as 125 individual strands, where each strand is modelled independently by a one-dimensional hydrodynamic simulation. The strands undergo small-scale episodic heating and are coupled together through the frequency distribution of the total energy input to the loop which follows a power law distribution with index ~ 2.29. The spatial preference of the swarm of heating events from apex to footpoint is investigated. From a theoretical perspective, the resulting emission measure weighted temperature profiles along these two extreme cases does demonstrate a possible observable difference. Subsequently, the simulated output is folded through the TRACE instrument response functions and a re-derivation of the temperature using different filter-ratio techniques is performed. Given the multi-thermal scenario created by this many strand loop model, a broad differential emission measure results; the subsequent double and triple filter ratios are very similar to those obtained from observations. However, any potential observational signature to differentiate between apex and footpoint dominant heating is possibly below instrumental thresholds. The consequences of using a broadband instrument like TRACE and Hinode-XRT in this way are discussed.

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arXiv2008

Hydrodynamic Simulation of a nano-flare heated multi-strand solar atmospheric loop

There is a growing body of evidence that the plasma loops seen with current instrumentation (SOHO, TRACE and Hinode) may consist of many sub-resolution elements or strands. Thus, the overall plasma evolution we observe in these features could be the cumulative result of numerous individual strands undergoing sporadic heating. This paper presents a short (10^9 cm ~ 10 Mm) ``global loop'' as 125 individual strands where each strand is modelled independently by a one-dimensional hydrodynamic simulation. The energy release mechanism across the strands consists of localised, discrete heating events (nano-flares). The strands are ``coupled'' together through the frequency distribution of the total energy input to the loop which follows a power law distribution with index alpha. The location and lifetime of each energy event occurring is random. Although a typical strand can go through a series of well-defined heating/cooling cycles, when the strands are combined, the overall quasi-static emission measure weighted thermal profile for the global loop reproduces a hot apex/cool base structure. Localised cool plasma blobs are seen to travel along individual strands which could cause the loop to `disappear' from coronal emission and appear in transition or chromospheric ones. As alpha increases (from 0 to 2.29 to 3.29), more weight is given to the smallest heating episodes. Consequently, the overall global loop apex temperature increases while the variation of the temperature around that value decreases. Any further increase in alpha saturates the loop apex temperature variations at the current simulation resolution. The effect of increasing the number of strands and the loop length as well as the implications of these results upon possible future observing campaigns for TRACE and Hinode are discussed.

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arXiv2002

One Parameter Solution of Spherically Symmetric Accretion in Various Pseudo-Schwarzschild Potentials

In this paper we have solved the hydrodynamic equations governing the spherically symmetric isothermal accretion (wind) onto (away from) compact objects using various pseudo-Schwarzschild potentials.These solutions are essentially one parameter solutions in a sense that all relevant dynamical as well as thermodynamic quantities for such a flow could be obtained (with the assumption of a one-temperature fluid) if {\it only one} flow parameter (temperature of the flow $T$) is given. Also we have investigated the transonic behaviour of such a flow and showed that for a given $T$, transitions from subsonic to the supersonic branch of accretion (wind) takes place at different locations depending on the potentials used to study the flow and we have identified these transition zones for flows in various such potentials.

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arXiv2002

Pseudo-Schwarzschild description of transonic spherical accretion onto compact objects

A number of `modified' Newtonian potentials of various forms are available in the literature which accurately approximate some general relativistic effects important for studying accretion discs around a Schwarzschild black hole. Such potentials may be called `pseudo-Schwarzschild' potentials because they nicely mimic the space-time around a non-rotating/slowly rotating compact object. In this paper, we examine the validity of the application of some of these potentials to study the spherically symmetric, transonic, hydrodynamic accretion onto a Schwarzschild black hole. By comparing the values of various dynamical and thermodynamic accretion parameters obtained for flows using these potentials with full general relativistic calculations, we have shown that though the potentials discussed in this paper were originally proposed to mimic the relativistic effects manifested in disc accretion, it is quite reasonable to use most of the potentials in studying various dynamical as well as thermodynamic quantities for spherical accretion to compromise between the ease of handling of a Newtonian description of gravity and the realistic situations described by complicated general relativistic calculations. Also we have shown that depending on the chosen regions of parameter space spanned by specific energy ${\cal E}$ and adiabatic index $γ$ of the flow, one potential may have more importance than another and we could identify which potential is the best approximation for full general relativistic flow in Scwarzschild space-time for particular values of ${\cal E}$ and $γ$.

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