arXiv subjects

Souvik Bose

Publications and source records attributed to Souvik Bose.

24 records · Page 2Linked to original sources

arXiv2019

Characterization and formation of on-disk spicules in the Ca II K and Mg II k spectral lines

We characterize, for the first time, type-II spicules in Ca II K 3934Å using the CHROMIS instrument at the Swedish 1-m Solar Telescope. We find that their line formation is dominated by opacity shifts with the K$_{3}$ minimum best representing the velocity of the spicules. The K$_{2}$ features are either suppressed by the Doppler-shifted K$_{3}$ or enhanced via an increased contribution from the lower layers, leading to strongly enhanced but un-shifted K$_{2}$ peaks, with widening towards the line-core as consistent with upper-layer opacity removal via Doppler-shift. We identify spicule spectra in concurrent IRIS Mg II k 2796Å observations with very similar properties. Using our interpretation of spicule chromospheric line-formation, we produce synthetic profiles that match observations.

astro-ph.SR↗

arXiv2019

Semi-empirical model atmospheres for the chromosphere of the sunspot penumbra and umbral flashes

Context. The solar chromosphere and the lower transition region is believed to play a crucial role in the heating of the solar corona. Models that describe the chromosphere (and the lower transition region), accounting for its highly dynamic and structured character are, so far, found to be lacking. This is partly due to the breakdown of complete frequency redistribution in the chromospheric layers and also because of the difficulty in obtaining complete sets of observations that adequately constrain the solar atmosphere at all relevant heights. Aims. We aim to obtain semi-empirical model atmospheres that reproduce the features of the Mg II h&k line profiles that sample the middle chromosphere with focus on a sunspot. Methods. We use spectropolarimetric observations of the Ca II 8542 A spectra obtained with the Swedish 1-m Solar Telescope (SST) and use NICOLE inversions to obtain semi-empirical model atmospheres for different features in and around a sunspot. These are used to synthesize Mg II h&k spectra using RH1.5D code, which we compare with observations taken with the Interface Region Imaging Spectrograph (IRIS). Results. Comparison of the synthetic profiles with IRIS observations reveals that there are several areas, especially in the penumbra of the sunspot, where most of the observed Mg II h&k profiles are very well reproduced. In addition, we find that supersonic hot downflows, present in our collection of models in the umbra, lead to synthetic profiles that agree well with the IRIS Mg II h&k profiles, with the exception of the line core. Conclusions. We put forward and make available four semi-empirical model atmospheres. Two for the penumbra, reflecting the range of temperatures obtained for the chromosphere, one for umbral flashes, and a model representative of the quiet surroundings of a sunspot. These are available in electronic as well as in table formats.

astro-ph.SR↗

arXiv2018

On the variability of the solar mean magnetic field: contributions from various magnetic features on the surface of the Sun

The solar mean magnetic field (SMMF) is referred to as the disc-averaged line-of-sight (LOS) magnetic field that also reflects the polarity imbalance of the magnetic field on the Sun. The origin of the SMMF has been debated over the past few decades, with one school of thought suggesting that the contribution to the SMMF is mostly due to the large scale magnetic field structure, also termed as the background magnetic field, whereas other and more recent studies have indicated that active regions have a major contribution to the observed SMMF. In this paper, we re-investigate the issue of the origin of the SMMF by decomposing the solar disk into plages, networks, sunspots and background regions, thereby calculating the variation in the observed SMMF due to each of these features. We have used full disk images from SDO/{\it AIA} recorded at 1600~Å\, for earmarking plages, networks and background regions and 4500~Å\, images for separating the sunspots. The LOS fields corresponding to each of these regions are estimated from the co-temporal SDO/{\it HMI} full disk magnetograms. The temporal variation of the SMMF shows a near one-to-one correspondence with that of the background field regions suggesting that they constitute the major component of the observed SMMF. A linear regression analysis based on the coefficient of determination shows that the background field dominates, and accounts for 89\% of the variation in the SMMF whereas the magnetic field from the other features accounts for the rest 11\%.

astro-ph.SR↗

arXiv2018

Role of the background regimes towards the Solar Mean Magnetic Field

The Solar Mean Magnetic Field (SMMF) is generally defined as disc-averaged line-of-sight (LOS) magnetic field on the sun. The role of active regions and the large-scale magnetic field structures (also called the background) has been debated over past few decades to understand whether the origin of SMMF is either due to the active regions or the background. In this paper, we have investigated the contribution of sunspots, plages, network regions and the background towards the SMMF using data from the SDO-AIA \& HMI, and found that 83\% of the SMMF is due to the background whereas the remaining 17\% originates from the active and network regions.

astro-ph.SR↗

arXiv2016

High Precision Full Stokes Spectropolarimetry of the Sun as a star-Instrument design aspects

The magnetic field plays a major role in governing the dynamics of the sun. Many interesting features like sunspots, flares, prominences, and Coronal Mass Ejections (CMEs) occur on its surface due to the dynamics associated with the magnetic fields. The magnetic activity exhibits spatial scales ranging from very fine scale (below the resolution limit of the current largest telescope) to large scale such as sunspots, active regions and the spatial scales as large as the sun itself. While the major efforts in building large telescopes is going on towards the goal of resolving smallest structure possible we propose here to measure the magnetic field on the global scale. For this purpose we propose an instrument to carryout high precision and high accurate spectropolarimetry of sun-as-a-star. In this thesis, we explore various instrumental design aspects that are necessary to make such observations. As part of the design consideration we have analysed a major noise source i.e. seeing induced cross-talk through simulation as well as using the measured scintillation data. Further, we have analysed full disk Stokes images from SOLIS/VSM and SDO/HMI for the purpose of getting a rough idea on the disk averaged Stokes signal level. The instrument design aspects are mainly constrained by the need to modulate the light before it enters the telescope and image the full sun as a point source. Modulating the light before it enters the telescope has the advantage of completely avoiding the instrumental polarization which will enable us to carry out high accurate polarimetric measurements. We have explored in this thesis various concepts of polarization modulator and spectral discriminator and worked out their suitability for the purpose of carrying out high precision and high accurate spectropolarimetry of sun-as-a-star at high spectral resolution.

astro-ph.IM↗

arXiv2014

Investigation of stochastic resonance (SR) near homoclinic bifurcation in a unijunction transistor (UJT) relaxation oscillator

A P-spice simulation followed by an experiment with a unijunction transistor (UJT) has been carried out to investigate stochastic resonance (SR) in which the response of a nonlinear system to a weak periodic input signal is amplified by an optimum level of noise. The experiments were carried out in the vicinity of homoclinic bifurcation and the quantification of SR has been done by normal variance (NV) and signal to noise ratio (SNR) techniques. We have also developed a tentative mathematical model based on the current-voltage characteristic of the UJT and obtained a second order differential equation that was solved using MATLAB to yield a response similar to the one observed experimentally.

nlin.AO↗