arXiv subjects

Sushree S. Nayak

Publications and source records attributed to Sushree S. Nayak.

9 recordsLinked to original sources

arXiv2026

Investigation of an X-class Flare and Quasi-Periodic Pulsations in NOAA AR 13842 using SDO, Aditya-L1 and Solar Orbiter

We investigate an X9.0 class flare (SOL2024-10-03T12:08) and associated quasi-periodic pulsations (QPPs) in NOAA active region (AR) 13842 using observations from the Solar Dynamics Observatory (SDO), Aditya-L1 and Solar Orbiter (SO). To explore the role of magnetic configuration in the flare, we employ non-force-free-field extrapolation technique for reconstructing a sequence of the coronal magnetic field throughout the flare. The observations reveal the emergence of intricate flare brightenings and ribbons. QPPs are observed with periods of $\approx$ 2 and $\approx$ 6 minutes in EUV, $\approx$ 7 minutes in UV, and $\approx$ 1 minute in X-rays. The extrapolation documents the presence of a flux rope, a sheared arcade, and a hyperbolic flux tube (HFT) enveloping the rope and the arcade. The sequence of the extrapolation illustrates an increase in the twist of the rope as it ascends towards a torus-unstable region, thereby rendering the rope a viable candidate for eruption. The sheared arcade appears to generate electric current through its foot-point's motion, which explains the sigmoidal brightening observed at the flare onset. The motion also seems to drive the non-parallel field lines closer together, heightening gradient in the magnetic field beneath the rope. Consequently, recurrent magnetic reconnection is anticipated within the sheared arcade --- explaining the ascent of the rope with its twist increment. The formation of parallel flare ribbons is attributed to these reconnections. Furthermore, reconnection at the HFT is proposed to lead the remote and quasi-circular ribbons. Crucially, repeated reconnections inside the arcade as well as at the HFT are speculated to cause the QPPs.

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arXiv2026

Magnetic Reconnection at Hyperbolic Flux Tube associated with a Confined Flare in NOAA Active Region 12268

In this paper, we identify the magnetic reconnections at the hyperbolic flux tube (HFT), aided by slipping reconnection at quasi-separatrix layers (QSLs), which are pivotal to the occurrence of a confined M2.1 class flare in NOAA active region 12268. The magnetic field topology before the flare's onset is obtained through a non-force-free-field extrapolation scheme that accommodates a non-zero Lorentz force. A key aspect is the presence of an HFT in the computational domain above the flaring region, along with two QSLs at the lower boundary. To simulate the dynamics of the active region, we conduct a data-constrained magnetohydrodynamics (MHD) simulation initiated by the extrapolated field. The dynamics captured in the simulation document the formation of a current sheet within the HFT configuration, leading to magnetic reconnection at the HFT. Additionally, we observe the slipping motion of the footpoints of the magnetic field lines in the QSLs at the bottom boundary, which indicates the occurrence of slipping reconnection in the QSLs. Importantly, the magnetic reconnection at the HFT is suggested to be the primary driver in the development of the intricate flare brightenings and the flare ribbons.

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arXiv2025

Data-Constrained Magnetohydrodynamics Simulation of a Confined X-class Flare in NOAA Active Region 11166

In this paper, we present a magnetohydrodynamics simulation of NOAA active region 11166 to understand the origin of a confined X-class flare that peaked at 23:23 UT on 2011 March 9. The simulation is initiated with a magnetic field extrapolated from the corresponding photospheric magnetogram using a non-force-free-field extrapolation technique. Importantly, the initial magnetic configuration identifies three-dimensional (3D) magnetic nulls and quasi-separatrix layers (QSLs), which nearly agree with the bright structures appeared in multi-wavelength observations. The Lorentz force associated with the extrapolated field self-consistently generates the dynamics that leads to the magnetic reconnections at the 3D nulls and the QSLs. These reconnections are found to contribute to the pre-flare activities and, ultimately, lead to the development of the flare ribbons. Notably, the anchored spine of the 3D null and the complete absence of flux rope in the flaring region are congruent with the confined nature of the flare. Furthermore, the simulation also suggests the role of reconnections at the 3D null with an open spine in the onset of a jet away from the flaring site.

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arXiv2023

Magnetohydrodynamics simulation of magnetic flux rope formation in a quadrupolar magnetic field configuration

Magnetic flux ropes (MFRs) play an important role in high-energetic events like solar flares and coronal mass ejections in the solar atmosphere. Importantly, solar observations suggest an association of some flaring events with quadrupolar magnetic configurations. However, the formation and subsequent evolution of MFRs in such magnetic configurations still need to be fully understood. In this paper, we present idealized magnetohydrodynamics (MHD) simulations of MFR formation in a quadrupolar magnetic configuration. A suitable initial magnetic field having a quadrupolar configuration is constructed by modifying a three-dimensional (3D) linear force-free magnetic field. The initial magnetic field contains neutral lines, which consist of X-type null points. The simulated dynamics initially demonstrate the oppositely directed magnetic field lines located across the polarity inversion lines (PILs) moving towards each other, resulting in magnetic reconnections. Due to these reconnections, four highly twisted MFRs form over the PILs. With time, the foot points of the MFRs move towards the X-type neutral lines and reconnect, generating complex magnetic structures around the neutral lines, thus making the MFR topology more complex in the quadrupolar configuration than those formed in bipolar loop systems. Further evolution reveals the non-uniform rise of the MFRs. Importantly, the simulations indicate that the pre-existing X-type null points in magnetic configurations can be crucial to the evolution of the MFRs and may lead to the observed brightenings during the onset of some flaring events in the quadrupolar configurations.

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arXiv2023

Coronal Magnetic Field Extrapolation and Topological Analysis of Fine-Scale Structures during Solar Flare Precursors

Magnetic field plays an important role in various solar eruptions like flares, coronal mass ejections, etc. The formation and evolution of characteristic magnetic field topology in solar eruptions are critical problems that will ultimately help us understand the origination of these eruptions in the solar source regions. With the development of advanced techniques and instruments, observations with higher resolutions in different wavelengths and fields of view have provided more quantitative information for finer structures. So it is essential to improve our method to study the magnetic field topology in the solar source regions by taking advantage of high-resolution observations. In this study, we employ a nonlinear force-free field (NLFFF) extrapolation method based on a nonuniform grid setting for an M-class flare eruption event (SOL2015-06-22T17:39) with embedded magnetograms from the Solar Dynamics Observatory (SDO) and the Goode Solar Telescope (GST). The extrapolation results employing the embedded magnetogram for the bottom boundary are obtained by maintaining the native resolutions of the corresponding GST and SDO magnetograms. We compare the field line connectivity with the simultaneous GST/H$α$ and SDO/AIA observations for fine-scale structures associated with precursor brightenings. Then we perform a topological analysis of the field line connectivity corresponding to fine-scale magnetic field structures based on the extrapolation results. The results indicate that by combining the high-resolution GST magnetogram with a larger HMI magnetogram, the derived magnetic field topology is consistent with a scenario of magnetic reconnection among sheared field lines across the main polarity inversion line during solar flare precursors.

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arXiv2022

Spectroscopic and imaging observations of transient hot and cool loops by IRIS and SDO

Coronal loops are basic building blocks of solar atmosphere and are observed on various length scales. However, their formation mechanism is still unclear. In this paper, we present the spectroscopic and imaging observations of small-scale transients and subsequent formation of transient loops. For the purpose, we have utilized the multi-wavelength observations recorded by AIA and IRIS-SJI, along with spectroscopic measurements provided by IRIS. For the photospheric magnetic field data, we obtained line-of-sight magnetogram data provided by HMI. Small-scale transients are simultaneously observed with several EUV and UV passbands of AIA and IRIS-SJI. HMI magnetogram provides evidence of negative flux cancellations beneath these transients. Differential Emission Measure (DEM) analysis shows that one of the transient attains temperature up to 8 MK whereas another one reaches only up to 0.4 MK. These transients further lead to the formation of small-scale loops with similar temperature distributions, and thus termed as hot and cool loops respectively. During the course of events, IRIS slit was rastering the region and thus provided spectroscopic measurements at both transients and associated loops. This enabled us to perform in-depth investigations of hot and cool loops. Using density sensitive O IV line pair, we obtained average electron densities along the hot and cool loop to be $10^{11.2}$ and $10^{10.8}$ cm$^{-3}$ respectively. Energy estimates suggest that flux cancellation can easily power the hot transient whereas is insufficient for cool transient. Life time estimates and magnetic field extrapolation suggest presence of small-scale and fine structures within these loops. Results provide crucial ingredients on the physics of loop formation and involved thermodynamics.

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arXiv2020

Magnetic reconnections in the presence of three-dimensional magnetic nulls and quasi-separatrix layers

Three-dimensional (3D) magnetohydrodynamic simulations are carried out to explore magnetic reconnections in the presence of 3D magnetic nulls and quasi-separatrix layers (QSLs). The initial magnetic fields are created by superposing uniform vertical magnetic fields of two different magnitudes on a linear force-free field. The interior of the numerical box contains two 3D nulls with separatrix domes separated by a quasi-separator (or hyperbolic flux tube) with QSLs. In the first simulation, the uniform vertical field is so large that the nulls are located at low heights and the domes are separate. Initially unbalanced Lorentz forces drive rotational flows that form strong electric currents and strong torsional fan reconnection at the 3D nulls and weak QSL reconnection at the hyperbolic flux tube. Flipping or slipping of field lines is observed in both cases. In the second simulation, with a weaker vertical field and larger domes, the separatrix surfaces meet at the central quasi-separator and their rotation drives stronger QSL reconnection than before.

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arXiv2018

Magnetohydrodynamic simulation of magnetic null-point reconnections in NOAA AR12192 initiated with an extrapolated non-force-free-field

Magnetohydrodynamics of the solar corona is simulated numerically. The simulation is initialized with an extrapolated non-force-free magnetic field using the vector magnetogram of the active region (AR) NOAA 12192 obtained on the solar photosphere. Particularly, we focus on the magnetic reconnections occurring close to a magnetic null-point that resulted in appearance of circular chromospheric flare ribbons on October 24, 2014 around 21:21 UT, after peak of an X3.1 flare. The extrapolated field lines show the presence of the three-dimensional (3D) null near one of the polarity inversion lines---where the flare was observed. In the subsequent numerical simulation, we find magnetic reconnections occurring near the null point, where the magnetic field lines from the fan-plane of the 3D null form a X-type configuration with underlying arcade field lines. The footpoints of the dome-shaped field lines, inherent to the 3D null, show high gradients of the squashing factor. We find slipping reconnections at these quasi-separatrix layers, which are co-located with the post-flare circular brightening observed at the chromospheric heights. This demonstrates the viability of the initial non-force-free field along with the dynamics it initiates. Moreover, the initial field and its simulated evolution is found to be devoid of any flux rope, which is in congruence with the confined nature of the flare.

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arXiv2018

Magnetic field topology from non-force free extrapolation and magnetohydrodynamic simulation of its eventual dynamics

Magnetic reconnections (MRs) for various magnetic field line (MFL) topologies are believed to be the initiators of various solar eruptive events like flares and coronal mass ejections (CMEs). Consequently, important is a thorough understanding and quantification of the MFL topology and their evolution which leads to MRs. Contemporary standard is to extrapolate the coronal MFLs using equilibrium models where the Lorentz force on the coronal plasma is zero everywhere, because either there is no current or the current is parallel to the magnetic field. In tandem, a non-force-free-field (NFFF) extrapolation scheme has evolved and allows for a Lorentz force which is non-zero only at the photosphere but asymptotically vanishes with height. The paper reports magnetohydrodynamic (MHD)- simulations initiated by NFFF extrapolation of the coronal MFLs for the active region NOAA 11158. Interestingly, quasi-separatrix layers (QSLs) which facilitate MRs are detected in the extrapolated MFLs. The AR 11158 is flare producing and, the paper makes an attempt to asses the role of QSLs in the flare onsets.

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