arXiv subjects

S. Krishna Prasad

Publications and source records attributed to S. Krishna Prasad.

At least 19 recordsLinked to original sources

arXiv2026

Sloshing Oscillations in coronal loops excited by successive M- and C-Class flares

Slow magnetoacoustic waves in hot coronal loops have remained a topic of considerable interest and debate over the past two decades. The periodic back-and-forth motion of plasma within a coronal loop, often initiated by a flare, is commonly referred to as sloshing oscillation. In the present study, we report unprecedented observations of sloshing oscillations in coronal loops excited by successive M- and C-class flares, using data from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO). A total of fifteen oscillation events were identified within seven distinct coronal loops, providing the rare opportunity to evaluate the influence of flare strength on the characteristics of the oscillations. Based on the appearance of the oscillations, their properties were extracted mainly from the AIA 131 and 94 Å channels. Additionally, we estimate the deprojected length of each loop by assuming a semi-circular geometry. Our results indicate considerable changes in the properties of oscillations from one flare to another, suggesting the role of individual flares in shaping the local physical conditions. The plasma temperature estimated from the loop length and oscillation period ranges from 9 to 31 MK. Additionally, we find that the damping times are not always longer in the colder 94 Å channel as previously observed. By combining the results obtained from all events, we study the inter-dependences between various parameters, including oscillation period, damping time, loop length, and plasma temperature, and discuss these results in the context of the theory of slow waves.

astro-ph.SR↗

arXiv2026

A universal scaling between damping time and period of quasi-periodic pulsations from solar EUV brightenings to X-ray stellar flares

Recent high spatial and temporal resolution extreme-ultraviolet (EUV) imaging observations have revealed that quasi-periodic pulsations (QPPs), a ubiquitous signature of impulsive energy release in solar and stellar flares, are also present in much smaller-scale coronal events known as EUV brightenings. Whether QPPs observed across such disparate spatial and energetic scales share a common physical origin remains an open question. Here we analyse 2,146 EUV brightenings observed with Solar Orbiter/EUI and 300 EUV solar flares observed with SDO/AIA, identifying 185 brightenings and 89 flares exhibiting statistically significant damped QPPs. We show that the relationship between damping time and oscillation period follows a common power-law scaling for EUV brightenings and EUV solar flares, consistent with previously reported X-ray QPPs spanning both solar and stellar flares. The persistence of this scaling over a wide range of energies and scales suggests that QPPs are governed by a common underlying physical mechanism.

astro-ph.SR↗

arXiv2025

Co-existence of longitudinal and transverse oscillations in polar plumes observed with Solar Orbiter/EUI

Magnetohydrodynamic (MHD) waves play a key role in heating the solar corona and driving the solar wind. Recent observations have shown the presence of slow magneto-acoustic and Alfvénic waves in polar plumes and inter-plumes. However, a complete understanding of wave dynamics in the polar regions has long been limited by the lack of simultaneous, high-resolution observations. In this study, we utilize high spatial (210 km per pixel) and high cadence (5s) dataset from the Extreme Ultraviolet Imager (EUI) aboard Solar Orbiter, acquired on 14 September 2021. Our findings reveal the simultaneous presence of slow magneto-acoustic and Alfvénic waves within the same polar plumes. For slow magneto-acoustic waves, the amplitudes of propagating disturbances are 1.4 to 3.2$\%$ of background intensity, with periodicities of 9 min, and the projected speed of these disturbances ranges between 115 to 125 kms$^{-1}$. The corresponding electron temperature in plumes ranges between 0.58 and 0.69 MK. The damping length of these propagating disturbances for five plumes is $\approx$2.4 to 7.1 Mm. The propagating disturbances are also detected in the fine-scale substructures within the plumes. Alfvénic waves, on the other hand, are detected with average displacement amplitude, periodicity, and velocity amplitudes of 165$\pm$82 km, 93$\pm$39 s, and 12$\pm$7 kms$^{-1}$ respectively. The ranges for displacement amplitude, period, and velocity amplitude are 50-600 km, 50-250 s, and 3-32 kms$^{-1}$ respectively. These results mark the first demonstration of Solar Orbiter/EUI's ability to simultaneously detect both slow magneto-acoustic and Alfvénic wave modes extending up to 20 Mm in polar plumes.

astro-ph.SR↗

arXiv2025

Understanding the magnetic field and plasma-$β$ along umbral fan loops traced using 3-min slow waves

The plasma-$β$ is an important fundamental physical quantity in solar plasma physics, which determines the dominating process in the solar atmosphere, i.e., magnetic or thermodynamic processes. Here, for the first time, we provide variations of magnetic field and plasma-$β$ along magnetically structured loops from the photosphere to the corona. We have selected several fan loops rooted in sunspot umbra observed simultaneously by the Interface Region Imaging Spectrograph and Solar Dynamics Observatory. The 3-min slow waves enabled us to trace and analyze several fan loops with cross-sectional areas in the lower atmosphere and locate their footpoints at the photosphere. We find the RMS magnetic field strengths in the range 1596-2269 G at the photospheric footpoints of the fan loops, which decrease rapidly to 158-236 G at the coronal footpoints. We estimated the plasma-$β$ at the photospheric and coronal footpoints in the range 0.2-0.5 and 0.0001-0.001, respectively. We found plasma-$β$$<$$1$ along the whole loop, whereas the plasma-$β$$\approx$$1$ layer is found to be at sub-photospheric heights. We compared our findings for isolated individual fan loops with a previously established model for active regions and found an almost similar pattern in variations with height, but with different plasma-$β$ values. Our results demonstrate the seismological potential of 3-min slow waves omnipresent in the umbral sunspot atmosphere to probe and map isolated loops and determine magnetic field and plasma-$β$ along these loops. The obtained parameters provide crucial ingredients for the theoretical modeling of the umbral atmosphere and wave dynamics along loops.

astro-ph.SR↗

arXiv2025

Nonlinearity of 3 minute Slow Magnetoacoustic Waves in the Sunspot Umbral Atmosphere

Slow magnetoacoustic waves with a 3 minute period are upward-propagating waves traveling through the density-stratified umbral atmosphere. The decreasing density causes their amplitude to increase, developing into nonlinear waves through steepening and eventually forming shocks. To investigate the vertical evolution of this wave nonlinearity, we utilized multi-wavelength data from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO), covering from the photosphere to the lower corona across 20 active regions. The steepening of the wave profile leads to the generation of higher harmonics. We quantify this using a nonlinearity index (NI), defined as the ratio of the amplitude of 2nd harmonic to the fundamental obtained using wavelet analysis. We find a characteristic pattern: nonlinearity increases from the photosphere through the lower chromosphere, peaking near the AIA 1700 Å formation height, and decreases at higher altitudes, notably in the AIA 304 Å channel. This trend indicates progressive wave steepening and subsequent energy dissipation before reaching the formation of AIA 304 Å, consistent with shock formation in the lower atmosphere. An additional rise in NI is observed at the AIA 131 Å channel, followed by a decline in AIA 171 Å, suggesting a 2nd phase of wave nonlinearity evolution in the lower corona. Based on the NI profile and the formation heights of these channels, we conjecture that nonlinear wave processes are most prominent between the AIA 1700 Å and AIA 304 Å formation layers and again between AIA 131 Å and AIA 171 Å.

astro-ph.SR↗

arXiv2025

Properties of slow magneto-acoustic waves observed simultaneously using Hi-C 2.1 and AIA

Propagating slow magneto-acoustic waves are commonly observed in different coronal structures but are most prominent in active region fan loops. Their rapid damping with damping lengths of the order of a wavelength has been investigated in the past by several authors. Although different physical mechanisms have been proposed, significant discrepancies between the theory and observations remain. Recent high-resolution observations captured simultaneously by two different instruments reveal distinct damping lengths for slow magneto-acoustic waves although their passbands are similar. These results suggest a possible contribution of instrumental characteristics on the measurement of damping lengths. Here, we analyse the behavior of slow waves using a different pair of instruments in order to check the prevalence of such results. In particular, the cotemporal observations of active region NOAA AR12712 by the High-Resolution Coronal Imager (Hi-C 2.1) and the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) are utilised. The estimated oscillation periods of slow magneto-acoustic waves identified from these data are 2.7{\,}$\pm${\,}0.2{\,}min from SDO/AIA, and 2.8{\,}$\pm${\,}1.2{\,}min from Hi-C 2.1. The corresponding propagation speeds are found to be 46.0{\,}$\pm${\,}1.7{\,}km{\,}s$^{-1}$ and 48.1{\,}$\pm${\,}0.6{\,}km{\,}s$^{-1}$, respectively. Damping lengths were calculated by two different methods, the Phase Tracking Method (PTM) and the Amplitude Tracking Method (ATM). The obtained values from PTM are 4.0{\,}$\pm${\,}2.1{\,}Mm and 4.1{\,}$\pm${\,}0.3{\,}Mm while those from ATM are 3.4{\,}$\pm${\,}1.0{\,}Mm and 3.7{\,}$\pm${\,}0.1{\,}Mm, respectively, for the AIA and Hi-C data. Our results do not indicate any notable difference in damping lengths between these instruments.

astro-ph.SR↗

arXiv2025

Height-Dependent Slow Magnetoacoustic Wave Amplitude and Energy Flux in Sunspot Atmospheres

Slow magnetoacoustic waves (SMAWs) have been considered in the past as a possible candidate for chromospheric heating. This study analyzes 20 active regions observed between 2012 and 2016 to examine the amplitude and energy flux variation of SMAWs in the umbral atmosphere. Six different wavelength channels from the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory, covering regions from the photosphere to the low corona, were utilized for this purpose. The wave amplitude estimations show a gradual increase in 3-minute oscillation amplitude, peaking between 700--900 km, followed by a steady decrease; at altitudes greater than 1800 km, the amplitude appears to increase and then decrease again. The corresponding energy flux, on the other hand, displays a steady and monotonous decrease with a significant reduction in value from approximately $3.32 \pm 0.50~\mathrm{kW,m^{-2}}$ near the photosphere to about $(6.47 \pm 3.16) \times 10^{-4}~\mathrm{W,m^{-2}}$ at an altitude of 2585 km. This decay may be attributed to radiative damping and shock dissipation in the lower altitudes, and thermal conduction and viscosity in the higher altitudes. The missing flux is a factor of 3--15 lower than that required to counterbalance the chromospheric radiative losses.

astro-ph.SR↗

arXiv2025

Static and hydrodynamic periodic structures induced by AC electric fields in the antiferroelectric SmZA phase

We report the effect of AC electric fields in the range of 0.1-300 kHz on planar antiferroelectric SmZA layers of DIO. Significant results are (a) primary bifurcation into a quasistationary periodic instability with its voltage threshold Uc and wave vector qc along the initial director being, respectively, quadratic and linear functions of f over 10-150 kHz, and with an azimuthal distortion of the director which changes sign between adjacent stripes, (b) transition from the modulated planar state to a homogeneous state at higher voltages, and (c) third bifurcation into travelling wave periodic state on further rise in U in the region 10-40 kHz. We interpret these findings as follows. The low voltage instability is very similar to that seen in the higher temperature apolar nematic phase, and is the electrohydrodynamic (EHD) instability possibly belonging to the region of dielectric inversion frequency. The azimuthal distortions of n result from an undulatory distortion of the SmZA layers in the book-shelf geometry. The intermediate homogeneous state of SmZA in which the periodic structure is absent results from a linear coupling between the layer polarization P and applied field E, giving rise to a scissoring type mutual P reorientation in adjacent layers. Finally, at even higher voltages, the medium goes over to a field induced transition to the ferroelectric nematic, with the polarization following the AC field, and the periodic EHD instability being similar to that of the dielectric regime. The polar vector symmetry of the medium leads in general to travelling waves.

cond-mat.soft↗

arXiv2024

On the formation height of low-corona and chromospheric channels of the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO)

The multi-wavelength data from the Solar Dynamics Observatory (SDO) is extensively used in studying the physics of the Sun and its atmosphere. In this study, we estimate the formation heights of low-corona and chromospheric channels of the Atmospheric Imaging Assembly (AIA) over the atmospheres of sunspot umbrae during the quiet condition period within 20 different active regions. The upward propagating slow magnetoacoustic waves (slow MAWs) of 3-min period, which are perpetually present in sunspots, are utilized for this purpose. Employing a cross-correlation technique, the most frequent time lag between different channel pairs is measured. By combining this information with the local sound speed obtained from the characteristic formation temperatures of individual channels, we estimate the respective formation heights. The median values of formation heights obtained across all active regions in our sample are 356, 368, 858, 1180, and 1470 km, respectively, for the AIA 1600 Å, 1700 Å, 304 Å, 131 Å, and 171 Å channels. The corresponding ranges in the formation heights are 247 $\--$ 453, 260 $\--$ 468, 575 $\--$ 1155, 709 $\--$ 1937, and 909 $\--$ 2585 km, respectively. These values are measured with respect to the HMI continuum. We find the formation height of UV channels is quite stable (between 250 $\--$ 500 km) and displays only a marginal difference between the AIA 1600 Å and 1700 Å during quiet conditions. On the other hand, the formation height of coronal channels is quite variable.

astro-ph.SR↗

arXiv2024

Modelling the propagation of slow magneto-acoustic waves in a multi-stranded coronal loop

We study the propagation properties of slow magneto-acoustic waves in a multi-thermal coronal loop using a 3D MHD model, for the first time. A bundle of 33 vertical cylinders, each of 100{\,}km radius, randomly distributed over a circular region of radius 1{\,}Mm is considered to represent the coronal loop. The slow waves are driven by perturbing the vertical velocity ($v_z$) at the base of the loop. We apply forward modelling to the simulation results to generate synthetic images in the coronal channels of SDO/AIA. Furthermore, we add appropriate data noise to enable direct comparison with the real observations. It is found that the synthetic images at the instrument resolution show non-cospatial features in different temperature channels in agreement with previous observations. Time-distance maps are constructed from the synthetic data to study the propagation properties. The results indicate that the oscillations are only visible in specific channels depending on the temperature range of plasma existing within the loop. Additionally, the propagation speed of slow waves is also found to be sensitive to the available temperature range. Overall, we propose that the cross-field thermal properties of coronal structures can be inferred using a combination of numerical simulations and observations of slow magneto-acoustic waves.

astro-ph.SR↗

arXiv2024

Multithermal apparent damping of slow waves due to strands with a Gaussian temperature distribution

Context. Slow waves in solar coronal loops are strongly damped. The current theory of damping by thermal conduction cannot explain some observational features.\n Aims. We investigate the propagation of slow waves in a coronal loop built up from strands of different temperatures. \n Methods. We consider the loop to have a multithermal, Gaussian temperature distribution. The different propagation speeds in different strands lead to an multithermal apparent damping of the wave, similar to observational phase mixing. We use an analytical model to predict the damping length and propagation speed for the slow waves, including in imaging with filter telescopes. \n Results. We compare the damping length due to this multithermal apparent damping with damping due to thermal conduction and find that the multithermal apparent damping is more important for shorter period slow waves. We have found the influence of instrument filters on the wave's propagation speed and damping. This allows us to compare our analytical theory to forward models of numerical simulations. \n Conclusions. We find that our analytical model matches the numerical simulations very well. Moreover, we offer an outlook for using the slow wave properties to infer the loop's thermal properties.

astro-ph.SR↗

arXiv2022

Observational and numerical characterization of a recurrent arc-shaped front propagating along a coronal fan

Recurrent, arc-shaped intensity disturbances were detected by EUV channels in an active region. The fronts were observed to propagate along a coronal loop bundle rooted in a small area within a sunspot umbra. Previous works have linked these intensity disturbances to slow magnetoacoustic waves that propagate from the lower atmosphere to the corona along the magnetic field. The slow magnetoacoustic waves propagate at the local cusp speed. However, the measured propagation speeds from the intensity images are usually smaller as they are subject to projection effects due to the inclination of the magnetic field with respect to the line-of-sight. Here, we aim to understand the effect of projection by comparing observed speeds with those from a numerical model. Using multi-wavelength data we determine the periods present in the observations at different heights of the solar atmosphere through Fourier analysis. We calculate the plane-of-sky speeds along one of the loops from the cross-correlation time lags obtained as a function of distance along the loop. We perform a 2D ideal MHD simulation of an active region embedded in a stratified atmosphere. We drive slow waves from the photosphere with a 3 minutes periodicity. Synthetic time-distance maps are generated from the forward-modelled intensities in coronal wavelengths and the projected propagation speeds are calculated. The intensity disturbances show a dominant period between [2-3] minutes at different heights of the atmosphere. The apparent propagation speeds calculated for coronal channels exhibit an accelerated pattern with values increasing from 40 to 120 km/s as the distance along the loop rises. The propagation speeds obtained from the synthetic time-distance maps also exhibit accelerated profiles within a similar range of speeds. We conclude that the accelerated propagation in our observations is due to the projection effect.

astro-ph.SR↗

arXiv2021

Novel data analysis techniques in coronal seismology

We review novel data analysis techniques developed or adapted for the field of coronal seismology. We focus on methods from the last ten years that were developed for extreme ultraviolet (EUV) imaging observations of the solar corona, as well as for light curves from radio and X-ray. The review covers methods for the analysis of transverse and longitudinal waves; spectral analysis of oscillatory signals in time series; automated detection and processing of large data sets; empirical mode decomposition; motion magnification; and reliable detection, including the most common pitfalls causing artefacts and false detections. We also consider techniques for the detailed investigation of MHD waves and seismological inference of physical parameters of the coronal plasma, including restoration of the three-dimensional geometry of oscillating coronal loops, forward modelling and Bayesian parameter inference.

astro-ph.SR↗

arXiv2021

MHD Waves in open coronal structures

Modern observatories have revealed the ubiquitous presence of magnetohydrodynamic waves in the solar corona. The propagating waves (in contrast to the standing waves) are usually originated in the lower solar atmosphere which makes them particularly relevant to coronal heating. Furthermore, open coronal structures are believed to be the source regions of solar wind, therefore, the detection of MHD waves in these structures is also pertinent to the acceleration of solar wind. Besides, the advanced capabilities of the current generation telescopes have allowed us to extract important coronal properties through MHD seismology. The recent progress made in the detection, origin, and damping of both propagating slow mangetoacoustic waves and kink (Alfvénic) waves is presented in this review article especially in the context of open coronal structures. Where appropriate, we give an overview on associated theoretical modelling studies. A few of the important seismological applications of these waves are discussed. The possible role of Aflvénic waves in the acceleration of solar wind is also touched upon.

astro-ph.SR↗

arXiv2021

Compressive oscillations in hot coronal loops: Are sloshing oscillations and standing slow waves independent?

Employing high-resolution EUV imaging observations from SDO/AIA, we analyse a compressive plasma oscillation in a hot coronal loop triggered by a C-class flare near one of its foot points as first studied by Kumar et al. We investigate the oscillation properties in both the 131{\,}Å and 94{\,}Å channels and find that what appears as a pure sloshing oscillation in the 131{\,}Å channel actually transforms into a standing wave in the 94{\,}Å channel at a later time. This is the first clear evidence of such transformation confirming the results of a recent numerical study which suggests that these two oscillations are not independent phenomena. We introduce a new analytical expression to properly fit the sloshing phase of an oscillation and extract the oscillation properties. For the AIA 131{\,}Å channel, the obtained oscillation period and damping time are 608$\pm$4{\,}s and 431$\pm$20{\,}s, respectively during the sloshing phase. The corresponding values for the AIA 94{\,}Å channel are 617$\pm$3{\,}s and 828$\pm$50{\,}s. During the standing phase that is observed only in the AIA 94{\,}Å channel, the oscillation period and damping time have increased to 791$\pm$5{\,}s and 1598$\pm$138{\,}s, respectively. The plasma temperature obtained from the DEM analysis indicates substantial cooling of the plasma during the oscillation. Considering this, we show that the observed oscillation properties and the associated changes are compatible with damping due to thermal conduction. We further demonstrate that the absence of a standing phase in the 131{\,}Å channel is a consequence of cooling plasma besides the faster decay of oscillation in this channel.

astro-ph.SR↗

arXiv2020

Evolution Of Downflows In The Transition Region Above A Sunspot Over Short Time-Scales

Downflows with high velocities occur in the transition region above many sunspots; however, how these signatures evolve over short time-scales in both spatial and spectral terms is still unknown. In this article, we investigate the evolution of downflows detected within the transition region on time-scales of the order minutes and search for clues as to their formation mechanisms in co-temporal imaging data. The high-resolution spectral and imaging data used to identify downflows here were sampled by IRIS on the 20th and 21st May 2015. Imaging data from the Hinode and SDO satellites provided context about the wider solar atmosphere. Four downflows were identified and analysed. The potential super-sonic components of these downflows had widths of around 2" and evolved over time-scales of the order minutes. The measured apparent downflow velocities were structured both in time and space, with the highest apparent velocities occurring above a bright region detected in Si IV 1400 Å images. Downflows with apparent velocities below 50 km s^{-1} were observed to extend a few arcsecs away from the foot-points suggesting the potential super-sonic components are linked to larger-scale flows. The electron density and mass flux for these events were within the ranges 10^{9.6}-10^{10.2} cm^{-3} and 10^{-6.81}-10^{-7.48} g cm^{-2} s^{-1}, respectively. Each downflow formed at the foot-point of thin `fingers' extending out around 3-5" in Si IV 1400 Å data with smaller widths (<1") than the super-sonic downflows. Downflows can appear, disappear, and recur within time-scales of less than one hour in sunspots. As potential super-sonic downflow signatures were detected at the foot-points of both extended fingers in Si IV 1400 Å SJI data and sub-sonic downflows in Si IV 1394 Å spectra, it is likely that these events are linked to flows within structures such as coronal loops.

astro-ph.SR↗

arXiv2020

Evolution Of Super-Sonic Downflows In A Sunspot

Super-sonic downflows have been observed in transition region spectra above numerous sunspots; however, little research has been conducted to date into how persistent these signatures are within sunspots on time-scales longer than a few hours. Here, we analyse the lead sunspot of AR 12526 to infer the properties and evolution of super-sonic downflows occurring within it. Sixteen large, dense raster scans sampled by IRIS are analysed. These rasters tracked the lead sunspot of AR 12526 at discrete times between the 27th March 2016 and the 2nd April 2016. One sit-and-stare observation acquired on the 1st April 2016 is also studied in order to analyse the evolution of super-sonic downflows on shorter time-scales. Super-sonic downflows are variable within this sunspot both in terms of spatial structuring and velocities. 13 of the 16 raster scans display some evidence of super-sonic downflows in the Si IV 1394 Å line co-spatial to a sustained bright structure detected in the 1400 Å slit-jaw imaging channel, with a peak velocity of 112 km s^{-1} being recorded on the 29th March 2016. Evidence for super-sonic downflows in the O IV 1401 Å line was found in 14 of these rasters, with the spatial structuring differing from that inferred from the Si IV 1394 Å line. In the sit-and-stare data, no dual flow is initially detected, however, a super-sonic downflow does develop after 60 minutes. This downflow accelerates from 73 km s^{-1} to close to 80 km s^{-1} in both the Si IV 1394 Å and O IV 1401 Å lines over the course of 20 minutes before the end of the observation. The morphology of these downflows evolved over the course of both hours and days and was often different in the Si IV 1394 Å and O IV 1401 Å lines. These events were found co-spatial to a bright region in the core of the Si IV 1394 Å line which appeared to form at the foot-points of coronal fan loops.

astro-ph.SR↗

arXiv2019

The Temperature-dependent Damping of Propagating Slow Magnetoacoustic Waves

The rapid damping of slow magnetoacoustic waves in the solar corona has been extensively studied in previous years. Most studies suggest that thermal conduction is a dominant contributor to this damping, albeit with a few exceptions. Employing extreme-ultraviolet (EUV) imaging data from SDO/AIA, we measure the damping lengths of propagating slow magnetoacoustic waves observed in several fan-like loop structures using two independent methods. The dependence of the damping length on temperature has been studied for the first time. The results do not indicate any apparent decrease in damping length with temperature, which is in contrast to the existing viewpoint. Comparing with the corresponding theoretical values calculated from damping due to thermal conduction, it is inferred that thermal conduction is suppressed in hotter loops. An alternative interpretation that suggests thermal conduction is not the dominant damping mechanism, even for short period waves in warm active region loops, is also presented.

astro-ph.SR↗