arXiv subjects

M. V. Sunil Krishna

Publications and source records attributed to M. V. Sunil Krishna.

7 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

Unraveling the Generation Mechanism of a Mid-Latitude Plasma Blob and the Evidence of Its Rare Interaction with a MSTID Phase Front

We report observations of two distinct nighttime F-region irregularities, plasma blob (localized density enhancement) and medium-scale traveling ionospheric disturbance (MSTID), in O(1D) 630.0 nm all-sky airglow images from Hanle (32.7°N, 78.9°E; Mlat~24.1°N), Ladakh, India, during the geomagnetically quiet (Ap=6) night of 06 July 2021. Global vertical total electron content (VTEC) maps revealed that the plasma blob developed beyond the southern edge of imager's field-of-view before appearing in airglow images and propagated predominantly westward, as confirmed from both the airglow and VTEC datasets. The existence of the plasma blob and MSTID outside the imager field-of-view was further confirmed by temporal VTEC fluctuations recorded by multiple GNSS receivers. Additionally, FORMOSAT-7/COSMIC-2 signal-to-noise ratio and ICON/MIGHTI wind profiles indicated the presence of sporadic-E (ES) layers at E-region near both the plasma blob and the MSTID. We propose that polarization electric field associated with either MSTID or ES-layers mapped along magnetic field lines to lower latitudes, driving upward plasma transport from F-peak region through vertical uplift of the F-layer. This F-layer uplift was confirmed by simultaneous in-situ O+/H+ density enhancements/reductions at LEO altitudes measured by FORMOSAT-7/COSMIC-2. Upward-transported plasma experienced reduced chemical loss at higher altitudes, producing localized VTEC enhancements (plasma blob). The plasma subsequently diffused along magnetic field lines to higher/lower latitudes/altitudes (~250 km), entering imager's field-of-view, where enhanced dissociative recombination of O2+ produced high intensity airglow region. Interestingly, interaction between the plasma blob and MSTID's plasma-depleted front caused gradual decay and bifurcation of the front due to plasma influx from the high-density blob region.

physics.space-ph↗

arXiv2026

Microwave Polar Brightening and Its Connection to Polar Coronal Holes

Polar brightening (PB) observed at microwave frequencies serves as an important probe to study the thermal and magnetic properties in the Sun's polar regions. Building on earlier studies that linked microwave PB to polar faculae, small-scale loops, and the polar coronal holes (PCHs), we present a comprehensive analysis of the long-term behaviour of 17 GHz microwave PB and its relation to polar magnetic field and coronal hole evolution. Using daily Nobeyama Radioheliograph observations spanning 1992 to 2018, we quantify microwave PB peak temperature variations and compare them with the temporal evolution of PCH area extracted from SDO/AIA-based SPoCA coronal hole catalogues during the period 2010-2018. We also examine the correspondence between microwave PB and the polar magnetic field to assess the nature of their association. Our results show a strong correlation between microwave PB peak temperature and PCH area, as well as with the polar magnetic-field strength. In addition, we found that regions of enhanced microwave emission are frequently associated with small-scale loop structures, consistent with Coronal Bright Points (CBPs), which are often associated with the eruption of jets. Overall, this study aims to investigate the impact of coronal holes, polar magnetic fields, and small-scale polar activity on polar brightening observed at 17 GHz and its long-term evolution.

astro-ph.SR↗

arXiv2026

Automatic Characterization of Mid-latitude Multiple Ionospheric Plasma Structures from All-sky Airglow Images using Deep Learning Technique

The F-region ionospheric plasma structures are propagating high and or low electron density regions in the Earth ionosphere. These plasma structures can be observed using ground based all-sky airglow imagers which can capture faint airglow emissions originating from the F-region of ionosphere. This study introduces a novel automatic method for determining the propagation parameters (horizontal velocity and orientation) of these multiple ionospheric plasma structures observed in O(1D) 630.0 nm all-sky airglow images from Hanle, India located in the mid-latitude region. We have used a deep learning-based segmentation model called YOLOv8 (You Only Look Once) to localize and BoT-SORT tracker to track individual mid-latitude ionospheric plasma structures. Three different automatic algorithms are used to characterize the observed plasma structures utilizing the segmented outputs from the YOLO model. Finally, an additional quality control step is introduced that filters the results from the three automatic algorithms and generates a flag to retain the most reliable estimate. The results of the proposed fully automated pipeline are systematically compared with a previously developed semi-automatic approach to assess the estimation efficacy. The automatic technique developed in this study is particularly valuable for all-sky airglow imaging systems having large datasets, where manual intervention or semi-automatic analysis is impractical.

physics.space-ph↗

arXiv2024

Evidence for the evolution and decay of an electrified Medium Scale Traveling Ionospheric Disturbances during two consecutive substorms: First results

Electrified Medium Scale Traveling Ionospheric Disturbances (EMSTIDs) is one of the prominent plasma structures that affect the propagation of high frequency radio waves. Overall, seasonal variation and propagation characteristics of the EMSTIDs are widely reported in literature. However, the effects of substorms on the formation and dissipation of the EMSTIDs are not well explored. In the present study, on a moderately geomagnetically active night of 26 October 2019 (Ap=24), the airglow imager over Hanle (32.7°N, 78.9°E; Mlat. ~24.1°N), India recorded the evolution and decay of an EMSTID in the O(1D) 630.0 nm airglow images in between 13.3 UT and 15.8 UT. In addition, during the same time, a steep rise and fall of the virtual base height of the ionospheric F-layer were also recorded by a nearby digisonde over New Delhi (28.70°N, 77.10°E; Mlat. ~20.2°N). The most important aspect of the event was the occurrence of the two consecutive substorms in between 13.3 UT and 15.8 UT. To the best of our knowledge, this is the first of its kind study where we report the role of interplanetary electric field (IEF) and substorm induced electric fields on the evolution and decay of the EMSTID. This study elicits effects of the externally imposed electric fields on the mid-latitude ionospheric plasma structures and provides insight into the complex coupling between auroral and low-mid latitude region.

physics.space-ph↗

arXiv2024

A rare simultaneous detection of a mid-latitude plasma depleted structure in O($^1$D) 630.0 nm and O($^1$S) 557.7 nm all-sky airglow images on a geomagnetically quiet night

In general, nighttime thermospheric 557.7 nm emission over mid-latitudes is predominantly masked by significantly larger mesospheric component, and hence, F-region plasma structures are rarely observed in this emission. This paper reports the first rare simultaneous detection of F-region plasma depleted structure in O($^1$D) 630.0 nm and O($^1$S) 557.7 nm airglow images from Hanle, India, a mid-latitude station (32.7°N, 78.9°E; Mlat. ~24.1°N) on a geomagnetically quiet night (Ap=3) of 26 June 2021. This indicates significant enhancement of thermospheric 557.7 nm emission. Interestingly, thermospheric 557.7 nm emission was not significant on the following geomagnetically quiet night as MSTID bands were only observed in 630.0 nm images. We show that enhanced dissociative recombination caused by descent of F-layer peak over the observation region coupled with the significant increase of the electron density at thermospheric 557.7 nm emission altitude enabled the detection of the plasma depleted structure on 26 June 2021.

physics.space-ph↗

arXiv2008

Testing of Solar2000 EUV flux model between 900-1350 A using Greenline Dayglow Emission

The contribution of photodissociation of molecular oxygen to the total volume emission rate of greenline dayglow emission at 5577 A is modelled in the present study. The Solar EUV radiation fluxes for the modelling are obtained from the Solar2000 V 2.25 model. The modelling has been done in the altitude range of 92 - 105 Km, where the photodissociation and the three body recombination are the main contributing processes to the greenline dayglow emission. The present results are discussed in the light of Wind Imaging Interferometer (WINDII) observations of greenline dayglow emission in the altitude range of 92 - 105 Km. It is found that the Solar2000 V2.25 flux model gives better agreement with the WINDII observations especially in the 92 - 96 Km where the earlier models predicted a very low emission rate. In the mesospheric emission peak region the present results are higher than the measurements and this discrepancy may possibly be attributed due to higher values of solar fluxes for those spectral lines which are main sources of the production of greenline dayglow emission. The present study suggests the reexamination of Solar2000 flux model at least for those spectral lines which are the main sources of greenline dayglow emission in mesospheric emission peak region.

physics.ao-ph↗