arXiv subjects

Hirdesh Kumar

Publications and source records attributed to Hirdesh Kumar.

5 recordsLinked to original sources

arXiv2026

Magnetoacoustic Shocks and Spectropolarimetric Signals in He I 10830 {\AA}

Umbral flashes are manifestations of magnetoacoustic shocks in the solar chromosphere. These phenomena are thought to influence the evolution of chromospheric umbral magnetic fields. However, the impact of these shocks on inferred chromospheric magnetic field oscillations remains unclear. We examined five different sunspots located near the solar disk center, observed with the GRIS instrument installed at the GREGOR telescope. The HAZEL2 Spectropolarimetric inversion code is used to obtain the photospheric and chromospheric line-of-sight velocities and magnetic fields in Si 10827 {\AA} and He 10830 {\AA} spectral lines, respectively, using various inversion strategies. In the inversions with one chromospheric component, three of the sunspots exhibit remarkably stronger magnetic fields accompanying the shocks, while the other two sunspots show striking reductions in the magnetic field. Alternatively, the Stokes profiles can be reproduced by models with two chromospheric slabs, one on top of the other, through two-component inversions. These inversions provide excellent fits even when magnetic field fluctuations are discarded by imposing a constant magnetic field during the whole temporal series. In this scenario, the observed Stokes profiles are interpreted as the result of strong velocity gradients, where the He 10830 {\AA} line is sensitive to both sides of the shock front. Both competing models explaining the spectral profiles during the shocks, either large magnetic field fluctuations or velocity gradients, are critically discussed.

astro-ph.SR↗

arXiv2024

Investigation of Phase Shift and Travel Time of Acoustic Waves in the Lower Solar Atmosphere Using Multi-Height Velocities

We report and discuss phase-shift and phase travel time of low-frequency ({\nu} < 5.0 mHz) acoustic waves estimated within the photosphere and photosphere-chromosphere interface regions, utilizing multi-height velocities in the quiet Sun. The bisector method has been employed to estimate seven height velocities in the photosphere within the Fe I 6173 {\AA} line scan, while nine height velocities are estimated from the chromospheric Ca II 8542 {\AA} line scan observations obtained from the Narrow Band Imager instrument installed with the Multi-Application Solar Telescope operational at the Udaipur Solar Observatory, India. Utilizing fast Fourier transform at each pixel over the full field-of-view, phase shift and coherence have been estimated. The frequency and height-dependent phase shift integrated over the regions having an absolute line-of-sight magnetic field of less than 10 G indicates the non-evanescent nature of low-frequency acoustic waves within the photosphere and photosphere-chromosphere interface regions. Phase travel time estimated within the photosphere shows non-zero values, aligning with previous simulations and observations. Further, we report that the non-evanescent nature persists beyond the photosphere, encompassing the photospheric-chromospheric height range. We discuss possible factors contributing to the non-evanescent nature of low-frequency acoustic waves. Additionally, our observations reveal a downward propagation of high-frequency acoustic waves, indicating the refraction from higher layers in the solar atmosphere. This study contributes valuable insights into the understanding of the complex dynamics of acoustic waves within different lower solar atmospheric layers, shedding light on the non-evanescent nature and downward propagation of the acoustic waves.

astro-ph.SR↗

arXiv2023

On the propagation of gravity waves in the lower solar atmosphere in different magnetic configurations

Gravity waves are generated by turbulent subsurface convection overshooting or penetrating locally into a stably stratified medium. While propagating energy upwards, their characteristic negative phase shift over height is a well-recognized observational signature. Since their first detailed observational detection and estimates of energy content, a number of studies have explored their propagation characteristics and interaction with magnetic fields and other wave modes in the solar atmosphere. Here, we present a study of the atmospheric gravity wave dispersion diagrams utilizing intensity observations that cover photospheric to chromospheric heights over different magnetic configurations of quiet-Sun (magnetic network regions), a plage, and a sunspot as well as velocity observations within the photospheric layer over a quiet and a sunspot region. In order to investigate the propagation characteristics, we construct two-height intensity - intensity and velocity-velocity cross-spectra and study phase and coherence signals in the wavenumber-frequency dispersion diagrams and their association with background magnetic fields. We find signatures of association between magnetic fields and much reduced coherence and phase shifts over height from intensity-intensity and velocity-velocity phase and coherence diagrams, both indicating suppression/scattering of gravity waves by the magnetic fields. Our results are consistent with the earlier numerical simulations, which indicate that gravity waves are suppressed or scattered and reflected back into the lower solar atmosphere in the presence of magnetic fields.

astro-ph.SR↗

arXiv2023

A study of the propagation of magnetoacoustic waves in small-scale magnetic fields using solar photospheric and chromospheric Dopplergrams: HMI/SDO and MAST observations

In this work, we present a study of the propagation of low-frequency magneto-acoustic waves into the solar chromosphere within small-scale inclined magnetic fields over a quiet-magnetic network region utilizing near-simultaneous photospheric and chromospheric Dopplergrams obtained from the HMI instrument onboard SDO spacecraft and the Multi-Application Solar Telescope (MAST) operational at the Udaipur Solar Observatory, respectively. Acoustic waves are stochastically excited inside the convection zone of the Sun and intermittently interact with the background magnetic fields resulting into episodic signals. In order to detect these episodic signals, we apply the wavelet transform technique to the photospheric and chromospheric velocity oscillations in magnetic network regions. The wavelet power spectrum over photospheric and chromospheric velocity signals show a one-to-one correspondence between the presence of power in the 2.5-4 mHz band. Further, we notice that power in the 2.5-4 mHz band is not consistently present in the chromospheric wavelet power spectrum despite its presence in the photospheric wavelet power spectrum. This indicates that leakage of photospheric oscillations (2.5-4 mHz band) into the higher atmosphere is not a continuous process. The average phase and coherence spectra estimated from these photospheric and chromospheric velocity oscillations illustrate the propagation of photospheric oscillations (2.5-4 mHz) into the solar chromosphere along the inclined magnetic fields. Additionally, chromospheric power maps estimated from the MAST Dopplergrams also show the presence of high-frequency acoustic halos around relatively high magnetic concentrations, depicting the refraction of high-frequency fast mode waves around vA ~ vs layer in the solar atmosphere.

astro-ph.SR↗

arXiv2020

On the seismic emission in sunspots associated with Lorentz force changes accompanying major solar flares

Solar flares are known to generate seismic waves in the Sun. We present a detailed analysis of seismic emission in sunspots accompanying M- and X-class solar flares. For this purpose, we have used high-resolution Dopplergrams and line-of-sight magnetograms at a cadence of 45 s, along with vector magnetograms at a cadence of 135 s obtained from Helioseismic and Magnetic Imager (HMI) instrument aboard the Solar Dynamic Observatory (SDO) space mission. In order to identify the location of flare ribbons and hard X-ray foot-points, we have also used H-alpha chromospheric intensity observations obtained from Global Oscillation Network Group (GONG) instruments and hard X-ray images in 12-25 KeV band from the Reuvan Ramaty High Energy Solar Spectroscopic Imager (RHESSI) spacecraft. The Fast Fourier Transform (FFT) technique is applied to construct the acoustic velocity power map in 2.5-4 mHz band for pre-flare, spanning flare, and post flare epochs for the identification of seismic emission locations in the sunspots. In the power maps, we have selected only those locations which are away from the flare ribbons and hard X-rays foot-points. These regions are believed to be free from any flare related artefacts in the observational data. We have identified concentrated locations of acoustic power enhancements in sunspots accompanying major flares. Our investigation provides evidence that abrupt changes in the magnetic fields and associated impulsive changes in the Lorentz force could be the driving source for these seismic emissions in the sunspots during solar flares.

astro-ph.SR↗