arXiv subjects

G. Sindhuja

Publications and source records attributed to G. Sindhuja.

5 recordsLinked to original sources

arXiv2025

Solar Cycle Variation of Sustained Gamma Ray Emission from the Sun

We investigated the occurrence rate of the sustained gamma ray emission (SGRE) events from the Sun using data obtained by Fermi Large Area Telescope (LAT) since its launch in 2008. Only 16 SGRE events were observed during the first 61 months of solar cycle (SC) 25, likely due to the solar array drive assembly's malfunction in 2018; 27 SGRE events were observed in SC 24 over the corresponding epoch. The average sunspot number (SSN) increased from 56.9 in SC 24 to 79.0 in SC 25. Fast and wide (FW) CMEs and decameter-hectometric (DH) type II bursts increased significantly in SC 25 by 29% and 33%, respectively when normalized to SSN. Therefore, we expect a higher number of SGREs in SC 25. We estimated the number of SGREs in SC 25 using three methods. (i) If the SGRE number varies commensurate with SSN, we should have 38 SGRE events in SC 25. However, FW CMEs and DH type II bursts in SC 25 were overabundant by 29% and 33%, so the number SGRE events should be 48 or 50. (ii) In SC 24, ~18% of FW CMEs and 27% of DH type II bursts were associated with SGRE events. At this rate SC 25 should have 48 and 49 SGRE events. (iii) Since SGRE events are invariably associated with >100 keV hard X-ray (HXR) bursts, we identified DH type II bursts associated with >100 keV HXR bursts from Fermi's Gamma ray Burst Monitor (GBM) during LAT data gaps. Almost all SGRE events in SCs 24 and 25, and 27 of the 79 LAT-gap type IIs were associated with HXR bursts of duration > ~5 min. These DH type II bursts are indicative of SGRE, bringing the total number of SGRE events to 43 (16 + 27). Thus, the three methods provide similar estimates of the number of SGRE events in SC 25. We, therefore, conclude that SC 25 is stronger than SC 24 based on the estimated number SGRE events.

astro-ph.HE↗

arXiv2025

A Tale of Two Shocks

Energetic particles in interplanetary space are normally measured at time scales that are long compared to the ion gyroperiod. Such observations by necessity average out the microphysics associated with the acceleration and transport of 10s - 100s keV particles. We investigate previously unseen non-equilibrium features that only become observable at very high time resolution, and discuss possible explanations of these features. We use unprecedentedly high-time-resolution data that were acquired by the in situ instruments on Solar Orbiter in the vicinity of two interplanetary shocks observed on 2023-11-29 07:51:17 UTC and 2023-11-30 10:47:26 UTC at $\sim 0.83$ astronomical units from the Sun. The solar-wind proton beam population follows the magnetic field instantaneously, on time scales which are significantly shorter than a gyro-period. Energetic particles, despite sampling large volumes of space, vary on remarkably short time scales, typically on the order of the convection time of their gyro-radius. Non-equilibrium features such as bump-on-tail distributions of energetic particles are formed by small-scale magnetic structures in the IMF. High-time-resolution observations show previously unobserved microphysics in the vicinity of two traveling interplanetary shocks, including ion reflection at a current sheet, which may explain where ions are reflected in shock acceleration.

astro-ph.SR↗

arXiv2023

Data processing of Visible Emission Line Coronagraph Onboard ADITYA L1

ADITYA-L1 is India's first dedicated mission to observe the sun and its atmosphere from a halo orbit around L1 point. Visible emission line coronagraph (VELC) is the prime payload on board at Aditya-L1 to observe the sun's corona. VELC is designed as an internally occulted reflective coronagraph to meet the observational requirements of wide wavelength band and close to the solar limb (1.05 Ro). Images of the solar corona in continuum and spectra in three emission lines 5303Å [Fe xiv], 7892Å [Fe xi] and 10747 [Fe xiii] obtained with high cadence to be analyzed using software algorithms automatically. A reasonable part of observations will be made in synoptic mode, those, need to be analyzed and results made available for public use. The procedure involves the calibration of instrument and detectors, converting the images into fits format, correcting the images and spectra for the instrumental effects, align the images etc. Then, develop image processing algorithms to detect the occurrence of energetic events using continuum images. Also derive physical parameters, such as temperature and velocity structure of solar corona using emission line observations. Here, we describe the calibration of detectors and the development of software algorithms to detect the occurrence of CMEs and analyze the spectroscopic data.

astro-ph.SR↗

arXiv2022

Temporal and latitudinal variations of Ca-K plage and network area: An implication to meridional flows

The Ca-K spectroheliograms obtained at the Kodaikanal observatory (KO) are used to generate a uniform time series using the equal contrast technique (ECT) to study the long and short-term variation in the solar chromosphere. The percentage of plage, Enhanced network (EN), Active network (AN), and Quiet network (QN) area at various latitudes is compared with the activity at 35$^{\circ}$ latitude and also with the sunspot number for the period of 1907 -- 1984. The values of phase differences indicate that the activity begins at $\sim$45$^{\circ}$ latitude and shift progressively to the lower latitude at a speed of $\sim$~9.4~m~sec$^{-1}$ . The shift speed slows down gradually and reaches $\sim$~3~m~sec$^{-1}$ at $\sim$5$^{\circ}$ latitude. No phase difference between the variations of Ca-K activity at 55$^{\circ}$, 65$^{\circ}$, and 75$^{\circ}$ latitude belts implies that changes in the activity are happening simultaneously. The analysis shows that the activity at polar latitude belts is anti-correlated with the sunspot number. This study indicates that a multi-cell meridional flow pattern could exist in the solar convection zone. One type of cell could transport the magnetic elements from mid-latitude to low-latitude belts through meridional flows, and the other one could be operating in the polar region.

astro-ph.SR↗

arXiv2015

Detection of high frequency oscillations and damping from multi-slit spectroscopic observations of the corona

During the total solar eclipse of 11 July 2010, multi-slit spectroscopic observations of the solar corona were performed from Easter Island, Chile. To search for high-frequency waves, observations were taken at a high cadence in the green line at 5303 A due to [Fe xiv] and the red line at 6374 A due to [Fe x]. The data are analyzed to study the periodic variations in the intensity, Doppler velocity and line width using wavelet analysis. The data with high spectral and temporal resolution enabled us to study the rapid dynamical changes within coronal structures. We find that at certain locations each parameter shows significant oscillation with periods ranging from 6 - 25 s. For the first time, we could detect damping of high-frequency oscillations with periods of the order of 10 s. If the observed damped oscillations are due to magnetohydrodynamic (MHD) waves then they can contribute significantly in the heating of the corona. From a statistical study we try to characterize the nature of the observed oscillations while looking at the distribution of power in different line parameters.

astro-ph.SR↗