Observations of the Quiet Sun during the Deepest Solar Minimum of the Past Century with Chandrayaan-2 XSM: Sub-A-class Microflares outside Active Regions

Abstract

Solar flares, with energies ranging over several orders of magnitude, result from impulsive release of energy due to magnetic reconnection in the corona. Barring a handful, almost all microflares observed in X-rays are associated with the solar active regions. Here we present, for the first time, a comprehensive analysis of a large sample of quiet-Sun microflares observed in soft X-rays by the Solar X-ray Monitor (XSM) on board the Chandrayaan-2 mission during the 2019–2020 solar minimum. A total of 98 microflares having peak flux below GOES A-level were observed by the XSM during observations spanning 76 days. By using the derived plasma temperature and emission measure of these events obtained by fitting the XSM spectra along with volume estimates from concurrent imaging observations in EUV with the Solar Dynamics Observatory/Atmospheric Imaging Assembly, we estimated their thermal energies to be ranging from 3 × 1026 to 6 × 1027 erg. We present the frequency distribution of the quiet-Sun microflares with energy and discuss the implications of these observations of small-scale magnetic reconnection events outside active regions on coronal heating.

Radio, X-ray and extreme-ultraviolet observations of weak energy releases in the `quiet' Sun

“…So, we calculated the maximum possible total energy of the type I bursts as E t ≈ 5×7×3600×8.1×10 22 14×14 ≈ 5.3×10 25 erg. This is in reasonable agreement with the range of energies (3×10 26 -7×10 27 erg) for the soft X-ray microflares reported by Vadawale et al (2021) since the authors had mentioned that their estimates represent upper limits. Note that the minimum possible energy of the type I bursts in the present case is E ≈ 8.1×10 22 erg.…”

“…Here n th is the number density of the background thermal electrons, n is the number density of the non-thermal electrons, V is the volume of the burst source, and E m is the mean energy of the individual electrons. In the present case n th = 7.9×10 7 cm −3 and E m ≈ 5 keV (Vadawale et al 2021). Assuming n/n th = 1.23×10 −7 at 80 MHz (Thejappa & Kundu 1991) and V = 10 30 cm 3 (corresponding to a density scale height of ≈ 10 10 cm in the solar corona), we find E ≈ 7.8×10 22 erg.…”

“…Recently Vadawale et al (2021) had reported observations of 'quiet' Sun X-ray microflares with the Chandrayaan-2/XSM during the solar minimum 2019-2020 (see for e.g. Ramesh et al (2020b)).…”

Evolution of Elemental Abundances During B-Class Solar Flares: Soft X-ray Spectral Measurements with Chandrayaan-2 XSM

“…As detailed observations have been lacking, it is not clear if the evolution of these smaller events follows the standard flare model. It is only recently, with the advent of better instrumentation, that in-depth studies of smaller flares are becoming feasible (Kuhar et al 2018;Mitra-Kraev and Del Zanna 2019;Athiray et al 2020;Cooper et al 2020;Glesener et al 2020;Duncan et al 2021;Vadawale et al 2021a). Lately, using X-ray time resolved spectroscopy, Narendranath et al (2020) have carried out abundance studies of flares as small as GOES B9-class.…”

NuSTAR observations of a repeatedly microflaring active region

“…Microflares have energies about 10 26 -10 28 erg and are identified to have <10 −6 W m −2 GOES (1-8 Å) soft X-ray flux, labelled as B, A, and sub-A class flares (Lin et al 1984;Fletcher et al 2011;Hannah et al 2011). Sub-A class microflares are not reliably detected by GOES but sub-A level events have been observed by more sensitive full-disk X-ray spectrometers identified to be located in the quiet Sun (Sylwester et al 2012;Vadawale et al 2021a) and ARs (Gburek et al 2011;Vadawale et al 2021b). However, a GOES equivalent class for sub-A class microflares can be calculated from their temperature and emission measure.…”