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

Abstract

The Solar X-ray Spectrometer (XSM) payload onboard Chandrayaan-2 provides disk-integrated solar spectra in the 1 -15 keV energy range with an energy resolution of 180 eV (at 5.9 keV) and a cadence of 1 second. During the period from September 2019 to May 2020, covering the minimum of Solar Cycle 24, it observed nine B-class flares ranging from B1.3 to B4.5. Using time-resolved spectroscopic analysis during these flares, we examined the evolution of temperature, emission measure, and absolute elemental abundances of four elements -Mg, Al, Si, and S. These are the first measurements of absolute abundances during such small flares and this study offers a unique insight into the evolution of absolute abundances as the flares evolve. Our results demonstrate that the abundances of these four elements decrease towards their photospheric values during the peak phase of the flares. During the decay phase, the abundances are observed to quickly return to their pre-flare coronal values. The depletion of elemental abundances during the flares is consistent with the standard flare model, suggesting the injection of fresh material into coronal loops as a result of chromospheric evaporation. To explain the quick recovery of the so-called coronal "First Ionization Potential (FIP) bias" we propose two scenarios based on the Ponderomotive force model.

Multiwavelength Observations by XSM, Hinode, and SDO of an Active Region. Chemical Abundances and Temperatures

“…During its disk crossing, this AR (the only one on the visible disc) produced many microflares. A few of the larger, B-class flares have already been analysed by Mondal et al (2021). In all those cases, the behaviour of the chemical abundances was similar, i.e.…”

“…In our previous study (Mondal et al 2021), we used XSM data to show for the first time that chemical abundances of low-FIP elements quickly decrease towards photospheric values in B-class flares, with a return towards coronal values during the gradual phase. AR 12759 produced many such flares during its disk crossing, but clearly such variations did not affect the abundances of the AR core.…”

“…We used XSM Data Analysis Software (Mithun et al 2021) to generate the level-2 science data product for the Solar observations by selecting the Good Time Intervals (GTI) based on the observing geometry and a few other instrumental parameters. To do a spectral analysis of the XSM spectra we used the "chisoth" model (Mondal et al 2021). This is a local model of X-ray spectral fitting package (XSPEC) (Arnaud 1996), meant for the spectral fitting of the observed X-ray spectrum.…”

“…XSM does not have enough sensitivity to measure chemical abundances for individual microflares, but is able to do so for B-class flares. Mondal et al (2021) presented a timeresolved spectroscopic analysis of several B-class flares occurring in AR cores. During the peak of the X-ray emission, temperatures reached 6-8 MK, whilst the chemical abundances of Mg, Al, and Si decreased towards their photospheric values.…”