Characteristics of solar microflares as seen in soft X-ray emission

Abstract

In this paper, we present the thermal and non-thermal characteristics of solar plasma producing microflares in 4–12 keV energy range. The X-ray spectra of 10 B-class solar microflares observed by the silicon (Si) detector (4–25 keV) on-board solar X-ray spectrometer (SOXS) mission were analysed in 4–12 keV energy range. We employed forward fitting for the spectral modelling of thermal and non-thermal components of X-ray spectra with isothermal, multithermal and single power-law functions in order to determine flare parameters. The fit results obtained from the combination of isothermal and single power-law functions yield a weighted mean value of emission measure \((\mathrm{EM})\approx 0.0203 \times 10^{49} \mathrm{cm^{-3}} \), plasma temperature [Case (1)] \( {T(1)} \approx 10.24\,{\mathrm{MK}}\) and non-thermal spectral index \( \gamma (1) \approx 3.90 \). The fit results obtained from the combination of multithermal and single power-law functions yield a weighted mean value of differential emission measure, \((\mathrm{DEM}) \approx 0.00116 \times 10^{49}\,\mathrm{cm^{-3}}\, \mathrm{keV^{-1}} \), plasma temperature [Case (2)], \({T(2)}\approx 12.90\,{\mathrm{MK}}\), thermal spectral index, \(\delta \approx 4.06\) and non-thermal spectral index, \( \gamma (2) \approx 3.81 \). Further, we obtained the mean value of conduction cooling time, \( \tau _{\mathrm{c}}(T)\approx 283\,{\mathrm{s}}\) at 11.6 MK, thermal energy, \({E_{\mathrm{th}}} \approx 0.50 \times 10^{29}{\mathrm{erg}}\) and thermal–non-thermal cross-over energy, \( \epsilon _{\mathrm{th}} \approx 9.23\) keV. In this analysis, the obtained results were found to be compatible with the earlier analysis carried out for the microflares through Reuven Ramety High Energy Solar Spectroscopic Imager (RHESSI), Solar Dynamics Observatory / Atmospheric Imaging Assembly (SDO/AIA) and NuSTAR observations. Here, we observed that EM decreases with increasing plasma temperature (T). We find that \(\tau _{\mathrm{c}}\)(T) scale with plasma temperature (T) with an inverse gradient exhibits time delay characteristic of the cooling process of plasma. The correlation of \( E_{\mathrm{th}} \) and temperature (T) shows moderate anticorrelation. The present analysis demonstrates the multithermal plasma model and conduction cooling process during high temperature of microflares (similar to large flares) followed by radiative cooling in post-flare.