Generalization of the Neupert effect over the Solar Flare Plasma Cooling

AWASTHI ARUN KUMAR, awasthi@prl.res.in, Physical Research Laboratory, India
Jain Rajmal, rajmal@prl.res.in, Physical Research Laboratory

Abstract
We investigate 10 M-class flares observed by the “Solar X-ray Spectrometer” (SOXS) onboard GSAT-2 Indian spacecraft to study the influence of the solar flare plasma cooling processes on the Neupert effect. It is widely known that the energetic electrons responsible for the hard X-ray (HXR) emission by thick-target bremsstrahlung are the source of heating and thereby causing chromospheric evaporation seen in soft X-rays (SXR). A close relationship between the non-thermal and thermal time profiles was found, which is known as the Neupert effect. Accordingly the time-derivative of thermal emission should mimic the non-thermal emission. This relationship, however, is strictly expected only for the asymptotic limit of very long cooling times while a physically more accurate model is necessary to generalize over finite cooling time. In this regard, we study the temporal evolution of X-ray emission observed by CZT detector of SOXS mission in 7-10 keV and 10-30 keV representing the SXR and HXR emission respectively. We model the cooling as a function of time by estimating the ratio of time-derivative of SXR with the HXR flux. We found that the ratio is exponentially decaying in rise phase of the flare evolution, which, however, starts to rise after the peak emission in HXR. This changeover represents the swapping of underlying process of cooling of the plasma from conductive to radiative. We estimate the conduction cooling time scale for the events under investigation and found to be varying between 65 and 525 s. However, the modeling of radiative cooling time scale has not been possible due to fast decay of HXR flux. In this regard, we study the radiative cooling time scale by employing H-alpha and SXR emission as both the emission are dominant emission in post-flare.