The Statistical Study Of The Neupert Effect In Solar Flares And The Test Of X-ray Imaging Algorithms

A large number of particles are accelerated to high energy in a short period of time duraing a solar flare.These particles then produce a series of flare phenomena and sec-ondary effects in the solar atmosphere.The observation of high-energy X-ray of the Sun can reveal the process of energy release and transmission of flares.The qualita-tive description of the Neupert effect is that there is a causal relationship between the pulse component(hard X-ray,microwave burst)and the gradual component(soft X-ray emission)in a flare.According to the quantitative description of the classic Neupert effect,SXR should reach maximum instantly at the end of HXR emission(sign of non-thermal electron injection).However,previous observations have found that for quite a number of flares the SXR peak time(t2)is significantly later than the end time of HXR(t1)(?=t2-t1,?>0),deviating from the classic Neupert effect.In order to study the events deviating from the classic Neupert effect or not,we checked the RHESSI and GOES events list from 2002 to 2015,and found out flare samples that have simple lightcurves at 25-50 keV together with other criteria.A total of 276 flare samples were finally selected.We have analysed the ? distribution of these flares,as well as the re-lationship between the loop length(represented by the distance between two footpoint sources d)and ?.We found that:(1)227 sample flares present ?>0,which means that about 82%of total samples deviate from the classical Neupert effect;(2)there is a roughly linear correlation between ? and d,that is,the longer the loop is,the later the maximum time of SXR with respect to the end of HXR is;(3)there seems to have a crit-ical distance,within which the classic Neupert effect works.We have also made some statistics of other flare parameters using these data,and found that the time of maxi-mum plasma temperature corresponds almost to the end time of hard X-ray(t1).These results confirm the necessity of modifying Neupert effect and it is helpful to understand the energy process in flare eruption.The Hard X-ray Imager(HXI)is a payload of ASO-S,the first space solar mission in China.It uses the spatial modulation Fourier transform imaging principle.In order to study the characteristics of different imaging algorithms and provide reference for the future development of ASO-S/HXI imaging algorithms,we selected three sources of different spatial forms(point,loop and foot-points),and tested 10 algorithms based on RHESSI software.We found that(1)Clean algorithm can eliminate sidelobes very well,but the display of source shape details is poor.(2)EM and MEM are more suitable for extended sources,such as loop sources.(3)Pixon,VIS CS and vis WV can be used when compact source and loop source exist at the same time.(4)Forward fit is constrained by the parameters selected.The image for loop and extended sources is poor,but the influence of compact source is little.(5)Different algorithms have little effect on photon flux.These results provide a reference for the development of HXI imaging algorithms,but due to the small number of samples,more research is still needed.