摘要：Aims. We constrain energy release and particle acceleration processes in
solar flares by means of comprehensively characterizing the physical parameters of both
the thermal plasma and the accelerated nonthermal particles using X-ray data. Our aim is
to bridge the gap between detailed case studies and large statistical studies.
Methods. We obtained time series of spectral fits and images for 24
flares ranging from GOES class C3.4 to X17.2 using RHESSI hard X-ray observations. These
data were used to derive basic physical parameters for the thermal plasma (using the
isothermal approximation) and the injected nonthermal electrons (assuming the thick-target
model). For the thermal component, this was supplemented by GOES soft X-ray data. We
derived the ranges and distributions of the various parameters, the scaling with flare
importance, and the relation between thermal parameters derived from RHESSI and GOES.
Finally, we investigated the relation between thermal and nonthermal parameters.
Results. Temperature and emission measure of the thermal plasma are
strongly correlated with the peak GOES X-ray flux. Higher emission measures result both
from a larger source volume and a higher density, with the latter effect being more
important. RHESSI consistently gives higher temperatures and lower emission measures than
GOES does, which is a signature of a multithermal plasma. The discrepancy between RHESSI
and GOES is particularly pronounced in the early flare phase, when the thermal X-ray
sources tend to be large and located higher in the corona. The energy input rate by
nonthermal electrons is correlated with temperature and with the increase rate of emission
measure and thermal energy.
Conclusions. The derived relations between RHESSI- and GOES-derived
thermal parameters and the relation between thermal parameters and energy input by
nonthermal electrons are consistent with a two-component model of the thermal flare
plasma. Both RHESSI and GOES observe a cooler plasma component (≈10−25 MK) that is generated by chromospheric
evaporation caused by a nonthermal electron beam. In addition, a hotter component
(≥25 MK) is only detected by RHESSI; this
component is more consistent with direct in situ heating of coronal plasma. With the
exception of the early impulsive phase, RHESSI observes a combination of the evaporated
and the directly heated component.
关键词：Sun: flares;Sun: X-rays, gamma rays;acceleration of particles
