一般认为,Langmuir波(LW)转换为电磁波是太阳Ⅲ型射电爆的产生机制。由电子束流不稳定性可以很容易地激发LW,正向LW和反向LW的相互作用被认为是产生Ⅲ型爆二次谐波的原因,但反向LW的色散方程和产生机制尚未得到充分研究。通过粒子(Particle In Cell,PIC)模拟研究了反向LW的产生机制,发现反向LW不能由电子束流直接激发,其能量基本上是由正向LW散射得到的。然而,电子束流对正向LW的二次谐波有直接放大作用。
With RHESSI data from five solar flares taken from beginning to end,we investigate the power conversion factorμdefined as the ratio of the time derivative of total thermal energy(ERHESSI+Erad+Econd)and the kinetic power(PRHESSI)of nonthermal electrons.Here, ERHESSI is the computed energy contained in thermal plasmas traced by RHESSI SXRs.Other two contributions(Erad and Econd)to the total energy are the energies lost through radiation and conduction,both of which can be derived from the observational data.If both are not considered,μis only positive before the SXR maximum.However,we find that for each flare studiedμis positive over the whole duration of the soalr flare after taking into account both radiation and conduction.Mean values forμrange from 11.7% to 34.6%for these five events,indicating roughly that about this fraction of the known energy in nonthermal electrons is efficiently transformed into thermal energy from start to end.This fraction is traced by RHESSI SXR observations;the rest is lost.The bulk of the nonthermal energy could heat the plasma low in the atmosphere to drive mass flows(i.e.chromospheric evaporation).
Based on Dulk and Marsh＇s approximate theory about nonthermal gyrosyn- chrotron radiation, one simple impulsive microwave burst with a loop-like structure is selected for radio diagnostics of the coronal magnetic field and column density of non- thermal electrons, which are calculated from the brightness temperature, polarization degree, and spectral index, as well as the turnover frequency, observed by using the Nobeyama Radioheliograph and the Nobeyama Radio Polarimeters, respectively. Very strong variations （up to one or two orders of magnitude） of the calculated transverse and longitudinal magnetic fields with respect to the line-of-sight, as well as the cal- culated electron column density, appear in the looptop and footpoint sources during the burst. The absolute magnitude and varied range of the transverse magnetic field are evidently larger than those of the longitudinal magnetic field. The time evolution of the transverse magnetic field is always anti-correlated with that of the longitudi- nal magnetic field, but positively correlated with that of the electron column density. These results strongly support the idea that quantifying the energy released in a flare depends on a reconstruction of the coronal magnetic field, especially for the trans- verse magnetic field, and they are basically consistent with the recent theoretical and observational studies on the photospheric magnetic field in solar flares.
