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 upon the observational data of the fast magnetic reconnection in the nearly collisionless magnetotail and the particle in cell (PIC)simulations on the electron acceleration in the reconnecting current sheet with guide magnetic field,we self consistently solved one dimension Vlasov equation with the magnetotail parameters and realistic mass ratio to explore the relationship between the anomalous resistivity and the induced electric field.As compared with theoretic formula for the current driven ion-acoustic and Buneman anomalous resistivity,the anomalous resistivity may result from the ion acoustic instability for small reconnecting electric field and the Buneman instability for large reconnecting electric field.The discrepancy between the theoretic results and numerical simulations may be caused by the high frequency instability that results from the deviation of electron distribution from Maxwellian one.These results are consistent with the early experimental results and favorable for the fast reconnection to take place.
Anomalous resistivity is critical for triggering fast magnetic reconnection in the nearly collisionless coronal plasma.Its nonlinear dependence on bulk drift velocity is usually assumed in MHD simulations.However,the mechanism for the production of anomalous resistivity and its evolution is still an open question.We numerically solved the one dimension Vlasov equation with the typical solar coronal parameters and realistic mass ratios to infer the relationship between anomalous resistivity and bulk drift velocity of electrons in the reconnecting current sheets as well as its nonlinear characteristics.Our principal findings are summarized as follows:1) the relationship between the anomalous resistivity and bulk drift velocity of electrons relative to ions may be described as ηmax = 0.03724 vvde 5.702 Ω m for vd/ve in the range of 1.4-2.0 and ηmax = 0.8746 vvde 1.284 Ω m for vd/ve in the range of 2.5-4.5;2) if drift velocity is just slightly larger than the threshold of ion-acoustic instability,the anomalous resistivity due to the wave-particle interactions is enhanced by about five orders as compared with classic resistivity due to Coulomb collisions.With the increase of drift velocity from 1.4ve to 4.5ve,the anomalous resistivity continues to increase 100 times;3) in the rise phase of unstable waves,the anomalous resistivity has the same order as the one estimated from quasi-linear theory;after saturation of unstable waves,the anomalous resistivity decreases at least about one order as compared with its peak value;4) considering that the final velocity of electrons ejected out of the reconnecting current sheet(RCS) decreases with the distance from the neutral point in the neutral plane,the anomalous resistivity decreases with the distance from the neutral point,which is favorable for the Petschek-like reconnection to take place.
