Impulsive chromospheric heating of two-ribbon flares by the fast reconnection mechanism

Abstract

Chromospheric heating of two-ribbon flares is quantitatively studied for different values of R0, the ratio of the chromospheric plasma density to the coronal one, on the basis of the spontaneous fast reconnection model. In general, occurrence of impulsive chromospheric joule heating is delayed for the larger R0 because of more Alfvén traveling time in the chromosphere. Once the chromospheric heating occurs, the temperature becomes more than 30 times its initial value for the case of R0=400 in a pair of layers of deep chromosphere, and the region of high temperature shifts upward and becomes broader with time, since the chromospheric thin layer of joule heating shifts upward according to a pileup of reconnected field lines in the flare loop; then, chromospheric evaporation grows and extends outward, and its velocity becomes comparable with the coronal downflow velocity inside the loop boundary. The impulsive chromospheric heating is caused by drastic evolution of the flare current wedge, through which some part of the coronal sheet current suddenly turns its direction to be concentrated into the chromospheric thin layer; simultaneously, a magnetohydrodynamic (MHD) generator arises ahead of the flare loop top to provide a new current circuit inside the large-scale flare current wedge. Hence, it is concluded that the powerful MHD generator, sustained by the fast reconnection jet, drives the flare current wedge to evolve, leading to the impulsive chromospheric heating.