Formation Of Thermal Front Research Articles

The role of electrostatic waves in the formation of thermal front in solar flares: 1-D PIC simulation

The electrostatic waves excited during the formation of the thermal front are studied using a one-dimensional electromagnetic particle-in-cell simulation. The thermal front is defined as a sudden decrease in the local electron temperature. The results suggest that the beam mode and the electron acoustic waves are first excited, and then ion acoustic waves are produced. The thermal front just forms in the region of the plasma density depression. The nonlinear evolution of ion acoustic waves can be a potential mechanism for the formation of the thermal front which confines hot electrons and reduces their cooling time. According to the evolution of electron dynamics in phase space, the thermal front actually reflects hot electrons with low kinetic energy through the double layer which is driven by the ion-acoustic waves. That is, the thermal front is actually a double layer.

THERMAL FRONTS IN SOLAR FLARES

We studied the formation of a thermal front during the expansion of hot plasma into colder plasma. We used a three-dimensional electromagnetic particle-in-cell model that includes inductive effects. In early phases, in the area of the expanding hot plasma, we found several thermal fronts, which are defined as a sudden decrease of the local electron kinetic energy. The fronts formed a cascade. Thermal fronts with higher temperature contrast were located near plasma density depressions, generated during the hot plasma expansion. The formation of the main thermal front was associated with the return-current process induced by hot electron expansion and electrons backscattered at the front. A part of the hot plasma was trapped by the thermal front while another part, mainly with the most energetic electrons, escaped and generated Langmuir and electromagnetic waves in front of the thermal front, as shown by the dispersion diagrams. Considering all of these processes and those described in the literature, we show that anomalous electric resistivity is produced at the location of the thermal front. Thus, the thermal front can contribute to energy dissipation in the current-carrying loops of solar flares. We estimated the values of such anomalous resistivity in the solar atmosphere together with collisional resistivity and electric fields. We propose that the slowly drifting reverse drift bursts, observed at the beginning of some solar flares, could be signatures of the thermal front.