Dissipation and reformation of thermal fronts in solar flares

Abstract

Thermal fronts are often used to explain the confinement of energetic electrons in the solar corona. In this paper, a 1-D particle-in-cell simulation model is used to explore the evolution of thermal fronts in the flaring magnetic loop. The numerical simulation starts with hot electrons in contact with background cold electrons along the magnetic field. The hot electrons transport along the magnetic field and induce a polarized electric field, drawing a return current from the background cold electrons. This return current can generate ion acoustic waves which could evolve into a double layer (DL). The DL is able to inhibit the transport of electron heat flux, which ultimately leads to the formation of a thermal front. However, the thermal front cannot persist for very long, since the DL will be dissipated by ambient cold protons. As a result, the cold protons trapped within the DL will be efficiently heated. After the vanishing of the first thermal front, hot electrons can again freely expand into the cold plasma, resulting in the growth of ion acoustic waves, which ultimately develop into a new DL. Then, a new thermal front will reform at a location farther into the regions of hot electrons. The new thermal front moves forward \(200 \lambda _{D}\) (\(\lambda _{D}\) is the cold electron Debye length) toward the regions containing hot electrons. The implications of simulation results to the observations of hard X-ray emission and confinement of energetic electrons in the corona are also discussed.