Expansion of Hot Plasma with Kappa Distribution into Cold Plasma

The X-ray emission of coronal flare sources can be explained by considering the kappa electron distribution. Motivated by this fact, we study the problem of how hot plasma with the kappa distribution of electrons is confined in these sources. For comparison, we analyze the same problem, but with the Maxwellian distribution. We use a 3D particle-in-cell code, which is large in one direction and thus effectively only one-dimensional, but describe all electromagnetic effects. In the case with the Maxwellian distribution, and in agreement with the previous studies, we show a formation of the double layer at the hot–cold transition region that suppresses the flux of hot electrons from hot plasma into the cold one. In the case with the kappa distribution, contrary to the Maxwellian case, we found that there are several fronts with the double layers in the hot–cold transition region. It is caused by a more extended tail in the kappa case than in the Maxwellian one. The electrons from the extended tail freely escape from the hot plasma into a cold one. They form a beam that generates the return current and also Langmuir turbulence, where Langmuir waves accumulated at some locations. At these locations, owing to the ponderomotive force, Langmuir waves generate density depressions, where the double layers with the thermal fronts that suppress the hot electron flux, are formed. We also show how protons accelerate in these processes. Finally, we compare the Kappa and Maxwellian cases and discuss how these processes could be observed.