Evolution of ion distribution functions during the “aging” process of thin current sheets Original Research Article

This paper is devoted to the analysis of ion distributions in the velocity space {vx, vy} of the self-consistent thin current sheets (TCSs) due to the influence of nonadiabatic effects. In the frame of 1D-model it is shown that nonadiabatic jumps of the approximate integral of motion Iz may lead to the trapping of the particle in the vicinity of the sheet and influence significantly the structure of newly formed current sheets supported by ions at Speiser orbits. The part of the Speiser population is diffusing into the quasi-trapped domain, where ions have so called “cucumber” orbits. The scattering process results in the appearance of ions with large magnetic moments, which may reduce the current in the center of the sheet. We consider this process as deterioration, or “aging”, that might finally lead to TCS disruption. In the course of TCS “aging”, ion distribution functions experience characteristic evolution, which we are analyzing in this paper. The plots of velocity distribution functions corresponding to three characteristic moments of TCS “aging” are presented: 1) “young” sheet with only Speiser ions; 2) “deteriorated” TCS with the scattered quasi-trapped population; 3) “old” TCS before the very moment of destruction when the population of quasi-trapped plasma is very abundant. It is shown that quasi-trapped population occupies the well-defined domains in the phase space which could be identified by the in situ measurements of ion distribution function in the vicinity of the field reversal and may indicate the stage of the temporal TCS dynamical evolution.