Plasma distribution solver for the field-aligned distribution of ionospheric/magnetospheric plasma related to the auroral electron acceleration process

Abstract:

FAST observation suggested the propagation of Alfven wave from the outer magnetosphere into the Earth’s auroral acceleration region [Chaston et al., 2002], resulting in the Alfvenic acceleration of auroral electrons. Juno revealed the significant roles of the Alfvenic acceleration in the Jupiter’s auroral regions [Mauk et al., 2017]. Despite the increasing attention to the Alfvenic acceleration process in considering the aurora formation of magnetized planets, physical processes controlling the characteristic energy and pitch angle distributions have been still unclear.

For the discussion of Alfvenic acceleration, the spatial distribution of multispecies ions and electrons along a field line is necessary to understand properties of Alfven waves. In the present study, based on the model used in Ergun et al. (2000) and Matsuda et al. (2010), we developed a Plasma Distribution Solver for the plasma distribution along a magnetic field line between the ionosphere and magnetic equator. The developed Plasma Distribution Solver iteratively computes both the spatial distribution of multispecies plasma and the electrostatic potential to satisfy Poisson’s equation, as shown in Fig. a) and b). We assume bi-Maxwellian distributions for the initial velocity distributions of plasmas at the ionospheric end and at the magnetic equator. By referring to both energy conservation law and adiabatic invariant, we determine the interval of integration in the velocity space for a certain location, and then integrate the distribution function at the boundary in the determined interval of the velocity space in order to obtain the number density there.

Using the developed Plasma Distribution Solver, we study the effects of the assumed boundary condition and the initial condition of the electrostatic potential. We show that the plasma distribution changes according to the assumed boundary condition. We also show that there can be multiple solutions under the same boundary condition and different initial gap position of the electrostatic potential.

We show results of the Plasma Distribution Solver and discuss the variations of the solution under different initial settings. The comparison with those obtained by the conventional plasma distribution model is made as well.