Numerical simulation of magnetospheric convection including the effect of field‐aligned currents and electron precipitation

Abstract

Early results of a new self‐consistent fluid model are presented for steady convection of the plasma from the geomagnetic tail through the Earth's inner magnetosphere below 10 Earth radii (RE), including its coupling with the ionosphere. This model computes the transport of both the ion and electron fluids and constitutes an important improvement of the fluid numerical model of Fontaine et al. (1985) (referred to as Paper 1), which simulated the convection of electrons only. The coupling with the ionospheric convection is effected by the precipitation of magnetospheric electrons, which enhances ionospheric auroral conductivities, and by the flow of region 2 field‐aligned currents, which connect the magnetospheric and ionospheric circuits. This last coupling was not considered in Paper 1, because region 2 field‐aligned currents are generated mainly by pressure gradients of the ion plasma that develop progressively during its motion. We also take into account the ion precipitation, which affects the generation of region 2 field‐aligned currents, but we neglect their relatively small effect on the ionospheric conductivities. We devoted much effort to produce a self‐consistent description of the transport of the ionospheric and magnetospheric plasmas, and their couplings. As a first step, in order to distinguish basic physical processes from effects of geometrical origin, we solved the equations for the case of a dipolar magnetic field as in Paper 1. As a consequence, we do not expect our results to reproduce very precisely the available observations but rather to provide only correct orders of magnitude. The numerical solution scheme of the hyperbolic equations governing the transport of both magnetospheric ions and electrons, of the elliptic equation of the ionospheric electrostatic potential, and of their coupling due to electron precipitation and region 2 field‐aligned currents, is basically the same as Paper 1. It makes use of the finite element method. The numerical model is run to simulate the evolution of the system from an initial state in which the inner magnetosphere is free of plasma, to a steady state situation in which the plasma has penetrated into the inner magnetosphere from the tail. Stable magnetic conditions corresponding to a Kp between 2 and 3 are considered to allow a comparison with observations. The formation of a belt of electron and ion precipitation in the auroral zone is globally reproduced by the model, but the electron fluxes are underestimated and the auroral ion zone is located equatorward with regard to the DMSP satellite statistical observations. The field‐aligned current region 2 computed by the model is comparable to the available statistical observations, apart from an underestimation of the amplitude. The convection structure is also found comparable to observations, thus improving the results of Paper 1. The amplitude and the direction of the major component of the electric field (the northward component being at least 4 times larger than the eastward component) is consistent with the observations, but the high latitude distribution of the eastward component is incorrectly predicted. The region 2 field‐aligned currents prevent the electric field from penetrating to midlatitudes and induce an eastward rotation of the overall convection electric field distribution. It is suggested that the ion precipitation rate computed under the strong pitch angle scattering is overestimated and that ion and electron precipitation contributions to the ionospheric conductances are of the same order of magnitude in the dusk sector.