Auroral Electrons Trapped and Lost: A Vlasov Simulation Study

Abstract:

In the upward current region of the aurora, about two thirds of the total voltage between the auroral ionosphere and the equatorial magnetosphere can be concentrated in a stationary double layer at an altitude of about one earth radius, as Vlasov simulations of the plasma on a magnetic field line have shown (Gunell, et al., Ann. Geophys., 31, 1227–1240, 2013).

We perform numerical experiments, changing the total voltage between the ionosphere and the equatorial magnetosphere during the course of the simulation. In the initial state, the total acceleration voltage is 3 kV and there is a double layer approximately 5000 km above the ionospheric end of the system. When the voltage is increased, electrons are trapped between the double layer and the magnetic mirror in a region of velocity space that initially was empty. When the voltage is decreased to its initial value these trapped electrons are released upwards. If the voltage is lowered first and then raised back to where it started, the newly trapped electrons remain trapped. As a consequence of the difference between the two cases, the electron pitch angle distribution, below the double layer, carries information about the recent history of the acceleration voltage. In both cases, most of the change in voltage, ΔV, is assumed by the double layer, in agreement with a study of Cluster data that could confine most of ΔV to altitudes below the spacecraft (Forsyth et al., JGR, 117, A12203, 2012). Hysteresis effects in the double layer position are seen in connection with the electron trapping.

This work was supported by the Belgian Science Policy Office through the Solar-Terrestrial Centre of Excellence and by PRODEX/Cluster contract 13127/98/NL/VJ(IC)-PEA 90316.