Abstract
Abstract. The plasma on an auroral field line is simulated using a Vlasov model. In the initial state, the acceleration region extends from one to three Earth radii in altitude with about half of the acceleration voltage concentrated in a stationary double layer at the bottom of this region. A population of electrons is trapped between the double layer and their magnetic mirror points at lower altitudes. A simulation study is carried out to examine the effects of fluctuations in the total accelerating voltage, which may be due to changes in the generator or the load of the auroral current circuit. The electron distribution function on the high potential side of the double layer changes significantly depending on whether the perturbation is toward higher or lower voltages, and therefore measurements of electron distribution functions provide information about the recent history of the voltage. Electron phase space holes are seen as a result of the induced fluctuations. Most of the voltage perturbation is assumed by the double layer. Hysteresis effects in the position of the double layer are observed when the voltage first is lowered and then brought back to its initial value.
Highlights
Auroral optical emissions are caused by electrons that have been accelerated by electric fields that are parallel to the magnetic field
The acceleration region extends from one to three Earth radii in altitude with about half of the acceleration voltage concentrated in a stationary double layer at the bottom of this region
The auroral current circuit can be viewed as being composed of four elements: a generator in the equatorial magnetosphere and a load in the ionosphere that are coupled together by the upward and downward current regions
Summary
Auroral optical emissions are caused by electrons that have been accelerated by electric fields that are parallel to the magnetic field. Laboratory experiments on parallel electric fields in magnetic mirror fields have been carried out in Q-machines (Sato et al, 1986, 1988) and modelled using particle in cell simulations (Ishiguro et al, 1995) It was found in experiments, simulations, and theory of discharge plasmas that electrons accelerated in an electric double layer can cause oscillating electric field spikes when entering a density gradient on its high potential side (Gunell et al, 1996a, b; Gunell and Löfgren, 1997; Löfgren and Gunell, 1998; Brenning et al, 2006). We perform numerical experiments on a system that contains trapped electrons in order to study how they are trapped and how they can be released again
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