Characteristics of the auroral particle acceleration in the upward and downward current regions

To understand auroral particle acceleration it is necessary to consider the entire auroral current circuit, from the generator in the outer magnetosphere to the auroral ionosphere, decoupled from each other by field-aligned potential drops in both the upward and downward current regions. This paper focusses on the characteristics of such potential drops responsible for the accelerated electrons that produce the aurora and for the upward acceleration of ions and electrons. First, some features of the converging electric field structures in the upward current region are discussed based predominantly on results from Viking observations within and above the acceleration region. These include the relationship between the high-altitude potential and the ionospheric potential associated with auroral arcs; ways to estimate the field-aligned potential drop from in situ satellite measurements of fields and particles; the role of low-frequency electric field fluctuations for acceleration and heating of auroral particles. A brief review is then given of recent Freja results on characteristics of the downward current region, such as the intense diverging electric field structures that give rise to upward acceleration of ionospheric electrons and perpendicular ion heating. Different theoretical models have been proposed to explain these phenomena. It is clear that the plasma density on auroral field lines and the ionospheric density play a fundamental role for where and when potential structures are formed in the upward and downward current region. For a current generator feeding the auroral currents, particle acceleration is necessary to maintain the current through regions of low plasma density. The altitude distribution of the field-aligned potential will thus depend strongly on the local ambient plasma conditions, and thus vary with local time, season and magnetic activity level. These characteristics could be investigated in detail by combining measurements from various spacecraft (such as Viking, Polar, Freja, FAST, and Astrid-2) at different altitudes with respect to the the field-aligned potential distribution.