Six auroral generators: A review

Abstract

[1] The paper reviews generator processes and configurations of six types of auroral arcs: embedded arcs, Alfvenic arcs, onset arcs, poleward arcs during substorms, auroral streamers, and auroral spirals. The arcs and generators are elements in global current systems, which are classified as Types I and II after Bostrom. The arcs may be dominated by transient processes or be quasi-stationary. The main emphasis of the paper is on the generator forces. They are pressure gradient forces, magnetic normal or shear stresses, or inertial forces. For three cases, the arcs embedded in the oval convection, the poleward arc during substorms, and the auroral streamers, simple expressions are presented of the currents injected into the ionosphere by the generator process, allowing quantitative evaluations. The relations leading from these currents to other key auroral quantities are summarized. The apparent conflict between the widths of Alfvenic arcs and the transverse scales required for energy coupling to the topside ionospheric plasma is solved by assuming current and field filamentation by multiple reflections in the ionospheric Alfven resonator. The substorm generator is described as a high-beta plasma layer arising from collapse of the tail current sheet. An essential element in this process is the shedding of excess flux tube entropy through energy dumping in the auroral acceleration process and ionospheric dissipation. The dynamics of this process needs further investigation. The physics of the connection between flow bursts in the tail plasma sheet and the flows associated with auroral streamers in the ionosphere is discussed. Regular auroral spirals and the westward traveling surge have in common a concentration of upward field-aligned current which demands strongly enhanced dissipation. They differ by the processes creating the upward current concentration.