Stable auroral red arcs, sinks for energy ofDstmain phase

Abstract

A new source of energy for the ionosphere during geomagnetic storms is discussed. Stable auroral red arcs, together with the atmosphere in their vicinity, constitute a major sink of energy for the Dst main phase of a magnetic storm. The intensity of the red arc, the electron density in the F region, the inferred temperature of electrons in the arc, the inferred transfer of energy from electrons and ions to neutral particles in the region, and the (uninhibited) decay rate of Dst main phase energy are found to be mutually consistent. Energetic trapped particles causing a major part of the Dst main phase are degraded in energy by Coulomb collisions with a ‘background’ magnetospheric ‘thermalized’ plasma of temperature about 10,000°K. The temperature of F-region electrons (about 3200°K in a 100 γ storm), sufficient to excite the arc, is maintained by heat conduction from the magnetosphere along the lines of force. Conduction in the electron-ion gas proceeds downward to a ‘transition’ level where transfer of heat to neutrals absorbs the heat flux. Below this height, conduction in the neutrals dominates. This source of heat causes the temperature of the neutral particles in the arc to rise by amounts of the order of 100°K in a 100 γ storm. Most of the reported properties of the arc are consistent with the theory. The present paper and a recent paper by the same author provide a coherent theory of (1) the main-phase decrease of the surface geomagnetic field during storms, (2) the depletion of ionization in the outer magnetosphere during storms, (3) the stable auroral red arc, and (4) Dst variations of the F region at mid-latitudes.