The light-ion trough, the main trough, and the plasmapause

Abstract

Extensive observations of midlatitude depletions in electron and total ion density by both direct and indirect techniques have prompted numerous studies of the possible association between these troughs, observed both in the F region and in the topside ionosphere, and the plasmapause. Within this work, one basic problem arises in that, although the plasmapause has been detected as a global phenomenon by both VLF and ion composition measurements, the electron and ion density troughs have been identified primarily as nightside features. This problem, as well as the difficulty in explaining various inconsistencies in relating the position of the plasmapause and the ionization trough, is explained by a close examination of the ion composition, generally unavailable in previous trough studies. In particular, ion composition results from the polar-orbiting Ogo satellites identify the persistence of a pronounced light-ion trough (LIT) in H+ and He+, often identified by order-of-magnitude decreases in the light-ion concentrations, that occurs within a few degrees of latitude and reaches residual concentration levels of 10²–10³ ions/cm³ near 60° dipole latitude. The LIT, observed both within the thermosphere and at high latitudes in the magnetosphere, has been correlated directly with the VLF whistler identification of the plasmapause. On the nightside near 1000 km a complex relationship is observed between the dominant ions O+ and H+. The rapidly changing mean ion mass observed at the plasmapause typically produces a resultant total ion density distribution of complexity sufficient to confuse correlative studies with the plasmapause, even though the LIT is very sharply defined within the ion composition. On the dayside, where O+ is typically the predominant ion near 1000 km, LIT structure is often hidden beneath the O+ layer, which may exhibit little or no depletion even though H+ and He+ show trough structure at the plasmapause. This analysis of the midlatitude topside ion composition indicates the need for caution both in studies of the plasmapause per se and in phenomenological studies such as the relationship between the subauroral red arc and the plasmapause, which are most likely dependent on the detailed information available within the ion composition.