ULF pulsation eigenperiods within the plasmasphere

Abstract

Physically realistic ion density models are applied to calculations of toroidal mode ULF pulsation eigenperiods within the plasmasphere. The model periods reflect the dominant effect of O + ions along the shorter field lines at lower latitudes, and of H + ions in the outer plasmasphere. In the absence of electromagnetic drifts, the largest periods occur during the day. Inclusion of previously measured azimuthal electric field values retains the daytime period maximum at latitudes below ∼35°, but results in a period minimum near mid-day at higher latitudes. The latitudinal period variation has a peak value near 20° where O + is dominant, a minimum near 35° and increasing periods at higher latitudes. Geomagnetic activity is simulated by reducing the ion densities in flux tubes at latitudes greater than 50°. While the H + population is replenished from the underlying ionosphere, the model periods have a corresponding increase, as does the rate of period increase with latitude. At mid latitudes (30–40°) where neither the O + nor H + ion populations dominate, the toroidal mode higher harmonics have irregularly spaced frequencies. The Alfvén velocity distribution along the mid-latitude field lines suggests the standing wave fields, and hence mapping factors between the equatorial plane and ionosphere, could be different to those employed at higher latitudes where H + is the dominant ion.