High-latitude electrodynamics: Electromagnetic energy dissipation and neutral wind feedback effects.

Abstract

Results of an experimental and theoretical investigation into the electromagnetic energy budget and the effects of the time dependent neutral wind flywheel on high-latitude electrodynamics are presented. This investigation is carried out using measurements obtained from the DE-2 satellite and numerical simulations from the National Center for Atmospheric Research Thermosphere Ionosphere General Circulation Model (NCAR-TIGCM). A one-dimensional hybrid satellite-track model has been developed to map the DE-2 satellite measurements into the low-altitude dynamo region. A validation study, made using measurements taken from an Incoherent Scatter Radar site during a DE-2 overflight, indicates that the model is able to determine the electric conductivities and Joule heating rate in the dynamo region to a reasonable accuracy. The model is then used to study the conversion of electromagnetic energy in the lower thermosphere. The electromagnetic energy input into the thermosphere is measured by the downward Poynting Flux at DE-2 satellite orbit. Good agreement was found between the amount of energy dissipated by Joule heating and that transferred into the thermosphere by Poynting flux during a southward IMF Bz condition. A relatively small part ($\sim$10%) of this electromagnetic energy is converted into the neutral mechanical energy. The mechanical energy stored in the neutral gas may maintain the high-latitude electric fields and/or drive the reversed currents during decaying magnetic activity. This has been called flywheel effect. A numerical experiment has been conducted to simulate this effect using the NCAR-TIGCM and its diagnostic package. The results of this simulation indicate that the height-integrated Hall currents, Poynting flux and field-aligned currents are dominated by neutral-wind-driven components for several hours after the end of main phase of the storm, if the magnetosphere is assumed to act as a perfect conductor with respect to the thermospheric dynamo. The direction of these currents and fluxes reverse from their normal direction. On the other hand, if it is assumed that magnetosphere acts as an insulator, this study indicates that the high-latitude neutral wind dynamo can generate about 20-30 KV polarization electric fields in the period immediately following the geomagnetic storm. Data from the DE-2 satellite have provided a demonstration of existence of the local flywheel effect and are in good agreement with model calculations.