Seasonal dependence of the electron density in the topside ionosphere

*Ryoichi Fujii[1]
STE Laboratory, Nagoya University[1]

The ionosphere was considered to play a passive role on the M-I coupling. Recent satellite observations, however, have shown that ionospheric thermal electrons are sometimes accelerated to a few hundred eV with carrying downward field-aligned currents. Such upward electron beams were observed almost exclusively in the winter hemisphere, suggesting that the ionosphere should play an important role in their formation. In this paper, based on an analysis of data obtained from the EISCAT radars, we present seasonal variations of plasma distributions in the topside ionosphere in order to better understand relationships between poorer electron density and the production of field-aligned electric field in winter.

The magnetosphere and ionosphere strongly interact each other, exchanging energy in the form of electromagnetic energy fluxes accompanied by currents and electric fields, of particle energy fluxes associated with plasma precipitation into the ionosphere and ionospheric plasma outflow into the magnetosphere, and/or of plasma wave energy fluxes. The ionosphere was previously considered to play a passive role on these energy and mass transfer processes called the M-I coupling. Recent observations, however, have clearly shown that this is not always the case but the ionosphere sometimes plays an active role on them. Indeed, observations with the FAST and Freja satellites have shown that ionospheric thermal electrons are sometimes accelerated to a few hundred eV ~ a few keV with carrying downward field-aligned currents. Such upward electron beams were observed almost exclusively in the winter hemisphere, suggesting that the ionosphere should play an important role in their formation. Not only the ionospheric conductivity but also the electron distribution in the topside ionosphere and bottomside magnetosphere should be heavily involved in the production process of these ionospheric electron beams associated with downward field-aligned currents, since a poorer electron density in the topside ionosphere can produce a larger field-aligned electric field to keep the current continuity. In this paper, based on an analysis of data obtained from the EISCAT radars, we present seasonal variations (solar zenith angle dependence) of plasma distributions in the topside ionosphere in order to better understand relationships between poorer electron density and the production of field-aligned electric field in winter.