The sink for outer belt electrons and the electrodynamics of the middle atmosphere

Abstract The middle atmosphere at the foot of the geomagnetic field lines of the outer belt of trapped radiation (about L =4) is a region of dramatically variable electron precipitation. Consequently, electrodynamic phenomena in the atmosphere of this region exhibit significant variation. Typically, the electron population of the outer belt increases by several orders of magnitude during geomagnetic activity associated with the passage of a solar sector boundary; then, over the course of the following week or so, this enormous quantity of trapped electrons precipitates into the atmosphere. However, due to the geometry of the geomagnetic field, this electron precipitation is not distributed evenly: it is concentrated in the sink for outer belt electrons in Antarctica above the Weddell Sea. This “sink” occurs because trapped electrons are scattered to lower equatorial pitch angles by wave–particle interactions. Individual scattering events are smaller than the longitudinal, or hemispheric, variations in the equatorial pitch angle at which electrons are lost into the atmosphere, thus precipitation occurs around 0° longitude in the southern hemisphere for eastward drifting trapped electrons. West of this location, electrons can be temporarily trapped at smaller equatorial pitch angles, but eastward drift transports these particles into the atmosphere, a phenomenon referred to as the drift loss cone. The diurnal modulation of electron precipitation induced by the effect of the dawn-to-dusk geoelectric field is discussed, as well. Experimental data that support this model of electron precipitation are presented. Also, the effect of the outer belt electron sink on the chemistry of the middle atmosphere is introduced.