INTERBALL-Auroral observations of 0.1-12 keV ion gaps in the diffuse auroral zone

Abstract. We examine ion flux dropouts detected by INTERBALL-Auroral upon traversal of the auroral zone at altitudes of \sim13 000 up to 20 000 km. These dropouts which we refer to as "gaps", are frequently observed irrespectively of longitudinal sector and appear characteristic of INTERBALL-Auroral ion spectrograms. Whereas some of these gaps display a nearly monoenergetic character (~12 keV), others occur at energies of a few hundreds of eV up to several keV. INTERBALL-Auroral data exhibit the former monoenergetic gap variety essentially in the evening sector. As examined in previous studies, these gaps appear related to transition from particle orbits that are connected with the magnetotail plasma source to closed orbits encircling the Earth. The latter gap variety, which spreads over several hundreds of eV to a few keV is often observed in the dayside magnetosphere. It is argued that such gaps are due to magnetospheric residence times well above the ion lifetime. This interpretation is supported by numerical orbit calculations which reveal extremely large (up to several tens of hours) times of flight in a limited energy range as a result of conflicting E × B and gradient-curvature drifts. The characteristic energies obtained numerically depend upon both longitude and latitude and are quite consistent with those measured in-situ.

Key words. Magnetospheric physics (auroral phenomena; plasma convection)