Relationship between field-aligned electron fluxes and field line topology at the tail lobe magnetopause: Geotail observations

Abstract

Using Geotail observations, we investigate characteristics of field-aligned, suprathermal electron fluxes in the transition region between the magnetosheath and the distant ( X < −100 R E) tail lobe in order to clarify their relationship with field line topologies. The behavior of 100–300 eV electrons in the boundary layer is closely correlated with the solar wind strahl electrons, suggesting that the electrons result from the direct entry of the solar wind electrons along reconnected field lines. A remarkable finding is the frequent occurrence of unidirectionally streaming electrons, which implies that field lines threading the boundary layer are of IMF-type, i.e., they are connected at one end to the sun and at the other end to the interplanetary medium beyond the Earth. The unidirectional electron flux usually coexists with double-component ion populations, one part of which streams faster than the magnetosheath flow. This fact again supports the view that the field lines are disconnected from the Earth through magnetic reconnection occurring on lobe open field lines, since the high-speed ion population would be generated tailward of the high-latitude reconnection site. We suggest that the tailward unidirectional electron fluxes are found in the boundary region when the northern (southern) lobe field lines are reconnected with the IMF in toward (away) sectors, while the sunward unidirectional electron fluxes are found when the northern (southern) field lines are reconnected with the IMF in away (toward) sectors. Our statistics show that the tailward and sunward unidirectional electron fluxes are encountered with similar occurrence probabilities. It is inferred that, in each hemisphere, high-latitude reconnection takes place for both IMF sectors, that is, regardless of the polarity of IMF B x .