Electron and ion signatures of field line topology at the low‐shear magnetopause

Abstract

Electrons above 50 eV are a sensitive indicator of field line topology at the magnetopause, particularly when the solar wind dynamic pressure is high and the shear across the boundary is low. AMPTE/CCE electron observations under conditions when these criteria are fulfilled indicate a clear topological transition from the magnetosheath to open field lines threading the magnetopause in the magnetosheath boundary layer (MSBL). Once across the magnetopause and in the low latitude boundary layer (LLBL), the fast moving electrons are no longer a good indicator of magnetic field topology. In particular, the counterstreaming electron observations in this region are not an indicator of a closed magnetic topology. Rather, the field topology continues to be open, and the counterstreaming occurs because electrons from the magnetopause region move rapidly enough along the LLBL magnetic field to make it to the ionosphere, mirror, and return to the observation point Slower moving ions provide important additional information on magnetic field topology in the LLBL. CCE observations discussed here indicate that two types of solar wind ion distributions are observed in this layer. One type consists of a single, heated distribution which resembles somewhat the electron distribution in the layer. The field‐aligned velocity of this distribution is near zero. The other type consists of a unidirectional streaming distribution. The field‐aligned velocity of this distribution is higher than in the adjacent magnetosheath. Combining these observations with magnetospheric ion observations (e.g., O+) in the LLBL and with electron observations in the MSBL, two distinct magnetic field topologies emerge for the low‐shear magnetopause. The first, which gives rise to single, low‐parallel‐velocity and heated solar wind ion distributions in the LLBL, is magnetic reconnection poleward of the cusp. The second, which gives rise to unidirectional streaming solar wind ion distributions in the LLBL, is magnetic reconnection equatorward of the cusp. This type of component reconnection may not be sustained in a quasi‐steady fashion.