Cluster observations of the entry layer equatorward of the cusp under northward interplanetary magnetic field

Abstract

Various boundary crossings in the vicinity of the high‐altitude cusp region were experienced by the Cluster spacecraft when the interplanetary magnetic field (IMF) was northward. In contrast to the southward IMF cases, in which a turbulent and diffusive entry layer is present equatorward of the cusp, a transition layer (without significant turbulence and diffusive properties) that shows clear differences in plasma parameters (sometimes step‐like profile) compared to the adjacent regions was observed. We suggest that this transition layer, which contains both magnetosheath and magnetospheric populations, is the entry layer during northward IMF conditions. This transition layer is possibly formed by dual‐lobe reconnection when the IMF is northward. The plasma property and the closed field line geometry of this layer indicate that it is possibly linked to the low‐latitude boundary layer. The width of this layer varies from 480 to 2200 km. The results support the notion that high‐latitude dual‐lobe reconnection is a potential mechanism of the transport of solar wind into the magnetosphere during northward IMF through the formation of a high‐altitude entry layer. The observations of different sublayers with evident density and temperature differences are consistent with the view that the reconnection process at the magnetopause is not steady.