Structure and dynamics in the polar cleft: Coordinated satellite and ground‐based observations in the prenoon sector

Abstract

Observations of electron and proton precipitation fluxes and Birkeland currents from Defence Meteorological Satellite Program spacecraft F7 and simultaneous ground‐based optical measurements from Svalbard (∼75° geomagnetic latitude) of dayside aurorae (∼0900 MLT) are presented. In these cases, the auroral emissions within the field of view can be separated into different latitudinal zones with corresponding structures in particle precipitation and Birkeland current. The temporal evolution of these structures is monitored by the ground‐based instruments, indicating the presence of both stationary cleft emissions produced by soft electron fluxes (energy of a few hundred eV) and multiple, discrete arcs and arc fragments of transient or more persistent nature, corresponding to the precipitation of keV electrons. The latter are associated with narrow sheets of upward flowing Birkeland currents and reduced proton precipitation, suggesting the presence of field‐aligned potential drops above the spacecraft. Assuming that these low‐altitude structures map to the low‐latitude boundary layer of the magnetosphere, they correspond to a strong filamentation of the boundary layer, in the prenoon sector. Field‐aligned current systems separated in latitude and associated with different particle populations, i.e., plasma‐sheet‐like and low‐latitude‐boundary‐layer‐like particles, respectively, are observed in one case, after a sharp interplanetary magnetic field Bz polarity change.