A narrow region of electron beams at the poleward edge of the cusp

Abstract

Viking electron energy spectrogram (ESP) and positive ion energy spectrogram (PISP) data show that there is a thin and faint but distinguishable region at the poleward edge of the cusp‐cleft region. This region is characterized by 100‐ to 300‐eV field‐aligned electrons with no striking ion feature except for coexisting low‐energy upward ions. These faint electron beams are almost always observed unless the interplanetary magnetic field (IMF) is southward, yet they are different from the polar arcs, boundary cusp electrons, and the merging type electron burst on merging field lines for the following reasons. First, the thickness of this region (very thin) and the electron beams' pitch angle distribution of this region are clearly different from those of the boundary cusp or the polar arc region. Second, these electron beams are observed during weak IMF when the ions do not show any merging‐associated energy dispersion. This region sometimes exists even when the merging line is located on the other (equatorward) side of the cusp, indicating that these electron beams are not the merging‐related electron burst. Third, this region is absent near local noon (1130‐1230 MLT). The electron beams are found rather on the morningside and the afternoonside. This special distribution is not expected for polar arcs or merging‐related electron bursts. The absence of the thin electron beam region near local noon indicates, together with its latitudinal location (always between the cusp and the polar cap or polar arcs), that the tail boundary layer can be a possible source of these electron beams. Since this newly found region is very thin and it is always located between the cusp and polar cap or polar arcs, we call it the “cusp poleward edge,” despite its absence near local noon.