Plasma waves in the dayside polar cap boundary layer: Bipolar and monopolar electric pulses and whistler mode waves

Abstract

We report four different types of plasma waves detected in and near the dayside polar cap boundary layer (PCBL) region at high altitudes (>6 RE). One wave type is narrowband whistler‐mode emission at frequencies just below fce (5.5 kHz). These emissions could be locally generated by resonant wave‐particle interactions involving an electron beam of ∼100 eV energy. A second type is a low frequency (200–300 Hz) whistler mode wave, which may be locally generated by ∼25 keV electrons or ∼45 keV ions. It is also possible that these latter waves are generated at low altitudes near the ionosphere and then converted from the ion cyclotron mode into whistler‐mode during propagation from the generation region to the spacecraft. Two further types of waves are large‐amplitude bipolar and monopolar solitary “electrostatic” waves. The bipolar wave structures are possibly generated all along the magnetic field lines in the field‐aligned current regions (at all local times). The monopolar structures could be evolved bipolar solitary waves. A one‐d schematic is presented to explain the paired monopolar structures as a result of splitting of an electron hole into two parts.