High‐frequency waves in the cusp/cleft regions

Abstract

Because of the high time resolution of many of its scientific instruments, the Viking satellite has detected the presence of intense wave bursts in the cusp/cleft regions, at altitudes between 1 and 2 Earth radii. These bursts are closely related to the acceleration processes of charged particles and are characterized by a strong plasma turbulence illustrated by sporadic and intense broadband electrostatic noise detected in the 4‐ to 700‐kHz frequency range of the high‐frequency wave receiver. The duration of such bursts is small, typically a few hundreds of milliseconds, corresponding to a spatial width less than 1 km at the satellite altitude. The low‐frequency part of this noise is always the most intense, with electric field amplitudes of a few tens of millivolts per meter. This noise is correlated with 0.1‐ to 1‐keV electron beams and with small‐scale field‐aligned currents and is always associated with low‐frequency (∼1 Hz) electric field fluctuations. Perpendicular acceleration of ambient plasma ions occurs in connection with the bursts. The impulsive broadband waves are detected at the equatorward edge of the polar cusp and more generally in the entire cleft region. The central part of the cusp is characterized by an increase of the frequency cutoff of the hiss emissions, related to the increase of the electron density. In addition, electrostatic electron cyclotron waves are also recorded. Terrestrial kilometric radiation is often detected in the cusp/cleft regions usually in close correlation with particle acceleration processes. The Viking satellite tends to have been mostly above the generation regions of these electromagnetic waves in the dayside auroral oval, which makes it difficult to localize precisely their source.