Characteristics of the ULF waves associated with upstream ion beams

Abstract

This report describes a new class of upstream wave with relatively high frequencies (∼1 Hz in the spacecraft frame of reference) and small amplitudes (1/2;γ peak to peak) compared with the more common larger amplitude (ΔB/B ∼ 1), low‐frequency (0.03 Hz) upstream waves. These waves were first noted in association with beams of ions reflected back upstream at the bow shock, appearing in conjunction with some but not all of such beams. Further work confirms the association of the waves with the beams: the presence of a beam appears to be a necessary condition for the observation of the waves. Beam presence is, however, not a sufficient condition for existence of the waves and no clear correlation has been found between beam properties and the presence or absence of the waves. We have used characterizations of the beams provided by the ISEE 1 and 2 LEPEDEA plasma analyzers along with wave profiles obtained by the ISEE 1 and 2 magnetometers in probing these relationships. In addition, measurements made by the magnetometers were used to determine the intrinsic properties of these waves, using the simultaneous two point measurements to calculate and eliminate Doppler shifting effects. Results demonstrate that these are right‐hand elliptically polarized whistler mode waves with plasma rest frame frequencies of 20–100 times the proton gyrofrequency and wavelengths ∼100 km. The waves tend to propagate in the plane formed by the local IMF direction and the solar wind at angles ∼45° with respect to the magnetic field direction. Group velocities estimated from cold plasma theory for waves of these frequencies traveling in the observed directions are often not sufficient to allow the waves to reach the spacecraft at the times of observation. This argues against the possibility that the waves could have propagated upstream to the spacecraft position from the bow shock itself.