Observations of the right‐hand resonant ion beam instability in the distant plasma sheet boundary layer

From the ISEE 3 distant tail prime intervals of study (days 24–43 and 169–193, 1983), 66 large amplitude wave events at the plasma sheet boundary layer were identified, analyzed and tabulated. It is found that (1) 83% of the wave events are clearly right‐hand polarized in the spacecraft frame (arguments are given as to why they must also be right‐hand polarized in the plasma frame), (2) the waves are propagating nearly parallel to the ambient magnetic field (θ kB < 15°), (3) they have a significant compressional‐to‐transverse amplitude ratio (typically 0.2–0.5), of which only part can be ascribed as being due to cross‐field propagation, (4) the waves have a maximum amplitude of 6–7 nT peak to peak (presumably caused by a presently unexplained saturation effect), (5) the waves have rest frame frequencies within the range 0.3 to 2.0 Ω + , and (6) the waves are strongly substorm dependent. Several case studies involving simultaneous energetic (>35 keV) ion measurements have been analyzed and are presented. Highly anistropic, tailward streaming, relatively cold ion beams (1000–100∶1 anisotropy ratio) are found to occur in near time coincidence with the waves. The ion beam properties are calculated and are compared with theoretical predictions of right‐hand resonant and nonresonant ion beam instabilities. It is demonstrated that for the cases examined, the measured beam and wave parameters are in agreement with the presence of the resonant instability. It is believed that this is the first simultaneous observation of the waves and the unscattered cold ion beam of the right‐hand resonant ion beam instability in either space plasma or laboratory measurements. Because of the absence of strong convective effects separating the waves and the generating ion beam, the plasma sheet boundary layer is an excellent region of space to study the details and development of this very important plasma instability.