Observations of bidirectional electrons in the distant tail lobes: GEOTAIL results

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ISEE‐3 demonstrated the relatively frequent occurrence of low‐energy (50 ∼ 500 eV) electron flux enhancements within the distant magnetotail lobes. These electrons were found to be “bidirectional” in the sense that intensities were highest parallel (and antiparallel) to the lobe magnetic field lines. It was concluded that these electrons must enter the tail lobes along open field lines at the distant magnetopause and that this population would constitute the source for polar rain electrons at low altitudes in the polar cap regions. Similar plasma electron features have been found using the Low Energy Particle (LEP) sensor system onboard the GEOTAIL spacecraft. Remarkably close correspondences of low‐energy (≲ 2 keV) ion tailward flow enhancements are found to occur when bidirectional electron fluxes suddenly intensify. Such ion plasma measurements were not available with ISEE‐3 and, thus, a new aspect of solar wind entry and overall plasma dynamics is revealed by these GEOTAIL measurements in the distant tail. Other ISTP data (e.g., IMP‐8 and CANOPUS) are used to analyze selected periods.