Tail lobe ion composition at energies of 0.1 to 16 keV/e: Evidence for mass‐dependent density gradients

A large set of energetic (0.1‐ to 16‐keV/e) ion composition data from the central magnetotail, obtained by the ISEE 1 spacecraft between 10 and 23 RE from Earth, is sorted according to measured total ion beta value, in order to investigate whether bulk properties of different ions vary in different ways between plasma sheet and lobes, as suggested by a comparison of certain experimental and theoretical results in the literature. Despite inevitable difficulties with extracting statistically valid data at very low beta (10−2 or lower), the results seem to support a recent theoretical model suggesting that lighter ions have a steeper density gradient than heavier ions, especially when comparing H+ ions with O+ ions. The results also indicate that ion velocity distributions are fairly isotropic even at low beta, at least those of H+ ions, although field‐aligned flows are common. The results are evaluated in the context of plasma transport and are found to lend some support to the notion that tail lobe convection may be directed inward from the dawn and dusk flanks.