Properties of He+ beams observed by Geotail in the lobe/mantle regions: Comparison with O+ beams

Abstract

In the lobe/mantle regions of Earth's magnetotail (<210 RE), the Geotail spacecraft sometimes observed multicomposition ion flows consisting of both ionospheric (H+/He+/O+) and solar wind (H+/He++) ions. Statistics on the He+ beams show that they were observed under similar conditions to those of the O+ beams reported previously. Namely, the heavy ion beams of ionospheric origin tend to exist during geomagnetically active periods in the mantle‐like regions of high plasma beta, where ions of solar wind origin are the major component. The total duration of identified He+ events amounts to 3% of the total observation time in the lobe/mantle region, while that of O+ events amounts to 13%. A remarkable point is that on a short timescale, the He+ and O+ beams often appear to exist nearly exclusively. That is to say, their densities sometimes vary in an opposite sense, even when He+ and O+ coexist. If the O+ and He+ ions are given the same energy in a source region, the initial distribution function of He+ has twice the peak velocity of that of O+, and their alternating appearance and density anticorrelation in the lobe/mantle are easily explained due to the velocity filter effect. Thus the anticorrelation of their densities may suggest that ionospheric ions have undergone an energization which leads to a different velocity for different ion species. A mechanism leading to the same velocity, on the other hand, would require an alternating enhancement of the He+ and O+ fluxes in a source region to explain the anticorrelation of the densities.