Solar wind composition changes across the Earth's magnetopause

Abstract

ISEE 1 and Active Magnetosphcric Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE) composition measurements are used to investigate the change in solar wind composition across the Earth's magnetopause. The combined data set of 59 magnetopause crossings covers 0500 to 1900 LT and −20° to +40° magnetic latitude. For each magnetopause crossing, the ratio of the He2+/H+ density in the magnetosheath and in the low latitude boundary layer (LLBL) were determined. In addition, some crossings on the duskside flank of the magnetosphere were sufficiently long to determine this density ratio in the current layer. Averaged over all local time, the He2+/H+ density ratio is observed to decrease by 40% from the magnetosheath to the low latitude boundary layer. The density ratio decreases by its greatest amount (47%) in the dawnside LLBL and decreases by its smallest amount (16%±14%) in the duskside LLBL. In addition, there is essentially no reduction in the density ratio (7%±12%) in the duskside current layer. Finally, even those events that show clear signatures of magnetic reconnection at the magnetopause exhibit an average decrease in the density ratio that is comparable to the decrease seen in the entire data set. This decrease in the He2+/H+ density ratio across the magnetopause is not due to the presence of a significant magnetospheric H+ population in the LLBL. One possible explanation for this composition change is a mass dependent reflection coefficient at the magnetopause. To be consistent with the observations, the reflection coefficient for He2+ at the magnetopause must be 40–50% higher than that for H+. If true, then the He2+ concentration must increase by about 20% in the magnetosheath boundary layer (MSBL) just sunward of the magnetopause.