The He~(++)/H~+ Density Ratio Across Earth's Subsolar Magnetopause and Its Implications for the Presence of a Mass‐Dependent Reflection Coefficient

摘要

When magnetic reconnection is occurring at Earth's magnetopause, incident magnetosheath ions are either reflected or transmitted, and the likelihood of each outcome is quantified by the reflection coefficient. This statistical study reexamines the question of whether or not the reflection coefficient is mass dependent by observing the He~(++)/H~+ density ratio at magnetopause crossings. High‐resolution data from the Hot Plasma Composition Analyzer on board the Magnetospheric Multiscale spacecraft are used from its Mission Phase 1a. We analyze these new measurements of the relative changes in the He~(++)/H~+ density ratio across the boundary layers of the magnetopause for a restricted set of solar wind conditions. From this analysis, we conclude that, for southward IMF, the reflection coefficient has no dependence on mass.