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MMS Observation of Shock‐Reflected He++ at Earth's Quasi‐Perpendicular Bow Shock

Specular reflection of protons at Earth's supercritical quasi‐perpendicular bow shock has long been known to lead to the thermalization of solar wind particles by velocity‐space dispersion. The same process has been proposed for He++ but could not be confirmed previously due to insufficient time resolution for velocity distribution measurements. We present observations and simulations of a bow shock crossing by the Magnetospheric Multiscale (MMS) mission on 20 November 2015 indicating that a very similar reflection process for He++ is possible, and further that the part of the incoming distribution with the highest probability of reflecting is the same for H+ and He++. However, the reflection process for He++ is accomplished by deeper penetration into the downstream magnetic fields.