The Extra‐Magnetospheric Ion Environment as Observed by the Magnetospheric Multiscale Mission Hot Plasma Composition Analyzer (MMS‐HPCA)

Observations within the solar wind (SW) environment outside of the Earth's magnetosphere were conducted on 6 December 2016, by the Hot Plasma Composition Analyzer of the Magnetospheric Multiscale mission. At the time of measurement, the SW was in a compressed state with a southbound interplanetary magnetic field of 9 nT and corresponding increases in proton density and dynamic pressure. High time resolution measurements of pickup ions in this region were made during the time when the Earth is situated in a solar gravitational focusing cone for interstellar neutrals. Proton measurements revealed a suprathermal ion population traveling with a maximum velocity that is two and a half times that of the SW velocity (Vsw). Helium ions exhibited pitch angle scattering that was uncorrelated with changes in the interplanetary magnetic field orientation. The time‐of‐flight measurements exhibited expected contributions from both protons and singly charged helium. Because the Magnetospheric Multiscale spacecraft are in the SW, counts from heavy SW ions (possibly Fe7+ or Fe8+) with M/Q = 8 AMU/e are observed with a mean energy/charge (E/Q) of 5.75 keV/e. Key Points Pickup ions of interstellar or inner source origin are observable with near‐Earth plasma instruments Short‐term PUI measurements can be used to survey composition variance in the Sun's gravitational focusing cone Analyzing PUI distribution functions for gyrotropy can hint at acceleration processes that occur within the solar system