MMS Observations of Accelerated Interstellar Pickup He + Ions at an Interplanetary Shock

Abstract

Interstellar pickup ions (PUI) are interstellar neutrals that have been ionized in transit through the heliosphere via charge exchange or photoionization. These new PUIs then “freeze” into the solar wind (SW) and move with the bulk SW velocity ( V SW ). They also begin to gyrate around the local magnetic field with V SW , resulting in a maximum PUI velocity of 2 ∗ V SW . Understanding how He + PUIs are accelerated at shocks in space provides valuable insights into shock dynamics and shock acceleration mechanisms. On 2018 January 8, while the Magnetospheric Multiscale (MMS) Mission was in the SW, it observed the forward shock of a corotating interaction region. In this work, we analyze the upstream and downstream velocity distributions of He + for this quasiperpendicular, marginally supercritical ( θ Bn = 67°, M A = 2.8) interplanetary shock. We derive average two-dimensional pitch-angle distributions in the field-aligned SW frame, as well as reduced one-dimensional velocity distributions for selected upstream and downstream intervals. We find that the shock accelerates He + largely perpendicular to the magnetic field, consistent with shock reflection. Furthermore, we derive a measured ratio of accelerated He + ions by estimating a maximum speed in the downstream frame, above which we assume that He + ions have been accelerated by their interaction with the shock. The measured acceleration ratio, R Measured  = 0.14, is then compared to a theoretical ratio derived from a simple model of the upstream distribution, R Theoretical  = 0.06.