MMS Observations of Energized He+ Pickup Ions at Quasiperpendicular Shocks

Abstract

Abstract Accelerated He+ pickup ions (PUIs) downstream of quasiperpendicular shocks are studied as a function of the fast-mode Mach number (Mf) and shock obliquity (θ Bn). We analyze 10 quasiperpendicular shocks with Mach numbers in the range [1, 7] observed by the Magnetospheric MultiScale (MMS) mission, and compare upstream and downstream He+ velocity distribution functions. For each shock event, we characterize the upstream PUI distribution and derive reduced 1D velocity distributions for the selected upstream and downstream intervals. We also compare the upstream-to-downstream ratio of spectral indices, computed from the He+ perpendicular distributions, to Mf and θ Bn. We find a positive correlation of this spectral index ratio and Mf, which suggests that perpendicular energization of He+ PUIs is enhanced as the shock becomes stronger. These results inform modeling efforts of PUIs and shock-acceleration processes, particularly those taking place at the termination shock.