The effect of Mach number on accelerated He+ pickup ions downstream of quasi-perpendicular shocks

Abstract:

Interstellar pick-up ions (PUI) are interstellar neutrals, which have been ionized in transit through the Heliosphere, via charge exchange or photoionization. These new PUI’s 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 2V_SW. Helium in the SW is typically doubly charged and in low abundance compared to bulk SW protons. On the other hand, singly charged Helium in the SW is typically of interstellar origin. Thus, He⁺ can be used to track the evolution of PUI distributions as they encounter various magnetic obstacles throughout the SW, such as shocks. Understanding how He⁺ PUIs are accelerated at shocks near 1 AU will provide valuable insights into shock dynamics and in particular, the physics surrounding the termination shock, which is expected to be heavily influenced by interstellar PUIs.

In this work we aim to characterize the nature of accelerated He⁺PUIs, downstream of quasi-perpendicular shocks, as a function of shock strength or Mach number (M_A) and shock obliquity (θ_Bn). We analyze a collection of quasi-perpendicular supercritical shock observations made by the MMS mission and compare upstream and downstream He⁺ velocity distribution functions (VDFs). For each shock event, we derive pitch angle distributions, as well as reduced 1-D velocity distributions, for the selected upstream and downstream intervals. We find that the shock accelerates He⁺ largely perpendicular to the magnetic field, consistent with shock reflection and shock drift acceleration mechanisms. Furthermore, we compute and compare the spectral index of accelerated He⁺ ions downstream of each shock in order to see how this index changes with M­_A and θ_Bn.