Ion Dynamics Across a Low Mach Number Bow Shock

A thorough understanding of collisionless shocks requires knowledge of how different ion species are accelerated across the shock. We investigate a bow shock crossing using the Magnetospheric Multiscale spacecraft after a coronal mass ejection crossed Earth, which led to solar wind consisting of protons, alpha particles, and singly charged helium ions. The three species are resolved upstream of the shock. The low Mach number of the bow shock enabled the ions to be partly distinguished downstream of the shock due to the relatively low ion heating. Some of the protons are specularly reflected and produce quasi-periodic fine structures in the velocity distribution functions downstream of the shock. Heavier ions are shown to transit the shock without reflection. However, the gyromotion of the heavier ions partially obscures the fine structure of proton distributions. Additionally, the calculated proton moments are unreliable when the different ion species are not distinguished by the particle detector. The need for high time-resolution mass-resolving ion detectors when investigating collisionless shocks is discussed. One of the ongoing challenges when investigating collisionless shocks is determining the energy partition between electromagnetic fields and different particle species. Resolving this question requires detailed observations of the electromagnetic fields and particle distributions, and is challenging when multiple ion species are present. We investigate a crossing of Earth's bow shock for unusual solar wind conditions; three ion species are observed in the solar wind and behind the bow shock, namely protons, alpha particles, and singly charged helium ions. We investigate the ion dynamics and show that a small fraction of protons are reflected by the electric field associated with the shock, which results in complex ion distributions. However, since the highest time-resolution ion detectors cannot distinguish between different ion species, the heavier ions partly obscure the fine structure of the protons. The heavier ions lead to errors when calculating the bulk properties (e.g., moments) of protons. These observations illustrate the need for high time-resolution ion detectors, which can distinguish different ion species when studying shocks. Protons, singly charged helium ions, and alpha particles are observed upstream and downstream of a bow shock crossing The alpha particles and helium ions partly obscure the fine structure of the downstream proton distributions High time-resolution mass-resolving ion detectors are needed to study the ion dynamics across collisionless shocks