The Role of Gyrating Ions in Reformation of a Quasi-parallel Supercritical Shock

Abstract Collisionless shocks in space and astrophysical plasmas mediate energy exchange between charged particles and fields in two or more plasma flows. In this study we analyze the evolution of ion distributions around a reformation cycle of a quasi-parallel shock. We use multipoint in situ observations in the foreshock region of the Earth’s bow shock of a transient foreshock structure as it generates a shock. We find that backstreaming ions in the foreshock create a density and magnetic field depletion known as caviton, which locally changes the shock geometry. Gyrating suprathermal ions that emerge within the caviton and reach the upstream edge of the core create a cross-field current imbalance that results in the nonlinear growth of a new shock layer. The new shock forms from the background foreshock fields over a distance of ∼6 ion inertial lengths ( l i ) and within 4.5–11.2 l i from the main bow shock. We find that plasma compression at the new thin shock layer is due to compactification of the cold upstream ion beam by high-amplitude magnetic field–aligned electrostatic fields. At later stages, the plasma compression expands to form a new sheath.