Ion injections and magnetic field oscillations near the high‐latitude magnetopause associated with solar wind dynamic pressure enhancement

Abstract

The characteristics of particles and magnetic fields associated with a step‐like enhancement in solar wind dynamic pressure are investigated by using observations from the Polar spacecraft. At the time, Polar was located near its apogee of ∼9 RE at high latitude in the afternoon sector. Impulsive, energy‐dispersed injections of ions were observed by Polar in both parallel and antiparallel pitch angle directions, with a much shorter duration in the parallel direction (toward the Earth) than in the antiparallel direction (away from the Earth). The energies of the injected ions ranged from ∼200 eV to a few keV, and they are considered of magnetosheath origin. The energy dispersion of the injected ions is consistent with the interpretation of the velocity filter or time‐of‐flight effect as the magnetosheath ions enter the magnetosphere and propagate downward and then are mirrored at low altitude where magnetic field lines converge. It is estimated that the entrance of the magnetosheath ions was located ∼3.9–7.5 RE away from Polar, and the distance from the spacecraft to the mirror point was ∼7.7–8.1 RE, corresponding to ∼830–3600 km in altitude. The Polar data clearly favor magnetic reconnection as the primary mechanism for the transport of magnetosheath plasmas into the magnetosphere. A good correspondence is found between the magnetic field oscillation and the intermittence of the ion injections, implying that the impulsive ion injections were a consequence of the intermittent magnetic reconnection at the high‐latitude magnetopause that had been modulated by compressional waves produced by the solar wind dynamic pressure enhancement.