Observations of oxygen ions in the dayside magnetosheath associated with southward IMF

Abstract

We present a case study of high energy oxygen ions (O⁺) observed in the dayside terrestrial magnetosheath, in the southern hemisphere. It is shown that the presence of O⁺ is strongly correlated to the IMF direction: O⁺ is observed only for B_z < 0. Three satellites observe O⁺ immediately at both sides of the magnetopause and about 2 R_E outside the magnetopause. These conditions indicate escape along open magnetic field lines. We show that if outflowing O⁺ is heated and accelerated sufficiently in the cusp, it takes 15-20 minutes for it to reach the magnetopause, allowing the ions to escape along newly opened field lines on the dayside. Earlier studies show evidence of strong heating and high velocities of O⁺ in the cusp and mantle at high altitudes, strengthening our interpretation. The observed magnetosheath O⁺ fluxes are of the same order as measured in the ionospheric upflow, which indicates that this loss mechanism is significant when it takes place.