He2+ and H+ dynamics in the subsolar magnetosheath and plasma depletion layer

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Abstract
Active Magnetospheric Particle Tracer Explorer (AMPTE)/Charge Composition Explorer observations downstream from the quasi‐perpendicular bow shock are combined with upstream observations from the AMPTE/UKS and IRM spacecraft to distinguish intrinsic changes in the solar wind H+ and He2+ plasma and magnetic field from changes inherent in the magnetosheath and near magnetopause region. Using these spacecraft, a plasma depletion layer and an external accelerated flow layer (on the magnetosheath side, sunward of the magnetopause) are identified in the magnetosheath near the magnetopause. Compared to the upstream region, the magnetosheath plasma has higher densities and temperatures and the same He2+/H+ density and average temperature ratios. Compared to the magnetosheath, the plasma depletion layer has decreased solar wind ion densities, increased magnetic field magnitude, lower and more anisotropic ion temperatures, a higher He2+/H+ density and temperature ratio, and an energetic H+ population leaked from the magnetosphere. Compared to the depletion layer, plasma the accelerated flow layer on the magnetosheath side of the magnetopause has an increased flux of energetic electrons and the addition of magnetosheath ions reflected off the magnetopause. The density, temperature, and magnetic field changes and the presence of leaked magnetospheric protons in the plasma depletion layer are consistent with the draping and pileup of the magnetic field sunward of but near the dayside magnetopause. Changes in the ion distributions in the accelerated flow layer are consistent with dayside magnetic reconnection. These results indicate that at times the plasma adjacent to the dayside magnetopause has significantly different characteristics from the magnetosheath or upstream solar wind plasma.