Superthermal ions detected in Venus dayside ionosheath ionopause and magnetic barrier regions

Abstract

A comprehensive morphological analysis of the dayside superthermal ion presence in the dayside ionopause region of Venus was made using the Orbiter Ion Mass Spectrometer (OIMS) measurements. Superthermal O⁺ ions with energies ranging from -15 to 90 eV relative to the spacecraft are observed on essentially every orbit of Pioneer Venus that penetrates into the dayside ionosphere. There is a strong inbound-outbound difference in both the number and intensity of the superthermal ions with most events detected on the inbound crossing of the ionopause. This is consistent with a net flow of plasma along the ionopause away from the subsolar region. The ambient energy (including thermal and/or bulk motions) of the detected ions is comparable to the ram energy of cold ions relative to the moving spacecraft. A comparison of the locations of the superthermal O⁺ ion occurrences with respect to other parameters shows that the superthermals are a characteristic of the ionopause transition separating the essentially stationary ionosphere plasma from the flowing ionosheath plasma as deduced by Taylor et al. (1980b) from individual examples. These superthermal ions are not often detected within the main ionosphere in the midday region. Except in a few rare instances of changing interplanetary magnetic field (IMF) conditions or at locations near the terminator, they are only present at altitudes within or above the magnetic barrier. The OIMS also detects superthermal H⁺ ions with an inbound outbound asymmetry in the ionosheath consistent with shocked ionosheath proton flows.