O⁺ outflow channels around Venus controlled by directions of the interplanetary magnetic field: Observations of high energy O⁺ ions around the terminator

Using the plasma and the magnetic field data measured by the ASPERA-4 (Analyzer of Space Plasma and Energetic Atoms) and the magnetometer (MAG) onboard Venus Express between June 2006 and December 2008, positions of high energy O⁺ fluxes (>100 eV) near the Venus terminator are examined for two different interplanetary magnetic field (IMF) configurations: IMF nearly perpendicular to the Venus-Sun line (perpendicular IMF case) and IMF nearly parallel to it (parallel IMF case). In most of the perpendicular IMF case, the high energy O⁺ fluxes are observed only near the magnetic poles. In contrast, they are observed regardless of a convection electric field around the terminator in most of the parallel IMF case. Energy of the flux depends on the convection electric field direction in the former case. In contrast, it has no dependence on the field direction in the latter case. We attribute these results to more complicated draping pattern of IMF around the ionosphere in the parallel IMF case than for that of the perpendicular IMF case. In the perpendicular IMF case, the IMF drapes around the ionosphere, forming a single plasma sheet. In contrast, in the parallel IMF case, the IMF drapes complicatedly, creating many antiparallel configuration of local magnetic field. Such multiple antiparallel configuration results in a local acceleration of O⁺ and more outflow channels. Thus, the ion acceleration region and its mechanism can be essentially different between the perpendicular IMF case and the parallel IMF case.