Magnetic field properties of the intermediate transition of the Venus ionosheath

Abstract

We report new results of a statistical analysis of the electric and magnetic field data of the Pioneer Venus Orbiter (PVO) on the intermediate transition of the Venus ionosheath. This transition marks the outer boundary of an expansion region along the flanks of the inner ionosheath, and is identified by simultaneous observations of weak plasma fluxes and characteristic 30 kHz electric field bursts measured, respectively, with the plasma analyzer and the electric field detector. In many cases, the magnetic field signature across the intermediate transition is characterized by a sudden decrease in intensity and an abrupt rotation so that the direction is more nearly aligned with the Sun‐Venus line. In other cases the 30 kHz bursts occur outside a region of enhanced magnetic field reminiscent of the magnetic barrier external to the Venus dayside ionopause. In these cases there is also a rotation of the magnetic field toward the Sun‐Venus line associated with the 30 kHz bursts, but, in general, the conditions change more gradually. These variations may result from the expansion of the shocked solar wind at and near the magnetic polar regions of the Venus ionopause around which the draped interplanetary magnetic field lines slip over the planet. The expansion of the ionosheath plasma may result from local heating processes associated with a friction‐like interaction between the shocked solar wind and the topside ionospheric plasma.