MAGNETIC-FIELD PROPERTIES OF THE INTERMEDIATE TRANSITION OF THE VENUSIONOSHEATH

We report new results of a statistical analysis of the electric and ma gnetic field data of the Pioneer Venus Orbiter (PVO) on the intermedia te transition of the Venus ionosheath. This transition marks the outer boundary of an expansion region along the flanks of the inner ionoshe ath, and is identified by simultaneous observations of weak plasma flu xes and characteristic 30 kHz electric field bursts measured, respecti vely, with the plasma analyzer and the electric field detector. In man y cases, the magnetic field signature across the intermediate transiti on is characterized by a sudden decrease in intensity and an abrupt ro tation so that the direction i s more nearly aligned with the Sun-Venu s line. In other cases the 30 kHz bursts occur outside a region of enh anced magnetic field reminiscent of the magnetic barrier external to t he Venus dayside ionopause. In these cases there is also a rotation of the magnetic field toward the Sun-Venus line associated with the 30 k Hz bursts, but, in general, the conditions change more gradually. Thes e variations may result from the expansion of the shocked solar wind a t and near the magnetic polar regions of the Venus ionopause around wh ich the draped interplanetary magnetic field lines slip over the plane t. The expansion of the ionosheath plasma may result from local heatin g processes associated with a friction-like interaction between the sh ocked solar wind and the topside ionospheric plasma.