PLASMA-WAVES AND FIELD-ALIGNED CURRENTS IN THE VENUS PLASMA MANTLE

Plasma waves, observed above the dayside Venus ionosphere by the Pione er Venus Orbiter (PVO) plasma wave instrument, have been attributed to whistler mode waves, lower hybrid waves, or ion acoustic waves. In or der to clarify the nature of the waves, we have performed both case st udy and statistical analyses of the plasma wave and magnetic field dat a. We find that the plasma wave data are well ordered by altitude with respect to the location where the PVO Langmuir probe, or orbiter elec tron temperature probe (OETP), measures a density of 100 cm(-3) known as the OETP ionopause. The dominant signature in the wave data appears to be a change in the instrument noise level because of changes in th e plasma Debye length. However, there is a burst of wave activity near the OETP ionopause. Also, we find that there is a rotation in the mag netic field at or near this location. By casting the magnetic field da ta. into a coordinate system ordered by the presumed magnetosheath flo w, we find that the rotation of the field tends to orient the field in a more flow-aligned direction at lower altitudes. We attribute: this to mass loading at lower altitudes. We further suggest that the field- aligned current associated with the field rotation corresponds to a sh ear Alfven wave standing in the magnetosheath flow. The field-aligned currents are present because of boundary conditions imposed on the flo w, and it is not clear if the waves are actually associated with the f ield-aligned currents or are simply coincidental. Since the waves are observed at the OETP ionopause, further progress in understanding thes e: waves will be made though determining what underlying plasma struct ure, if any, is related to the OETP ionopause, which is defined by a s pecific instrument threshold. Nevertheless, our study confirms that th e wave activity, field-aligned currents, and OETP ionopause all occur within the plasma mantle above the ionosphere. As such, the plasma wav es are not an energy source for the dayside ionosphere.