Pioneer Venus Orbiter magnetic field and plasma observations in the Venus magnetotail

Abstract

This study uses Pioneer Venus orbiter (PVO) magnetometer and plasma analyzer measurements to investigate the draped‐field tail of Venus with an emphasis on determining the magnetic field and plasma conditions within the various tail regions and their dependence upon interplanetary magnetic field (IMF) orientation. For this purpose PVO orbits during which the spacecraft's high inclination trajectory took it through the central magnetotail were identified. The criteria used to select these orbits were the existence of relatively complete magnetic field and plasma data during crossings of the magnetotail which lasted more than 3 hours. Examination of observations from four PVO tail seasons in 1981–1983 produced 12 orbits meeting these requirements. Analysis of the observations taken during those orbits indicates that the distribution of plasma within the magnetotail is highly asymmetric and controlled by the orientation of the IMF. In the plasma sheet and adjacent lobe regions downstream of the Venus hemisphere over which the solar wind motional electric field, Esw = −Vsw × Bsw, is directed away from the planet, PVO observed increasing fluxes of H+ and O+ as the spacecraft moves away from the tail axis toward the outer boundary of the tail. The greatest concentration of these O+ ions was found in the vicinity of the cross‐tail current layer downstream of this Venus hemisphere. No O+ ions were observed outside of the magnetotail based upon the magnetic field data and the definitions adopted in this study. However, only slow, <310 km/s, antisolar directed O+ is detectable by the PVO plasma analyzer and this factor may be the reason for the frequent observation of O+ in the cross‐tail current and the absence of measurable O+ outside of the magnetotail where the bulk flow speeds are significantly slower than in the distant ionosheath. Downstream of the Venus hemisphere over which the solar wind motional electric field is directed in toward the planet, PVO does not usually observe significant fluxes of E/Q = 0–8 kV ions, except sometimes directly adjacent to the outer boundary of the tail. The outer boundary of the Venus magnetotail downstream of this hemisphere is typically a well‐defined magnetopauselike current layer. The magnetotail‐ionosheath interface downstream of Venus hemisphere over which the solar wind motional electric field is outward, however, is very broad and resembles a slow mode expansion fan with field strength slowly decreasing and plasma density gradually increasing as PVO moves outward and enters the ionosheath. These results are interpreted as being due to the more efficient pick‐up of newly ionized atmospheric neutrals over the Venus hemisphere where the initial gyromotion takes the newly created ions away from the dense, lower atmosphere where they might be lost due to scattering (Cloutier et al., 1974). The implications of these findings for the formation and maintenance of the Venus magnetotail are discussed.