Observations of large scale steady magnetic fields in the nightside Venus ionosphere and near wake

Abstract

Pioneer Orbiter Magnetometer observations during the few minutes surrounding nightside periapsis traversals generally show a weak average field of magnitude <10 gammas between ∼200 km and the periapsis altitude of ∼150 km except when either 1) the local solar wind dynamic pressure is high or 2) the spacecraft is in a ∼70° wide solar zenith angle range, which includes the midnight meridian and is centered west of it at ∼1 hr local time. Both the direction and magnitude of the low altitude weak field vary irregularly along the orbit on apparent spatial scales of 1‐1000 km, the peak value of the field magnitude remaining under ∼20 gammas. When the local solar wind dynamic pressure exceeds ∼5 × 10−8 dynes cm−2 average low altitude fields of up to ∼40 gammas with a primarily horizontal orientation are observed in the regions adjacent to the terminator. In contrast, within ∼35° of the antisolar point where the average magnetic field is typically higher (∼10‐20 gammas), the field magnitude does not exhibit a similar correlation with incident solar wind dynamic pressure. Moreover, it is found that the radial component may exceed 60% of the total field magnitude. Closer inspection of the data near the antisolar point indicates that the radial fields are frequently related to the observation at higher altitudes of extended regions of radial field that reverse direction along the orbit. These results are consistent with the interpretation that a magnetotail‐like feature is sometimes present down to ionospheric altitudes on the nightside of Venus.