Venus ionospheric “clouds”: relationship to the magnetosheath field geometry

Abstract

Venus plasma “clouds,” apparently detached regions of ionospheric plasma above the main ionosphere of Venus observed by the Langmuir probe on the Pioneer Venus orbiter, are thought to be evidence for bulk removal of ionospheric ions by the solar wind interaction. In addition to the primary feature of an electron density enhancement above the ionopause, the characteristic signature of a cloud may include a change in the sign of the Bx (sunward/antisunward) component of the surrounding magnetosheath magnetic field. The plasma wave instrument also sometimes observed enhanced electric field noise in the clouds. In an effort to learn more about clouds and their cause(s), we analyzed the magnetic field control of cloud positions relative to the planet by rotating their observed locations into a coordinate system in which all of the transverse upstream (interplanetary) magnetic fields were aligned. The results indicate that clouds are scattered around the periphery of the planet in the terminator plane. There is no evidence of a concentration of clouds where the magnetosheath magnetic field is most strongly “draped,” as suggested by some earlier studies of selected occurrences. On the other hand, statistics show that the change in the orientation of the transverse upstream magnetic field between the inbound and outbound bow shock crossings, for the orbits where clouds are seen, is approximately 30° greater than the average upstream change at Venus over the same time intervals. The Spreiter and Stahara gasdynamic model of the magnetosheath provides further support for the idea that a rotation in the upstream transverse magnetic field accompanies cloud formation. Modeling of the magnetic field time series in the magnetosheath for a “cloud” orbit is possible only when a sequence of upstream fields is used to reconstruct the observed time series. Thus, clouds are associated not simply with normal magnetosheath field draping but with interplanetary field orientation changes.