Venus tail rays were first observed in the Pioneer Venus Langmuir probe data. These rays appear to be filaments of ionospheric plasma extending far downstream from the nightside ionosphere. The characteristic signatures of tail rays are in many ways similar to those of plasma ''clouds'' observed above the terminator ionopause. These signatures include a localized electron density enhancement, and occasional magnetic field reversal in the Bx component and/or enhanced electric field noise. Here, selected Pioneer Venus orbits from the interval between orbits 700 and 3500 during the period November 1980 to May 1988 are surveyed for tail ray structures as seen in electron density time series from the Orbiter Langmuir probe. After rotating the upstream transverse magnetic field [interplanetary magnetic field (IMF)> to point in the same direction, the resulting distribution of tail rays is found to be scattered rather than organized around a central ''plasma sheet'' in the plane perpendicular to the IMF. If tail rays originate from clouds, this lack of organization is consistent with the observed scatter of cloud locations around the planet in the terminator plane. On the other hand, statistical analyses indicate that tail ray formation does not appear to require an extraordinary degree of rotation of the IMF during the pass, unlike the clouds.

The steadiness of the IMF during passes containing tail rays is further supported by gas dynamic magnetosheath field modeling. Only one upstream magnetic field vector is needed as input to simulate the magnetometer time series for the magnetosheath during orbits on which tail rays are seen. Thus, although tail rays and clouds exhibit similar plasma and field signatures, there is no conclusive evidence that tail rays evolve from clouds. The results of the present study suggest that tail rays are a normal feature of the steady solar wind interaction with Venus but are not generally associated with a central tail plasma sheet. Instead, a terminator source, or multiple sources, is suggested. ¿American Geophysical Union 1991