A teardrop-shaped ionosphere at Venus in tenuous solar wind

A very tenuous solar wind regime, following a series of large coronal mass ejections, impacted Venus during early August, 2010. STEREO-B downstream from Venus observed that the solar wind density at Earth orbit dropped to similar to 0.1 #/cm(3) and persisted at this value over 1 day. A similar low value was observed at Earth in 1999 and has attracted comprehensive attention (Lazarus, A.J., 2000. Solar physics: the day the solar wind almost disappeared. Science 287, 2172-2173.), especially its consequences on Earth's ionosphere and magnetosphere (Lockwood, M., 2001. Astronomy: the day the solar wind nearly died. Nature 409, 677-679.). We now have an opportunity to examine the response of Venus' ionosphere to such a tenuous solar wind. After Venus Express spacecraft entered the ionosphere near the terminator, it continuously sampled O+ dominated planetary plasma on the nightside till it left the optical shadow region when Venus Express was located at 2 R-V (Venus' Radii) to the Venus center and 1.1 R-V to the Sun-Venus line. Moreover, the O+ speed was lower than the gravitational escape speed. We interpret this low-speed O+ as a constituent of the extended nightside ionosphere as a consequence of long-duration (18 h) tenuous solar wind, because the very low dynamic pressure enhances the source and reduces the sink of the nightside ionosphere. Though the full extent of the nightside ionosphere is not known due to the limitation of spacecraft's trajectory, our results suggest that the global configuration of Venus' ionosphere could resemble a teardrop-shaped cometary ionosphere. (C) 2012 Elsevier Ltd. All rights reserved.