Observations of the Venus Dramatic Response to an Extremely Strong Interplanetary Coronal Mass Ejection

On 2011 November 5, Venus Express observed the impact of an extremely strong interplanetary coronal mass ejection (ICME) on Venus. As a result, the Venusian induced magnetosphere dramatically fluctuated during the ICME passage: the bow shock was compressed and broadened by the sheath and the body of the ICME, respectively; an atypically strong magnetic barrier (over 250 nT) of Venus was detected; and the plasma sheet in the magnetotail flapped so rapidly that it was crossed by Venus Express 5 times within 1.5 minutes. The ionosphere was totally magnetized because of the very high magnetic pressure of the induced magnetosphere. However, the altitude of the ionopause did not decrease with respect to those in neighboring orbits, which is inconsistent with the ionopause descents reported by previous studies. We found that the ionosphere was greatly excited by the ICME as evidenced by the much higher heavy ion density. That is why the balance between the ionospheric thermal pressure and the strong magnetic pressure can be maintained at a relatively high altitude. We propose that a much stronger massloading effect resulting from the excited ionosphere is responsible for the anomalously high magnetic barrier because much more magnetic field lines were anchored. Our results also suggest that such ICMEs that can excite the ionosphere are substantially efficient in enhancing the atmospheric loss of Venus.