  We present initial analysis and conclusions from plasma observations made
during the reported "Mars plume event" of March - April 2012. During this
period, multiple independent amateur observers detected a localized,
high-altitude "plume" over the Martian dawn terminator [Sanchez-Lavega et al.,
Nature, 2015, doi:10.1038/nature14162], the cause of which remains to be
explained. The estimated brightness of the plume exceeds that expected for
auroral emissions, and its projected altitude greatly exceeds that at which
clouds are expected to form. We report on in-situ measurements of ionospheric
plasma density and solar wind parameters throughout this interval made by Mars
Express, obtained over the same surface region, but at the opposing terminator.
Measurements in the ionosphere at the corresponding location frequently show a
disturbed structure, though this is not atypical for such regions with intense
crustal magnetic fields. We tentatively conclude that the formation and/or
transport of this plume to the altitudes where it was observed could be due in
part to the result of a large interplanetary coronal mass ejection (ICME)
encountering the Martian system. Interestingly, we note that the only similar
plume detection in May 1997 may also have been associated with a large ICME
impact at Mars.
