A giant aneurysm of plasma distorted Venus's atmosphere on 22 March 2008, a space-weather event known as a hot flow anomaly (HFA). Such events occur when electric fields associated with the Sun's solar wind create a fast-growing bulge that is filled with hot ions from the bow shock, a boundary between the solar wind and the planet.

Venus joins Earth, Saturn and Mars as planets where HFAs have been observed, say Glyn Collinson at the Goddard Space Flight Center in Greenbelt, Maryland, and his colleagues, who used data from the European Space Agency's Venus Express satellite to identify the 2008 event.

On Earth, HFAs can cause auroras. The authors suggest that HFAs would be even more disruptive to Venus's atmosphere because the planet lacks a magnetic field, meaning that the bow shock, and so the HFA, would be much closer in.

J. Geophys. Res. http://dx.doi.org/10.1029/2011JA017277 (2012)