New observations of Venus have shown that the planet’s ionosphere on its night side resembles a comet’s tail. The observations were made by the European Space Agency’s (ESA) Venus Express satellite.

The ionosphere is the region of electrically charged gas high in a planet’s atmosphere. The shape and density of an ionosphere are determined, in part, by the internal magnetic field of its planet. Earth has a strong internal magnetic field, which makes its ionosphere stable. Venus, on the other hand, does not have an internal magnetic field. Its ionosphere has now been found to be shaped by interactions with solar wind.

The new data was obtained in August 2010, when solar wind density dropped to only 0.1 particles per cubic centimeter for around 18 hours. Researchers were able to use this window to observe Venus’ ionosphere balloon out towards its nightside.

“The teardrop-shaped ionosphere began forming within 30–60 minutes after the normal high pressure solar wind diminished. Over two Earth days, it had stretched to at least two Venus radii into space,” said Yong Wei, lead author of a paper on the new research and a scientist at the Max Planck Institute for Solar System Research.

Before now, researchers had debated how solar wind could affect the way ionospheric plasma moves from one side of Venus to the other. The plasma normally flows from the dayside to the nightside of Venus in a thin channel in the ionosphere. Until now, it was unknown whether in low solar wind conditions the flow would increase due to reduced pressure or decrease due to reduced force pushing plasma through the channel.

“We now finally know that the first effect outweighs the second, and that the ionosphere expands significantly during low solar wind density conditions,” said Markus Fraenz, co-author of the paper.

(Image courtesy ESA)