European Space Agency's Venus Express has made unique observations of Venus during a period of reduced solar wind pressure, that the planet's ionosphere balloons out like a comet's tail on its nightside.
For Earth, which has a strong magnetic field, the ionosphere is relatively stable under a range of solar wind conditions.
But by comparison, Venus does not have its own internal magnetic field and relies instead on interactions with the solar wind to shape its ionosphere.
Venus Express' new results have revealed for the first time the effect of a very low solar wind pressure on the ionosphere of an unmagnetised planet.
The observations were made in August 2010 when NASA's Stereo-B spacecraft calculated a drop in solar wind density to 0.1 particles per cc, around 50 times lower than normally observed this persisted for about 18 hours.
As this significantly reduced solar wind hit Venus, it's ionosphere ballooned outwards on it's 'downwind' nightside, much like the shape of the ion tail seen streaming from a comet under similar conditions.
"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," says Yong Wei of the Max Planck Institute for Solar System Research in Germany, lead author of the new findings.
The new observations settle a debate about how the strength of the solar wind affects the way in which ionospheric plasma is transported from the dayside to the nightside of Venus.
A similar effect is also expected to occur around Mars, the other non-magnetised planet in our inner Solar System.(ANI)