The ionosphere is a region of weakly electrically charged gas high above the main body of a planet's atmosphere. Its shape and density are partly controlled by the internal magnetic field of the planet, as occurs with Earth's ionosphere.
However, Venus does not have its own internal magnetic field and relies instead on interactions with the solar wind to shape its ionosphere, scientists with the European Space Agency's Venus Express mission said.
Observations made during a period when the sun's solar wind was greatly reduced revealed the planet's ionosphere balloons outwardly on the planet's "downwind" night side, much like the shape of the ion tail seen streaming from a comet under similar conditions, they said.
"The teardrop-shaped ionosphere began forming within 30-60 minutes after the normal high pressure solar wind diminished," said Yong Wei of the Max Planck Institute for Solar System Research in Germany, lead author of the new findings.
"Over two Earth days, it had stretched to at least two Venus radii into space," he said in a release from the Paris headquarters of the ESA.
Researchers said a similar effect likely occurs around Mars, the other non-magnetized planet in our inner solar system.
"We often talk about the effects of solar wind interaction with planetary atmospheres during periods of intense solar activity, but Venus Express has shown us that even when there is a reduced solar wind, the sun can still significantly influence the environment of our planetary neighbors," Hagan Sondheim, ESA's Venus Express project scientist, said.