ESA's Venus Express spacecraft has spotted the planet blowing back million-mile-per-hour solar winds in weather explosions similar to what happens outside Earth's magnetic shield.

Solar winds bombard the Earth every day, but the torrential gales are stopped about 44,000 miles away from the planet when they hit the magnetic shield known as the magnetosphere.

Venus doesn't have any magnetic shield, so the Sun's winds are free to hit the surface and cause massive weather explosions known as hot flow anomalies (HFAs).

The solar wind isn't a one-directional gust - instead it's blustery because its own magnetic fields contain discontinuities, so the gale abruptly and sharply changes directions. At times, the wind forms a puddle on top of the Venusian bow shock, the point where the supersonic solar blast slows down and diverts around the planet.

These puddles collect into pools of plasma that can expand to the size of Earth, and then explode out from the bow shock.

HFAs also happen around Earth and have been spotted at Mars and Saturn as well, but this is the first time boffins have collected strong evidence for them at Venus.

How Venus explodes the solar winds off its ionosphere. Credit: NASA/Collinson

Scientists were tipped off that HFAs might be happening at Venus when NASA's Messenger satellite spotted what amounted to a suggestive magnetic signature that hinted at HFA activity. But Messenger, which is really studying Mercury not Venus, didn't have the instruments to detect the temperature inside the signature and figure out if it was hot.

So boffins from NASA's Goddard Space Flight Centre set out to analyse data from the Venus Express to search for the space weather. Express isn't really set up for that kind of investigation either, but it is capable of measuring magnetic fields and the charged particles of a solar wind, which was enough to confirm a HFA around the planet in March 2008.

The reason the researchers are so interested in the phenomenon is because of its effects on a planet with no magnetic shield. On Earth, HFA explosions off the magnetosphere can create a downdraft that compresses the entire shield for minutes at a time, causing charged particles to tumble along magnetic lines and fall through to the atmosphere as dayside aurora at the poles.

"At Earth, HFAs have a big effect, but don't necessarily rule the roost," said Glyn Collinson, lead author of the study. "But at Venus, since the HFA happens right up next to the planet, it is going to have a more dramatic effect on the system."

The boffins can't fully understand what happens to Venus in the downdraft of an HFA without direct observations, but they can take an educated guess.

The bow shock around the planet is the boundary between the blast of solar wind and Venus' ionosphere, the layer of charged particles in its atmosphere. This layer changes in height easily in response to the environment, so the boffins reckon that it could shoot up in the downdraft of an HFA, where the flow of material away from the surface acts like a hoover pulling it up.

Finding HFAs at Venus, the first planet without a magnetic field where they've been spotted, also implies that they could be happening at all planets in the solar system, or even in other systems as well.

The study was published in the Journal of Geophysical Research. ®