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Atmospheric "explosions"
on Venus

Posted: 07 March 2012

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A phenomenon known as a hot flow anomaly (HFA), which causes a temporary reversal of the solar wind that normally moves past a planet thanks to its protective magnetosphere, has been detected on planet Venus, a surprising find given the planet's lack of magnetic field.

Planet Earth is protected by a magnetic "bubble" – the magnetosphere – which acts as a defence shield against the solar wind. The magnetosphere usually forces the solar wind to flow around the planet but sometimes conditions allow the solar wind to enter this bubble. When discontinuities in the solar wind remain in contact with a planet's bow shock – where the supersonic solar wind slows down abruptly and diverts around the planet – a pool of hot, 10 million degree plasma can collect.

When discontinuities in the solar wind remain in contact with a planet's bow shock, they can collect a pool of hot particles that becomes a hot flow anomaly (HFA). An HFA on Venus most likely acts like a vacuum, pulling up parts of the planet’s atmosphere. Image: NASA/Collinson.

“These plasma particles are trapped in place,” says David Sibeck, of NASA’s Goddard Space Flight Center. “They make a big puddle that gets bigger and bigger, sending out its own shock waves. Hot flow anomalies release so much energy that the solar wind is deflected, and can even move back toward the Sun. That’s a lot of energy when you consider that the solar wind is supersonic – traveling faster than the speed of sound – and the HFA is strong enough to make it turn around.”

HFAs occur at Earth on the order of roughly one a day, and they are also seen on Saturn, and possibly on Mars (Mars Global Surveyor found indications of a martian HFA, but did not have all the instruments needed to conclusively prove it). Hints of an HFA at Venus were detected by NASA's MESSENGER satellite, which is currently in orbit around Mercury, alerting scientists to look through ESA's Venus Express satellite data. While HFAs are detectable most days on Earth, Venus' day is 243 Earth days, making the task somewhat challenging.

Glyn Collinson, first author on the paper due to appear in the Journal of Geophysical Research – Space Physics and also from Goddard, looked for signs that the spacecraft had passed through an HFA. This would be revealed by a sharp change in magnetic fields, with a lower density region "inside". Without instruments specifically designed to detect this phenomenon, the search turned up many potential events, but one stuck out from the rest, with the signature hot plasma found for an event occurring on 22 March, 2008.

Venus Express has been in orbit around Venus since 2006. Image: ESA - D. DUCROS.

"HFAs happen really fast – the whole thing took maybe only five seconds to form before slamming into the Venus Express," Collinson tells Astronomy Now. These atmospheric "explosions" last for many minutes on Earth, shaking particles along magnetic field lines and causing them to fall into the atmosphere near the magnetic poles to create aurorae. What the effects are for a planet without a magnetic field can only be speculated. "Without a magnetic field, HFAs will form directly on top of the ionosphere," explains Collinson. The ionosphere is the upper layer of the atmosphere and is filled with charged particles. "HFAs will likely disrupt the sensitive pressure balance between the solar wind and the ionosphere, maybe driving huge upwellings in the upper layers of the atmosphere like a gigantic tsunami, although you’d probably not be able to measure any difference at the ground level."

Since the new data confirms that HFAs can occur on a planet without a magnetic field, scientists will be on the look out for similar effects at other bodies in the Solar System, and in extrasolar systems too. "It’s fascinating that HFAs can form at such a small obstacle in the solar wind: Venus is ten times smaller an obstacle to the solar wind than the bubble created by Earth’s magnetic field," adds Collinson. "Now that they’ve been seen at the Earth, Saturn, Venus and probably Mars, I think it’s likely you’re going to find them everywhere, and not just confined to our Solar System. We are only really beginning to unravel the mysteries of how the Sun and space weather interacts with the Earth and the other planets. We’ve only known about HFAs for maybe 20 years. Imagine, such a violent and dramatic event going unnoticed for so long. What else is out there?!"