March 6, 2012
NASA Researchers Study Explosions On Venus
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Venus, often referred to as Earth´s sister planet due to its similar size, gravity, and composition, has been discovered by astronomers to also have a type of space weather event that is commonly found on Earth: a hot flow anomaly or HFA.HFAs are generally associated with the magnetosphere of a planet, such as Earth. But where Venus has no magnetosphere, the phenomenon surprised astronomers.
The astronomers, reporting their findings from a recent study in the Journal of Geophysical Research, said they have found clear evidence of an HFA on Venus. These anomalies cause a temporary reversal of solar wind that normally moves past a planet, causing material to flood backward, said David Sibeck, a scientist at NASA´s Goddard Space Flight Center in Greenbelt, Maryland.
Sibeck, who studies HFAs on Earth and is a coauthor of the new study, said these 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.”
Space weather is milder at Venus than at Earth, but because it has no magnetic field, the anomaly occurs much closer to the planet´s surface.
Glyn Collinson, a Goddard scientists and lead author of the study, said HFAs occur on average one per day on Earth. “They´ve been seen at Saturn, they may have been seen at Mars, and now we´re seeing them at Venus. But at Venus, since there´s no protective magnetic field, the explosion happens right above the surface of the planet.”
The first notion that Venus had hot flow anomalies occurred in 2009 when NASA´s Messenger satellite, flying by Earth´s sister on its way to study Mercury, spotted a possible HFA. But because Messenger´s instruments could only measure a magnetic signature and not detect the temperature of the material inside, the data received from Messenger was only speculative.
Collinson, looking for better evidence, turned to Venus Express, a European Space Agency (ESA) satellite. While Venus Express was not designed to study space weather, Collinson believed the instruments on board could be used to detect HFAs. Using the ESA satellite, Collinson began looking for telltale signs of an HFA from a few days worth of data.
“That may not sound like much,” he said. “But a day on Venus is 243 Earth days.”
Collinson looked for a pattern of magnetic change that would indicate the satellite traveled through a HFA as it occurred. He likened the experience of a satellite traveling through an HFA to that of a bullet being shot through a hot-air balloon: The bullet picks up a moment of heat in an otherwise consistently temperate journey.
With the satellite, the heat also comes with other characteristics. The boundaries of the apparent HFA show an abrupt change in the magnetic fields, and the inside is less dense than the outside. Collinson´s search was able to turn up significant, but not conclusive, finds.
But continued research eventually proved worthwhile. A combination of magnetic and plasma data showed that a Venusian HFA did occur on March 22, 2008.
Collinson and colleagues used the Venus Express data and compared it with the known physics of the Earth to show how an HFA forms at the second planet from the Sun. The moving solar wind harbors discontinuities in its attendant magnetic fields where they can change direction, sharply and abruptly. These discontinuities can sometimes align with the flow of the solar wind, where they remain in contact with what´s called the bow shock -- the place where the supersonic solar wind slows down abruptly and diverts around the planet. If such a discontinuity travels slowly across the bow shock it allows time to trap particles, collecting pools of 10 million degree plasma that can expand to be as big as Earth.
“These plasma particles are trapped in place,” says Sibeck. “They make a big puddle that gets bigger and bigger, sending out its own shock waves. Everything downstream from that bubble is going to be different than what´s upstream.”
The downstream disturbances are what make HFAs interesting. The eruptions create disturbances that extend far beyond the local disruption of the explosion. The eruptions of solar material can compress the entire magnetosphere around Earth for minutes at a time, shaking the particles along magnetic lines and causing them to fall into Earth´s atmosphere near the magnetic poles to create daytime auroras.
The researchers said it is difficult to understand what the HFAs do in the non-magnetized environment around Venus because the current data sets provided by Venus Express are not enough to make direct observations of such an event.
Collinson and his colleagues, however, made some educated guesses.
“At Earth, HFAs have a big effect, but don´t necessarily rule the roost,” said Collinson. “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 bow shock on Venus serves as the boundary between incoming solar wind, and the planet´s ionosphere. The boundary changes in height easily in response to the environment, and so the researchers believe it would also respond strongly to the presence of an HFA. Since an HFA causes material to retreat backward, away from the planet, it may operate almost like a vacuum cleaner, pulling that bow shock further away from the planet, causing the ionosphere to swell as well.
If HFAs can occur on a planet without a magnetosphere, scientists believe they may well occur on planets throughout the solar system, and throughout other solar systems as well.
Image 1: Interaction between Venus and the solar wind. Credit: ESA/C. Carreau
Image 2: 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. Credit: NASA/Collinson
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