Extreme Space Weather Event Detected at Mercury

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A vast and powerful space weather event has been observed at the solar system’s innermost planet for the first time.

NASA’s Messenger satellite, which has been in Mercurian orbit since 2011, detected what is known as a hot flow anomaly (HFA) emanating from the tiny world’s bow shock. HFAs are common at Earth’s bow shock and they have also been detected at Mars, Venus and Saturn, but to detect the phenomenon around Mercury is a space weather boon for scientists.

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“Planets have a bow shock the same way a supersonic jet does,” said Vadim Uritsky at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “These hot flow anomalies are made of very hot solar wind deflected off the bow shock.”

All the planets in the solar system are bathed in a constant flow of plasma carried by the million-mile-an-hour solar wind. As this stream of energized gas encounters a planet with a global magnetic field (in the case of Earth’s magnetosphere) or a planet without a magnetic field (like Venus), the stream of particles flow around the immovable object like a stream of water flowing around a rock.

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Solar wind particles ‘bunch up’ to form a standing shock wave upstream of their planetary encounter — a feature known as a bow shock. It is thought that an HFA occurs when a pocket of solar wind particles get trapped in a pocket at the bow shock, becoming heated and energized. At a certain threshold, the pocket blasts the trapped particles energetically back into space back toward the sun, against the direction of the solar wind. The energy of an HFA event is impressive, often propelling ionized particles against the flow of the solar wind for a considerable distance.

HFAs occur rapidly and can have ranging effects on planetary magnetospheres. Around Earth, for example, HFAs will cause a ‘ripple effect’ through the magnetosphere, impacting the dynamics of geomagnetic storms. Around Venus, observations have shown that HFAs not only impact the planet’s bow shock, they can also have a ‘vacuum effect,‘ causing anomalies in the planet’s ionosphere.

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But Messenger’s new observations of Mercury’s HFA provides new insight to space weather effects close to the sun and adds more detail to our understanding of how these explosive events occur and their scale. It is known that the HFAs generated by Venus’ bow shock measure around 600 miles across, whereas Saturn’s HFAs are 60,000 miles across — therefore the size of the bow shock appears to have a part to play in the size of the resulting HFA, a factor this new Mercury research will complement.

Messenger was able to detect the Mercury HFA through the magnetic field that was generated by the vast electric current sheet that was created and the heated particles that were generated in the HFA pocket of plasma. Associated turbulence was also detected by the spacecraft.

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