Mercury's magnetosphere fends off the solar wind

U-M's Fast Imaging Plasma Spectrometer (FIPS) on Jan. 14 took the first direct measurements of Mercury's magnetosphere to determine how the planet interacts with the space environment and the Sun.
 

The solar wind, a stream of charged particles, fills the entire solar system. It interacts with all planets, but bears down on Mercury, 2/3 closer than the Earth to the Sun.

Earth's magnetosphere is strong enough to protect us from the solar wind's radiation, but Mercury's magnetic field is comparatively weaker.

"From our magnetic measurements, we can tell that Mercury is managing to stand up to a lot of the solar wind and protect the surface of the planet, at least in some spots. Even though the magnetic field was weak, it was enough," said Thomas Zurbuchen, FIPS instrument project leader and a professor in the U-M Department of Atmospheric, Oceanic and Space Science.

Zurbuchen said scientists can tell Mercury is putting up a good fight because instruments detected a layer of much slower-moving magentospheric plasma around the planet.

It's possible that the magnetosphere shield has holes. Scientists found ions in the magnetosphere that may have been knocked off the surface by the solar wind at the poles, for example. The source and chemical composition of the ions is still unclear, Zurbuchen said. The particles could also be from the planet's thin atmosphere.

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/ Carnegie Institution of Washington
High resolution image

The plot shows the measured magnitude of the magnetic field of Mercury as MESSENGER executed its first flyby of that planet. MESSENGER's Magnetometer (MAG) provided definitive identification of all boundaries of the Mercury magnetosphere system, consistent with the observations made with the Fast Imaging Plasma Spectrometer (FIPS) on the Energetic Particle and Plasma Spectrometer (EPPS) instrument, and revealed a much more quiescent system than was seen during the first Mariner 10 flyby. This state of the system was also consistent with the absence of energetic particles as documented by the Energetic Particle Spectrometer (EPS) portion of MESSENGER's EPPS instrument. Mercury lacks radiations belts similar to the Van Allen belts at the Earth discovered by James Van Allen with a simple particle experiment on Explorer I launched 50 years ago.

"Mercury's magnetosphere is more similar to Earth's than we might have thought," Zurbuchen said.

The spacecraft did find one major difference. Mercury has no Van Allen Belts, wing-shaped regions of energetic particles trapped by Earth's magnetic field.

"We flew through the region they would be in and they just weren't there," Zurbuchen said. "It could be that they're intermittent, but when we were there, they weren't."

Mercury and Earth are the only two terrestrial planets in the solar system with magnetospheres produced by an intrinsic magnetic field

This was the first of three planned flybys of Mercury. MESSENGER is scheduled to enter orbit in 2011.

Source: University of Michigan
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