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The different components of Mercury's magnetosphere result from the complex and dynamic interactions between Mercury's magnetic field and the solar wind. Figure courtesy of J. A. Slavin, NASA Goddard Space Flight Center. |
Mercury's magnetic field and the resulting magnetosphere, caused by the interaction of Mercury's magnetic field with the solar wind, are unique in many ways. Perhaps one of the most noteworthy observations about Mercury's magnetic field is the simple observation that the small planet has one. Mercury's magnetic field is similar to the "dipole" shape of Earth's magnetic field, which resembles the field that would be produced if there was a giant bar magnet at the center of the planet. In contrast, Venus, Mars, and the Moon do not show evidence for intrinsic dipolar magnetic fields, but the Moon and Mars have evidence for local magnetic fields centered on different rock deposits.
Earth's magnetic field is very dynamic and constantly changing in response to activity of the Sun, including the solar wind and solar flares. We see the effects of these dynamics on the ground as they affect power grids and electronics, causing blackouts and interference with radios and telephones. Mercury's magnetic field was shown by Mariner 10 to experience similar dynamics; understanding those variations will help us to understand the interaction of the Sun with planetary magnetospheres in general, including that of Earth. Although Mercury's magnetic field is thought to be a miniature version of Earth's, Mariner 10 didn't measure Mercury's field well enough to characterize it. There is even considerable uncertainty in just how strong Mercury's field really is. These basic uncertainties in the nature of Mercury's magnetic field have led to uncertainties in understanding the source of the magnetic field. Some theories have proposed that Mercury's magnetic field is actually a relic field and not one that is actively being generated today. Advocates for an active global magnetic field on Mercury, arising from fluid motions in an outer liquid portion of Mercury's metal core, debate the fraction of the core that is molten as well as whether the field is driven by compositional or thermal differences. However, these different ideas for the driving force behind Mercury's magnetic field predict slightly different field geometries, so careful measurements by spacecraft can distinguish among current theories.
MESSENGER's magnetometer will characterize Mercury's magnetic field in detail from orbit over four Mercury years (each Mercury year equals 88 Earth days) to determine its exact strength and how its strength varies with position and altitude. The effects of the Sun on magnetic field dynamics will be measured by the magnetometer and by an energetic particle and plasma spectrometer. MESSENGER's highly capable instruments and broad orbital coverage will greatly advance our understanding of both the origin of Mercury's magnetic field and the nature of its interaction with the solar wind.
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