University of Minnesota
School of Physics & Astronomy

Physics and Astronomy Colloquium

Wednesday, November 11th 2009
3:35 pm:
Speaker: Brian Anderson, John Hopkins University Applied Physics Lab
Subject: Lessons in extreme solar wind-planetary interactions at Mercury: Results from the three MESSENGER flybys in anticipation of orbital observations in 2011
Refreshments served in Room 216 Physics after colloquium

The most stringent tests of physical theories are often provided by extremes. The planet Mercury’s interaction with the solar wind provides one such case for magnetospheric physics. Mercury is unique in that it possesses no significant ionosphere, and owing to the planet’s comparatively small magnetic moment, yielding a surface field one percent of Earth’s, Mercury’s magnetosphere is small and entirely dominated by the solar wind interaction. In solar orbit from 0.31 to 0.47 AU heliocentric distance, Mercury is also exposed to solar wind densities four to ten times higher than at Earth, an interplanetary magnetic field (IMF) about three times stronger, and a predominantly radial IMF, which changes the basic nature of the bow shock structure and dynamics. Finally, length and time scales that are widely separated at Earth and other solar system magnetospheres merge in the Mercury system. Energetic (>20 keV) protons and even thermal heavy ions have gyroradii comparable to the radius of Mercury’s magnetosphere. The largest-scale fluid-mode waves have transit times through the magnetosphere comparable to ion gyro-frequencies. Thus the convenient scale ordering that applies to other systems breaks down at Mercury. This seminar provides a summary of the initial findings from NASA’s Mercury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission. Launched in August 2004, the MESSENGER spacecraft has successfully completed six planetary gravity assist maneuvers, one at Earth, two at Venus, and three at Mercury, placing the spacecraft on target for insertion into orbit about Mercury in March 2011. Results from the three Mercury flybys reveal a rapid-fire electrodynamic system with a magnetic recirculation timescale of only tens of seconds, magnetic reconnection events at least an order of magnitude more intense than at Earth, and large-scale boundary structures unique to Mercury. The correspondence with ion-kinetic/electron-fluid simulations give tantalizing suggestions of new physical insights that will be gained once the comprehensive survey in Mercury orbit begins in 2011.

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