MESSENGER Observations of Extreme Loading and Unloading of Mercury’s Magnetic Tail

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Science  15 Jul 2010:
DOI: 10.1126/science.1188067


During MESSENGER’s third flyby of Mercury, the magnetic field in the planet's magnetotail increased by factors of 2 to 3.5 over intervals of 2 to 3 min. Magnetospheric substorms at Earth are powered by similar tail loading, but the amplitude is ~10 times less and typical durations are ~1 hour. The extreme tail loading observed at Mercury implies that the relative intensity of substorms must be much larger than at Earth. The correspondence between the duration of tail field enhancements and the characteristic time for the Dungey cycle, which describes plasma circulation through Mercury’s magnetosphere, suggests that such circulation determines substorm time scale. A key aspect of tail unloading during terrestrial substorms is the acceleration of energetic charged particles, but no acceleration signatures were seen during the MESSENGER flyby.