Researchers working with NASA's Mercury Surface, Space Environment,Geochemistry, and Ranging (MESSENGER) spacecraft report thefrequent detections of Kelvin-Helmholtz (KH) waves at the edge ofthe innermost planet's magnetosphere. In six different sets of magnetic field measurements made by theorbiter as it passed through Mercury's magnetopause, the boundarythat separates the planet's magnetosphere from the solar windplasma in the magnetosheath, Sundberg et al. detect the magneticfield oscillations characteristic of fully developed KH waves. Kelvin-Helmholtz waves form when fluids of different speeds travelalongside each other - in this case, the magnetosphere andmagnetosheath plasmas - and promote mixing of the plasmas on largerspatial scales, and shorter time scales, than diffusive transport.The observations, which span the first 88 days of MESSENGER's timein orbit, bring Mercury alongside Earth, Saturn, and Venus asplanets for which such Kelvin-Helmholtz waves are of importance. The waves seen at Mercury's magnetopause, however, differ markedlyfrom those at Earth's. |
The authors' KH wave observations were allmade in the postnoon and duskside region of Mercury'smagnetosphere, whereas at Earth, KH waves are seen farther towardthe nightside on both flanks. Moreover, the measured waves had periods averaging 10-20 seconds,whereas the periods of their terrestrial counterparts are severalminutes. Also, the amplitudes of the measured magnetic field oscillationswere 2-3 times larger than those seen at Earth. Wave growth at themagnetopause is known to be an important mechanism for transportingmaterial across the largely impermeable boundary, and the authorspropose that these newly identified Kelvin-Helmholtz waves could bethe source of plasma for Mercury's dayside boundary layer,discovered previously by the MESSENGER mission.
Torbjorn Sundberg: Heliophysics Science Division, NASA GoddardSpace Flight Center, Maryland, USA; Scott A. Boardsen: HeliophysicsScience Division, NASA Goddard Space Flight Center, Maryland, USAand Goddard Earth Sciences and Technology Center, University ofMaryland, Maryland, USA; James A. Slavin, Thomas H. Zurbuchen, andJim M.
Raines: Department of Atmospheric, Oceanic and SpaceSciences, University of Michigan, Michigan, USA; Brian J. Andersonand Haje Korth: The Johns Hopkins University Applied PhysicsLaboratory, Maryland, USA; Sean C. Solomon: Department ofTerrestrial Magnetism, Carnegie Institution of Washington, Districtof Columbia, USA. "MESSENGER orbital observations of large-amplitude Kelvin-Helmholtzwaves at Mercury's magnetopause" Journal of Geophysical Research-Space Physics,doi:10.1029/2011JA017268, 2012.
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