Physics
Scientific paper
May 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agusm.p33b..05a&link_type=abstract
American Geophysical Union, Spring Meeting 2009, abstract #P33B-05
Physics
2724 Magnetopause And Boundary Layers, 5421 Interactions With Particles And Fields, 5443 Magnetospheres (2756)
Scientific paper
The two MESSENGER flybys of Mercury revealed the presence of a pronounced boundary layer structure just inside the dayside magnetopause boundary in the morning sector. During both flybys a step decrease of 25 to 30 percent in the magnetic field magnitude occurred on the outbound trajectory about 5 minutes prior to the magnetopause crossing. Corresponding step-wise changes in the distributions of protons in the energy range 0.1 to 10 keV were also observed. Interpreted in terms of spatial structure these observations correspond to a boundary layer approximately 2000 km thick, which is appreciable compared with the small size of Mercury's magnetosphere. Significantly, comparable boundary layers were seen on the outbound legs of both MESSENGER flybys, the first occurring for northward interplanetary magnetic field (IMF) and the second for southward IMF. Signatures of intense magnetopause and tail reconnection during the second flyby imply that rapid convection was then occurring. Thus, whatever process generated the boundary layer seen during the second flyby must act rapidly and be continuously effective even in the presence of strong convective flows. Several possibilities for the generation of the layer include cusp entry and subsequent drift of solar wind plasma, mass loading of ions originating from the planetary surface or the exosphere, and processes associated with reconnection and viscous mixing known to occur at Earth. Detailed examination of the signatures of the layer in magnetic field and ion observations permit an assessment of whether the same generation process acted during both encounters as well as the identification of key signatures that can be used to discriminate among different layer formation processes.
Acuña Mario Humberto
Anderson Benjamin J.
Gloeckler George
Gold Robert E.
Ho George C.
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