Physics
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmsm23a0310s&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #SM23A-0310
Physics
2712 Electric Fields (2411), 2721 Field-Aligned Currents And Current Systems (2409), 5421 Interactions With Particles And Fields, 5443 Magnetospheres (2756), 6235 Mercury
Scientific paper
Field-aligned currents (FACs) play a fundamental role in the response of planetary magnetospheres to changes in the external stresses exerted by the solar wind and to processes - such as magnetic reconnection and photo-ion pickup - that redistribute stress internal to the magnetosphere. At the Earth and the outer planets, these field-aligned currents close at low altitudes in planetary ionospheres. Mercury is unique in having a well developed magnetosphere, but lacking sufficient atmosphere to support an ionosphere. How field-aligned current systems driven by Mercury's magnetosphere close at low altitudes is still an unsolved mystery. We suggest that surface charging and the secondary emission of electrons likely play a crucial role in closing the Hermean magnetospheric current systems, particularly on the nightside. Any object in a plasma and exposed to UV and X-rays, such as the surface of Mercury, will charge up to an equilibrium electric potential, such that the net incident current is zero. These currents come from photoemission of electrons, plasma ions and electrons, and secondary electrons. Secondary electrons are knocked out of the surface by incident plasma electrons, with yields typically peaking for incident energies ~400 eV. This will be an important factor where field-aligned currents from the magnetosphere meet the surface. Initial calculations indicate that the surface exposed to the hot plasma sheet on the nightside of Mercury may charge negative up to ~ 2 kilovolts. The secondary electron conductivity resulting from this charging process integrated over 1000 km could be of order 10 S. We will discuss the details of this mechanism, expand on the initial predictions of this model, and address some of the implications for Mercury's magnetosphere.
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