Physics
Scientific paper
May 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agusmsm54a..06d&link_type=abstract
American Geophysical Union, Spring Meeting 2008, abstract #SM54A-06
Physics
2724 Magnetopause And Boundary Layers, 2728 Magnetosheath, 2784 Solar Wind/Magnetosphere Interactions, 7526 Magnetic Reconnection (2723, 7835), 7827 Kinetic And Mhd Theory
Scientific paper
Our current understanding of magnetic storm triggering invokes magnetic reconnection as the basic solar wind- magnetosphere coupling mechanism. It is clear that the storm time ring current enhancement cannot be understood as a simple superposition of substorm injections (e.g., McPherron [AGU monograph 98, 131, 1998]). Instead, the storm time ring current is now thought to be produced by the global convection electric field which results from a prolonged southward IMF and the associated enhanced dayside reconnection rate. Gonzalez [Planet. Space. Sci., 38, 627, 1990]) has reviewed various solar wind magnetosphere coupling functions which have appeared in the literature, arguing that they are all special cases of a general expression (e.g., Sonnerup [J. Geophys. Res., 79, 1546, 1974]) for the rate of component reconnection at the dayside magnetopause. Sonnerup [1974] assumed that the dayside reconnection X line is parallel to the local current density, so that there is a critical IMF clock angle below which reconnection is geometrically impossible. Further, in the special case for which the magnitudes of the IMF and magnetospheric magnetic fields are equal, the reconnection electric field is proportional to sin2(θc/2), where θc is the IMF clock angle. Thus, the Sonnerup [1974] formula seems to explain the rapid decrease in solar wind magnetosphere coupling efficiency with decreasing IMF clock angle. In this talk, we revisit the role of dayside magnetic reconnection in the triggering of magnetic storms, taking into account the global, three-dimensional nature of the magnetopause magnetic field topology. We find that in contrast to the Sonnerup [1974] model -- in which two-dimensional reconnection theory is applied to a local patch of the magnetopause -- the three-dimensional X line is not parallel to the local current density. Further, we argue that the magnetopause has a null-null separator topology for all northward IMF clock angles (save due north, when the topology is structurally unstable). Since the magnetic field projected onto a plane perpendicular to the separator line has an X type topology (by definition), the Sonnerup [1974] constraint does not apply: magnetic reconnection is geometrically possible for all nonvanishing clock angles. The subsolar reconnection electric field does not exhibit the half-wave rectifier behavior which one expects from the Sonnerup constraint. Thus, we argue that steady separator reconnection at the subsolar magnetopause cannot explain the half-wave rectifier response of the magnetosphere to the solar wind electric field.
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