Physics
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufmsm42d..06v&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #SM42D-06
Physics
2778 Ring Current, 2788 Storms And Substorms
Scientific paper
The size of a magnetic storm is usually defined by the maximum depression of the horizontal magnetic field at the Earth's equator, as measured by the Dst index. Storms with peak negative Dst of 100 nT or more are classed as intense, and storms with peak -Dst over 300 nT occur in general only a few times per solar cycle. The question arises, what is the largest depression of the geomagnetic field that could possibly occur? Since the storm-time depression results from inflation of the geomagnetic field by plasma in the magnetosphere, an upper limit should be given by the maximum amount of plasma that can be contained on closed magnetic field lines, but Stewart and Parker (J.Geophys.Res. 72, 5287, 1967) have shown that (specialized but exact) MHD equilibrium solutions exist with a purely radial field configuration, i.e. a depression equal to the Earth's main magnetic field (effective -Dst 31000 nT). To obtain a more meaningful upper limit, additional constraints must be invoked. The Dessler-Parker-Sckopke theorem, in its exact form that takes surface terms into account, implies that plasma pressure contributes to -Dst only if it is enhanced over the ambient solar wind and magnetotail values. This enhancement is produced by adiabatic compression as plasma is transported inward into flux tubes of decreasing volume. I argue that the total pressure of compressed plasma and magnetic field perturbation cannot exceed, in order of magnitude, the minimum pressure of the Earth's dipole field at each flux tube. The largest possible field depression at Earth can then be estimated by setting the plasma pressure to the maximum value everywhere (the result of a postulated super-effective transport process) and applying the Dessler-Parker-Sckopke theorem. The calculation will be presented; preliminary estimates of the upper limit to -Dst are about 2000 to 2500 nT.
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