Authors: Joe Borovsky, Joachim Birn; Aaron Ridley
Affiliation: Los Alamos National Laboratory; University of Michigan
Cross correlating magnetospheric-event catalogs with solar-wind measurements, we have determined the types of solar wind that give rise to global sawtooth oscillations, periodic substorms, randomly occurring substorms, and SMC events. Statistically, the solar wind that drives sawtooth oscillations has low density, strong southward IMF, normal to fast speed, and low-levels of upstream MHD turbulence. This "sawtooth solar wind" is a low-Mach-number wind that creates a magnetosheath with extremely low beta values. Using NASA/CCMC, several 3D MHD simulations of the solar wind driving the magnetosphere have been run. Some runs utilized solar-wind conditions that should produce sawtooth oscillations; other runs deviated from these conditions. For sawtooth solar wind, the simulations reveal the following findings. (1) The low-beta magnetosheath is asymmetric about the Earth; the orientation of the IMF affects the shape of the bow shock and the flow pattern of the magnetosheath. (2) The magnetotail is short and unflared, in agreement with a force-balance model. (3) Sunward flow of plasma in the magnetotail bifurcates around the Earth to produce a flattened sunward-flowing plasma sheet in the dipolar magnetosphere. In this plasma sheet the weakening of the magnetic-field strength and the distortion of magnetospheric field lines into a "stretched" configuration extends from the nightside well sunward of the dawn and dusk terminators. An equatorial current system associated with this stretching forms a "horseshoe" of current around the nightside of the dipole. (4) Open field lines are adjacent to this flattened plasma sheet. Dipole tilt and the twisting of this plasma sheet by the IMF can bring lobe field lines to the equator in the middle magnetosphere. (5) If the IMF is not steadily southward, the plasma sheet in the dipole is not flattened and the field lines in the dipole are not stretched.