Global Sawtooth Oscillations of the Magnetosphere during Large Storms

During strong geomagnetic storms, the dipolar portions of the magnetosphere can undergo large-amplitude, azimuthally invariant global oscillations with periods that ranges from 2-4 hours. Such oscillations occurred during the July-2000, the August-2000, and the March-2001 storms. In plots of the energetic-electron and energetic-ion fluxes versus time, these oscillations appear as periodic "sawtooths", with the fluxes ramping down over a timescale of 2-4 hours and then jumping back up on a timescale of a few minutes to 20 minutes. With multiple satellites at geosynchronous orbit, the oscillations are seen simultaneously and in phase at all local times. The energetic-particle sawtooth is associated with a global stretching and rapid relaxation (sawtooth crash) of the geomagnetic field. During the stretching phase the field at geosynchronous orbit appears tail-like at all local times. During the sawtooth crash the swing in the field direction can be 60 degrees. Multisatellite observations show that the sawtooth crash commences at dusk and propagates rapidly in a westward direction around the global magnetosphere. The sawtooth crash is sometimes associated with a sudden decrease by several 10's of nT in the magnitude of the Dst index. As yet the importance of this sawtooth cycle to the physics of geomagnetic storms is not known.