SPRG Seminars
October 30, 2012:
"Exploring the mechanism of substorm onset: A study of substorm timing sequence and the effect of solar wind variations"
N. Lin, SSL/UC Berkeley, and Y. Song, University of Minnesota
In previous studies of substorm onset mechanism, an onset of disturbances in a certain location in space (reconnection or current disruption) is often assumed to trigger an explosive energy release of the entire magnetotail, while observed disturbances in other locations are considered as the consequence of the triggering process as the perturbations propagate. By plotting the distribution of observed disturbances in the space-time diagram, we have examined the timing sequence of the disturbances for a number of carefully studied substorm events. We found that the time delays between disturbance events observed at different locations in space and between events in space and on the ground are often shorter than those expected from MHD wave propagation between the regions, which may imply that the disturbance onsets in different active regions do not have a simple causal relationship between them. These observations cannot be well explained by the traditional single-region triggering assumption.
We have suggested an alternative mechanism emphasizing that the substorm onset is a result of Alfvénic interactions in the global current system including the tail and magnetopause current sheets as well as the auroral field-aligned current region in M-I coupling system. The solar wind conditions and their variations strongly affect Alfvénic interactions between the solar wind and the magnetosphere, and thus affect substorm associated disturbance onsets occurring in the magnetotail. Net momentum transport across the magnetopause provides an electromagnetic, viscous-like, tailward drag force in the tangential direction and stretches the magnetotail. In a quasi-static case, the drag force tends to be balanced by earthward force acting on the magnetotail. Reduction of solar wind momentum input breaks the force balance on the stressed magnetotail and provides favorable conditions for excitation and intensification of magnetotail disturbances and for the substorm onset.
The theoretical foundation of the global Alfvenic interaction in the substorm process is basic dynamic theories which include newly developed theory of the generation of field-aligned currents and parallel electric fields, and the nonlinear Alfvenic interaction..