Observations of Dayside Convection Reduction Leading to Substorm Onset

Details Observations of Dayside Convection Reduction Leading to Substorm Onset Observations of Dayside Convection Reduction Leading to Substorm Onset

Dec 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufmsm52b..06l&link_type=abstract

American Geophysical Union, Fall Meeting 2002, abstract #SM52B-06

Physics

2463 Plasma Convection, 2712 Electric Fields (2411), 2736 Magnetosphere/Ionosphere Interactions, 2788 Storms And Substorms

Scientific paper

We have used SuperDARN radar measurements of ionospheric plasma flow on the dayside to evaluate the extent to which substorm onset is the result of interplanetary magnetic field (IMF) changes that lead to a reduction in the strength of magnetospheric convection and the timing of such convection changes relative to onset. By using direct observations of convection, we avoid the uncertainties in previous studies of substorm triggering arising from their use of IMF measurements from locations away from the magnetosphere. Our analysis of the dayside plasma flows indicates that the vast majority of well-defined substorm onsets are associated with a reduction in the strength of large-scale convection that is imparted to the magnetosphere from the solar wind. We also find that the convection reduction imparted to the magnetosphere initiates a few minutes prior to substorm onset. This is the same time relative to onset that, based on auroral observations, the nightside magnetosphere undergoes a transition to an unstable configuration. This suggests that the convection reduction is responsible for the transition to instability that leads to the substorm expansion phase. Our results are consistent with the idea that, under enhanced magnetospheric convection, energy builds up within the nightside magnetosphere and can reach a stable equilibrium where energy losses balance further energy input. If the energy accumulated on the nightside during a period of enhanced convection exceeds that of the equilibrium configuration after a reduction in the strength of convection, the nightside magnetosphere undergoes a transition to instability when the strength of convection is reduced and the excess energy is released as the substorm expansion phase. These results indicate that the question of how this instability arises and develops after a decrease in convection is a critical outstanding question for understanding the substorm expansion phase.

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