Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsm41a1844s&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #SM41A-1844
Physics
[2736] Magnetospheric Physics / Magnetosphere/Ionosphere Interactions, [2744] Magnetospheric Physics / Magnetotail, [2784] Magnetospheric Physics / Solar Wind/Magnetosphere Interactions, [2790] Magnetospheric Physics / Substorms
Scientific paper
Substorms are the dynamic response of the magnetosphere-ionosphere (M-I) system to external perturbations in the solar wind driving conditions. During substorms, the magnetotail is often far from thermal equilibrium. The interaction between the solar wind and the M-I coupling system must have an Alfvenic nature, since the generation of parallel electric fields associated with the breakdown of the frozen-in condition and auroral acceleration is caused by the Alfvenic, not dissipative, interaction, and the Poynting flux is carried by Alfven waves. Thus, the generation of magnetospheric transients and their ionospheric signatures is the result of Alfvenic interactions in the global driven system, which are largely controlled by the dynamic driven conditions of the solar wind but are also influenced by internal changes of the system, such as, the formation of auroral arcs and the changes in ionospheric parameters. From the above considerations, first, we suggest that the ultimate cause of the substorm onset and tail energy release is a decrease in momentum transfer from the solar wind due to the change of solar wind parameters. During the growth phase, energy and momentum transfer between the solar wind and magnetosphere via MHD mesoscale Alfvenic interactions throughout the magnetopause current sheet, which stresses the magnetotail and leaves it susceptible to further dynamical interactions. A decrease in momentum transfer produces a strong earthward body force acting in the whole magnetotail within a short time period, exciting fast mode waves, causing plasma flows and other magnetotail transients. These transient flows are the consequence of the breakdown of the frozen-in condition in multiple localized regions. As a consequence, the whole magnetotail tends to return to a more dipolar configuration releasing the previously stored free magnetic energy. During these processes and the further reconfiguration of the plasma sheet, Alfven waves carrying field aligned currents can be generated which lead to the subsequent auroral substorm development in the global M-I coupling system seen in the expansion phase. There is a preconditioning time period that lasts from the decrease in momentum transfer to substorm onset. The time scale for the preconditioning stage is determined by the external driven conditions, the inertial time scale for earthward moving of the tail plasma and the Alfvén transit time for M-I coupling. By analyzing the change of forces and torques due to the formations of arcs, we will explain how the formation of auroral arcs triggers a further release of the tail energy in M-I coupling system.
Lin Nan
Lysak Robert L.
Song Yushu
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