Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsm41a1835l&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #SM41A-1835
Physics
[2744] Magnetospheric Physics / Magnetotail, [2784] Magnetospheric Physics / Solar Wind/Magnetosphere Interactions, [2790] Magnetospheric Physics / Substorms
Scientific paper
We have examined the relationships between disturbance onsets in the magnetotail and the corresponding variations of the solar wind energy input within 1 hour prior and after the times of the space onsets. The space disturbances examined include dipolarization signatures, intensification of magnetic field fluctuations, and sudden occurrences of plasma flows, which are associated with auroral substorm events. The solar wind energy input rate is calculated as the ɛ coupling function [Perreault and Akasofu, 1978]. To remove short time scale variations of the solar wind we have taken three minute integration of ɛ as the measurements of the solar wind energy input. A total of 77 space events are surveyed. We found that prior to the occurrences of the space disturbance onsets the solar wind energy input may reach a maximum value which is enough to power an auroral substorm (corresponding to ɛ >10^11 J/s). Most of the space onsets (~ 90%) occurred during the decrease of the solar wind energy input. The reduction period has no dependence on the location where the space onsets occurred. The amount of energy reduction seems to increase with the maximum energy input level before the onsets. The above findings may be understood in terms of the interaction between the solar wind and the magnetosphere. The transfer of the solar wind energy and momentum is the results of nonlinear Alfvénic interactions between fast mode waves and the current sheets throughout the magnetopause and in the tail. A net momentum transport in the tangential direction across the magnetopause provides electromagnetic viscous-like drag force and stretches the magnetotail. The drag force tends to be balanced by the resultant force of JxB and -grad_P(thermal) forces acting on the magnetotail, which is directed earthward. The reduction of the solar wind momentum input breaks the force balance on the stressed magnetotail and provides favorable conditions for the excitation and intensification of magnetotail disturbances, which may develop into magnetospheric substorms. We have also calculated the coupling function (ρV^2)^(-1/3)VB^2sin^4(θc/2) [Vasyliunas et al. , 1982], which includes an explicit dependence on ρV^2, and the newly developed coupling function, V^(4/3)BT^(2/3)sin^(8/3)(θc/2) [Newell et al. 2007], to be compared with the ɛ function . The comparison shows that the variations of the three coupling functions are very consistent, which suggests that the variations in ρV^2 is not significant for this study. The comparison justifies that the use of the ɛ parameter is good enough for the purpose of this study without involving more sophisticated formulation.
Angelopoulos Vassilis
Frey Harald U.
Lin Nan
Lysak Robert L.
Mende Stephen B.
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