Investigating the role of the entropy parameter in plasma sheet dynamics

Details Investigating the role of the entropy parameter in plasma sheet dynamics Investigating the role of the entropy parameter in plasma sheet dynamics

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufmsm14b..05y&link_type=abstract

American Geophysical Union, Fall Meeting 2011, abstract #SM14B-05

Physics

[2723] Magnetospheric Physics / Magnetic Reconnection, [2760] Magnetospheric Physics / Plasma Convection, [2764] Magnetospheric Physics / Plasma Sheet, [2788] Magnetospheric Physics / Magnetic Storms And Substorms

Scientific paper

Representing a combination of mass and entropy, the entropy parameter PV5/3 is approximately conserved for plasma sheet flux tubes and proves very useful for understanding plasma sheet dynamics. (Here P is plasma pressure and V is the volume of a flux tube containing one unit of magnetic flux). Under quasi-static-equilibrium conditions, PV5/3 determines Birkeland currents and interchange instability. It is consequently a key parameter for physical interpretation of results from RCM and RCM-E. The appropriate generalization of PV5/3 for conditions when P is not constant along a field line is the 5/3 power of the flux-tube integral of P3/5. That parameter is conserved in ideal MHD and is useful in physical interpretation of MHD simulations of the magnetosphere. We present recent computer experiments to investigate how the values of PV5/3 in the plasma sheet can affect the plasma transport and field configuration during various geomagnetic active times. A comparative study of RCM-E simulations shows that persistent steady magnetospheric convection during strong polar cap potential drops is possible if the flux tubes in the magnetotail are substantially depleted along a sector with very wide local times; otherwise, the magnetic field will gradually become highly stretched if the inner magnetosphere is fed with relatively high entropy plasma, resembling the substorm growth phase. In the end of the growth phase, resistive MHD simulations using OpenGGCM indicate that the violation of frozen-in-flux condition can give rise to the formation of a bubble (lower PV5/3 than its neighbors) earthward of a blob (higher PV5/3 than its neighbors). Both OpenGGCM and RCM-E results show that the earthward motion of the bubble and the tailward motion of the blob lead to a reduction of the normal magnetic field between them, which thins the current sheet rapidly. Substorm injection simulations are carried out using RCM-E by placing bubbles on the tailward boundary for both non-storm and storm times. A variety of aspects associated with the bubble injections will be discussed, including the classic substorm injection boundary problem, the reconfiguration of large-scale current systems and the magnetic disturbance in ground magnetograms.

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