Entropy Properties of Bubble Penetration in the Plasma Sheet

Details Entropy Properties of Bubble Penetration in the Plasma Sheet Entropy Properties of Bubble Penetration in the Plasma Sheet

Dec 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsm52a..04s&link_type=abstract

American Geophysical Union, Fall Meeting 2010, abstract #SM52A-04

Physics

[2736] Magnetospheric Physics / Magnetosphere/Ionosphere Interactions, [2740] Magnetospheric Physics / Magnetospheric Configuration And Dynamics, [2760] Magnetospheric Physics / Plasma Convection, [2764] Magnetospheric Physics / Plasma Sheet

Scientific paper

A longstanding question exists about how transport in the plasma sheet proceeds to avoid pressure imbalance. Various mechanisms have been proposed to explain how pressure is relieved. One mechanism involves rapid earthward convection of flux tubes of under-dense, cold plasma (“bubbles”). Quantifying the conservation of specific and total entropy along the bubble trajectory provides a direct assessment of the validity of entropy assumptions used by global MHD models, which are fundamental building blocks of the Geospace General Circulation Model (GGCM). The paramount objective of this study is to quantify the contribution of inertial effects to releasing pressure in the plasma sheet. The study follows individual bubble/streamer trajectories with ionospheric and magnetospheric observations to determine whether the total entropy (S) and specific entropy (s) are conserved along the bubble trajectories. If entropy is not conserved, the objective then is to determine whether s and S vary randomly, or whether they depend on location, solar wind or magnetic conditions, temperature, or density. We identify the plasma properties of bubbles in the magnetosphere with THEMIS and in the ionosphere with FAST, DMSP and all-sky imagers in order to address the following two questions: (1) Is penetration of bubbles controlled by entropy equalization in the plasma sheet? (2) Can bubbles contribute in a non-negligible way to reducing the entropy of the plasma sheet? We estimate s and S along the trajectory of the flux tubes during THEMIS major tail alignments and observed auroral streamer propagation. Instances are identified during weakly driven geomagnetic conditions where bubble penetration conserved total entropy. For the cases analyzed thus far, the entropy of the bubbles in the farthest region of the plasma sheet is typically ~67% smaller than the average entropy of the ambient plasma sheet. In some cases the entropy reduction was found to be as much as ~95%. At the closest sampled distance, the bubble entropy is only ~10% smaller. Entropy equalization does appear to apply in most cases. A profile of S versus radial distance calculated from three examples shows that S(bubble)/S(ambient) drops more rapidly between THEMIS-B and THEMIS-C than between THEMIS-C and THEMIS-D,-E.

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