Entropy conservation and rate of propagation of bubbles in the Earth's magnetotail: A case study

Abstract

Earthward propagation of low-density flux tubes, called “plasma bubbles,” is a candidate mechanism to reduce total entropy in the plasma sheet. MHD models predict that bubbles conserve total entropy (S) as they propagate earthward, that the amount of density depletion governs the speed of propagation, and that bubbles slow down as their S nears the ambient plasma sheet S. The main objective of the present study is to test these predictions. Multiple instances of bursty flows with low entropy content were analyzed using in situ and all-sky observations during a major THEMIS (TH) alignment. Twelve bubbles identified at TH-B altitude were tracked earthward to determine if they convected past TH-C, -D or -E and TH-A. The analysis shows that the mean plasma sheet S along the THEMIS alignment is not conserved in general, as it decreases with decreasing distance. However, S is nearly constant along the bubbles' trajectories. Furthermore, S of the bubbles is one standard deviation smaller than the mean ambient plasma sheet S. Therefore, bubbles tend to decrease the S of the plasma sheet. An inverse relation of density and S of bubbles with velocity is consistent with theoretical predictions. The mean specific entropy (s) of the bubbles tends to be one standard deviation larger than the mean plasma sheet s. Therefore, bubbles tend to increase the plasma sheet s. A DMSP-THEMIS coincidence during the sampled period shows how plasma measurements from low-orbiting satellites combined with all-sky images can be used to track the propagation of bubbles.