No abstract

Collisionless shocks heat electrons in the solar wind, interstellar blast waves, and hot gas permeating galaxy clusters. How much shock heating goes to electrons instead of ions, and what plasma physics controls electron heating? We simulate 2-D perpendicular shocks with a fully kinetic particle-in-cell code. For magnetosonic Mach number Mms ∼ 1-10 and plasma beta βp < ∼ 4, the post-shock electron/ion temperature ratio Te/Ti decreases from 1 to 0.1 with increasing Mms. In a representative Mms = 3.1, βp = 0.25 shock, electrons heat above adiabatic compression in two steps: ion-scale E = E • b accelerates electrons into streams along B, which then relax via twostream-like instability. Shock rippling also allows quasi-static shock-normal electric fields to heat electrons; we find that quasi-static fields generally contribute half of the electron heating beyond adiabatic compression.

The rheology of particle-laden fluids with a yield stress, such as mud or crystal-rich magmas, controls the mobility of bubbles, both the size needed to overcome the yield stress and their rise speed. We experimentally measured the velocities of bubbles and rigid spheres in mud sampled from the Davis-Schrimpf mud volcanoes adjacent to the Salton Sea, Southern California. Combined with previous measurements in the polymer gel Carbopol, we obtained an empirical model for the drag coefficient and bounded the conditions under which bubbles overcome the yield stress. Yield stresses typical of mud and basaltic magmas with sub-mm particles can immobilize millimeter to centimeter sized bubbles. At Stromboli volcano, Italy, a vertical yield stress gradient in the shallow conduit may immobilize bubbles with diameter 1 cm and hinder slug coalescence.