Universidad Castilla La Mancha
Wednesday, July 13, 2016
ABSTRACT: In 1808, Poisson realized that the conservation equations of a fluid through an interface offer 2 solutions: one continuous, when nothing changes, and another one, discontinuous, where there is a jump in density, velocity and pressure. Nearly 70 years later, Mach proved the second option was more than a mathematical curiosity, by providing the first picture of a shock wave moving through a fluid. Of course, the real world does not display any mathematical discontinuity. Since binary collisions are the only microscopic mechanisms capable of altering the fluid properties, the shock transition region in a fluid is a few mean-free-paths thick.
How is it then that the Earth bow shock in the solar wind has its front about 100 km thick, while the mean-free-path at the same location is about the Sun-Earth distance? It just happens that plasmas, that is, charged fluids, can sustain shock waves through purely collective electromagnetic means, even in the absence of binary collisions. The talk will review our current knowledge of these entities, focusing in particular on the way they can be formed, and their fulfillment of the Rankine-Hugoniot conditions.