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Graduate Student Seminar - 12/01/2017

"Entropy, Irreversibility, and the Madness of Collisionless Shocks"

by James Juno

Friday, December 1, 2017 -- 12:00 p.m.
Large Conference Room, 1207 Energy Research Facility

Advisor:  Professor William Dorland

Abstract: Shock waves, waves traveling faster than the local speed of sound in the medium, are everywhere in the universe, and often invoked to explain phenomena such as energetic particle spectra and the large temperatures of astrophysical plasmas. Normally, shock waves are intricately linked not just with the heating of the gas or plasma, but the irreversible heating of the system, due to the increase in the entropy associated with a shock wave passing through a medium. This poses a somewhat philosophical, but also practical, question about shocks in astrophysical plasmas, where the collisional mean free path is much larger than the shock ramp, thus implying viscosity and thermal conductivity are not responsible for the dynamics of the shock. If a “collisionless” shock heats the plasma, is that heating a reversible process? While mathematically this may be true, I will show that functionally this will not be the case in a subclass of collisionless shocks, where the magnetic field is perpendicular to the shock normal, due to physical process present downstream of a shock which ultimately make it inevitable that the heating of a plasma via a shock will eventually be an irreversible process.

For additional information about the IREAP Graduate Student Seminars, contact Linus Feder.

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