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Unformatted text preview: Astro 405/505, fall semester 2005 Homework, 5th set, return before Friday, November 18, 4pm. Dont forget to give your name. Problem 9: The thickness of collisionless shocks Collisionless shocks have a thickness that is given by the mean free path for the interaction between the upstream and the downstream plasmas. For a homogeneous interstellar magnetic field B 3 G, an ISM temperature T 10 4 K, one hydrogen atom per cc in the upstream medium, and a supernova remnant shock with V s 10 4 km/s calculate the sonic and the Alfv enic Mach numbers, M s and M A . Are we dealing with a strong shock, for which both numbers would be much larger than unity? Estimate the shock thickness as a function of the resonant fluctuations in the magnetic field, B . To make the blastwave picture of supernova remnants viable, the shock thickness must be much smaller than the physical size of the remnant. Turn this into a requirement for B ....
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