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The Balmer line profiles of nonradiative supernova remnant shocks provide themeans to measure the post-shock proton velocity distribution. While mostanalyses assume a Maxwellian velocity distribution, this is unlikely to becorrect. In particular, neutral atoms that pass through the shock and becomeionized downstream form a nonthermal distribution similar to that of pickupions in the solar wind. We predict the H alpha line profiles from thecombination of pickup protons and the ordinary shocked protons, and we considerthe extent to which this distribution could affect the shock parameters derivedfrom H alpha profiles. The Maxwellian assumption could lead to an underestimateof shock speed by up to about 15%. The isotropization of the pickup ionpopulation generates wave energy, and we find that for the most favorableparameters this energy could significantly heat the thermal particles.Sufficiently accurate profiles could constrain the strength and direction ofthe magnetic field in the shocked plasma, and we discuss the distortions from aGaussian profile to be expected in Tycho's supernova remnant.

Non-Maxwellian Proton Velocity Distributions in Nonradiative Shocks