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In interstellar and intergalactic space, densities are far lower and shock speeds far higher than anything that can be produced in a terrestrial laboratory. Electrons and ions at these shock fronts are heated by plasma waves and magnetic turbulence rather than by direct collisions. The relative degree to which electrons and ions are heated at the shock transition is one of the important unsolved problems in plasma astrophysics. Furthermore, the lack of temperature equilibrium between electrons and persists far downstream from the shock, strongly impacting the optical, X-ray and UV emission from young supernova remnants (SNRs). Some SNRs in the non-radiative stage of evolution exhibit faint optical spectra dominated by the Balmer emission lines of hydrogen (i.e., they are ‘Balmer-Dominated’). These Balmer lines, which consist of broad and narrow components, arise in a very thin ionization layer immediately behind the shock, and offer rare and valuable diagnostics of the poorly understood physics of the shock transition. I will describe some of the latest results in the study of Balmer-dominated shocks, including how the Balmer line spectra can provide a sensitive probe of the physics of electron-ion temperature equilibration. Recent work suggests that the mechanisms heating the electrons and ions are also connected to cosmic ray acceleration in supernova remnants up to the knee in the cosmic ray spectrum at 1e15 eV. |
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