Plasma Dynamic Determination of Shock Thickness in an Ionized Gas.

It is shown that the principal interaction in a shock front in a tenuous ionized gas is a plasma interaction in which the ions transfer their translational energy to the plasma oscillations. The effect is, of course, of interest in auroral theories and in solar radio noise. It turns out to be the dominant effect in determining shock thicknesses nearly everywhere that the gas density is less than the solar photospheric value of 1016 atoms/cm3. We estimate that shock thicknesses in interstellar H ii regions and in interplanetary space will be 1 km or less, even though the mean free paths may be 108 km or more and the ion Larmor radius 100 km.