Physical conditions in a hydrogen gas heated by suprathermal protons

A self-consistent calculation has been made of the physical conditions in a pure hydrogen gas that is heated by suprathermal protons (energies greater than 200 keV). The geometry adopted is that of a plane-parallel slab with suprathermal particles incident normally. Lower and upper limits were set on the degree of ionization and temperature of the gas by calculating conditions as a function of penetration distance for (1) an optically thin gas where all radiation immediately escapes, and (2) an optically thick gas where Lα photons are trapped and destroyed on the spot by two-photon emission. The adjustable parameters used were the initial energy of the suprathermal protons, the fast-particle density, and the ambient gas density. Even with the on-the-soit approximation and no dispersion for the initial fast proton energies, conditions are roughly uniform throughout the gas. For both cases, when the gas is not completely ionized, the calculated electron temperatures are on the order of 104 K and the degrees of ionization are usually less than 10 percent. When the suprathermal particle density is greater than about 10-6 of the gas density, the gas becomes completely ionized