A statistical model of the collision term in the Boltzmann equation is introduced which is similar in concept to the well-known Krook model but yields the correct Prandtl number of ⅔ for a monatomic gas. A new method for constructing kinetic models is presented which is based on the information theory concept of entropy or uncertainty. This method, which is applicable to quite general systems in nonequilibrium statistical mechanics, is used to construct statistical models for multicomponent gas mixtures and for gases consisting of molecules with internal degrees of freedom.

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In addition to the communication applications that stem from the generation of field‐aligned irregularities, ionospheric heating by powerful HF waves also holds promise for establishing new techniques in D‐, E‐, and F‐region aeronomy. This paper concentrates on the application of the theory of plasma heating by ohmic and nonlinear dissipation to the special conditions of the lower ionosphere where the electron collision and disturbing wave frequencies are comparable and the upper ionosphere where electron thermal transport is large and restricted by the earth's magnetic field. We predict the magnitude and distribution of large‐scale changes in the electron temperature and density in both the D and F layers under a variety of conditions. These results are compared with direct measurements or inferred changes whenever possible. We also examine the effects of specular‐height mismatch, multiple frequency operation involving heater‐power sharing, and antenna beam tilt on the total cross section for field‐aligned scattering. The analysis is based on the concept that the energy‐density distribution on the heater reflectrix computed by means of a ray‐tracing technique can be related directly to the density‐fluctuation intensity through an appropriate scattering model. Lastly, potential applications to aeronomy are discussed, particularly in regard to D‐region measurements of the effective recombination coefficient magnitude and temperature dependence and the relative water cluster ion concentration.

Recent experimental investigations into laser−induced damage in the alkali halides have indicated that the damage is due to avalanche breakdown in its dc limit. Numerical computations for NaCl have been carried out that show that the ionization rate can be accurately computed by a classical diffusion or Fokker−Planck approximation to the Boltzmann equation for ionization rates on the order of 1012 sec−1. For hot electrons it is necessary to add a contribution to the collision frequency that is due to the deformation potential in order to explain the dc character of breakdown by 1.06−μm radiation.

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