On the theory of classic mesodiffusion

A simple model of the classical random walk of particles with a constant sp eed and anisotropic angular distribution is used to study the characteristi c features of mesodiffusion, that is, of an intermediate stage between the ballistic regime (short times) and ordinary diffusion (long times). In the extreme case of anisotropy, namely, walking along a straight line, the proc ess can be described by the telegraph equation, whose solution contains del ta -functions accounting for the ballistic component. As the anisotropy bec omes less pronounced, the delta -singularity transforms into a frontal burs t (the quasi-ballistic component), beyond which the distribution can be sat isfactorily described by the telegraph approximation. In the other extreme case of isotropic walking, the frontal burst disappears and the telegraph a pproximation, contrary to general belief, proves to be cruder than the diff usion approximation. (C) 2001 MAIK "Nauka/Interperiodica".