Modulational instability of two colliding whistlers

It is shown that the dynamics of two colliding whistlers is governed by a pair of coupled nonlinear Schroedinger equations. The latter exhibit a modulational instability of constant amplitude whistlers that are nonlinearly interacting through quasistationary density perturbations created by the combined action of the ponderomotive forces of two whistlers. It has been found that the growth rate of the modulational instability increases with the intensity of any of the colliding whistlers, but decreases with the wave number per unit length of the modulated wave packet