Small-scale magnetic field-aligned density irregularities excited by a powerful electromagnetic wave

A set of equations describing the evolution of diffusive plasma density and temperature perturbations, and upper hybrid waves, coupled by a dipole electromagnetic pump field is derived. For the first time, instability from infinitesimal perturbations (soft excitation) and instability from a finite level of perturbations (hard excitation) are contained in the same model. Conditions for the formation of density irregularities and the irregularity spectrum in the nonlinearly stabilized instability state in the different excitation regimes are discussed. The results are of relevance for the small-scale magnetic field-aligned irregularities and a number of associated phenomena observed to be excited by a powerful high frequency electromagnetic wave transmitted into the ionospheric plasma.