THE THERMAL SELF-FOCUSING INSTABILITY NEAR THE CRITICAL SURFACE IN THE HIGH-LATITUDE IONOSPHERE

A fully nonlinear development of the thermal self-focusing instability of high power radio waves in the ionosphere in the region near the cr itical surface is the subject of the present study. In the simulation model studied, a high-powered radio wave in the frequency range 5 - 10 MHz, with a 1% amplitude modulation, is launched vertically. In the h igh latitude geometry this represents a direction antiparallel to the magnetic field which is almost vertically downwards. The modulated wav e undergoes strong self-focusing at the critical surface, where the gr oup velocity of the wave goes to zero. The scale size of the structure s transverse to the magnetic field is controlled by the wave intensity and the die-action effects. The large parallel thermal conduction lea ds to the diffusion of these irregularities into the underdense and ov erdense plasma in narrow filaments. The depletion in the density in th e overdense plasma allows propagation of the wave to higher altitude a bove the original critical surface and hence into the overdense plasma .