Low frequency waves during RF heating of the ionosphere: numerical simulations

Radio frequency heating of the ionosphere produces local plasma heating and the resulting pressure gradient leads to plasma currents. When the heating is modulated the time varying current can excite waves of frequency close to the modulation frequency and propagate away from the heating region. The generation of the waves by a modulated heating of the F-region ionosphere is modeled using numerical codes of wave propagation in the ionosphere with the conducting ground as the lower boundary and the magnetosphere as the top boundary. The diamagnetic current due to the pressure gradient resulting from the localized RF heating oscillates at the modulation frequency and excites hydromagnetic waves, mostly the magnetosonic mode. As these waves propagate away from the heated region in the F-region it encounters regions of different conductivity, driving an oscillating Hall current in the E-region where Hall conductivity is dominant. These currents produce shear Alfven waves which propagate along the field lines. Simulations of RF heating with modulation frequencies in the range 2 - 10 Hz in the high- and mid-latitude ionosphere provide the wave propagation characteristics which depend on the ionospheric conductivity, modulation frequency and size of the heated region. In the high-latitude case the wave propagation is simulated using an essentially vertical magnetic field and the parameters corresponding to the HAARP heater experiments are used. The measurements on the ground during these experiments agree well with the simulation results. The mid-latitude case is simulated using a code that uses a dipole magnetic field in polar coordinates. With a source located at L = 1.6 and altitude of 300 km the EMIC and whistler waves are generated and the field-aligned waves propagate to the conjugate region. The characteristics of these waves depend on the modulation frequency, and in the case of modulation at 10 Hz the EMIC waves encounter the resonance layer. The whistler waves on the other hand propagate along the field lines to the conjugate region. These simulations correspond to the ionospheric heating by the Arecibo facility.