Low Frequency Waves During RF Heating of the Ionosphere
Monday, 15 December 2014
Radio frequency heating of the ionosphere produces local plasma heating and the resulting pressure gradient leads to plasma currents. Modulated heating excites waves at the modulation frequency which propagate away from the heating region. The wave generation modulated heating of the F-region ionosphere is modeled using a numerical code 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 propagating along the field lines. Simulations with modulation frequencies in the range 2 – 10 Hz in the high- and mid-latitude ionosphere show their dependence on the ionospheric conductivity, modulation frequency and size of the heated region. In the high-latitudes the wave propagation is simulated using an essentially vertical magnetic field. For the mid-latitudes a dipole magnetic field in polar coordinates is used. With a source at L = 1.6 and altitude of 300 km the EMIC and whistler waves are generated, and field-aligned waves propagate to the conjugate region. In the case of modulation at 10 Hz the EMIC waves encounter the resonance layer, while the whistler waves propagate along the field lines to the conjugate region. These simulations correspond to the ionospheric heating by the Arecibo facility.