OBSERVATIONS AND SIMULATIONS OF VLF HARMONIC-GENERATION WITH THE HIGH-POWER AURORAL SIMULATION

We report experimental results from the high-power auroral simulation facility showing that the lower D region can be heated so as to produc e either a single, narrowband ELF/VLF radiation signal at the modulati on frequency or a signal containing the modulation frequency and many odd harmonics. The measurements are compared to a new 2 1/2-dimensiona l simulation code that models the lower D region and the Earth ionosph eric waveguide. The simulation spectra show good agreement with the ob servations. Rapidly switching the HF power off and on leads to perturb ing currents in the D region that vary in time in a square wave type p attern. This generates an ELF/VLF wave with high, odd harmonics observ ed up to n=13 (f = nf(0,) where f(0) is the modulating frequency). A m ore smoothly changing power variation generates a narrowband signal at f(0). Simulations show how the D region affects the spectral shape. F rom these results we find that the heating should occur at a low altit ude in the D region (approximately 75 km). Using the simulation result s, we determine that the D region current needed to produce the magnet ic field spectrum with the odd harmonics has a square wave time variat ion.