PROPAGATION TO THE GROUND AT HIGH-LATITUDES OF AURORAL RADIO NOISE BELOW THE ELECTRON GYROFREQUENCY

The propagation of auroral radio noise at frequencies below the ionosp heric electron gyrofrequency (f(ce) approximate to 1.4 MHz) at high la titudes is investigated using the HOTRAY ray tracing code. Two mechani sms are described whereby energy in this frequency range may access th e ground. It is assumed that the radiation is generated by an upgoing loss cone. in regions of depleted electron density above the ionospher e according to the theory of Wu et al. (1989). It is demonstrated that since the waves are generated with f(pe) < f < f(ce), where f(pe) is the electron plasma frequency, they correspond to Z mode waves. Ray tr acing shows that for a density model where f(pe) < f(ce) at high altit udes and f(pe) > f(ce) in the ionosphere and for a dipole. magnetic. f ield, Z mode waves are reflected in the. topside. ionosphere. and do n ot have. access to the ground. However, Z mode waves can access the se cond radio window where energy can be mode converted into whistler mod e. waves which do propagate to the ground. Typically, the range of ini tial wave normal angles is a few degrees and the resulting latitudinal spread of emissions on the ground is a few degrees. This suggests tha t any emissions observed on the. ground should be closely confined in latitude to the generation region. For a density model where f(pe) < f (ce) throughout the ionosphere it is shown that Z mode. waves can prop agate through the ionosphere and hence to the ground for f(Lcut) < f < f(ce) where f(Lcut) is the left-hand cutoff frequency. Due. to refrac tion these waves are spread over a few tens of degrees in latitude but the spread should decrease with decreasing frequency. This suggests t here is low-frequency cutoff to this mechanism below which waves can o nly reach the ground by mode conversion.