Propagation to the ground at high latitudes of auroral radio noise below the electron gyrofrequency

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Abstract
The propagation of auroral radio noise at frequencies below the ionospheric electron gyrofrequency (ƒce ≈ 1.4 MHz) at high latitudes is investigated using the HOTRAY ray tracing code. Two mechanisms are described whereby energy in this frequency range may access the ground. It is assumed that the radiation is generated by an upgoing loss cone in regions of depleted electron density above the ionosphere according to the theory of Wu et al. (1989). It is demonstrated that since the waves are generated with ƒpe < ƒ < ƒce, where ƒpe is the electron plasma frequency, they correspond to Z mode waves. Ray tracing shows that for a density model where ƒpe < ƒce at high altitudes and ƒpe > ƒce in the ionosphere and for a dipole magnetic field, Z mode waves are reflected in the topside ionosphere and do not have access to the ground. However, Z mode waves can access the second radio window where energy can be mode converted into whistler mode waves which do propagate to the ground. Typically, the range of initial wave normal angles is a few degrees and the resulting latitudinal spread of emissions on the ground is a few degrees. This suggests that any emissions observed on the ground should be closely confined in latitude to the generation region. For a density model where ƒpe < ƒce throughout the ionosphere it is shown that Z mode waves can propagate through the ionosphere and hence to the ground for ƒLcut < ƒ < ƒce where ƒLcut is the left‐hand cutoff frequency. Due to refraction these waves are spread over a few tens of degrees in latitude but the spread should decrease with decreasing frequency. This suggests there is low‐frequency cutoff to this mechanism below which waves can only reach the ground by mode conversion.

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