The propagation of auroral hiss observed on the ground as deduced from direction-finding measurements

Abstract

Based on the direction-finding results of auroral VLF hiss at Syowa (geomag. lat. −70°), an extensive ray tracing analysis, and an estimation of transmission loss in the simulated auroral ionosphere, the propagation characteristics of ground-based auroral hiss in the magnetosphere and ionosphere are deduced ; incoherent transmission of impulsive hiss with a wide frequency range (⩾ 100 kHz) from localized exit regions at the ionospheric level almost coincident with some localized regions of bright electron auroras; non-ducted propagation of continuous hiss with a narrow frequency range (⩽20 kHz) after emerging from a duct exit at higher altitudes (3000–5000 km); and coherent transmission from an exit region at the ionospheric level between Syowa and the location of a quiet auroral arc appearing poleward far from Syowa. Coupled with previous spacebased observations and model calculations of the auroral hiss power flux spectrum, we see that impulsive hiss emissions generated in a wide range of higher to low altitudes by the beam amplification are trapped in irregularities of decreased electron density outside the auroral arc at lower altitudes, propagate down to the ionosphere and are transmitted to the ground, and that after emerging from a duct exit at altitudes of 3000–5000 km, continuous hiss emissions are propagated in a non-ducted mode down to the ionosphere at different latitudes, corresponding to the initial wave normals at the duct exit, and are transmitted to the ground.