An explanation of ground observations of auroral hiss: Role of density depletions and meter-scale irregularities

Auroral hiss is one of the most intense whistler mode plasma wave phenomena observed both on the ground at high latitudes and on spacecraft in the aur oral zone. Propagation of auroral hiss from its source region to the ground is poorly understood. The standard whistler mode propagation in a smooth m agnetosphere predicts that auroral hiss generated at large wave-normal angl es along the auroral field lines by Cerenkov resonance cannot penetrate to the ground. We show that the presence of density depletions along the field lines in the auroral zone and meter-scale density irregularities at altitu des < 5000 km at high latitude permits the auroral hiss propagation to the ground. In our mechanism the auroral hiss generated at high altitudes (> 50 00 - 20,000 km) propagates to lower altitudes (< 3000 - 5000 km) in two mod es: (1) a ducted mode guided by field-aligned density depletions and (2) a nonducted mode. The hiss with large wave-normal angle arriving at < 5000 km altitude is scattered by meter-scale irregularities, and about 0.1% to 10% of the scattered hiss has small wave-normal angles which can penetrate to the ground. Our mechanism explains the following features of auroral hiss o bserved on the ground: (1) the characteristic spectra of continuous and imp ulsive auroral hiss, (2) the upper and lower frequency cutoffs, (3) the dis persion of impulsive auroral hiss, (4) the location of ionospheric exit poi nts of auroral hiss with respect to visible aurora, and (5) the 2-5 order o f magnitude intensity decrease of auroral hiss observed on the ground relat ive to that observed on spacecraft. Based on the model presented here, we p rovide methods to infer parameters of density depletions and intensity of l ower hybrid waves stimulated by auroral kiss from the ground measurements o f auroral hiss together with optical and radar measurements.