Investigation of auroral hiss observations on the ground: Application to remote sensing of auroral magnetosphere.

摘要

Observed both on the ground at high latitudes and on spacecraft in the auroral zone, auroral hiss (AH) emissions (∼1 kHz to ∼1 MHz) are intense electromagnetic emissions emitted from the auroral region. Standard whistler mode propagation theory in a smooth magnetosphere predicts that AH generated at large wave-normal angles along the auroral field lines by Cerenkov resonance cannot penetrate to the ground. This thesis presents a new mechanism of AH propagation to the ground in which presence of density depletions along the field lines in the auroral zone and meter-scale density irregularities at altitudes 5000 km at high latitude permits the AH propagation to the ground. In the proposed mechanism AH generated at high altitudes (>5000–20,000 km) propagates to lower altitudes (3000–5000 km) in two modes, the ducted mode and the non-ducted mode, with large wave-normal angles. At altitudes 5000 km meter-scale irregularities scatter the hiss into electrostatic waves with large wave-normal angles that are reflected into the magnetosphere and electromagnetic waves with small wave-normal angles that can penetrate to the ground. The AH propagation model proposed in this thesis also explains the spectral characteristics of AH including the upper and lower frequency cutoffs, the dispersion of AH, the location of ionospheric exit points of AH with respect to visible aurora, and the 2–5 orders of magnitude difference in the power spectral density ratio measured on satellites versus ground. The new understanding of AH permits the determination of AH source region, energetic electron parallel resonance energy, and cold plasma electron concentrations along field lines. Analysis of AH spectra, recorded at South Pole (July 09, 1996 0005 UT), using the model developed in this thesis shows that: (a) AH source region altitude for frequencies 7–9 kHz should be >16,000 km while for frequencies 12–20 kHz it should be 8000 km, (b) parallel resonance energy of the energetic electrons generating the frequencies should be 1 keV, and (c) cold plasma electron concentration along the field line Λ = 79° should be ∼100 el cm−3 at 12,740 km altitude.