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M.J. Kosch
Dept. of Communication Systems, Lancaster University, Lancaster, UK.
Black auroras are spatially well-defined regions within a uniform diffuse auroral background where the optical emission is absent. They typically appear post-magnetic midnight and during the substorm recovery phase, and are extremely common over Tromso. Their mechanism is unknown. Non-sheared black auroras are most probably not associated with field-aligned currents. The apparent motion of the black aurora is most probably controlled by the drift of particles in the magnetosphere and not ExB drift in the ionosphere. The drift speed is therefore dependent on the energy of the precipitating particles and therefore the height of the diffuse background. The mechanism for black aurora is most probably active within the magnetosphere and substorm associated plasma turbulence.
Fast Auroral Waves have been reported only 10 times since 1938. The optical signature appears as a wave-like structure propagating at 10-300 km/s. In 1987 a further 9 Super Fast Auroral Wave events, with speeds of 500-1000 km/s were recorded using an all-sky TV camera propagating equatorward from auroral arcs.
The altitude, thickness and intensity are estimated at 96 km, <10 km and <1 kR at 427.8 nm. The mechanism of auroral waves is unknown but may due to a leaky Ionospheric Alfven Resonator.
High-latitude ionospheric modification facilities may be used to modulate the Hall conductance, which modulates the electrojet current, and in turn radiates VLF whistler waves. Assuming the VLF waves are ducted along the magnetic field lines, the frequency of the VLF wave is modulated in time such that Doppler-shifted cyclotron resonance between the wave and gyrating electrons is maintained along a large fraction of the magnetic field line for a selected particle energy. The electron travel time is a function of particle energy, which is selected for best optical detection upon precipitation into the atmosphere, pitch angle, which is selected to be at the loss cone, and magnetic field strength. The VLF wave travel time is a function of frequency, which we calculate, electron gyro-frequency, which is a function of magnetic field strength, and plasma frequency, which we model. The frequency-time modulated VLF wave patterns have been successfully implemented at HAARP in Alaska.
Björn Gustavsson
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