Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmsm41a1677b&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #SM41A-1677
Physics
[2407] Ionosphere / Auroral Ionosphere, [2441] Ionosphere / Ionospheric Storms, [2790] Magnetospheric Physics / Substorms, [6984] Radio Science / Waves In Plasma
Scientific paper
Auroral Medium Frequency (MF) burst is a naturally occurring spontaneous impulsive radio emission observed at ground level between 800 and 4500 kHz. MF burst has been shown to be associated with auroral substorm onset, commonly observed coincidentally with auroral roar and hiss, with typical durations of a few minutes, amplitudes of ~10-14 V^2 /m^2 Hz, and measured to be left hand polarized. Despite several suggested theories, the exact generation mechanism for MF burst still remains a mystery. Recent experimental findings combined with a discussion of normal wave modes available in the auroral ionosphere allow us to constrain potential modes of generation. Normal modes available in the auroral ionosphere (Omega_ce = ~800-1600 kHz, omega_pe = ~100-6000 kHz between 100 and 1000 km altitude) include R-X, L-O, Z (L-X), Langmuir-upper hybrid, electron cyclotron, electron acoustic, and electron cyclotron sound. Each mode is considered in light of experimental evidence, including accessibility to the L-O mode for propagation to ground with left hand polarization, and requirements for excitation by an auroral electron beam (100-10s of keV). A linear mode conversion process is also preferred over non-linear due to their inefficiency. Specific scenarios include: mode conversion of Langmuir waves at a range of altitudes on either the top or bottom side F-region, short wavelength (thermal branch) Langmuir waves, electron acoustic waves, electron cyclotron sound waves, direct generation of the L mode, and refraction and conversion of Z / upper hybrid waves - similar to that of auroral roar.
Bunch Nicholas L.
Labelle James W.
Weatherwax Allan T.
Yoon Peter H.
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