Abstract
[1] Medium Frequency Burst (MFB) is a broadband auroral radio emission observed at ground-level at the onset of auroral substorms. Recent high-resolution measurements show it to be composed of fine structures with “backwards seven”-shaped leading edges when displayed as a frequency-time spectrogram. With a simple one-dimensional model, the group delay of Langmuir waves originating on the F-region topside and converting linearly to L-mode can be made to match the shape of the leading edge by adjusting the topside density profile, but for plausible density profiles the fit implies relatively low energy electron beams (few hundred keV). If this mechanism explains the emissions, it would be consistent with their excitation by relatively low energy Alfvénically accelerated electron beams in the leading edges of poleward-expanding auroral arcs during substorm onset.
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