Experimental tests of a topside generation mechanism for auroral medium frequency radio emissions

Medium frequency (MF) burst is a ground‐level, impulsive auroral radio emission at 1.3–4.5 MHz associated with substorm onset. Using continuous wave measurements, Bunch and LaBelle (2009) discovered that MF burst consists of a structured feature that appears as a “backwards seven” on a frequency‐time spectrogram. LaBelle (2011) suggested that this fine structure results from Langmuir waves on the topside of the ionosphere that mode‐convert into L‐mode electromagnetic waves. This theory predicts that the lower frequency boundary of a “backwards‐seven” fine structure must exceed the maximum L‐mode cutoff ( f L ) along the wave propagation path. To test this prediction, we analyzed 186 fine structures from two sites over multiple seasons. To account for the possibility of wave refraction into regions of lower density, the values for f L were calculated assuming that the maximum plasma frequency ranged from 0–5% below the upper frequency boundary of the observed fine structure; for these conditions 96–100% of the fine structures had a lower frequency boundary above f L . The theory further predicts that the MF burst upper frequency boundary must lie below the maximum ionospheric plasma frequency along the source magnetic field line. We analyzed fifteen MF bursts that occurred while the Sondrestrom Incoherent Scatter Radar (ISR) measured the local ionospheric density. In all cases, the upper frequency boundary was below the maximum ionospheric plasma frequency measured by the radar. These two studies are consistent with the idea that MF bursts originate as Langmuir waves on the topside of the ionosphere that subsequently mode‐convert to L‐mode waves.