Title: "Ground and Space Observations of Auroral Medium Frequency Radio Emissions"

Abstract:
The auroral zone is the source of multiple kinds of radio emissions that can be observed in space and at ground-level. The study of radio emissions offers a way to remotely sense space plasma processes and, in the case of auroral emissions, to use the auroral ionosphere as a large-scale plasma physics laboratory.
 
Medium frequency (MF) burst is an impulsive radio emission at 1.5-4.5 MHz observed at ground-level. Its generation mechanism is unknown, and it is often associated with the onset of substorms. Using fine structure measurement of MF bursts reported by Bunch and LaBelle (2009), LaBelle (2011) proposed that MF bursts originate as Langmuir/Z waves on the topside of the ionosphere that subsequently mode-convert to L-mode waves and propagate to ground-level.
 
We present three studies that build on the work of LaBelle (2011). First, theory predicts that the upper frequency of MF burst must be below the maximum ionospheric plasma frequency (fpe) and that  the lower frequency boundary must be above the maximum Z-mode cutoff (fz). Analysis of full waveform radio measurements taken over multiple seasons along with radio measurements made in conjunction with measurements of fpe by the Sondrestrom Incoherent Scatter Radar indicate that these two conditions hold for all cases where the maximum ionospheric plasma frequency is allowed to vary by 5%.
 
Second, we investigate bursty MF waves observed by the DEMETER satellite that resemble MF burst. Similar to MF burst, the waves are broadband and impulsive. They also show a similar amplitude and correlation with auroral hiss, another natural radio emission that is observed with MF burst at ground-level. There are some differences in the emission’s magnetic local time distribution and relationship to substorms, but these may be caused by a combination of orbital and propagation effects. Although it is uncertain, the balance of evidence suggests these waves are MF bursts.
 
Finally, we examine the propagation, Landau damping, and mode conversion of Langmuir/Z waves in a model ionosphere.  Full wave simulations of the propagation of Langmuir waves in a reasonable topside ionospheric electron density profile are consistent with the idea that the fine structure of MF burst could be caused by Langmuir wave propagation effects on the topside ionosphere. Simulations of the bottomside mode conversion, combined with estimates of the Landau damping, indicate that the total power attenuation of MF burst in the ionosphere ranges from 53-97%, which is within the range of power attenuation that would allow the waves to be observed at ground-level.