Study of median frequency bursts and anomalous ISR echoes

Abstract:

Median frequency bursts (MF bursts) are a type of broadband natural auroral radio emission, the production mechanism of which is not well understood. They are a signature of substorm onset, and could result from a radiation mechanism applicable elsewhere in geospace. Recently, a correlation between MF burst and certain anomalous incoherent scatter radar (ISR) echoes, interpreted as cavitating Langmuir turbulence, are found in a limited number of events. By employing radio data from Toolik Lake Observatory (68.630692, -149.610636) and radar data from the Poker Flat Incoherent Scatter Radar (PFISR), we investigate, more thoroughly, the correlation between the MF bursts and the anomalous scattering events.

We identify 146 MF bursts from spectrograms of the Toolik Lake receivers for cross examination with radar data. We find that 73 of the MF burst events occur within less than ~1 hour from at least one anomalous scattering event. However, close examination of radio spectrograms and radar data reveals that for only eight MF burst events do the anomalous radar echoes exactly coincide with radio emission detected at Toolik Lake, and in only one of these cases is the radar echo likely due to cavitating Langmuir turbulence. Nevertheless, these eight events enable us to investigate possible relationships between the anomalous scatter events and the fine frequency and temporal structure of the MF burst. Interestingly, in one case on October 19, 2017, cavitating Langmuir turbulence coincides with extension of MF burst to unusually low frequencies below the local electron gyrofrequency. In summary, there appears to be a significant correlation between these two ionospheric phenomena, but it remains unknown whether there is a causative link between them.