Tracking Traveling Ionospheric Disturbances through Doppler-shifted AM radio transmissions

Abstract. Six specialized radio receivers were developed to measure the Doppler shift of amplitude modulation (AM) broadcast radio carrier signals due to ionospheric effects. Five were deployed approximately on a circle at a one-hop distance from an 810-kHz clear-channel AM transmitter in Schenectady, New York, and the sixth was located close to the transmitter, providing a reference recording. Clear-channel AM signals from New York City and Connecticut were also received. The experiment confirmed detection of travelling ionospheric disturbances (TIDs) and measurement of their horizontal phase velocities through monitoring variations of the Doppler shift of reflected AM signals imparted by vertical motions of the ionosphere. Comparison of thirteen events with simultaneous global navigation satellite system (GNSS) based TID measurements showed generally good agreement between the two techniques, with differences attributable to differing sensitivities of the techniques to wave altitude and characteristics within a complex wave environment. Detected TIDs had mostly southward phase velocities, and in 4 cases they were associated with auroral disturbances that could plausibly be their sources. A purely automated software technique for event detection and phase velocity measurement was developed and applied to one year of data, revealing that AM Doppler sounding is much more effective when using transmitter signals in the upper part of the AM band (above 1 MHz) and demonstrating that the AM Doppler technique has promise to scale to large numbers of receivers covering continent-wide spatial scales.