Statistical studies of auroral MF burst emissions observed at South Pole Station and at multiple sites in northern Canada

摘要

Auroral MF burst is a broadband impulsive natural radio emission of auroral origin observed at ground level in the frequency range 0.8–4.5 MHz. Application of a semiautomated analysis method to data collected at South Pole Station, 74° magnetic latitude, results in a database of frequencies, amplitudes, and universal times of MF burst emissions observed during 1 January–31 December 2003. Besides confirming that MF burst emissions are observed at ground level only during darkness and that magnetic local time plays a significant role in controlling the MF burst occurrence rate, which maximizes in the pre-magnetic-midnight hours, these data reveal that the frequencies of MF burst emissions tend to be higher just after sunset and before sunrise than they are during midwinter. This observation supports a link between the emission frequency and the maximum plasma frequency in the ionosphere. South Pole data also show evidence for MF burst extending to frequencies well below the ionospheric gyrofrequency, which brings up the possibility that they are partly in the whistler mode. Analysis of optical and riometer data from one extra-low-frequency MF burst example suggests that MF burst is associated with a relatively high-energy (>10 keV) auroral electron beam. Data from multiple Canadian observatories during 1997–1998 show that the MF burst occurrence rate maximizes near 74° magnetic latitude. With observatories spaced by 260–585 km, about 10% of the MF burst events are observed at two or more stations. Among coincident events, about 3/4 occur first at the lower-latitude station and shift to the higher-latitude station. The average duration of the MF burst events detected at two or more observatories increases with latitude.