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

Abstract

[1] 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.

Experimental tests of the generation mechanism of auroral medium frequency burst radio emissions

“…The resulting frequencies would range from the plasma or upper hybrid frequency at the F peak, of order foF2, to the plasma frequency at an altitude where collisions prevent the effective excitation of the waves, or the frequency corresponding to the peak plasma frequency in a blanketing E layer if one exists. This mechanism fits properties of MF burst known from previous observations, including: the frequency range of 1.3–4.5 MHz, which is similar to the typical range of plasma and upper hybrid frequencies in the F layer; the polarization of the emissions which is consistent with LO mode; the association with auroral activity and substorm onsets, when auroral electron beams penetrate the ionosphere and excite Langmuir and upper hybrid waves there; and extension of MF burst emissions to higher frequencies when conditions favor higher densities, such as early and late winter conditions at South Pole [ LaBelle et al , 2005] or times when ISR records increases in density [ LaBelle et al , 1997]. Open questions are whether the electron beams persist to sufficiently low altitudes with sufficient free energy to excite Langmuir or upper hybrid waves on the F‐region bottomside, and whether the beam can persist to excite waves over a sufficiently broad altitude range to explain the broadband nature of the MF burst.…”

“… Shepherd et al [1997] show that MF bursts are left‐hand polarized. LaBelle et al [1997] also presented waveform measurements, though restricted to a 10‐kHz bandwidth, which suggested sub‐structure with timescales as short as 100–300 μ s. LaBelle et al [2005] reported statistical and case studies of MF burst events at South Pole Station and several sites in northern Canada, in which they confirmed MF burst to be observed at ground level only during darkness, and that their occurrence rate is controlled heavily by local magnetic time, maximized during premidnight hours; a case study of an event observed at five different Canadian observatories shows that the MF burst intensity shifts poleward during the course of the event.…”

First observations of 4fce auroral roar emissions

“…[10] The 4f ce roar did not accompany any other types of MF/HF auroral radio emissions (2f ce roar, 3f ce roar and MF burst), which are detected only during local darkness and are most common in the premidnight hours [e.g., Weatherwax et al, 1995;LaBelle et al, 1997;LaBelle and Weatherwax, 2002;LaBelle et al, 2005]. This difference is causallyrelated to dependence of ionospheric radio wave absorption on frequency: during daylight hours, photoionization enhances electron densities in the ionospheric D-and E-regions to increase radio wave absorption, which is severer for lower frequency waves.…”

Auroral radio emission and absorption of medium frequency radio waves observed in Iceland

“…MF burst shows a broadband impulsive feature with a bandwidth of 1-2 MHz in a frequency range of 0.8-4.5 MHz (LaBelle et al, 2005). The polarization character of 2 f ce roar and MF burst have been identified as left-handed polarized waves by Shepherd et al (1997), implying that they should propagate in the L-O mode in the ionosphere.…”