Fine Structure of Auroral Roar - Part I


James LaBelle, P. I.

Beginning in September 1994, a narrowband downconverting receiver was installed at Circle Hot Springs, Alaska, to obtain high-frequency and time resolution measurements of selected 10-kHz bands in the medium frequency range. The results show that the auroral roar emission is composed of numerous discrete structures which exhibit complicated frequency-time characteristics. These discrete structures bear a superficial resemblance to the fine structure of AKR [Gurnett and Anderson, 1981] and to discrete emissions at VLF [Helliwell, 1965]. Implications of the fine structure for existing models of auroral roar generation are numerous. No theory has been proposed for the roar which explains the newly discovered fine structure.

The high resolution roar signal was analyzed by translating the 2.85 MHz center frequency to baseband. The resulting "audio" signal can be played and processed using conventional audio analysis tools.

10 second (80k) sample
55 second (428k) complete event

The discovery of fine structure in auroral roars radically alters the previous perception of these emissions and must be addressed by any theories of their generation. Even the name "auroral roar" may turn out to be misleading, since the two events detected so far resemble VLF chorus more than roar when played through a loudspeaker; perhaps the name "HF chorus" provides a more apt description of these particular examples. All models which have been introduced in the literature require serious modifications to explain the fine structure. The auroral roar fine structures superficially resemble VLF emissions and AKR, but they apparently originate at far lower altitudes, and hence in a more collisional plasma, than these other emissions. The process acting at such low altitudes to produce these fine structures is completely unknown, and the possibility to use features of the emissions to sense the auroral ionosphere remotely remains unexplored.

See Part II for more on auroral roar fine structure.

References

Gurnett, D. A., and R. R Anderson, The kilometric radio emission spectrum: Relationship to auroral acceleration processes, in Physics of Auroral Arc Formation, Geophys. Monogr. Ser., vol. 25, edited by S.-.I. Akasofu and J. R. Kan, p. 341, AGU, Washington, D. C., 1981.

Helliwell, R. A., Whistlers and Related Ionospheric Phenomena, Stanford Univ. Press, Standford, Calif., 1965.

LaBelle, J., M. L. Trimpi, R. Brittain, and A. T. Weatherwax, Fine structure of auroral roar emissions, J. Geophys. Res., 100, 21953, 1995.

Shepherd, S. G., J. LaBelle, and M. L. Trimpi, Further investigation of auroral roar fine structure, J. Geophys. Res., 102, 2219, 1998.

Weatherwax, A. T., Ground-based observations of auroral radio emissions, Ph.D. Thesis, Dartmouth College, Hanover, N.H., 1994.