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Circle Hot Springs, AK

Latitude/Longitude:
(65.50º N, 215.30º E)

Mag Coordinates:
(66.27º, Midnight MLT = 10:60 UT)

Instruments    Selected Data    Selected Publications   

Main Science

The receiving system at Circle Hot Springs detected the first auroral roar emissions recorded since the phenomenon was initially discovered and briefly investigated (in 1979-1982 by the University of Minnesota group). The significant discovery: Auroral roar occurs not just near 3 MHz, as previously observed, but also at 4.0-4.5 MHz. We speculate that the 3 MHz emission is near two times the electron cyclotron frequency in the source region, and the 4.0-4.5 MHz emission is near three times the cyclotron frequency. See Weatherwax et al., 1993 for details.

The first statistics of auroral roar emissions in Alaska, and the first statistics of 3f_ce auroral roar emissions, come from Circle Hot Springs. Emissions favor premidnight/midnight local time and winter/equinoctial conditions. The emissions were most common in March-April in 1994-5. The most common emission frequency matches 2f_ce at 275 km in the case of 2f_ce-roar and matches 3f_ce at somewhat higher altitudes in the case of 3f_ce roar. One case study shows a correlation between time variability of auroral roar radio emission and optical aurora emissions measured with a meridian-scanning photometer. See Weatherwax et al. 1995 for details.

A new type of auroral radio emission, MF-burst, was discovered at Circle Hot Springs. MF-burst is a broadband impulsive emission which occurs for a few minutes around substorm onsets. It is strongly correlated with LF impulsive auroral hiss (another signature of substorm onset). MF-burst usually occurs either at 1.5-2.5 MHz or at 3.0-4.5 MHz; when it occurs across the whole band, there is often a null in the spectrum near 3 MHz, the frequency of auroral roar emissions. See Weatherwax et al., 1994 for details.

The downconverting receiver operated at Circle Hot Springs revealed for the first time the fine structure of auroral roar emissions: The emission is often not homogeneous over a ten kilohertz band as implied by low resolution measurements, but consists of  discrete fine structures which change in frequency with time. See LaBelle et al., 1995 for details.

Instruments | Top

Dartmouth Programmable Frequency Receiver (PFR)

This receiving system consists of a loop antenna of approximately 10 square meters.  The antenna response is a dipole, with the null in the horizontal plane oriented such as to eliminate the largest source of local interference. A low-noise preamplifier at the antenna has frequency response 100 kHz to above 5 MHz, and transmits these signals through a 50-ohm coaxial cable to the observatory as little as a few hundredfeet or as much as a mile away, depending on the station.

The PFR is a superheterodyne receiver tunable to 0-5 MHz using IF frequency of 10.7 MHz and crystal filter with bandwidth 7.5 kHz. The local oscillator is controlled directly by a PC running DOS. In the standard mode, frequency is stepped from 30 kHz to 5 MHz in 10-kHz steps, repeating the 498-frequency sequence each 2 seconds. Other programs are used on occasion, including faster frequency switching. In the standard mode, data are collected 20-24 hrs/day, archived on disk in the PC at the station, backed up onto CD-ROM monthly by a local operator,  and mailed to Dartmouth (except at Arviat and Taloyoak, where data are backed up annually onto tape by visiting personnel from SED Inc.)

For more information, see: Weatherwax, A.T., Ground-based observations of auroral radio emissions, Ph.D. thesis, Dartmouth College, Hanover, N.H., 1994.

Selected Data | Top

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

Publications | Top

39. LaBelle, J., and A.T. Weatherwax, Ground-based observations of LF/MF/HF radio waves of auroral origin, Proceedings of the 1992 Cambridge Workshop in Geoplasma Physics, T. Chang, editor, Scientific Publishers, Cambridge, Mass., p. 223, 1993.

40. Weatherwax, A.T., J. LaBelle, M.L. Trimpi, and R. Brittain, Ground based observations of radio emissions near 2f_ce and 3f_ce in the auroral zone, Geophys. Res. Lett., 20, 1447, 1993.

44. Weatherwax, A.T., J. LaBelle, and M.L. Trimpi, A new type of auroral radio emission at 1.4-3.7 MHz observed from the ground, Geophys. Res. Lett., 21, 2753, 1994.

45. LaBelle, J., A.T. Weatherwax, M.L. Trimpi, R. Brittain, R.D. Hunsucker, and J.V. Olson, The spectrum of LF/MF/HF radio noise at ground level during geomagnetic substorms, Geophys. Res. Lett., 21, 2749, 1994.

49. Weatherwax, A.T., J. LaBelle, M.L. Trimpi, R. Brittain, R.A. Treumann, and J. Minow, Statistical and case studies of 2f_ce and 3f_ce auroral roar from Alaska, J. Geophys. Res., 100, 7745, 1995.

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

66. LaBelle, J., Review of recent ground-level observations of terrestrial auroral radio emissions, Planet. Radio Emissions IV, ed. by H.O. Rucker, et al., Austrian Acad. Sci. Press, p. 283, 1997. 

90. LaBelle, J., and R.A. Treumann, Auroral Radio Emissions, 1. Hisses, Roars, and Bursts, to appear in Space Sci. Rev., 2002.