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Gillam, Manitoba

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Selected Publications

Main Science

Gillam is part of a meridional chain of six observatories running from Gillam, Manitoba, in the south to Resolute Bay in the north. These are: Gillam, Churchill, Arviat, Baker Lake, Taloyoak, and Resolute Bay.  For periods of time, identical receiving instrumentation has been operated simultaneously at all six sites.  See Data Availability Chart.

Three example results from this chain:

1. The latitude dependence of the frequency and occurrence rate of auroral roar emissions has been determined. The frequency of auroral roar emissions varies linearly with the strength of the Earth's magnetic field. This provides strong evidence that the auroral roar emission are associated with harmonics of the gyration frequency of electrons around the magnetic field, which also varies linearly with field strength. See Hughes and LaBelle, 1998 for details.
2. The occurrence rate of auroral roar peaks near 75 degrees magnetic latitude, a few degrees poleward of the typical position of the aurora. We believe that the emissions actually occur in the auroral zone, generated by the auroral electrons, but the conditions for the waves to propagate to the ground are more favorable poleward of the aurora rather than in the aurora or equatorward of it. The peak in occurrence north of the auroral zone represents the product of these two functions, generation peaking in the aurora but propagation conditions favorable in the polar cap. See Hughes and LaBelle 1998 and Shepherd et al. 1999 for details.
   
   Furthermore, as the large scale current structures associated with the aurora shift north or south in latitude, the station at which the strongest auroral roar signals are observed similarly shifts north and south in latitude. 
   See Shepherd et al. 1999 for details.
   
3. Auroral absorption, which is absorption of subionospherically propagating HF radio signals due to ionization at low altitudes under the aurora, can be measured by monitoring signal strengths of multiple subionospherically propagating HF signals at different stations An existing empirical model for auroral absorption does a pretty good job predicting the absorption of HF signals observed along our meridional chain, and our data may be used to improve the model. See Greenberg and LaBelle 2002 for details.

Instruments | Top

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 hundred feet 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.

Photos | Top

Publications | Top

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.

69. Hughes, J., and J. LaBelle, The latitude dependence of auroral roar emissions, J. Geophys. Res., 103, 14910, 1998.

76. Shepherd, S.G., J. LaBelle, C.W. Carlson, and G. Rostoker, The latitudinal dynamics of auroral roar emissions, J. Geophys. Res., 104, 17217, 1999.

87. Greenberg, E.M., and J. LaBelle, Measurement and modeling of auroral absorption of HF radio waves using a single receiver, to appear in Radio Sci., 2002.

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

Note: Numbers refer to Full Publication List

Also Note: Most abstracts are freely available through NASA's Astrophysics Data System Bibliographic Services (ADS). Some are available locally. Full texts are only available to users within institutions that subscribe to the corresponding web-based journal.