Home -> Ground Based Reseach -> Northern Research Sites -> Churchill, Manitoba

Churchill, Manitoba

Latitude/Longitude:
(58.76º N, 265.36º E)

Mag Coordinates:
(68.83º, Midnight MLT = 6:37 UT)

Instruments    Selected Publications
Selected Data    Photos

Main Science

Churchill 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.

Other significant results achieved at Churchill:

1. Operation of a down converting receiver under operator control for several weeks in 1996 allowed us to accumulate an atlas of fine structure of auroral roar emissions, showing the wide range of types of fine structure of this auroral emission. Among the surprising findings are an asymmetry between increasing versus decreasing frequency tones, and the extreme narrowness of some of the features (as narrow as less than ten Hertz, out of 2.8 MHz!) See Shepherd et al., 1998 for details. See the Atlas to examine more examples of auroral fine structures and hear corresponding sounds.
2. Operation of a polarization receiver for one full year (1997) revealed ;for the first time that auroral roar emissions are left hand polarized. This eliminates RX-mode cyclotron maser mechanism as a generation mechanism, and favors mode conversion radiation as the source. Hundreds of 2f_ce roar were all found to be left-hand polarized. ; A few examples of 3f_ce auroral roar were also measured by the system to be left-hand polarized. See Shepherd et al. 1997 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 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.

Downconverting Receiver (DCR)

This receiver uses the same preamplifier and antenna as the PFR, a 10 m² magnetic loop antenna oriented vertically. The downconverting receiver translates MF waves at a selected center frequency to baseband, and the output is recorded on audio frequency analog tapes with ~ 12-kHz bandwidth. A double-conversion quadrature IF stage is used to define the bandwidth. Any 10 kHz-band from 10 kHz to 5 MHz may be selected. The selected band appears at the output in the frequency range from 2 to 12 kHz. An analog filter at the tape recorder input compensates for the preemphasis built into the audio tape recorder, and an inverse filter applied upon playback produces an approximately equalized instrument response up to 12 kHz. A center frequency near 2.85 MHz was often selected, because it is optimum for auroral roar at Circle Hot Springs and because active experiments are sometimes conducted with the nearby high-power auroral simulation (HIPAS) facility on that frequency.

At Circle Hot Springs, the tape recorder was programmed to operate for 90 minutes around local midnight, the most probable time to detect auroral roar. Operation was limited to two nights per week on average due to constraints of the local operators.

At Churchill, audio output from the DCR together with the real-time visual display of the output of the PFR, also operated at the site, was used to tune the DCR manually in real-time. So that events could be matched from the two receivers, audio time stamps from the PFR were mixed with the DCR output signal and recorded onto the cassette tapes every minute. In addition, the DCR was equipped with a microphone so voice announcements about the tuning of the DCR and notes on the visible aurora could be recorded.

For more information, see:

• Shepherd, S.G., J. LaBelle, and M.L. Trimpi, The polarization of auroral roar emissions, Geophys. Res. Lett., 24, 3161, 1997.
• Shepherd, S.G., J. LaBelle, and M.L. Trimpi, Further observations of auroral roar fine structure, J. Geophys. Res., 103, 2219, 1998.
• Shepherd, S.G., Auroral Ionospheric Electron Gyroharmonic Radio Emissions: Observation and Theory, PhD. Thesis, Dartmouth College, 1998

Complete List of Dartmouth Instruments

Polarization Receiver

The polarization experiment consists of two perpendicular 10 m² loop antennas, one oriented N-S, the other E-W.

In the polarization detector, a 90º phase lag is introduced into the signal from the N-S loop. On alternate sweeps this signal is inverted, effectively shifting the phase of the N-S loop signal from ñ 90º to +90º relative to the E-W loop. The input to the receiver is the shifted and switched N-S loop signal summed with the signal from the E-W loop. If the original signals induced in the antenna loops are equal in amplitude but differ in phase by exactly 90º, as would be expected for vertically incident right- or left-circularly polarized waves, the input signal to the receiver alternates between zero and twice the induced signal strength. On the other hand, a linearly polarized signal induces in-phase signals in the antenna loops, which result in a constant input signal to the receiver, there being no difference between shifting the N-S signal forward or backward in phase in this case. To assist in data analysis, a marker is recorded during part of the noninverted sweep to allow sweep identification, and a calibration signal that simulates a received right-circularly polarized signal is periodically inserted into the antennas. In order to determine polarization using this technique, the measured signals must be relatively constant in amplitude and polarization during two consecutive measurements (~2 s); signals whose amplitude varies faster than that may register a false or indeterminate polarization. The sense of polarization (right or left) is determined by noting the relative signal strengths of the two sweeps and comparing that to the marker.

The interpretation above assumes that the electronics are perfect. It is difficult to shift a single signal by 90º over a wide bandwidth, but using an all-pass filter, it is easy to shift both the N-S and E-W loop signals such that the phase of the N-S loop signal lags the E-W loop signal by ~90º over a range of approximately two decades. The error in phase shift over the 0.05-5.0 MHz frequency range is less than ten degrees, implying that the maximum amplitude difference between consecutive sweeps for either right- or left-circularly polarized vertically incident signals is about 20 dB which is less than the observed differences of the real signals.

For more information, see:

• Shepherd, S.G., J. LaBelle, and M.L. Trimpi, The polarization of auroral roar emissions, Geophys. Res. Lett., 24, 3161, 1997.

Complete List of Dartmouth Instruments

Selected Data | Top

These figures show spectrograms of one hour of data from two of our sites located in Arviat, Northwest Territories and Churchill, Manitoba. Three types of emissions are shown: Auroral Roar, MF Bursts, and Auroral Hiss. Each 2 second sweep is displayed as a vertical stripe of grayscale with the darker pixels corresponding to more intense signals.

Photos | Top

Selected Publications | Top

65. Shepherd, S.G., J. LaBelle, and M.L. Trimpi, The polarization of auroral roar emissions, Geophys. Res. Lett., 24, 3161, 1997.

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.

68. Shepherd, S.G., J. LaBelle, and M.L. Trimpi, Further observations of auroral roar fine structure, J. Geophys. Res., 103, 2219, 1998.

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 the 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.