Instrumentation

Several types of radio receivers have been developed at Dartmouth College and deployed in the auroral zones. Two such receivers were used during this campaign to investigate roar fine structure: a programmable stepped-frequency receiver (PSFR) and a tunable downconverting receiver (DCR).

The PSFR, described elsewhere [e.g., Weatherwax 1994], is a versatile receiver tuned by a personal computer which also records the receiver output. The PSFR sweeps 0.03-5.00 MHz every 2 s in 10-kHz steps. Since November 1991, Dartmouth has operated a PSFR at Churchill in a semiautomatic manner in which data are collected, digitized, and stored by the computer and sent back to us on a biweekly basis. Churchill is located at $58.76^{\circ}$N, $265.92^{\circ}$E, and $69.2^{\circ}$ invariant latitude, and midnight magnetic local time occurs at 0635 UT.

The tunable DCR translates the received signal to base band allowing the output to be recorded on standard 90-min audio cassette tapes with ~12-kHz bandwidth. The operator may tune the DCR to any desired 10-kHz band between 10 kHz and 5 MHz. Audio output from the DCR in conjunction with the real-time visual display of the output of the PSFR was used to tune the DCR. So that events could be matched from the two receivers, audio time stamps from the PSFR 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.

The campaign window was chosen to occur near the equinox and during a peak in the geomagnetic activity as measured by the $Kp$ index because previous studies have shown that occurrences of auroral roar tend to maximize at these times [Weatherwax et al., 1995]. During the entire campaign the PSFR operated for 20 hours per day centered on local midnight (0600 UT). DCR recordings of radio emissions were made for 19 nights between March 30 and April 17. On a typical night, observing began at ~1900 LT (0100 UT) and ended near local midnight (0600 UT) unless roar was observed after ~2300 LT in which case the observing window was extended until quiet radio conditions existed for a full hour. When auroral roar was detected with the PSFR, the DCR cassette tape was started, and the DCR was tuned to the desired frequency. The audio output of the DCR allowed interesting portions of the auroral roar to be sampled. During prolonged periods of radio quiet the tape was stopped, but often the tape was left recording during these quiet periods between roar to capture the onset of some events. In addition, after the normal observing hours the DCR was set to a likely frequency and left to run in automatic mode for 90 min each night.

Auroral roar was recorded on 11 of the 19 nights of observation. Of 36 audio cassettes recorded in the field, 17 contain auroral roar events for an estimated 600 min of total event time. Thus the campaign was successful in increasing by several orders of magnitude the quantity of high-time and high-frequency resolution auroral roar data. The data revealed a much greater variety of fine structure features than had been recorded previously.