LaBelle et al. [1995] report two auroral roar events measured in September and October, 1994, with a downconverting receiver operated in a semi-automatic mode in Circle Hot Springs, AK (see part I). These recordings revealed for the first time that auroral roar is composed of multiple narrowband features previously unresolved by stepped frequency receivers. To increase the number of examples of auroral roar fine structure we operated the same downconverting receiver at the Northern Studies Centre in Churchill, Manitoba, during a three week campaign in April, 1996. One of us (SGS) controlled the center frequency of the downconverter according to information provided by a collocated swept frequency receiver. The goal of this campaign, based on the two events from 1994, was to collect numerous examples of auroral roar fine structure so we could perform statistical studies of duration and drift of individual features.
From an initial survey it is apparent that a major classification can be made based on the appearance of the fine structure. That is, the auroral roar, when viewed at high time and frequency resolution, falls into one of two broad categories: structured or unstructured. An unstructured event appears as a uniform enhancement of noise although sometimes it appears patchy in time and/or frequency. No amount of frequency or temporal magnification can resolve spectral features. Structured events contain at least some spectral and temporal features which distinguish them from noise or unstructured events.
Figure 1a shows a spectrogram of an auroral roar recorded during the campaign with the PSFR. The horizontal white lines running through the figure indicate the frequency setttings of the DCR. The bandwidth of the DCR is roughly the vertical width of the PSFR pixel. Figure 1b shows DCR output starting at 0218:56 UT as indicated by the second set of vertical lines in Figure 1a. Figure 1b illustrates an example of unstructured roar fine structure. The record begins at a time of relative radio quiet at 2.97 MHz and then switches into unstructured roar fine structure at 2.86 MHz 10 seconds into the record. Figure 1c shows DCR output starting at 0208:16 UT as indicated by the first set of vertical lines in Figure 1a. Figure 1c shows an example of structured roar. This example illustrates that auroral roar is often comprised of many different types of features: rising, falling, stationary, wavy, short, and long.
Structured auroral roar comprises many spectral and temporal features as shown in the Atlas pages 1, 2, 3, 4, 5. These examples were chosen from expanded survey plots to show a particular spectral or temporal feature. Each frame contains 3 seconds and 11 kHz of DCR data displayed at identical contrast levels to aid in distinguishing different types of features. Note that the center frequencies are not identical in all plots.
The structured features can be classified according to their duration, frequency drift, and grouping with like features. Within these classifications a nearly continuous spectrum of features were recorded. The duration of a particular spectral feature is an obvious distinction. The time durations ranges over three orders of magnitude from the minimum measurable limit of tens of milliseconds to tens of seconds. A random survey of a subset of structured events reveals that the majority of auroral roar fine structure features are typically less than a few seconds in duration and predominantly less than 1 second.
Another characteristic feature is the frequency drift of fine structures. Features are seen to drift in a variable manner sometimes nearly sinusoidal and with constant slopes that can be upwards, downwards, or flat. The features with constant drifts vary greatly from ~ -800 kHz/s to ~ +100 kHz/s with the majority of the steepst sloping features being negative.
Solitary features, those occurring by themselves during a time much longer than the feature duration, are observed in less than 1\% of the structured examples of the digitized data. Features more often occur in groups with similar characteristics. These groups contain up to several hundred features sometimes spaced as close as a few hundred Hertz.
The minimum bandwidth of auroral roar fine structure is an important clue in determining the generation mechanism. Any model proposed must provide explanations for the coherence of the emissions. Some mechanisms may be elimated on the basis that they provide no feasible explanation of the observed fine structure. To improve the upper bound of the minimum bandwidth of roar fine structure, it is necessary to analyze stationary features. The measured bandwidth of one particular fine structure feature is 6 Hz which is the resolution limit of the FFT. The actual bandwidth of the signal may be less than 6 Hz but not greater.
Difficulties in locating a feature which remains stationary for long enough to give better frequency resolution prevent further refinement of the minimum bandwidth. Tape flutter and wow in the recording equipment are ~5 Hz which prevents further refinement even if more stationary features are found thus allowing longer FFTs.
No theory addresses the generation of auroral roar fine structure. For similar fine structure in AKR, Calvert [1982] suggests a laser-feedback mechanism in which the walls of a density cavity feed a portion of the electromagnetic energy back into the region where electron energy is converted to waves via the cyclotron maser instability. For the X- or O-modes at ionospheric altitudes, this idea seems implausible for a cavity with vertical (field-aligned) walls, because it requires too large a cavity. If the walls are ~15° from vertical, a laser feedback mechanism with the X- or O-mode can function in principle, but it is unlikely that the cyclotron maser instability is the source of the energy because its growth rate is too low. It remains a theoretical challenge to explain these fine structure features of auroral roar emissions.
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., 103, 2219, 1998.
Weatherwax, A. T., Ground-based observations of auroral radio emissions, Ph.D. Thesis, Dartmouth College, Hanover, N.H., 1994.