Auroral kilometric radiation: Wave modes, harmonics, and source region electron density structures

Abstract

A number of dramatic changes are observed in the characteristics of auroral kilometric radiation (AKR) as the source region plasma to gyro‐frequency ratio fN/fH varies from 0.1 to 1.3. Most notable of these is a change from right hand polarized extraordinary (X) mode AKR dominance to left hand ordinary (O) mode AKR dominance as fN/fH varies from smaller to larger values. In addition to X and O mode AKR, Z (the slow branch of the X mode) and whistler (W) mode are also observed. The Z mode is observed over all fN/fH values, is often confined to the frequency region between fN and fH, and is typically slightly less intense than the O mode. The W mode is confined to frequencies less than fH/2, suggesting that it is the result of field‐aligned ducted signals reaching the satellite from a source at lower altitudes. Harmonic AKR bands, which appear to be of natural rather than instrumental origin and appear to be associated with the O as well as the X mode, are commonly observed. The second harmonic X mode appears to be due to propagating signals, whereas the higher harmonics appear to be confined to low‐density source regions. The second harmonic associated with the O mode is observed mainly at frequencies above the ambient 2fH value when fN/fH is large and below 2fH when fN/fH is small. The observations were obtained from some 200 ionograms from 12 passes of the ISIS 1 satellite through AKR source regions. Inferences concerning wave mode identification are based on comparisons of the observed AKR frequencies with sounder‐induced plasma resonances and wave cutoffs of ionospheric reflection traces. Electron density contours from the satellite altitude down to the altitude of the F layer ionization peak were obtained from the sounder data on each pass. Source encounters were made on nearly all of these passes, and both the minimum AKR source altitudes and the corresponding fN/fH values (which correspond to maximum values) were determined. The deduced (fN/fH) max is always less than 0.4 and is typically less than 0.2 during the generation of X mode AKR. For O mode AKR, on the other hand, (fN/fH) max approaches 0.9. The corresponding AKR source region minimum altitudes extend down to 2400 km. The (fN/fH) max values are in excellent agreement with previously published results of the maximum instability temporal growth rates, obtained from the AKR Doppler‐shifted cyclotron mechanism, as a function of fN/fH. The latitudinal extent of the low‐density, i.e., Ne < 100 cm−3, AKR source regions ranged from a few degrees to more than 20° along the satellite orbit (88° inclination). Within the wide density depletions, i.e., those that extended over many degrees, there were no large enhancements of the ambient fN/fH value. These results, which were limited by the ∼100 km spatial resolution corresponding to the spacing between topside sounder ionograms, were substantiated on a much finer spatial scale (∼10 km for Ne or 1/10 km for ΔNe) using in situ data from the on‐board Langmuir probe which were available for 10 of the 12 passes. The observed lack of large density enhancements within AKR source region density cavities provides additional confidence that the observed intense AKR is cyclotron X mode radiation rather than plasma frequency O mode radiation. The observed Ne enhancements, however, were larger than those required to support a feedback mechanism in the source region.