High Resolution Doppler Spectrum Measurements of 3-meter Auroral Irregularities at Large Magnetic Aspect Angles

Abstract

Doppler spectral properties of 50 MHz auroral echoes observed at several magnetic aspect angles are presented. The measurements were made with high resolution CW Doppler systems at 10 different aspect angles ranging between 1 and 14 degrees, during a series of experiments carried out the summers of 1981, 1982 and 1983 in western Canada. The extensive data set was representative of several backscatter periods and different geomagnetic conditions. We found that three basic spectral types associated with different irregularity groups and classified as type 1, 2 and 3, existed throughout the aspect angle range. Type 1 and 2 spectra are reminiscent of those observed in both the equatorial and auroral backscatter at small aspect angles, whereas type 3 is a newly established spectral signature possibly related to electrostatic ion cyclotron waves. The evidence suggests that 3 m primary and secondary plasma waves can exist in aurora with k||/k⊥ wavenumber ratios perhaps larger than 0.25. Statistical analysis of the data showed the magnetic aspect angle effect upon the Doppler spectrum properties to be weak for all types of irregularities at 50 MHz. It is now clear that equatorial E region instability theories, which had been applied directly to aurora, need important modifications, and possibly the inclusion of field aligned current effects, to account for the magnetic aspect angle observations.