High-Latitude Distributions of Plasma Waves and Spatial Irregularities from DE 2 Alternating Current Electric Field Observations

An 18-month data base from the Dynamics Explorer 2 AC electric field spectrometers is used to obtain average high-latitude magnetic local time (MLT) versus invariant latitude (INL) distributions of signal intensities in 12 frequency bands between 4 Hz and 512 kHz. Three distinctly different distributions are obtained, corresponding to (1) Doppler-shifted signals from spatial structures in the electric field (i.e., irregularities) and Alfven waves between 4 and 512 Hz, (2) ELF waves between 256 Hz and 4.1 kHz, and (3) VLF waves between 4.1 and 64 kHz with extensions into the 128-512 kHz band. The ELF and VLF distributions closely resemble previously published results based on more limited sampling. Comparable distributions for the seven channels between 4 and 512 Hz, showing a prominent zone of maximum intensities at 72.5-degrees-80-degrees INL between 0500 and 1300 MLT, have not previously been reported. The power law frequency dependence of average power spectral densities (PSDs) between 4 and 512 Hz is also mapped in MLT-INL coordinates. At all locations, two power law indices (slopes) are required to closely fit the PSDs with an inverted knee joining the two slopes in the 32-64 Hz band. This knee band corresponds to the range of O+ cyclotron frequencies encountered, and it lends credence to Gurnett et al.'s (1984) contention that Alfven waves are an essential ingredient in explaining the low-frequency in situ satellite signals which were previously attributed to polarization fields accompanying spatial irregularities in plasma densities. However, other aspects of the 4-512 Hz observations, including seasonal variations, favor the earlier spatial irregularity interpretation. As discussed, the difficulties encountered in seeking interpretations exclusively in terms of either spatial irregularities or Alfven waves can be resolved with a synthesis approach requiring both types of signals. It is proposed that the averaged intensities and corresponding spectral characteristics in the 4-512 Hz band represent the consequence of intermittently superimposing shear Alfven waves on a spatially irregular medium. There are then three principal contributions: (1) an omnipresent 4-512 Hz signal from Doppler-shifted responses to 2000-15 m spatial irregularities having an average power law spectral index near -1.9, (2) intermittent signals from locally generated shear Alfven waves having maximum power at frequencies of <4 Hz and average power law spectral indices of less-than-or-equal-to (-2.8) extending only to f(c)(O+), and (3) spatial irregularity modulations of shear Alfven waves originating both locally and in the distant magnetosphere.