Comparative in situ studies of the unstable day-time equatorial E-region

Abstract

Plasma waves measured by probes on sounding rockets are used to characterize the unstable equatorial E-region and to reflect the changing irregularity morphology with respect to altitude within the layer. We present measurements from three sounding rockets launched at the geomagnetic equator from Punta Lobos. Peru, including both a strong and a mild electrojet experiment conducted at midday and a weak electrojet experiment conducted in the late afternoon. The observed irregularities are analyzed in relation to simultaneous measurements of the electron number density and to either the measured or inferred profiles of the electron current density. The linear growth rate for the combined two-stream and gradient drift instabilities is computed using these profiles and the changing unstable wavenumber regimes are then compared to the power spectra of the wave observations as a function of altitude. We have allowed for long wavelength waves in the growth rate and have included the effect of recombination. In each case, the waves are observed only in the altitude regions, which, on the basis of the growth rate, are predicted to be unstable for horizontally propagating waves. Further, although the conversion of observed frequency to wavenumber is not definitive, the theoretical range of wavenumbers that will be unstable agrees at least qualitatively with the corresponding frequencies which have associated fluctuations displayed in spectrograms of the in situ time series measurements. In the case of the strong electrojet experiment, both the growth rate calculations and the wave observations show a region of high frequency (short wavelength) oscillations in the upper portion of the layer, where the medium was unstable to the primary two-stream instability. In the mild electrojet experiment, current measurements show that the two-stream threshold was not met, which is supported by the absence of observed high frequency oscillations for this flight. The medium may still have been unstable to this process as a secondary mechanism, as suggested in the 3 m backscatter radar data, implying that the large scale wave electric fields were on the order of or greater than, the polarization electric field. Where the payloads encountered a positive (upwards) gradient in electron density, all three rockets show a strong low frequency component which we attribute, in general, to the gradient drift instability.