Vertical-10 rocket measurements of mid-latitude, nighttime F region irregularities

The results of a rocket experiment carried out in the midlatitude nighttime ionosphere revealed an isolated plasma-density depletion between 700 and 1100 km. A substantial enhancement of intensity of irregularities coinciding with the depletion was observed over a broad band of irregularity sizes from tens of meters to several hundred kilometers. The power spectra of irregularities show a break in slope occurring at irregularity size 10-15 km. At larger sizes the spectrum is very steep, with the spectral index close to 4. Smaller irregularities have a shallow slope, and the spectral index is close to 1. No correlation between the spectral index and irregularity amplitude is found. It is suggested that the plasma depletion and large-scale irregularities are slow-mode hydromagnetic waves, stimulated by gravity waves in the lower atmosphere and propagating along magnetic-field lines. Medium-scale irregularities observed inside the depletion could be produced by the E x B gradient-drift-instability mechanism. A steep spectrum, however, indicates that the nonlinear evolution of the midlatitude topside irregularities can be quite different from those in other regions of the ionosphere. A potential production mechanism of small-scale irregularities could involve a high velocity stream of cold plasma from the plasmasphere to the ionosphere, as observed in the experiment. 27 references