High‐latitude F region irregularities observed simultaneously with ISIS 1 and the Chatanika radar

Abstract

Simultaneous measurements of the high‐latitude ionosphere were made by the Chatanika radar and the ISIS 1 topside sounder and cylindrical electrostatic probe (CEP) when the satellite was at a height of about 700 km. The variation of enhancements and valleys of electron density with invariant latitude at the satellite, on the scale of a few hundred kilometers, agrees reasonably well with the radar results, even though the satellite displacement changed from almost coincident to about 500 km magnetic west of the radar scan path. These density enhancements comprise a collection of large‐scale (tens of kilometers) irregularities or ‘blobs.’ Evidence for the formation of these blobs was obtained from the sounder. Natural, intense radio noise (hiss) was observed on the sounder ionograms recorded on the poleward side of the auroral zone, indicating the presence of precipitating kiloelectron volt electrons. A few minutes after these ionograms were recorded the radar scanned the same invariant latitude region and a large‐scale ionization enhancement, or ‘blob,’ not present 13 minutes earlier, had developed in the bottomside F layer. ISIS 1 telemetry dropouts resulting from medium‐scale irregularities, and enhanced irregularities down to about 200 m, observed with the CEP, are observed in association with blobs. Scatter signatures observed on the ISIS ionograms may be associated with large‐scale irregularities that can be identified in the radar data. The scattering detected by the sounder is caused by 30‐ to 60‐m wavelength irregularities (small‐scale) that presumably are created by instabilities operating on the edges of the large‐scale irregularities. The small‐scale irregularities, however, are observed on both poleward and equatorward directed gradients. If the gradient‐drift instability is responsible for these irregularities, then they either convect to the point of observation, are due to zonal gradients, or are field‐aligned extensions of irregularities generated at lower heights.