High‐latitude ionospheric perturbations and gravity waves: 1. Observational results

The Saskatoon HF radar has been used to observe ionospheric perturbations and gravity waves at high latitudes. The observations on December 29, 1993, taken during an extended period of northward interplanetary magnetic field, are analyzed in detail. Two distinct intervals of backscatter are received, a near‐range interval of ground scatter echoes from 900 to 1500 km and a far‐range interval of ionospheric scatter from 2000 to 2600 km. The ground scatter intensity shows strong quasi‐periodic enhancements caused by Earth‐reflected gravity waves. The ionospheric echoes, which form a circular band of radius about 600 km, occur in the auroral zone below 80° magnetic latitude, just equatorward of the convection reversal. They cover a magnetic longitude interval from −60° to 80°, corresponding to a magnetic local time period from about 1100 to about 1800 LT. From the shape of the Earth‐reflected gravity wave intensity bands on the latitude‐time plots, those gravity waves can be traced back to the ionospheric perturbation region, which lies in the high‐latitude echo zone, namely the auroral zone. This is confirmed by a two‐dimensional cross‐correlation between the time series for the intensity of the gravity‐wave modulated ground scatter and the auroral zone ionospheric scatter. That correlation shows evidence for a time delay corresponding to the Earth‐reflected gravity wave propagation time between the regions. Also, the correlation peaks for a range separation about 1100 km, which implies that the normal one‐and‐a‐half‐hop propagation mode usually used to explain the presence of the far‐range auroral echoes cannot be operative and that the Pedersen ray “trapped” ionospheric mode is responsible for the observations.