New techniques in ground‐based ionospheric sounding and studies

Rapid progress in the integrated circuit market has led to new advanced techniques in the remote probing of the ionosphere with HF radio waves. The classical ionosonde which measured virtual height as a function of frequency expanded into a geophysical research tool by measuring all the observables contained in the electromagnetic signals reflected from the ionosphere: amplitude, phase, Doppler, incidence angle, and polarization. A receiving antenna array and high speed digital processing provide the desired spatial and temporal resolution. The current emphasis is on both the on‐line and off‐line postprocessing of the multiparameter ionogram data to extract the geophysically important ionospheric characteristics: the vertical electron density profiles, horizontal gradients (tilts and waves), plasma drift, the mid‐latitude F region trough, and auroral and equatorial spread F . Digital ionosonds deployed in the polar cap and the auroral zone have helped to obtain a better understanding of some of the high‐latitude features, and measurements of the equatorial spread F have shown the development and motion of the F region bubbles. HF coherent radar techniques for studying ionospheric irregularity structures measure the velocity of irregularities with scale sizes of one half the radio wavelength. They have mainly been used in the northern auroral zone.