Investigation of heater induced irregularities in the high latitude ionosphere.

The results of artificial modification experiments in which the electron temperature and density of the high latitude ionosphere are intentionally altered by means of high power, HF radio waves are presented. The modified F-region was probed by means of low power diagnostic radio signals. The measurement of the amplitude and spectral content of the diagnostic signals yield information on the spatial development of both large scale, isotropic structures and small scale, strongly field aligned irregularities (FAI's), induced in the ionospheric plasma by the high power (heating) waves. In addition to this, the temporal development has been investigated for the first time. Observation of the ionospherically reflected heating wave provides new insight into the heater wave self-depletion processes and the growth and saturation mechanisms of the FAI's. The experimental observations indicate that anomalous absorption of electromagnetic waves, due to scattering from small scale FAI's plays an important role in the ionospheric modification processes at high latitudes. The scale lengths of the irregularities, along and across the magnetic field, have been deduced from several different experiments. These include the measurement of anomalous absorption of an O-mode diagnostic signal, the first investigation of the spectral indices derived from the power law spectra of X mode diagnostic signals, measurement of the ionospheric drift velocity and the first ever co-ordinated EISCAT-heater studies. In the latter case, the EISCAT VHF/UHF radar facility provided an additional diagnostic tool to measure the electron temperature, (Te), enhancement during periods of heating. Derivation of the relaxation time (rate constant) of the observed Te enhancement provides an estimate of the rate of diffusion of plasma across the magnetic field. Many of the features reported were observed for the first time and provided considerable insight into the plasma physics of the heating process.