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Abstract. A statistical study of the occurrence of ground and ionospheric backscatter within the ®elds-of-view of the CUTLASS HF radars, at an operating frequency of 10 MHz, during the ®rst 20 months of operation has been undertaken. The diurnal variation of the occurrence of backscatter and the range at which such backscatter is observed is found to be highly dependent on seasonal changes of the ionospheric electron density in both the E and F region, determined from ionosonde observations. In general, ionospheric backscatter is observed at far ranges during the local day in winter months and at near ranges during the local night in summer months. The Iceland radar observes more nearrange E region backscatter than the Finland radar as a consequence of its more zonal look-direction. The dependence of the occurrence of backscatter on geomagnetic activity and radar operating frequency are also investigated. The occurrence of ground and ionospheric backscatter is discussed in terms of HF propagation modes and ionospheric electron densities as well as geophysical processes. A brief assessment of the possible impact of solar cycle variations on the observations is made and frequency management is discussed. Such a study, with its focus on the`instrumental' aspect of backscatter occurrence, is essential for a full interpretation of HF coherent radar observations.

Abstract. Interferometric techniques allow the Super-DARN coherent HF radars to determine the elevation angles of returned backscatter, giving information on the altitude of the scatter volume, in the case of ionospheric backscatter, or the reflection altitude, in the case of ground backscatter. Assumptions have to be made in the determination of elevation angles, including the direction of arrival, or azimuth, of the returned signals, usually taken to be the forward look-direction (north) of the radars, specified by the phasing of the antenna arrays. It is shown that this assumption is not always valid in the case of ground backscatter, and that significant returns can be detected from the backward look-direction of the radars. The response of the interferometer to backscatter from behind the radar is modelled and compared with observations. It is found that ground backscatter from a field-ofview that is the mirror image of the forward-looking field-of-view is a common feature of the observations, and this interpretation successfully explains several anomalies in the received backscatter.

Simultaneous observations are presented of stimulated electromagnetic emission (SEE) spectra and the anomalous absorption of a diagnostic HF wave, caused by powerful radio waves generated by the Troms0 heating facility. The pump frequency was varied in small steps around the third, fourth, and fifth harmonic of the electron gyrofrequency. Systematic variations with pump frequency were observed in the anomalous absorption data and the entirety of SEE properties. The anomalous absorption exhibits a pronounced minimum at a certain pump frequency near (or at) the gyroharmonic in question. Of the SEE properties the strength and discreteness of the downshifted maximum and the width of the downshifted continuum minimize together with the anomalous absorption. Other SEE properties (namely the broad upshifted maximum, the downshifted peak, and the broad symmetrical structure) exist only in well-defined narrow pump frequency ranges near the absorption minimum. Significant quantitative and qualitative differences are found between the SEE properties around the third gyroharmonic on the one hand, and the fourth and fifth gyroharmonics on the other hand. frequency. There is no other constraint on the pump frequency tee. It has been found, however, that the character of the SEE spectra changes strongly when re0 approaches an integer multiple of the electron gyrofrequency, re e • nil e [Leyser et al., 1990]. Whereas some of the SEE features become particularly strong or weak when tOo is changed in small steps (--•2,r x 10 kHz) around nile, others exist only when the condition tee -• nil e is met. This experimental finding most naturally points to the importance of electron Bernstein waves for the SEE phenomenon. High-frequency electrostatic waves propagating perpendicular to the external magnetic field are thought to be generated by linear mode conversion of the electromagnetic pump wave at small-scale field-aligned density irregularities, so-called striations [MjOlhus, 1990, and references •herein]. The same striations are also thought to be responsible for the anomalous absorption imposed upon an O-mode wave prop-Copyfight 1994 by the American Geophysical Union. Paper number 94JA00023. 0148-0227/94/94JA-00023505.00 agating through an ionospheric volume affected by the pump wave [Graham and Fejer, 1976]. Naturally, the pump itself is also subject to this anomalous absorption, giving rise to a self-absorption effect. The phenomenon of anomalous HF wave absorption (or wideband attenuation, as it was called previously) has been discovered in ionospheric modification experiments performed near Boulder, Colorado [e.g., Utlaut and Vialette, 1974]. This phenomenon has also been the research subject in a series of experiments conducted at TromsO in which several new features were found [Stubbe et al., 1982; Kapka et al., 1982; Jones et al., 1983, 1984]. In these earlier experiments, systematic pump frequency changes in small steps around nil e were not made, and simultaneous SEE experiments not performed.

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