Jicamarca and Altair radar data [Woodman and scale corresponds approximately to the echo LaHoz, 1976; Tsunoda et al., 1979; Tsunoda and intensity in decibels relative to the incoherent White, 1981], recent rocket results [Szuszczewicz scatter level at an electron density of 106 cm-3 et al., 1980; Rino et al., 1981; Kelley et al., at the same range.

Abstract. The shape of the electron energy distribution has long been a central question in the field of highfrequency radio-induced optical emission experiments. This report presents estimates of the electron energy distribution function, f e (E), from 0 to 60 eV, based on optical multiwavelength (6300, 5577, 8446, 4278Å) data and 930-MHz incoherent scatter radar measurements of ion temperature, electron temperature and electron concentration. According to our estimate, the electron energy distribution has a depression at around 2 eV, probably caused by electron excitation of vibrational states in N 2 , and a high energy tail that is clearly supra-thermal. The temporal evolution of the emissions indicates that the electron temperature still plays an important role in providing electrons with energies close to 2 eV. At the higher energies the electron energy distribution has a nonthermal tail.

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Precisely simultaneous radar and satellite measurements at the altitude of reflection of a strong HF heating wave above the Arecibo Observatory were made on June 7, 1977. Parametric instabilities produce strong enhancements in the plasma line and ion line incoherent scatter radar echoes. These echoes also exhibit periodic deep fading that is attributed to a self‐focusing instability. This explanation was confirmed by the in situ observation of electron density fluctuations with peak‐to‐peak amplitudes reaching at least 3% and a spatial dependence that corresponded closely to the radar fading pattern, at least for irregularity wavelengths ranging from a few hundred meters to a few kilometers. The correspondence implies that the radar fading is associated with the convection of the density irregularities through the radar beam. The radar and satellite observations also provided values for the important parameters of the ambient ionosphere, making possible a quantitative comparison of the data with existing theories of the self‐focusing instability. In particular, the agreement with the theory of Cragin et al. (1977) is fairly good.

An ionospheric HF‐modification experiment was carried out near Tromsø, Norway, using the Max‐Planck‐Institut für Aeronomie (MPI) heating facility and the EISCAT 933‐MHz incoherent scatter radar (ISR). The MPI heater was normally operated at 4.04 MHz and modulated 20‐s on, 40‐s off. The ISR observed waves propagating parallel to B0, and chirped as well as normal plasma line observations were performed. Heater‐induced plasma lines were observed only in the first 10‐s integration interval, indicating a strong overshoot. These lines are unusual in that multiple simultaneous lines were observed, normally originating within one kilometer of the critical region but sometimes from lower heights, and that the frequency of the most constant line is offset some 250 kHz from the heating frequency, with the other lines occurring at greater frequency offsets. The natural, photoelectron‐enhanced plasma line was not observed; however, the background plasma was diagnosed via ion line observations and comparisons to chirped observations performed at EISCAT in May 1986 indicate that increased Landau damping may be responsible for both the strong induced‐line overshoot and the lack of a distinct natural line. Finally, ion power profile observations show the existence of a topside enhanced ion line at the critical density corresponding to the heater frequency. We believe this is due to strong O to Z‐mode coupling parallel to B0 and a low ƒ0F2.