The electrostatic dispersion relation for a magnetized electron plasma has two solutions, one of which reduces to the plasma oscillations for parallel propagation and gives rise to the wellknown plasma lines in the power spectrum of incoherent scattering. The other solution gives rise to an extra set of lines in the spectrum. Under suitable conditions these new "resonance lines" have an energy content comparable to the corresponding plasma lines and are easily observable. The enhancement of these lines due to secondary electrons is calculated.

We present ion composition results for several sporadic E layers observed during a Norwegian special program carried out by the European Incoherent Scatter Facility (EISCAT) UHF radar in August 1984. A range resolution of 600 m was used. Eight sporadic E layers at altitudes 95–114 km were studied using various fitting strategies for obtaining the ion composition. A fixed temperature was assumed in some of the strategies; in others the temperature was fitted together with the composition. Heavy ions, most probably Fe+, were found to be present in most layers with relative abundances of 30–80%. In one case, the composition analysis shows that essentially no heavy ions are present. This layer is supposed to be composed of light metal ions, such as Mg+. In one particular event a layer is seen to separate into two sheets with a height difference of 2–3 km, and the ion compositions in the two parts are found to be different. Possible explanations of this phenomenon are discussed.

Abstract. The plasma dispersion function and the reduced velocity distribution function are calculated numerically for any arbitrary velocity distribution function with cylindrical symmetry along the magnetic ®eld. The electron velocity distribution is separated into two distributions representing the distribution of the ambient electrons and the suprathermal electrons. The velocity distribution function of the ambient electrons is modelled by a near-Maxwellian distribution function in presence of a temperature gradient and a potential electric ®eld. The velocity distribution function of the suprathermal electrons is derived from a numerical model of the angular energy¯ux spectrum obtained by solving the transport equation of electrons. The numerical method used to calculate the plasma dispersion function and the reduced velocity distribution is described. The numerical code is used with simulated data to evaluate the Doppler frequency asymmetry between the up-and downshifted plasma lines of the incoherentscatter plasma lines at dierent wave vectors. It is shown that the observed Doppler asymmetry is more dependent on deviation from the Maxwellian through the thermal part for high-frequency radars, while for lowfrequency radars the Doppler asymmetry depends more on the presence of a suprathermal population. It is also seen that the full evaluation of the plasma dispersion function gives larger Doppler asymmetry than the heat ow approximation for Langmuir waves with phase velocity about three to six times the mean thermal velocity. For such waves the moment expansion of the dispersion function is not fully valid and the full calculation of the dispersion function is needed.

This paper reports the first EISCAT two‐radar plasma line experiment measuring photoelectron‐enhanced plasma lines with the UHF (933 MHz) and VHF (224 MHz) radars. The experiment shows that near noon at midsummer, simultaneous plasma lines can be continuously detected with the two radars in the altitude region 140–280 km. Such observations allow the electron temperature to be determined independently of observations of the ion part of the incoherent spectrum. Examples are given to illustrate this application.

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