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The soft particle spectrometer on the Isis 2 spacecraft, which is in a nearly circular orbit at 1400‐km altitude, frequently observes fluxes of upcoming ions with a conical pitch angle distribution and pitch angles usually in the range 90°–135°. The upcoming ions are usually observed near the boundaries of the auroral precipitation zone and primarily in the winter nighttime at the Isis 2 altitude. The observations are interpreted in terms of electrostatic ion cyclotron waves, which heat the ions to superthermal energies transverse to the earth's magnetic field. Subsequently, the ions are accelerated away from the earth by the earth's magnetic field.

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The present paper is a summary of studies carried out from Viking measurements on the propagation and the generauon of the auroral kilometric radiation (AKR hereafter). Advantage is taken of the spin modulation of the AKR observed as Viking was rotating in the cartwheel mode. This, together with the study of the cutoff of the various spectral components, confirms that low-amplitude Z and O modes are generated at the same time as a larger-amplitude X mode. Hence Z, O and X mode AKR is all generated by the same sources. The spectrum of the dominant polarization, the X mode, usually contains several spectral peaks. An AKR source crossing is characterized by a minimum in the frequency of the lowest-frequency peak (fpeak) and by a maximum of its amplitude. About 50 AKR source crossings are used to demonstrate that fpeak approaches fce, the electron gyrofrequency: (fpeak-fce)/fce •-0.025 in AKR sources. Similarly, the lo•v-frequency cutoff of the AKR is found, on average, to coincide with fce. The density inside AKR sources is determined from four sets of independent measurements, namely (1) the upper frequency cutoff of the hiss, (2) the relaxation sounder, (3) the Langmuir probe, and (4) particle measurements. It is shown that an AKR source coincides with a strong depletion in the density of the cold/cool electrons that becomes comparable to or less than the density of energetic electrons (E > 1 keV). The total density inside AKR sources is of the order of 1 cm -3, typically a factor 5 to 10 below that of the surrounding regions. AKR sources are found to coincide also with an acceleration region characterized by a potential drop of > 1 kV, both below and above the spacecraft. Evidence for this comes from the observation of electrons accelerated above the spacecraft and ions accelerated below it. In addition to a strong depletion in the density of the cool electrons, particle measurements on Viking give evidence of several possible free energy sources that could drive unstable the AKR, namely (1) a loss cone, (2) a hole for parallel velocities smaller than that of the observed downgoing electron beam, and (3) a trapped electron component for a pitch angle a •-90% The trapped electron component, bounded at low perpendicular energies (a few keVs) by an enhanced loss cone, is observed inside, and only inside, AKR sources. It is therefore concluded that the corresponding o•fio•v.L > 0, for small parallel velocities, is the free energy source that drives unstable the cyclotron maser. 11,657 11,658 Roux ET AL.: AURORAL KILOMETRIC RADIATION SOURCES