Intense ultralow‐frequency waves are commonly observed on GEOS 1 and 2 spacecraft, around FHe+, the helium gyrofrequency. These waves were identified as ion cyclotron waves (ICW's) by Young et al. (1981), who showed their close connection with a sufficient amount of He+ in the magnetospheric plasma. Motivated by these observations, we present a ray tracing study of ULF waves below FH+, the proton gyrofrequency. In the presence of He+ the dispersion relation of ULF waves is split into three branches, and we have studied the ray paths for these three branches. Of particular interest is the ion cyclotron branch, which is left handed above the new crossover frequency Fcr introduced by the presence of He+ ions. This mode is amplified, in the equatorial region, by anisotropic (T⊥ > T∥) energetic protons. It is well guided along field lines, suffers a polarization reversal when F = Fcr locally, and continues to be guided up to the point where F = Fbi, the bi‐ion hybrid frequency. Then it is reflected and returns to the equator, where it is amplified again. Thus such waves undergo several bounces through the amplifying region without significant drift, either azimuthal or radial. This mirroring effect, which can only occur when minor ions, such as He+, are present, is believed to be crucial for ICW amplification. We also show that for each such equatorial crossing the parallel wave number is conserved while the perpendicular one progressively increases. As a consequence, ICW's become quasi‐electrostatic, which enables them to acquire a small but finite parallel electric field; it is suggested that this electric field can in turn accelerate thermal electrons parallel to B.

This work is a continuation of paper 1 (Young et al., 1981) and is devoted to the generation process of ion cyclotron waves (ICWs) and the acceleration of He+ ions up to suprathermal energies. Simultaneous measurements are used from the ion composition experiment (0 < E < 16 keV), the energetic particle experiment (24 < E < 3 300 keV), and the ULF wave experiment (0.2–10 Hz) on board the GEOS 1 and GEOS 2 spacecraft. General characteristics of the local time distribution of ICWs will be presented and compared with those of the thermal anisotropy of energetic protons and the He+ abundance. Further calculations of the convective growth rate are conducted by applying two different methods, both of which are based upon the measured proton fluxes. The generation conditions of the ICWs in the presence of He+ ions will be investigated and three possible explanations will be discussed: (1) enhanced convection growth rates, (2) lowering of the threshold for absolute instabilities, and (3) change of the ICWs ray path (laser‐like effect). Finally, it is shown that the flux of suprathermal He+ ions is modulated at the ICW frequency. Owing to nonlinear effects, part of the energy of the energetic protons is transfered via the ICWs to the He+ ions that are essentially accelerated in the direction perpendicular to the static magnetic field. Then in the otherwise collisionless plasma the friction between energetic anisotropic protons and thermal He+ ions is achieved through the ICWs.

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ELF emissions observed on the low-altitude AUREOL 3 satellite are seen in association with H + ions at large pitch angle. The flux in the upward direction for -120 ø pitch angle is found to be equal to or larger than the flux at -60 ø pitch angle, which provides evidence for transverse acceleration at or below the spacecraft. These emissions have a sharp lower-frequency cutoff of the transverse components of the electric field and a narrow peak at, or, more precisely, just below the proton gyrofrequency f.+. This narrow peak is more easily seen on the parallel component and appears as a narrow line on the spectrogram of this component. A statistical study of the occurrence of this line at f -f.+ is presented. It is shown that this line is observed at relatively high invariant latitude within the light ion trough •vhere a strong depletion of thermal H + ions occurs. Detailed analysis of ELF waves observed just below fa+ demonstrates that they propagate in the left-hand mode. These observations are interpreted as a signature of mode conversion from a fast magnetosonic mode into a slow proton cyclotron mode. It is suggested that this slow proton cyclotron •vave can accelerate protons up to a few hundreds electron volts in the transverse direction. This mode conversion process can operate over a much broader of large altitude range than covered by AUREOL 3; it is a likely candidate for explaining the formation of H + conics. Theoretical calculations that support the above conclusions are given in a companion paper by Le Qu6au et al. (this issue). 1. INTRODUCHON Ion conics result from the combination of a transverse acceleration, followed by an upward motion driven by the magnetic mirror force. Ion conics therefore play a key role in the population of the magnetosphere with ions extracted from the ionosphere. Many papers have given experimental evidence [Ungstrup et al., 1979; Klurnpar, 1979; Yau et al., 1984, 1985a, b; Klurnpaz; 1986 and references therein] and theoretical explanations [Lysak et al., 1980; Okuda andAshour-Abdalla 1981, 1983; Ashour-Abdalla and Okuda, 1984; Chang et al. 1986; Andr• et al. 1990 and references therein] for the formation of the conical pitch angle distribution of ions. The narrowness of observed angular distributions suggests that a heating mechanism should be able to accelerate ions in a relatively narrow angular range around 90*. Several mechanisms have been proposed in the literature to heat the H +, He +, and O + ions in the perpendicular direction. Various scenarios have been proposed. Ungstrup et al. [1979], Olatda and Ashour-Abdalla [1981], and many others have suggested that electrostatic ion cyclotron waves (EICWs), driven unstable by a field-aligned current, heat the ions in the transverse direction. The main difficulty with this mechanism is that electrons diffuse rapidly in velocity space, in the V// direction, thus leading to a plateau.

Norris et al. E1983] have recently shown Norris et al. r1983], who found evidence of fieldaligned electrons having about 20 eV, during ICW's events. In order to clarify the role played by various parameters upon the efficiency of this acceleration process we have again displayed F•. and F; in Figure 3, the former quantity being plotted f'•r E# ='10 •V/m and E//= I •V/m, respectively. In the latter case m = 1; tl•'us the adiabaticity conditions is marginally fulfilled. Accordingly we conclude that for El/-= I •V/m the acceleration process discussed here !•ecomes inefficient. The above threshold (E# = I •V/m) has been obtained for a given set of valO'es of X , •/, and N . ß ß O O Above th•s threshold the maximum value of the parallel