In order to explain the ion temperature anisotropies and differential speeds, as observed in solar wind high-speed streams, a fluid type model is presented that takes into account ion heating and acceleration by resonant wave-particle interactions. Ion-cyclotron and fast magnetosonic waves propagating away from the sun parallel to the interplanetary magnetic field are considered. The radial evolution of the spectral wave energy density and of the bi-Maxwellian model ion distributions is calculated self-consistently for a spherically symmetric solar wind geometry. Numerical results are given for the dependence on heliocentfic radial distance for the ion parallel and perpendicular temperatures and ion speeds relative to their center of mass frame. The respective ion flow speeds in the inertial frame are also calculated, based on momentum equations that include the self-consistent temperature gradients. It is shown that transfer of wave momentum to the ions can lead to a preferential acceleration of the alpha particles with respect to the protons. Owing to the combined action of the ion cyclotron and magnetosonic waves, the alphas are accelerated to a differential speed of about the local Alfvb. n speed in close accord with in situ observations. By damping of wave energy the heavier ions are also preferentially heated with the result that alpha particle thermal speeds become equal or slightly larger than proton thermal speeds. Typical signatures in ion temperature anisotropies (like T•,ñ > T•,•) as predicted by the model agree fairly well with the observations in fast streams. The results are discussed with respect to the effects of various boundary conditions and the inhomogeneity of the expanding solar wind plasma. 1. INTRODUCTION Observations in the solar wind have revealed pronounced differences in the velocity distributions and plasma parameters of hydrogen and helium ions. The most striking feature is that frequently the helium ion bulk speed V, considerably exceeds the proton speed Vp [Formisano et al., 1970; Asbridge et al., 1974, 1976; Hirshberg et al., 1974; Ogilvie, 1975; Bosqued et al., 1976; Griinwaldt and Rosenbauer, 1978; Neugebauer, 1981; Marsch et al., 1982a, 1981]. Because Coulomb friction cannot even account for speeding up the heavier minor ions to the bulk speed of the ambient protons [Geiss et al., 1970] preferential acceleration forces have to be invoked to explain a differential speed AV.p = V. -v. >0. The observed, at times very close, positive correlation of Av,p with the local Affv6n velocity VA [Asbridge et al., 1976; Marsch et al., 1981, 1982a] strongly suggests that interaction of outward propagating electromagnetic waves with the particles and in particular with the helium ions might lead to Av,p > 0. Nonresonant acceleration by the Alfv6n wave pressure gradients [Hollweg, 1974; Chang and Hollweg, 1976] affects the solar wind constituents equally if V, = Vp. Thus it cannot explain Av,p > 0. Therefore, forces due to resonant wave particle interactions are required to accelerate minor io...

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Investigations on avalanches of large carrier numbers (> t0 s) show the following characteristic features of the streamer mechanism, known as ,,Kanal-Aufbau".I. An avalanche can only start a streamer, if the carrier number of the avalanche has reached a certain critical value nkrit. Having surpassed n~i t the probability of this streamer formation increases monotonously with growing carrier number. The applied field E 0 is kept constant.2. The probability of streamer formation for a constant carrier number of the avalanche increases with the applied field. The critical carrier number can therefore be reduced by increasing the applied field.3. As a consequence of this behaviour of individual avalanches the mean probability of an electron running through the gap to start a streamer increases if the mean value of amplification (e c~d) becomes higher or (what is in this case identical) the applied field is increased. There is a critical mean value of amplification, below which it is impossible to observe any streamer formation.4. If the carrier number of an avalanche reaches n ~ 10 s a process comes into force which is characterized by an increase of ionisation. w 1. Einleitung und Prinzip der MessungIn I i wurde berichtet, dab eine Lawine sehr hoher Verst~rkung (> l0 s) eine rasche Stromsteigerung (Durchschlag) einleiten kann, ohne dab yon der Kathode startende Sekund~irlawinen zu beobachten sind. Die Stromsteigerung setzt mit starker Streuung naeh etwa 20 bis 300 nsec ein. Die Deutung dieser Beobachtungen erfolgte im Bild des Kanalaufbaus, in dem angenommen wurde, dab in die naeh Ablauf der Elektronen an der Anode verbMbende positive Ionenwolke weitere Elektronen einlaufen, die durch gasionisierende Strahlung in der Umgebung der Ionenwolke gebildet werden. Es mug hierbei vorausgesetzt werden, dal3 die Anregungswahrscheinlichkeit ftir die gasionisierende Strahlung durch die Feldaufsteilung in der N~ihe der Ionenwolke vergr6Bert wird. Dutch eine Folge yon solchen durch die gasionisierende Strahlung erzeugten Lawinen werden die Voraussetzungen ftir den Start eines zur Kathode laufenden Kanals (Plasmaschlauch) geschaffen, i PFAUE, J., u. H. RA~TI~R: Z. Physik 153, 523 (1959).

Abstract. We report on experimental electronic stopping cross sections of elements and compounds for Li ions. They are measured with the Inverted Doppler Shift Attenuation (IDSA) method with an accuracy of the order of 1 }o-In cases where comparable data from other authors exist, agreement within the limits of error is obtained. Systematic and pronounced deviations up to about 30% from Lindhard's theory are found. It turns out that Bragg's rule is generally not valid for stopping cross sections in the velocity range 1.5.108 cm/sec to 4.8 -l0 s cm/sec.

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