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A slow forward shock-type discontinuity observed by Helios 1 on day 147, 1980, at 0.31 AU is analyzed in detail, employing high-resolution (40.5 s) plasma and magnetic field measurements. It is shown within the bounds of uncertainties in the various parameters (1) that this event is not an MHD discontinuity, as the total pressure balance is violated and as the entropy increases across this structure even after inclusion of the alpha particles, (2) that the Hada-Kennel relations, i.e., Ti/Te < 1 and/3 < 1, are satisfied upstream of this event even after inclusion of the alpha particles, indicating that steepening of MHD slow waves dominates Landau damping, (3) that the Rankine-Hugoniot relations are fulfilled to an absolute difference of <10 -5 by employing any possible combination of an upstream with a downstream time-averaged data set, (4) that the MHD evolutionary conditions are satisfied for any solution to the jump conditions, and (5) that the shock speed, the amplitudes, and the relative amplitudes associated with this event are absolutely comparable to those derived earlier for fast mode shock waves within 1 A U. It is shown that this event is an almost quasi-perpendicular slow forward MHD shock wave. INTRODUCTIONFirst observational evidence on the possibility that slow mode MHD shock waves might actually be present in the interplanetary medium was given by Chao and Olbert [ 1970], who presumably identified two slow forward shocks, and by Burlaga and Chao [1971], who "most probably" found one reverse and one forward slow shock. More recently, Feldman et al. [1984] and Smith et al. [1984] provided partial evidence that the lobe-plasma sheet boundary in the central part of the distant geomagnetic tail is often a slow mode shock. Up to now, these examples of interplanetary slow shocks could be regarded as the more serious candidates in the sense (1) that all the plasma and magnetic field parameters necessary for their identification have been taken into account, and (2) that it has been shown that the observations do, within their bounds of uncertainties, fulfill both the Rankine-Hugoniot relations and the "evolutionary conditions" [e.g., Chao, 1970] for MHD shock waves.Two further yet rather essential tests in association with slow shocks, which actually have to be carried out before the optimization of the space probe data to the shock model, have, however, not been realized by these authors. First is to explicitly show that their events are not MHD discontinuities. A more comprehensive study of several slow mode shock-type discontinuities observed by the Helios space probes within 1 AU has unfortunately revealed that by including the alpha particles many of these events are MHD discontinuities rather than shock waves [e.g., Hsieh and . Second is to prove that by following the relations derived by Hada and Kennel [1985] the plasmadynamical conditions of the upstream medium do actually allow for a steepening rather than for a damping of MHD slow waves.

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Applying the superposed epoch analysis technique to 16 and to 31 well-defined, nonshock-associated stream-stream interaction regions observed by the Helios spacecraft in the distance ranges 0.3 to 0.4 AU and 0.9 to 1.0 AU, respectively, we obtain the average azimuthal variation in the solar wind density, velocity and temperature, in the magnetic field strength, and in the total proton plasma plus magnetic field pressure across CIRs at these two radial distances separately. For the radial evolution of these interaction regions we find by comparison: (1) due to compressional and rarefactional effects the amplitudes of all parameterg.•p question taken along the leading as well as along the trailing part of the CIR are steadily increasing'•ith the most pronounced increase in the pressure; (2) at the same time even the leading portion of the velocity profile steepens; (3) simultaneously, the positions in azimuth of the overall maximum values of the solar wind density and temperature, of the magnetic field strength and of the plasma plus magnetic field pressure are getting steadily lined up in longitude; (4) at the same time the leading portions of all profiles are steepening into discontinuous, shocklike structures. Thus, this analysis provides observational evidence for the following results obtained earlier from numerical simulation studies: Stream steepening does occur within 1 AU, and the probability of corotating shocks to form is, on average, much higher beyond than at or within 1 AU.

Applying a superposed epoch analysis to the Mariner 5 plasma and magnetic field observations of 13 corotating high speed solar wind streams, we obtain the average azimuthal distribution of all relevant parameters of the background interplanetary medium, as well as those of superimposed Alfvén waves. Using these measurements in model calculations allows us to determine the radial and azimuthal variation of the background and fluctuation parameters between 1 and 5 AU, and thus to calculate the cosmic ray diffusion coefficient κ from the plasma and field properties. The calculation of κ assumes that quasi‐linear wave‐particle interaction theory is applicable, and that the Alfvén waves responsible for the scattering are propagating in the azimuthally varying solar wind according to geometrical optics. The consequences of these calculations regarding the occurrence of solar wind stream associated Forbush decreases are discussed.

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For an observer in space the intensities and anisotropies of solar cosmic-ray events are governed by the duration and the functional shape of the injection processes near the Sun and by the propagation along the interplanetary magnetic field from the Sun to the observer. We study the influence of four different types of solar injections (Gaussian, exponential, step-function and coronal diffusion), and of a purely diffusive interplanetary propagation, where the diffusion coefficient has a power law dependence on the radial distance from the Sun, K = Mr ~, on both the time-intensity and the time-anisotropy profiles at 1 AU. The main results are as follows: A slow quasi-exponential decay of the intensity can be modelled in some cases; all finite injections produce high anisotropies during the main phase of an event; an 'effective' solar injection length can be determined from simultaneous inspection of the intensities and anisotropies; the intensities and anisotropies do to first order not depend on the analytic shape of the various injection profiles. The model is applied to the November 18, 1968 solar event as observed by Pioneer 9 in the 7.5-21.5MeV and 21.5-60MeV energy channels. We obtain local diffusion coefficients in the range M= (2.5-5)x 1021 cm 2 s -1 and injection periods of the order of 10-20 hr. Closer inspection reveals the change of interplanetary propagation conditions during the event.

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