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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 power, coherence and magnetic and cross helicity spectral analysis of the solar wind plasma and magnetic field obtained by Helios‐1 and ‐2, we investigate the nature of MHD fluctuations occuring upstream and downstream of a parallel, supercritical, turbulent, of a quasi‐parallel, supercritical, turbulent, and of a quasi‐parallel, subcritical, laminar fast‐forward shock wave. The main results of our investigation are as follows: (1) The spectral slopes and powers vary significantly with the length of the data interval analyzed. The difference upstream of the parallel shock was such that the ratio of the axisymmetric spatial diffusion coefficients of a 1‐MeV proton, calculated from the 86.4‐min and 10.8‐min spectra using Morfill and Scholer's (1977) formula, is 11.82. (2) Counterstreaming Alfvén waves were identified immediately upstream and downstream of the quasi‐parallel, supercritical shock. (3) Fast magnetoacoustic waves were identified upstream of the quasi‐parallel, subcritical shock and far upstream of the quasi‐parallel, supercritical shock coinciding with the inclusion of a small transverse discontinuity in the spectral analysis. (4) Compressional turbulence which is not characteristic of either of the magnetoacoustic modes was observed downstream of the parallel, supercritical and quasi‐parallel subcritical shocks. As regards the determination of the parallel diffusion coefficient from power spectra of the observed magnetic field fluctuations, we may conclude from our results that this procedure is susceptible to major errors due to the following reasons: First, there is no complete theory for the diffusion of energetic particles in regions where the MHD fluctuations cannot be labeled Alfvénic or magnetoacoustic. Second, equations for particle motion in a field of counterstreaming Alfvén waves (second Fermi process) are also lacking. Third, even the “Alfvénic” fluctuations are never 100% Alfvénic; i.e., a non‐Alfvénic component is usually present. Such “mixing” is also not considered in the present theories of energetic particle propagation. Fourth and last, it is not at all clear how much data before/after the shock waves should be analyzed to compute values of the parallel diffusion coefficient (κ∥) or mean free path for scattering (λ∥,s) of an energetic particle. We show here that spectral analysis of the fluctuations 10.8 min., 43.2 min. and 86.4 min. upstream of the parallel shock yields reasonable, but very different, values of κ∥.

First observational evidence is presented for low‐frequency slow mode MHD turbulence in the solar wind (ƒ ≤ 5 × 10−3 Hz, δB‐δN coherence greater than 0.6) by employing power and coherence spectral analysis of the Helios high time resolution solar wind plasma and magnetic field observations. This turbulence occurs directly behind an interplanetary slow forward shock wave at 0.31 AU. As the foreshock plasma conditions are such that steepening of slow waves is favored in comparison to Landau damping, the observations presented here do in addition provide unique support to the theoretical results of Hada and Kennel (1985). It is also shown that the turbulence upstream is predominantly Alfvénic (ƒ≲ 10−2 Hz, 10−2 Hz, δB‐δV coherence greater than 0.94) and propagating away from the Sun (shock). Thus we provide an example of an interplanetary slow shock where the turbulence upstream and downstream is completely different in nature. Moreover, the “evolutionary conditions” for slow shocks indicate that the downstream (upstream) turbulence could not have been generated by an interaction and wave mode conversion of the upstream (downstream) turbulence with and by the slow shock, respectively.

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