A study of MHD fluctuations upstream and downstream of quasi‐parallel interplanetary shocks

Abstract

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∥.