Three ways to improve the theory and therefore the understanding of the physical processes in the interplanetary medium (during both quiet and disturbed periods of solar activity) are suggested. They are: (1) the development and consequently the use ofhigher order moments (fluid) equationsas well as of more realisticclosure conditionsandtransport coefficientsfor the macroscopic description of the solar wind ; (2) the undertaking ofcomputer simulation experimentson the nonlinear collective

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... tion process through particle-wave-particle interaction due to the plasma electromagnetic instabilities which may develop under conditions prevailing in the solar wind; and (3) the consideration of collective interactions in the evaluation of the transport coefficients, as deduced from the quasi-linear theory and computer simulation experiments, and their incorporation into the higher order moment equations.