Plasma field characteristics of directional discontinuities in the interplanetary medium

We examine the plasma and magnetic field changes occurring across 1359 directional discontinuities taken from interplanetary data spanning almost four solar rotations. The plasma field characteristics of these events exhibit a distinct variation with large-scale solar wind velocity. At low velocities, tangential discontinuities appear to predominate. At higher velocities a substantial and increasing fraction of directional discontinuities exhibits the plasma field properties expected of outwardly propagating rotational discontinuities. The results of Sari (1972,1975) and of the present study suggest that in the calculation of propagation diffusion coefficients for low-energy cosmic rays, the effects of directional discontinuities should be subtracted from the magnetic fluctuation spectrum during relatively quiet wind conditions. It is not clear that such subtraction is necessary during more disturbed periods