Nature and origin of directional discontinuities in the solar wind

Less than 25% of the ‘directional discontinuities’ (directional changes >30° in less than 30 sec) identified by Burlaga in Ness's interplanetary magnetic field data for the period December 18 to 25, 1965, are rotational discontinuities. Thus it is inferred that most of these directional discontinuities are tangential. The Pioneer 6 magnetic field and plasma data are not consistent with the hypothesis that directional discontinuities originate primarily by the collision of fast streams with slower plasma, since the rate of occurrence of discontinuities in regions of increasing bulk speed is only a little higher than the rate elsewhere. The rate at which discontinuities are seen at 0.82 AU is somewhat lower than at 1 AU, but the difference may be due to the lower data quality and increase in the number of data gaps when the spacecraft is far from the earth. The distributions of the change in direction of the field across the discontinuity and of the ‘normals’ (B1×B2/|B1×B2|) are very similar at ≈0.8, ≈0.9, and ≈1.0 AU. This suggests that most discontinuities originate within 0.8 AU and do not evolve appreciably between 0.8 and 1.0 AU.