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We have determined the structure and nature of 'discontinuities' in the interplanetary magnetic field at I AU in the period March 18 to April 9, 1971, by using high-resolution magnetic field measurements from Explorer 43. The discontinuities that were selected for this analysis occurred under a variety of interplanetary conditions at an average rate of 0.5/hour. This set does not include all discontinuities that were present, but the sample is large, and it is probably representative. Both tangential and rotational discontinuities were identified. Tangential discontinuities were observed every day, even among Alfvfinic fluctuations. In particular, on one day during which Alfvfinic fluctuations were intense and persistent in a high-speed stream, tangential discontinuities (TD's) were seen throughout the day at an average rate of 0.5/hour; rotational discontinuities (RD's) were also observed during this day at a higher than usual rate, the ratio of TD's to RD's being approximately one. The structure of most of the current sheets was simple and ordered; i.e., the magnetic field usually changed smoothly and monotonically from one side of the current sheet to the other. The thickness distributions of the TD's and RD's with very smooth current sheets were similar. The average thickness of the RD's was 1200 km (13 proton Larmor radii), and the average thickness of the TD's was 1300 km (12 proton Larmor radii).

Analysis of Imp 1 (Explorer 43) interplanetary plasma and magnetic field fluctuations on a scale of 1 hour for the period March 18 to April 7, 1971, reveals that linearly and circularly polarized Alfvén waves were rarely present. Fluctuations having most of the characteristics of large amplitude ‘Alfvén waves’ were observed ≈40% of the time and moved away from the sun nearly along B. These were not pure transverse Alfvén waves, however, because they were accompanied by nonzero fluctuations in the magnetic field intensity . No simple relation between ‘Alfvén waves’ and streams was found.

Simple models are described which investigate the combined effects on solar wind He++ of resonant and nonresonant acceleration by left‐hand transverse waves. The principal points are the following. (1) For a wide range of parameters, (υα – υp) at 1 AU is close to the effective phase speed of the left‐hand waves. (2) The most important factor in determining υα/υp at 1 AU is whether the high‐frequency left‐hand waves are predominantly outward propagating, inward propagating, or a mix of both. The resonant acceleration may be more important than the effects of heating or stream‐stream interactions. (3) Reasonable values of υα/υp at the sun (and of nα/np at 1 AU) are obtained for a power law index α ≅ 1.5 in the wave power spectrum if the effective phase speed of the resonant waves near the sun is not small in comparison to the Alfvén speed there. This requires a substantial level of high‐frequency power in outward going waves at the sun, which cannot come from heat‐conduction‐driven instabilities. (4) The present models do not allow one to decide whether the coronal He++ abundance is greater or less than that at 1 AU. (5) Some of the models show a positive correlation between nα/np and υp at 1 AU, roughly as has been observed. (6) The models suggest that variations in nα/np at 1 AU can result from variations in the wave properties near the sun and not necessarily from variations in the coronal abundance. (7) Some models indicate that (υα/υp) may decrease with increasing r in the vicinity of 1 AU. (8) The resonant acceleration is more efficient than Coulomb friction in the sense that it does not exhibit a runaway effect. (9) Observations of minor species may be used to deduce wave properties and plasma processes in the solar wind.

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