Coronal streamers in the solar wind at 1 AU

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Abstract
Examination of solar wind plasma data obtained by the Los Alamos experiments on Imp 6, 7, and 8 during the 1971–1978 interval has revealed a frequent association between minimums in helium abundance and maximums in proton density. These events occur at low flow speeds and are strongly correlated with polarity reversals in the interplanetary magnetic field. A large fraction of these high proton density‐low helium abundance events are examples of noncompressive density enhancements (NCDE), i.e., large positive density signals not readily associated with stream‐stream interactions. The cleanest examples of these events often occur at well defined sector boundaries; complex, multiple events with 2 or more peaks in proton density and lasting ∼3–7 days are, however, common, and are associated with multiple field polarity reversals. When mapped back to the sun, assuming constant speed along a stream tube, these high proton density ‐ low helium abundance events associated with magnetic field reversals usually correspond to intersections of the spacecraft trajectory with the mid line of a coronal streamer belt that encircles the sun. The duration or multiplicity of these 1 AU events is generally correlated with the local tilt of the middle of the streamer belt to the solar equator. These events thus appear to be the 1 AU extensions of coronal streamers. A fine scale warping of the streamer belt on a longitudinal scale of ∼10° and varying thickness and density of the streamer belt at 1 AU adequately account for the variety of low helium abundance ‐ high proton density events observed. It is not yet understood why the helium abundance is low within coronal streamers at 1 AU.