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.
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Autocorrelation analyses of K-coronameter observations made at Haleakala and Mauna Loa, Hawaii, during 1964-1967 have established average yearly rotation rates of coronal features as a function of latitude and height above the limb. At low latitudes the corona was found to rotate at the same rate as sunspots but at higher latitudes was consistently faster than the underlying photosphere. There were differences as large as 3-4 % in the rate at specific latitudes fi'om year to year and between the two hemispheres. In 1967 a nearly constant rotation was found for heights ranging from 1.125 to 2.0 R0. For 1966 there was a more complicated pattern of height dependence, with the rate generally decreasing with height at low latitudes and increasing at high latitudes.
Solar wind speeds measured from the Helios 1 solar probe between 0.31 and 1 AU and the earth‐orbiting Imp 7/8 satellites have been correlated with coronal holes as determined from K coronal brightness measurements. In March 1975 during perihelion passage, Helios 1 traversed the range of heliographic latitudes from −6° to +6° in a period of only 20 days. During that time the space probe crossed the northern boundary of the high‐speed stream associated with an equatorward extension of the south polar coronal hole. While this same stream continued to be observed by Imp satellites at −5° latitude, it was no longer observable from Helios 1 at +5° latitude. The conclusion is that sharp boundaries separate high‐speed flows from the surrounding solar wind. The thickness of the boundary in latitude appears to be narrower than about 10°. The local latitudinal gradient in flow speed is at least 30 km s−1 deg−1. With less confidence it can be deduced from the Helios 1 data that the actual gradients at 0.31 AU might even exceed 100 km s−1 deg−1.
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