Large‐scale interplanetary magnetic fields: Voyager 1 and 2 observations between 1 AU and 9.5 AU

Authors:
  • LBL. F. Burlaga
    L. F. Burlaga
  • LKL. W. Klein
    L. W. Klein
  • RLR. P. Lepping
    R. P. Lepping
  • KBK. W. Behannon
    K. W. Behannon
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Abstract
The strength B of the interplanetary magnetic field observed by the Voyager spacecraft between 1 AU and ∼9.5 AU was found to decrease with distance R from the sun as B = 4.75 (1 + R²)1/2/R², in agreement with the spiral field model. Between August 1977 and July 1979, when solar activity was increasing, corotating flows were observed at an average rate of at least 1 every 20 days, but the flows were evolving with time and seldom recurred from one solar rotation to the next without change. Many transient flows were also observed in this period. Large‐scale fluctuations in B with respect to the average spiral field were observed in association with interplanetary shocks and corotating stream interfaces, and these fluctuations varied with time in association with changes of the flows. The amplitude of the fluctuations in B relative to the mean field was large. There was a tendency for it to increase with distance to 5 AU, but the temporal variations were comparable to or larger than the radial variations. At large distances, B and the plasma density increased together, consistent with the idea that the structure of the outer heliosphere may be determined by stream interactions. The width of interaction regions increased with R owing to expansion, and closely spaced interaction regions often coalesced. A four‐sector pattern was observed from day 267, 1977, to approximately day 173, 1978, followed by a two‐sector pattern which lasted to at least day 179, 1979. In the interval with four sectors, there were usually several small‐amplitude peaks in B together with many transient streams and shocks on each solar rotation, whereas in the interval with two sectors there were one or two maxima in B together with interfaces and shock pairs on each solar rotation. Thus, the relatively abrupt change in sector pattern was accompanied by a change in the pattern of fluctuations in B and a change in the nature of the dominant flows.

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