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Recent observations of the solar wind in the outer heliosphere

Observations made during recent years from the Voyager and Pioneer spacecraft in the distant heliosphere continue to show interesting and unexpected features. Although the solar wind number density shows considerable variation from day to day, the radial variation of the proton number density (near...

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Bibliographic Details
Published in:Advances in space research 1995-01, Vol.16 (9), p.77-84
Main Authors: Lazarus, A.J., Belcher, J.W., Paularena, K.I., Richardson, J.D., Steinberg, J.T.
Format: Article
Language:English
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Summary:Observations made during recent years from the Voyager and Pioneer spacecraft in the distant heliosphere continue to show interesting and unexpected features. Although the solar wind number density shows considerable variation from day to day, the radial variation of the proton number density (near the ecliptic) is consistent with a R −2 decrease. The temperature of the wind varies approximately as R −0.5; it seems clear that wind at higher heliographic latitudes is hotter. There is good evidence for the slowing down of the wind as it propagates into the distant heliosphere; the reduction in speed is of the order of 30 km/s and is consistent with the change expected from pickup of interstellar neutrals. Even at 40 AU, velocity structures at roughly the solar rotation period are evident. Beginning in 1987, a 1.3-year period also became apparent; observations from IMP 8 near 1 AU confirm that long-period variation and thus suggest it is of solar origin. The distant spacecraft provide near-ecliptic comparisons with the Ulysses observations and help identify when Ulysses moved into southerly latitudes dominated by polar coronal holes. Local structures in the distant wind provide challenges to our understanding of the structure of the wind and its relationship to the higher-energy particles.
ISSN:0273-1177
1879-1948
DOI:10.1016/0273-1177(95)00317-8