INTENSITY VARIATIONS OF GREATER-THAN-70-MEV COSMIC-RAYS MEASURED BY PIONEER-10, VOYAGER-1 AND VOYAGER-2 AND IMP IN THE HELIOSPHERE DURING THE RECOVERY PERIOD FROM 1992-1995
Wr. Webber et Ja. Lockwood, INTENSITY VARIATIONS OF GREATER-THAN-70-MEV COSMIC-RAYS MEASURED BY PIONEER-10, VOYAGER-1 AND VOYAGER-2 AND IMP IN THE HELIOSPHERE DURING THE RECOVERY PERIOD FROM 1992-1995, Geophysical research letters, 22(19), 1995, pp. 2669-2672
Using data from the >70-MeV channel on the Pioneer 10 and Voyager 1 &
2 spacecraft we have examined the gradients and intensities of cosmic
rays in the outer heliosphere between 1992 and 1995. During this time
there is a slow increase of intensity towards the next maximum in simi
lar to 1997 and the radial gradient is small decreasing to similar to
0.8%/AU at the end of this period. The latitude gradient has also decr
eased to only similar to 0.1-0.2%/AU but remains negative as it was in
the previous Ii-year cycle. These gradients are similar to those for
>70-MeV particles reported recently in the inner heliosphere but quite
different from the positive gradients observed at this time throughou
t the heliosphere by the Ulysses and Voyager spacecraft for anomalous
cosmic rays. Overall the cosmic ray intensity is almost uniform in rad
ius, azimuth and latitude at this time in the outer heliosphere - and
far lower than the estimated interstellar intensity. The intensities a
re, in fact, much lower than would be expected based on data from the
previous cycle. This, along with the very slow increase that is observ
ed, suggests that, at the same radial distance, the intensity in 1977
will be much less than in the previous cycle in 1987 - resulting in a
large 22-year modulation wave. The data show some features which sugge
st that large scale drift patterns, which change when the solar magnet
ic polarity changes every 11 years, play a important role in the overa
ll intensity distribution in the outer heliosphere - but specific deta
ils, particularly in the present positive polarity cycle beginning in
1990 are not well predicted by current models.