Muon Observations |
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Authors: | Duldig Marc L |
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Institution: | (1) Australian Antarctic Division, Kingston, Tasmania, 7050, Australia |
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Abstract: | Muon observations are complementary to neutron monitor observations but there are some important differences in the two techniques.
Unlike neutron monitors, muon telescope systems use coincidence techniques to obtain directional information about the arriving
particle. Neutron monitor observations require simple corrections for pressure variations to compensate for the varying mass
of atmospheric absorber over a site. In contrast, muon observations require additional corrections for the positive and negative
temperature effects. Muon observations commenced many years before neutron monitors were constructed. Thus, muon data over
a larger number of solar cycles is available to study solar modulation on anisotropies and other cosmic ray variations.
The solar diurnal and semi-diurnal variations have been studied for many years. Using the techniques of Bieber and Chen it
has been possible to derive the radial gradient, parallel mean-free path and symmetric latitude gradient of cosmic rays for
rigidities <200 GV. The radial gradient varies with the 11-year solar activity cycle whereas the parallel mean-free path appears
to vary with the 22-year solar magnetic cycle. The symmetric latitudinal gradient reverses at each solar polarity reversal.
These results are in general agreement with predictions from modulation models. In undertaking these analyses the ratio of
the parallel to perpendicular mean-free path must be assumed. There is strong contention in the literature about the correct
value to employ but the results are sufficiently robust for this to be, at most, a minor problem. An asymmetric latitude gradient
of highly variable nature has been found. These observations do not support current modulation models.
Our view of the sidereal variation has undergone a revolution in recent times. Nagashima, Fujimoto and Jacklyn proposed a
narrow Tail-In source anisotropy and separate Loss-Cone anisotropy as being responsible for the observed variations. A new
analysis technique, more amenable to such structures, was developed by Japanese and Australian researchers. They confirmed
the existence of the two anisotropies. However, they found that the Tail-In anisotropy is asymmetric and that both anisotropies
had different positions from the prediction.
Most 27-day modulations are observed at neutron monitor rigidities but not so readily at higher rigidities. An exception to
this is the Isotropic Intensity Wave modulation observed in the early 1980s and again in 1991. This modulation is very strongly
related to the heliospheric sector structure and implies a significantly different cosmic ray density on either side of the
neutral sheet.
The interpretation of most cosmic ray modulation phenomena requires good latitude coverage in both hemispheres. The closure
of many muon observatories is a matter of concern. In the northern hemisphere a few new instruments are being constructed
and spatial coverage is barely adequate. In the southern hemisphere the situation is far worse with the possibility that within
a decade only the Mawson observatory in Antarctica will still be in operation.
This revised version was published online in August 2006 with corrections to the Cover Date. |
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