Latitude survey investigation of galactic cosmic ray solar modulation during 1994-2007
2016
The Galactic cosmic ray spectrum exhibits subtle variations over the 22 yr solar magnetic cycle
in addition to the more dramatic variations over the 11 yr sunspot cycle. Neutron monitors are
large ground-based detectors that provide accurate measurements of variations in the cosmic ray
flux at the top of the atmosphere above the detector. At any given location the magnetic field of
the Earth excludes particles below a well-defined rigidity (momentum per unit charge) known as
the cutoff rigidity, which can be accurately calculated using detailed models of the geomagnetic
field. By carrying a neutron monitor to different locations, e.g., on a ship, the Earth itself serves
as a magnet spectrometer. By repeating such latitude surveys with identical equipment a sensitive
measurement of changes in the spectrum can be made. In this work, we analyze data from the
1994 through 2007 series of latitude surveys conducted by the Bartol Research Institute, the University
of Tasmania, and the Australian Antarctic Division. We confirm the curious "crossover"
in spectra measured near solar minima during epochs of opposite solar magnetic polarity, and
show that it is related directly to a sudden change in the spectral behavior of solar modulation at
the time of the polarity reversal, as revealed from contemporaneous variations in the survey data
and a fixed station. We suggest that the spectral change and crossover result from the interaction
of effects due to gradient/curvature drifts with a systematic change in the interplanetary diffusion
coefficient caused by turbulent magnetic helicity.
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