Decadal trends in the diurnal variation of galactic cosmic rays observed using neutron monitor data
The diurnal variation (DV) in galactic cosmic ray (GCR) flux is a widely observed phenomenon in neutron monitor data. The background variation considered primarily in this study is due to the balance between the convection of energetic particles away from the Sun and the inward diffusion of ener...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-07-01
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| Series: | Annales Geophysicae |
| Online Access: | https://www.ann-geophys.net/35/825/2017/angeo-35-825-2017.pdf |
| Summary: | The diurnal variation (DV) in galactic cosmic ray (GCR) flux is a widely
observed phenomenon in neutron monitor data. The background variation
considered primarily in this study is due to the balance between the
convection of energetic particles away from the Sun and the inward diffusion
of energetic particles along magnetic field lines. However, there are also
times of enhanced DV following geomagnetic disturbances caused by coronal
mass ejections or corotating interaction regions. In this study we investigate
changes in the DV over four solar cycles using ground-based neutron monitors
at different magnetic latitudes and longitudes at Earth. We divide all of the
hourly neutron monitor data into magnetic polarity cycles to investigate
cycle-to-cycle variations in the phase and amplitude of the DV. The results
show, in general, a similarity between each of the <i>A</i> < 0 cycles and <i>A</i> > 0
cycles, but with a phase change between the two. To investigate this further,
we split the neutron monitor data by solar magnetic polarity between times
when the dominant polarity was either directed outward (positive) or inward
(negative) at the northern solar pole. We find that the maxima and minima of
the DV changes by, typically, 1–2 h between the two polarity states for
all non-polar neutron monitors. This difference between cycles becomes even
larger in amplitude and phase with the removal of periods with enhanced DV
caused by solar wind transients. The time difference between polarity cycles
is found to vary in a 22-year cycle for both the maximum and minimum times of
the DV. The times of the maximum and minimum in the DV do not always vary in
the same manner between <i>A</i> > 0 and <i>A</i> < 0 polarity cycles, suggesting a
slight change in the anisotropy vector of GCRs arriving at Earth between
polarity cycles. Polar neutron monitors show differences in phase between
polarity cycles which have asymptotic directions at mid-to-high latitudes.
All neutron monitors show changes in the amplitude of the DV with solar
polarity, with the amplitude of the DV being a factor of 2 greater in <i>A</i> < 0 cycles than <i>A</i> > 0 cycles. In most cases the change in timing of the
maximum /minimum is greatest with the stations' geomagnetic cut-off rigidity
shows little variation in the DV phase with latitude. We conclude that the
change in the DV with the dominant solar polar polarity is not as simple as a
phase change, but rather an asymmetric variation which is sensitive to the
neutron monitor's asymptotic viewing direction. |
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| ISSN: | 0992-7689 1432-0576 |