Decadal trends in the diurnal variation of galactic cosmic rays observed using neutron monitor data

Simon Thomas,Mike Lockwood,Mathew Owens,Chris Owen

doi:10.5194/angeo-35-825-2017

Abstract

Abstract. 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 A < 0 cycles and A > 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 A > 0 and A < 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 A < 0 cycles than A > 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.

Highlights

Galactic cosmic rays (GCRs) are high-energy ions that originate from galactic sources such as supernovae
In agreement with previous studies, we find that most neutron monitors (NMs) have a peak in the variation at 14:00–17:00 magnetic local time (MLT), which falls close to the range found by Pomerantz and Duggal (1970) of 15:00–18:00 MLT
Dτmax is longer than dτmin for Oulu, Deep River, Thule and South Pole. These results indicate that the change in diurnal variation (DV) between polarity cycles in not a change in phase, but rather an asymmetric change in the GCR flux arriving at any given NM

Summary

Introduction

Galactic cosmic rays (GCRs) are high-energy (typically classified as > 100 MeV) ions (predominantly protons) that originate from galactic sources such as supernovae. McMurdo NM is shown as it provides an example where the phase of the DV is offset and the amplitude is small compared to all of the other NMs. Figure 2 shows the same variations as Fig. 1 with the data divided by polarity cycle, as defined in Thomas et al (2014a), where A+1 and A+2 are those with positive polarity of the prevailing solar polar field and A−1 and A−2 are those with negative polarity.

Results

Conclusion