Abstract
The average characteristics of year-to-year variability of Galactic cosmic ray (GCR) flux measured in one mid-latitude neutron monitor stations (Newark) and high latitude station (Apatity) have been studied under different planetary disturbance (Kp) conditions. The year-to-year variability which oscillates in response to solar cycle was analyzed using Fourier technique and the amplitude of variation was obtained using data for 1980–2005. There is a noticeable trend in the difference between the amplitudes of the year-to-year variation of the two stations. The difference is highest during low Kp conditions and lowest during high Kp condition. There is generally lesser association of GCR with solar wind (SW) flow pressure and density as the Kp index increases. Similar feature is observed with the interplanetary magnetic field IMF (total). These observations have important implications for our present understanding of the effect of solar activity to variability in GCR flux.
Highlights
The variability of cosmic ray of Galactic origin near Earth is primarily driven by heliospheric factors and geomagnetic factors
The natures of variability of Galactic cosmic rays (GCR) have been studied in short term scales and long term scales [1,2,3,4], and these have revealed significant information about important processes involved in the transportation of GCR from source to the neutron monitor stations
Neutron monitors (NM) from lower latitudes exhibited higher contribution from the 11-year phase variation which is controlled by the diffusion associated with the change in strength of the interplanetary magnetic field [9]
Summary
The variability of cosmic ray of Galactic origin near Earth is primarily driven by heliospheric factors (through the interplanetary anisotropy in the heliospheric magnetic field) and geomagnetic factors (by the geomagnetic rigidity cutoff which is latitude dependent). Kudela and Storini [10] studied the variability of cosmic rays and its association with geomagnetic activity using the disturbance storm time index (Dst) and the southward component of the heliospheric magnetic field (Bz) component of the interplanetary magnetic field for four middle and high cutoff rigidity neutron monitors. They found high negative correlation (−0.86) between GCR intensity and interplanetary magnetic field (IMF) strength for 1 month lag between the two parameters. The global Kp index was obtained from ftp://ftp.ngdc.noaa.gov
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