The effects of solar wind on galactic cosmic ray flux at Earth

G D Ihongo,C H.-T Wang

doi:10.1007/s10509-015-2628-5

Abstract

The amount of solar wind produced continuously by the sun is not constant due to changes in solar activity. This unsteady nature of the solar wind seems to be responsible for galactic cosmic ray flux modulation, hence the flux of incoming galactic cosmic rays observed at the top of the Earth’s atmosphere varies with the solar wind reflecting the solar activity. The aforementioned reasons have lead to attempts by several researchers to study correlations between galactic cosmic rays and the solar wind. However, most of the correlation studies carried out by authors earlier are based on the analyses of observational data from neutron monitors. In this context, we study the effects of solar wind on galactic cosmic ray flux observed at \(r \approx 1~\mbox{AU}\), using a theoretical approach and found that the solar wind causes significant decreases in galactic cosmic ray flux at \(r \approx1~\mbox{AU}\). A short time variation of the calculated flux is also checked and the result is reflected by exposing a negative correlation of the solar wind with the corresponding galactic cosmic ray flux. This means that the higher the solar wind the lower the galactic cosmic rays flux and vice-versa. To obtain a better understanding, the calculated flux and its short time variation at 1 AU are compared to data that shows a good fit to the model making it possible to establish a statistically significant negative correlation of \(-0.988\pm0.001\) between solar wind variation and galactic cosmic rays flux variation theoretically.

Highlights

Modeling galactic cosmic rays is a highly non-trivial task which often requires a numerical simulation of the complex phenomena usually observed in the heliosphere
A short time variation of the calculated flux is presented in Fig. 2 right panel alongside the solar wind profile where a negative correlation between galactic cosmic ray flux variation and solar wind variation is observed. This is consistent with the negative correlation between solar wind variation and the corresponding galactic cosmic ray variation reported by other authors earlier (Sabbah 2000; Mishra and Mishra 2006; Firoz et al 2010; Alania et al 2011; Modzelewska and Alania 2013; Kojima et al 2015). This means that the higher the solar wind the lower the galactic cosmic ray flux and vice-versa
There have been several reports on cosmic rays and solar wind correlation from different researchers where negative correlations were found between cosmic rays and solar wind using observational data from ground based neutron monitors and experiments

Summary

Introduction

Modeling galactic cosmic rays is a highly non-trivial task which often requires a numerical simulation of the complex phenomena usually observed in the heliosphere This can mainly be described in four processes namely (Parker 1965; Sabbah 2000; Bazilevskaya et al 2013; Ihongo and Wang 2015; Aslam and Badruddin 2015) convection in the radial expanding solar wind, diffusion in the heliospheric magnetic field, particle drifts due to magnetic field irregularities and momentum change (or adiabatic cooling). If reasonable assumptions are implored, considering only the diffusion and convection processes, the complex phenomena can be reduced to a more simplified scenario which can be modeled analytically as in the force field and convection-diffusion models (CaballeroLopez and Moraal 2004; Bazilevskaya et al 2013; Ihongo and Wang 2015) Among these processes, the solar wind and magnetic field scattering are known to predominantly modulate the flux of galactic cosmic rays (Firoz et al 2010; Alania et al 2011; Modzelewska and Alania 2013). In view of the above, this paper is mainly concerned with the effects of solar wind on galactic cosmic ray flux

Results

Discussion

Conclusion