On the relationship of the 27-day variations of the solar wind velocity and galactic cosmic ray intensity in minimum epoch of solar activity

TL;DR

This study models the 27-day variation of galactic cosmic ray intensity based on solar wind velocity and magnetic field data, revealing that sector structure has minimal impact and that the variation is primarily driven by the product of solar wind speed and magnetic field strength.

Contribution

The paper introduces a 3-D model linking solar wind velocity and magnetic field to cosmic ray variations, demonstrating sector structure's limited influence and high correlation with modulation parameters.

Findings

Sector structure has minimal effect on cosmic ray variation.

High correlation (0.98) between theoretical and observed 27-day variations.

Inverse correlation (-0.91) between cosmic ray intensity and the product of solar wind velocity and magnetic field.

Abstract

We study the relationship of the 27-day variation of the galactic cosmic ray intensity with similar changes of the solar wind velocity and the interplanetary magnetic field based on the experimental data for the Bartels rotation period 2379 of 23 November 2007-19 December 2007. We develop a three dimensional (3-D) model of the 27-day variation of galactic cosmic ray intensity based on the heliolongitudinally dependent solar wind velocity. A consistent, divergence-free interplanetary magnetic field is derived by solving Maxwells equations with a heliolongitudinally dependent 27-day variation of the solar wind velocity reproducing in situ observations. We consider two types of 3-D models of the 27-day variation of galactic cosmic ray intensity - (1) with a plane heliospheric neutral sheet, and (2)- with the sector structure of the interplanetary magnetic field. The theoretical calculation shows that the sector structure does not influence significantly on the 27-day variation of galactic cosmic ray intensity as it was shown before based on the experimental data. Also a good agreement is found between the time profiles of the theoretically expected and experimentally obtained first harmonic waves of the 27-day variation of the galactic cosmic ray intensity (correlation coefficient equals 0.98 0.02). The expected 27-day variation of the galactic cosmic ray intensity is inversely correlated with the modulation parameter z (correlation coefficient equals -0.91 0.05) which is proportional to the product of the solar wind velocity V and the strength of the interplanetary magnetic field B (z VB). The high anticorrelation between these quantities indicates that the predictable 27-day variation of the galactic cosmic ray intensity mainly is caused by this basic modulation effect.