Dependence of the 27-Day Variation of Cosmic Rays on the Global Magnetic Field of the Sun

TL;DR

This study investigates how the 27-day variation of cosmic rays is influenced by the Sun's magnetic field and solar wind, using a 3-D model that aligns well with observational data across different solar activity periods.

Contribution

The paper introduces a 3-D model of GCR variation incorporating solar wind speed and magnetic field variations, explaining the polarity dependence of cosmic ray fluctuations.

Findings

Higher heliolongitudinal asymmetry in solar wind speed increases GCR variation amplitude.

Polarity dependence of GCR variation is weaker during the 2007-2009 solar minimum.

Model results agree with neutron monitor data, requiring increased diffusion coefficient for recent minimum.

Abstract

We show that the higher range of the heliolongitudinal asymmetry of the solar wind speed in the positive polarity period (Apos) than in the negative polarity period (Aneg) is one of the important reasons of the larger amplitudes of the 27-day variation of the galactic cosmic ray (GCR) intensity in the period of 1995-1997 (Apos) than in 1985-1987 (Aneg). Subsequently, different ranges of the heliolongitudinal asymmetry of the solar wind speed jointly with equally important corresponding drift effect are general causes of the polarity dependence of the amplitudes of the 27-day variation of the GCR intensity. At the same time, we show that the polarity dependence is feeble for the last unusual minimum epoch of solar activity 2007-2009 (Aneg); the amplitude of the 27-day variation of the GCR intensity shows only a tendency of the polarity dependence. We present a three dimensional (3-D) model of the 27-day variation of GCR based on the Parkers transport equation. In the 3-D model is implemented a longitudinal variation of the solar wind speed reproducing in situ measurements and corresponding divergence-free interplanetary magnetic field (IMF) derived from the Maxwells equations. We show that results of the proposed 3-D modeling of the 27-day variation of GCR intensity for different polarities of the solar magnetic cycle are in good agreement with the neutron monitors experimental data. To reach a compatibility of the theoretical modeling with observations for the last minimum epoch of solar activity 2007-2009 (Aneg) a parallel diffusion coefficient was increased by 40 percent