On the description of the GCR intensity in the last three solar minima

TL;DR

This paper analyzes heliospheric parameters affecting GCR intensity during the last three solar minima, proposing a model that explains observed variations and the unusually soft GCR spectra in the recent minimum.

Contribution

It introduces a model with specific parameters to describe GCR intensity modulation across different solar minima, including the recent unusual minimum.

Findings

The model successfully describes GCR intensity during typical minima.

A reduction in the modulation region size explains the soft GCR spectra in 2009.

Changes in the diffusion coefficient's rigidity dependence are supported by solar wind observations.

Abstract

We discuss the main characteristic features in the heliospheric parameters important for the GCR intensity modulation for the last three solar minima (1986--1987, 1996--1997 and 2008--2009). The model for the GCR intensity modulation is considered and the set of the model parameters is chosen which allows the description of the observed GCR intensity distributions at the moments of the maximum GCR intensity in two solar minima (1987 and 1997) normal for the second half of the last century. Then we try to describe with the above model and set of parameters the unusually soft GCR energy spectra at the moments of the maximum GCR intensity in the last solar minimum between cycles 23 and 24 (2009). Our main conclusion is that the most simple way to do so is to reduce the size of the modulation region and, probably, change the rigidity dependence of the diffusion coefficient. The change of both parameters is substantiated by the observations of the solar wind and heliospheric magnetic field.