The effect of magnetic field modifications on the modulation of cosmic rays in the heliosphere

TL;DR

This study uses a numerical model to compare how different modifications of the heliospheric magnetic field affect cosmic ray modulation, highlighting the physical basis and observational support for the Smith-Bieber modification as most effective.

Contribution

It introduces a comparative analysis of three magnetic field modifications and advocates for the Smith-Bieber modification based on physical and observational grounds.

Findings

Smith-Bieber modification best reproduces observed cosmic ray spectra.

Jokipii-Kota modification lacks strong physical motivation.

Model results align with PAMELA data during solar minimum.

Abstract

A numerical model for the solar modulation of cosmic rays, based on the solution of a set of stochastic differential equations, is used to illustrate the effects of modifying the heliospheric magnetic field, particularly in the polar regions of the heliosphere. To this end, the dfferences in the modulation brought about by each of three choices for the heliospheric magnetic field, i.e. the unmodified Parker field, the Smith-Bieber modified field, and the Jokipii-Kota modified field, are studied. It is illustrated that both the Jokipii-Kota and Smith-Bieber modifications are effective in modifying the Parker field in the polar regions. In addition, it is argued that the modification of Smith and Bieber is based on observational evidence and has a firm physical basis, while these motivations are lacking in the case of the Jokipii-Kota modification. From a cosmic ray modulation point of view, we found the Smith-Bieber modification to be the most suitable choice for modifying the heliospheric magnetic field. The features and effects of these three modifications are illustrated both qualitatively and quantitatively. It is also shown how the Smith-Bieber modified field can be applied in cosmic ray modulation models to reproduce observational cosmic ray proton spectra from the PAMELA mission during the solar minimum of 2006 - 2009. These results are compared with those obtained in previous studies of this unusual solar minimum activity period and found to be in good qualitative agreement.