Unfolding Drift Effects for Cosmic Rays over the Period of the Sun's Magnetic Field Reversal

TL;DR

This study uses a 3-D numerical model to analyze how cosmic ray modulation processes, especially particle drift, change during the Sun's magnetic polarity reversal from 2011 to 2015, comparing simulations with AMS observations.

Contribution

It provides detailed simulation results of cosmic ray modulation during a solar magnetic polarity reversal, highlighting the impact on particle drift and mean free paths.

Findings

Drift effects become negligible during polarity reversal.

Mean free paths decrease significantly during reversal.

Drift scale begins recovering after reversal.

Abstract

A well-established, comprehensive 3-D numerical modulation model is applied to simulate galactic protons, electrons and positrons from May 2011 to May 2015, including the solar magnetic polarity reversal of Solar Cycle 24. The objective is to evaluate how these simulations compare with corresponding AMS observations for 1.0-3.0 GV, and what underlying physics follows from this comparison in order to improve our understanding on how the major physical modulation processes change, especially particle drift, from a negative to a positive magnetic polarity cycle. Apart from their local interstellar spectra, electrons and positrons differ only in their drift patterns, but they differ with protons in other ways such as their adiabatic energy changes at lower rigidity. In order to complete the simulations for oppositely charged particles, antiproton modeling results are obtained as well. Together, the observations and the corresponding modeling indicate the difference in the drift pattern before and after the recent polarity reversal and clarify to a large extent the phenomenon of charge-sign dependence during this period. The effect of global particle drift became negligible during this period of no well-defined magnetic polarity. The resulting low values of all particles' MFPs during the polarity reversal contrast their large values during solar minimum activity, and as such expose the relative contributions and effects of the different modulation processes from solar minimum to maximum activity. We find that the drift scale starts recovering just after the polarity reversal, but the MFPs keep decreasing or remain unchanged for some period after the polarity reversal.