Addressing the Impact of Solar Modulation Systematic Uncertainties on Cosmic-Ray Propagation Models

TL;DR

This paper analyzes how solar modulation uncertainties affect cosmic-ray propagation models using AMS-02 data over a solar cycle, highlighting the limitations of current models and the need for future measurements to improve precision.

Contribution

It provides a comprehensive evaluation of solar modulation effects on cosmic-ray modeling, comparing multiple models and emphasizing the importance of time-dependent data.

Findings

Force-field approximation works well outside solar maximum for certain models

Simple modulation models cause unphysical results during high solar activity

Systematic uncertainties of 10-15% limit current cosmic-ray propagation precision

Abstract

We perform a comprehensive analysis of cosmic-ray propagation using the time-dependent AMS-02 flux measurements covering a full solar cycle, with particular emphasis on the role of solar modulation. We fit two representative Galactic propagation scenarios, convection- and re-acceleration-dominated models, in combination with three solar modulation prescriptions: the standard force-field approximation, an extended force-field model with a rigidity break, and the heliospheric propagation code $\texttt{HelMod}$. The inclusion of time-resolved antiproton data provides a unique probe of charge-sign-dependent modulation effects and low-energy systematics. We find that the force-field approximation can describe positively charged nuclei reasonably well outside the solar maximum in convection-dominated models, but fails during periods of high solar activity and for antiprotons at all times. In re-acceleration scenarios, strong degeneracies between solar modulation and low-energy propagation lead to unphysical results when simple modulation models are employed. Across all models, we identify systematic uncertainties of order 10-15% in the reconstructed local interstellar spectra and propagation parameters, driven by limitations in current solar modulation modelling. Compared to the percent level error of current measurements, these uncertainties significantly limit the precision of cosmic-ray studies. Future time-dependent measurements spanning a full 22-year solar cycle will be crucial to reduce these uncertainties.