Cosmic rays are charged particles that reach us from distant stellar explosions. But before hitting a detector, they must pass through the bubble of the solar wind. The Sun breathes: it inflates this bubble, then contracts it. Particles have to fight through a changing magnetic field, like a swimmer against a current that alternately strengthens and weakens.
Scientists took data from the AMS-02 detector on the International Space Station over an entire 11-year solar activity cycle. They tested how well different models describe the way the Sun distorts the particle flux. The simple model, like a current map that ignores obstacles, worked only in quiet years and only for positively charged nuclei. For antiprotons, it was wrong at all times.
Errors pile up: in the end, scientists get a cosmic ray map shifted by 10–15%. It's like measuring a river's depth without knowing that the water level changes with the seasons. To see the true picture, observations spanning a full 22-year cycle are needed — then the Sun's breathing can be separated from distant sources.
as written by the authors arXiv:2602.17759
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.
Antiprotons are rare visitors: there are a million times fewer of them than ordinary protons, but they are precisely the ones sensitive to the direction of the Sun's magnetic field.