Spectrum and anisotropies of Galactic cosmic rays: a laboratory for magnetic fields

TL;DR

This review discusses recent advances in understanding Galactic cosmic rays, highlighting spectral features, anisotropies, and the potential of cosmic rays as a tool to study Galactic magnetic fields, emphasizing current knowledge gaps and future prospects.

Contribution

It synthesizes recent observational and theoretical developments, emphasizing the role of cosmic rays as a laboratory for probing Galactic magnetic fields and identifying key unknowns.

Findings

Spectral breaks in cosmic ray nuclei have been observed.

Anisotropies in cosmic ray arrival directions have been detected.

Galactic magnetic field structure limits predictive models.

Abstract

Much has been learned about Galactic cosmic rays in the past decade: On the observational side, the spectra of cosmic ray nuclei have been directly measured with high precision, resolving chemical composition up to TV rigidities. At even higher rigidities, direct detection is making contact with indirect observations from air shower arrays. A number of breaks have been found in the nuclear spectrum, which was previously thought to be a pure power law up to the knee. Data from air shower arrays also show interesting features in the arrival directions of cosmic-ray nuclei. On the theoretical side, more sophisticated models are able to explain the various spectral breaks either with transitions between different classes of sources or with changes in the transport regime. Yet, it has become clear that our ignorance of the structure of the Galactic magnetic fields, both on large and small scales, is limiting precision predictions. Turning this problem into an opportunity though, we can use Galactic cosmic rays as a laboratory for the study of Galactic magnetic fields. In this review talk, delivered at the 39th International Cosmic Ray Conference (ICRC2025), I have summarised what is known about the spectrum and anisotropies of Galactic cosmic rays, what is not known yet and what can be learnt in the future.