Standard Self-Confinement and Extrinsic Turbulence Models for Cosmic Ray Transport are Fundamentally Incompatible with Observations

TL;DR

This paper demonstrates that standard models of cosmic ray transport, including self-confinement and extrinsic turbulence, are incompatible with observed cosmic ray spectra and behaviors, highlighting the need for alternative scattering mechanisms.

Contribution

The study shows fundamental incompatibilities of existing CR transport models with observations, and suggests the necessity for new scattering drivers across multiple scales.

Findings

Extrinsic turbulence models predict incorrect spectral shapes and scalings.

Self-confinement models cannot reproduce observed CR spectral features and are unstable.

Standard models are incompatible with observed CR spectra, requiring new scattering mechanisms.

Abstract

Models for cosmic ray (CR) dynamics fundamentally depend on the rate of CR scattering from magnetic fluctuations. In the ISM, for CRs with energies ~MeV-TeV, these fluctuations are usually attributed either to 'extrinsic turbulence' (ET) - a cascade from larger scales - or 'self-confinement' (SC) - self-generated fluctuations from CR streaming. Using simple analytic arguments and detailed live numerical CR transport calculations in galaxy simulations, we show that both of these, in standard form, cannot explain even basic qualitative features of observed CR spectra. For ET, any spectrum that obeys critical balance or features realistic anisotropy, or any spectrum that accounts for finite damping below the dissipation scale, predicts qualitatively incorrect spectral shapes and scalings of B/C and other species. Even if somehow one ignored both anisotropy and damping, observationally-required scattering rates disagree with ET predictions by orders-of-magnitude. For SC, the dependence of driving on CR energy density means that it is nearly impossible to recover observed CR spectral shapes and scalings, and again there is an orders-of-magnitude normalization problem. But more severely, SC solutions with super-Alfvenic streaming are unstable. In live simulations, they revert to either arbitrarily-rapid CR escape with zero secondary production, or to bottleneck solutions with far-too-strong CR confinement and secondary production. Resolving these fundamental issues without discarding basic plasma processes requires invoking different drivers for scattering fluctuations. These must act on a broad range of scales with a power spectrum obeying several specific (but plausible) constraints.