Reconciling cosmic-ray transport theory with phenomenological models motivated by Milky-Way data

TL;DR

This paper examines whether current microphysical theories of cosmic-ray transport, such as self-confinement and turbulence, can be aligned with phenomenological models that fit Milky Way data, highlighting significant theoretical gaps.

Contribution

It analyzes the compatibility of microphysical CR transport theories with phenomenological models and discusses the need for improved understanding of MHD turbulence.

Findings

Multi-phase ISM causes varied CR transport mechanisms.

Models with combined scattering mechanisms require fine-tuning.

Fast-mode turbulence damping challenges existing assumptions.

Abstract

Phenomenological models of cosmic-ray (CR) transport in the Milky Way (MW) can reproduce a wide range of observations assuming that CRs scatter off of magnetic-field fluctuations with spectrum $\propto k^{-\delta}$ and $\delta \sim [1.4,1.67]$. We study the extent to which such models can be reconciled with current microphysical theories of CR transport, specifically self-confinement due to the streaming instability and/or extrinsic turbulence due to a cascade of MHD fast modes. We first review why it is that on their own neither theory is compatible with observations. We then highlight that CR transport is a strong function of local plasma conditions in the multi-phase interstellar medium (ISM), and may be diffusive due to turbulence in some regions and streaming due to self-confinement in others. A multi-phase combination of scattering mechanisms can in principle reproduce the main trends in the proton spectrum and the boron-to-carbon ratio (B/C). However, models with a combination of scattering by self-excited waves and fast-mode turbulence require significant fine-tuning due to fast-mode damping, unlike phenomenological models that assume undamped Kolmogorov turbulence. The assumption that fast modes follow a weak cascade is also not well justified theoretically, as the weak cascade is suppressed by wave steepening and weak-shock dissipation even in subsonic turbulence. These issues suggest that there may be a significant theoretical gap in our understanding of MHD turbulence. We discuss a few topics at the frontier of MHD turbulence theory that bear on this (possible) gap and that may be relevant for CR scattering.