# Galactic Cosmic-ray Scattering due to Intermittent Structures

**Authors:** Iryna S. Butsky, Philip F. Hopkins, Philipp Kempski, Sam B. Ponnada, Eliot Quataert, Jonathan Squire  
**arXiv:** [2308.06316](https://arxiv.org/abs/2308.06316)  
**Date:** 2023-08-11  
**Summary page:** https://www.summarizepaper.com/en/paper/2308.06316v1/

## Key points

- Study focuses on behavior of cosmic rays (CRs) in the interstellar medium (ISM)
- Traditional models struggle to reproduce observables related to CR residence times and scattering rates
- Authors propose "patchy" CR scattering models where CRs are scattered by intermittent strong structures
- Mean free path between scattering structures must be around 10 parsecs at GeV energies
- Constraints derived for sizes, properties, mass/volume-filling factors, and number density of scattering structures
- Various candidate structures considered, ruling out macroscopic ones but smaller structures remain viable
- Future observational constraints discussed to test patchy CR scattering models

## Summary

The study focuses on the behavior of cosmic rays (CRs) in the interstellar medium (ISM). CRs with energies much lower than a teraelectronvolt (TeV) play a significant role in the ISM. Traditional models assume weak and uniform magnetic fluctuations that scatter CRs in a homogeneous ISM. However, these models struggle to reproduce basic observables such as the dependence of CR residence times and scattering rates on rigidity. To address this issue, the authors propose a new category of "patchy" CR scattering models. In these patchy models, CRs are predominantly scattered by intermittent strong scattering structures with small volume-filling factors. These structures have a simple size distribution constraint, where larger scattering structures are rarer but can scatter a wider range of CR energies. The authors find that to reproduce empirically-inferred CR scattering rates, the mean free path between scattering structures must be around 10 parsecs at GeV energies. The study derives constraints on the sizes, internal properties, mass/volume-filling factors, and number density of these scattering structures to ensure they are both physically and observationally consistent. The authors consider various candidate structures at different scales, including large-scale structures like H II regions and small-scale structures like intermittent turbulent structures or those associated with radio plasma scattering. While many macroscopic candidates can be ruled out as primary CR scattering sites based on their analysis, several smaller structures remain viable and warrant further theoretical investigation. The paper also discusses future observational constraints that could potentially test these patchy CR scattering models. Overall, this study provides insights into the behavior of cosmic rays in the interstellar medium and proposes new models that better explain observed phenomena related to their residence times and scattering rates.