What in the Galaxy is Scattering Cosmic Rays?

Cosmic rays with energies << TeV affect galaxy evolution on all scales, from ionizing protoplanetary disks and molecular clouds to driving galactic outflows that alter the gas phase hundreds of kiloparsecs from the galactic disk. All models of cosmic-ray physics on “macro” scales (> pc) are sensitive to the assumed models of cosmic-ray scattering on “micro” scales ( au), which are observationally and theoretically unconstrained. These effects are amplified in the circumgalactic medium, where models that fit existing data can vary by many orders of magnitude in their predictions for the cosmic-ray transport rate. Traditional first-principles models, which assume these magnetic fluctuations are weak and uniformly scatter CRs in a homogeneous ISM, struggle to reproduce basic observables such as the dependence of CR residence times and scattering rates on rigidity. In this talk, I will explore a new category of 'patchy' CR scattering models, wherein CRs are predominantly scattered by intermittent strong scattering structures with small volume-filling factors. These models produce the observed rigidity dependence with a simple size distribution constraint, such that larger scattering structures are rarer but can scatter a wider range of CR energies. I will present the physical properties any such structures would need to successfully scatter low-energy CRs and discuss future observational constraints that could test these models.