Benedikt Schroer

Phenomenological studies of cosmic-ray self-confinement often hinge on the linear theory for the growth rate of the streaming instability and for the damping rate of the generated magnetic modes. Largely different expressions exist, especially for the rate of nonlinear Landau damping, which is often assumed to be the most important damping mechanism in warm ionized plasmas. Using hybrid-PIC simulations in the resonant streaming instability regime, we present a comprehensive assessment of nonlinear Landau damping and show that the damping rate at a given scale depends on the power in magnetic fields on larger scales. Furthermore, we find that an inverse cascade develops, which produces magnetic fields on scales larger than the resonant ones. Here we extend previous results obtained for a mono-energetic distribution of non-thermal particles to the case of broader CR distributions, as a first step towards developing phenomenological models. Pre-existing turbulence of Alfvénic nature at large scales severely affects the damping of waves produced by low-energy CRs; depending on its amplitude, such a turbulence may inhibit the growth of streaming instability so that CRs are either self-confined at all energies or not at all.

Anomalies in cosmic-ray (CR) fluxes, such as the rising positron flux and the flat antiproton-to-proton ratio, have called into question the standard halo model of CR transport and supposedly support alternative models, such as the nested leaky box model. Here, we test such a model in terms of both primary cosmic-ray spectra, spectra of stable and unstable nuclei and antimatter production. We find the standard version of the model should be considered as ruled out as it is in direct conflict with several observational facts and current data. We further show that the flat antiproton-over-proton ratio proves that the Galactic residence time cannot be energy-independent.

We present the first investigation of the role of non-linear Landau damping in self-generated cosmic-ray transport in conditions appropriate for the Galactic halo using hybrid particle-in-cell simulations. We find reduced CR drift speeds due to scattering, that however, remain super-Alfvénic due to damping. The non-linear Landau damping leads to heating of the background plasma and initiates an inverse cascade, producing perturbations on non-resonant large scales, a result with many potential implications for CR transport.

The secondary/primary cosmic-ray ratios and the diffuse backgrounds of gamma rays and neutrinos provide us with complementary information about the transport of Galactic cosmic rays (CRs). We used the recent measurement of the diffuse gamma ray background in the ∼TeV − PeV range by LHAASO and of the very high-energy diffuse neutrino background from the Galactic disc by IceCube to show that CRs may be accumulating an approximately energy independent grammage X ∼ 0.4 g cm⁻², in regions where gamma rays and neutrinos are produced with a hard spectrum, resembling the source spectrum. We speculate that this grammage reflects the early stages of cosmic ray transport around sources, in what are referred to as cocoons, where particles spend ∼0.3 Myr before starting their journey in the Galactic environment.

We revisit the predictions of the nested leaky box model in detail, in terms of both primary cosmic-ray spectra, spectra of stable and unstable nuclei and antimatter production (positrons and antiprotons). We conclude that the model is in direct conflict with several observational facts and at least in its vanilla version should be considered as ruled out by current data. We also speculate on some possibly interesting implications of the idea that at least a fraction of Galactic grammage may not be accumulated in the journey of cosmic rays in the interstellar medium but rather inside the sources or in regions around them. These speculations will become increasingly more relevant with the higher precision data becoming available at high energies.

A critical component of particle acceleration in astrophysical shocks is the nonresonant (Bell) instability, where the streaming of cosmic rays (CRs) leads to the amplification of magnetic fields necessary to scatter particles. In this work we use kinetic particle-in-cell simulations to investigate the high-CR-current regime, where the typical assumptions underlying the Bell instability break down. Despite being more strongly driven, significantly less magnetic field amplification is observed than in low-current cases, an effect due to the anisotropic heating that occurs in this regime. We also find that electron-scale modes, despite being the fastest growing, mostly lead to moderate electron heating and do not affect the late evolution or saturation of the instability.

We present the first assessment, using hybrid PIC simulations, of the role of nonlinear Landau damping in the process of self-generated scattering in a high β plasma, conditions appropriate for CR scattering in the halo of the Galaxy. The drift speed of CRs reduces due to scattering, but remains super-Alfvénic due to damping. Furthermore, this damping process manifests itself in the form of heating of the background plasma. We also show that the damping leads to an inverse cascade process, consisting of producing nonresonant large scale modes, a novel result with many potential phenomenological implications.