Cosmic rays in the immediate local interstellar medium

• Applicants: Professor Dr. Dieter Breitschwerdt; Professor Dr. Philipp Mertsch
• Subject Area: Astrophysics and Astronomy
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 490751943

Precise observations of cosmic rays (CRs) are only available from our position in the Galaxy and are affected by the structure of our local environment: The Earth is situated in the heliosphere, which suppresses fluxes at energies smaller than a few GeV. The heliosphere itself is embedded in a low-density superbubble, called the Local Bubble (LB). The exact history and spatial distribution of nearby sources are also unknown, leading to sizeable uncertainties of the spectra at high and very low energies. Recently, the first direct observations outside the heliosphere have become available at energies below a GeV thanks to the two Voyager probes which have crossed into interstellar space. We note that compared to the dimensions of the LB, these measurements are still only probing the immediate local interstellar medium (ISM).Diffuse emission, most prominently from CR nuclei producing gamma-rays, and ionisation rates in diffuse molecular clouds (MCs) remotely probe CRs elsewhere in the Galaxy. This approach gives complementary constraints, but is affected by uncertainties in the astrophysics and interstellar chemistry involved. While the inferences from the gamma-ray measurements have displayed some interesting anomalies, the ionisation rates show an even stronger deviation, with the ones inferred from MCs elsewhere in the Galaxy one to two orders of magnitude large than what is inferred from the locally measured fluxes. The discrepancy is even stronger for measurements of ionisation rates close to the galactic centre which are almost four orders of magnitude larger than the local rate!For the resolution of this oftentimes ignored anomaly, we consider mainly two scenarios: First, the fluxes of CRs at energies below a GeV fluctuate spatially within the Galaxy. This is due to the combined effect of the discrete nature of sources and the limited range due to ionisation and Coulomb losses at low energies. If the locally observed flux were at the low end of the distribution of fluxes or the observed ionisation rates were biased towards the high end of the distribution, this could explain the discrepancy. We will predict the distribution of fluxes and statistically evaluate the compatibility between local spectra and ionisation measurements.Second, the flux of local CRs could be suppressed due to the presence of the LB. On small scales, CR transport is anisotropic due to the presence of the regular magnetic field. With little observational constraints on the regular component of the LB, we will instead consider a variety of analytical and numerical models, and study the diffusion of primary and secondary CRs. Eventually, we will combine both scenarios in a consistent global picture.The synergy between the Paris partners, experts on diffuse emission and ionisation rates, the Aachen ones, experts on CR transport, and the Berlin ones, experts on hydrodynamical simulations of the ISM, will guarantee resolving one of the most puzzling anomalies in CRs.

• DFG Programme: Research Grants
• International Connection: France
• Cooperation Partners: Dr. Stefano Gabici; Dr. Andrea Goldwurm; Dr. Régis Terrier