Data from the Voyager probes have provided us with the first measurement of cosmic ray intensities at MeV energies, an energy range that had previously not been explored. Simple extrapolations of models that fit data at GeV energies, e.g., from AMS-02, however, fail to reproduce the Voyager data in that the predicted intensities are too high. Oftentimes, this discrepancy is addressed by adding a break to the source spectrum or the diffusion coefficient in an ad hoc fashion, with a convincing physical explanation yet to be provided. Here, we argue that the discrete nature of cosmic ray sources, which is usually ignored, is instead a more likely explanation. We model the distribution of intensities expected from a statistical model of discrete sources and show that its expectation value is not representative but has a spectral shape different from that for a typical configuration of sources. The Voyager proton and electron data are however compatible with the median of the intensity distribution. 

The 3D distribution of gas in our Galaxy is an important ingredient for understanding the Galactic distribution of cosmic rays. Atomic hydrogen (HI), for example, could be studied with sky maps of 21-cm emission line resulting from the spin-flip transition of hydrogen atoms and, thanks to Galactic rotation, the line is Doppler shifted. If we assume a model for the velocity field, data from gas line surveys can be used to build models for the 3D distribution of HI. However, given our vantage point in the Galaxy, such a model suffers from a number of ambiguities. For instance, the peculiar motion of gas, that is random motions of gas on top of the large-scale gas flow due to stellar winds or supernova explosions, normally results in artificial structures stretching along many lines of sight in the 3D gas map (also referred to as the finger-of-god effect). In one of my recent works, my collaborator and I suggest that this effect could be cured if we take into account the spatial correlations of gas which ensures in a probabilistic sense that the gas density could not vary dramatically between two adjacent points in space. The reconstructed map shows structures on a variety of scales and, in particular, the gas on large scales seems to follow the spiral arms (fitted independently with data of molecular masers associated with very young high-mass stars). The 3D rendering of the gas map could be found in the attached  movie. 

The standard picture of cosmic-ray transport suggests that these particles execute random walks due to scattering on magnetic field turbulence which results in the highly isotropic cosmic ray arrival directions. High-statistics observatories like IceCube and HAWC have however observed significant deviations from isotropy down to very small angular scales. While large scale multipoles arise naturally, for example due to the earth's motion relative to the isotropic cosmic ray distribution, there is no intuitive mechanism to account for the observed anisotropies at smaller angular scales. This work provides one of the first analytical calculation of the angular power spectrum assuming a physically motivated model of the magnetic turbulence and this model seems to be in good agreement with numerical simulations.

Cosmic rays are believed to play an essential role in determining the chemistry and the evolution of molecular clouds. This is because they are usually considered to be the main ionization agent of these star-forming regions. We have recently studied such a hypothesis from a theoretical point of view for the case of diffuse molecular clouds using the one-dimensional transport equation under the assumption that the cosmic-ray spectra measured locally are representative of the Galactic ones. Interestingly, it is found that cosmic ray density inside the cloud is significantly reduced and, thus, the predicted ionization rate is around 10 to 100 times smaller than the observational data. Potential explanations of this finding are the fluctuations in the Galactic cosmic-ray spectra and additional sources of low-energy cosmic rays.