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Cosmic rays up to at least PeV energies are usually described in theframework of an elementary scenario that involves acceleration by objects thatare located in the disk of the Milky Way, such as supernova remnants or massivestar-forming regions, and then diffusive propagation throughout the Galaxy.Details of the propagation process are so far inferred mainly from thecomposition of cosmic rays measured near the Earth and then extrapolated to thewhole Galaxy. The details of the propagation in the Galactic halo and theescape into the intergalactic medium remain uncertain. The densities of cosmicrays in specific locations can be traced via the gamma rays they produce ininelastic collisions with clouds of interstellar gas. Therefore, we analyze 73months of Fermi-LAT data from 300 MeV to 10 GeV in the direction of severalhigh- and intermediate-velocity clouds that are located in the halo of theMilky Way. These clouds are supposed to be free of internal sources of cosmicrays and hence any gamma-ray emission from them samples the large-scaledistribution of Galactic cosmic rays. We evaluate for the first time thegamma-ray emissivity per hydrogen atom up to ~7 kpc above the Galactic disk.The emissivity is found to decrease with distance from the disk, which providesdirect evidence that cosmic rays at the relevant energies originate therein.Furthermore, the emissivity of one of the targets, the upperintermediate-velocity Arch, hints at a 50% decline of the cosmic-ray intensitywithin 2 kpc from the disk.

Tracing the propagation of cosmic rays in the Milky Way halo with Fermi-LAT observations of high- and intermediate-velocity clouds