  Cosmic rays up to at least PeV energies are usually described in the
framework of an elementary scenario that involves acceleration by objects that
are located in the disk of the Milky Way, such as supernova remnants or massive
star-forming regions, and then diffusive propagation throughout the Galaxy.
Details of the propagation process are so far inferred mainly from the
composition of cosmic rays measured near the Earth and then extrapolated to the
whole Galaxy. The details of the propagation in the Galactic halo and the
escape into the intergalactic medium remain uncertain. The densities of cosmic
rays in specific locations can be traced via the gamma rays they produce in
inelastic collisions with clouds of interstellar gas. Therefore, we analyze 73
months of Fermi-LAT data from 300 MeV to 10 GeV in the direction of several
high- and intermediate-velocity clouds that are located in the halo of the
Milky Way. These clouds are supposed to be free of internal sources of cosmic
rays and hence any gamma-ray emission from them samples the large-scale
distribution of Galactic cosmic rays. We evaluate for the first time the
gamma-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 provides
direct evidence that cosmic rays at the relevant energies originate therein.
Furthermore, the emissivity of one of the targets, the upper
intermediate-velocity Arch, hints at a 50% decline of the cosmic-ray intensity
within 2 kpc from the disk.
