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It is widely accepted that cosmic rays (CRs) up to at least PeV energies areGalactic in origin. Accelerated particles are injected into the interstellarmedium where they propagate to the farthest reaches of the Milky Way, includinga surrounding halo. The composition of CRs coming to the solar system can bemeasured directly and has been used to infer the details of CR propagation thatare extrapolated to the whole Galaxy. In contrast, indirect methods, such asobservations of gamma-ray emission from CR interactions with interstellar gas,have been employed to directly probe the CR densities in distant locationsthroughout the Galactic plane. In this article we use 73 months of data fromthe Fermi Large Area Telescope in the energy range between 300 MeV and 10 GeVto search for gamma-ray emission produced by CR interactions in several high-and intermediate-velocity clouds located at up to ~ 7 kpc above the Galacticplane. We achieve the first detection of intermediate-velocity clouds in gammarays and set upper limits on the emission from the remaining targets, therebytracing the distribution of CR nuclei in the halo for the first time. We findthat the gamma-ray emissivity per H atom decreases with increasing distancefrom the plane at 97.5% confidence level. This corroborates the notion that CRsat the relevant energies originate in the Galactic disk. The emissivity of theupper intermediate-velocity Arch hints at a 50% decline of CR densities within2 kpc from the plane. We compare our results to predictions of CR propagationmodels.

Fermi-LAT Observations of High- and Intermediate-Velocity Clouds: Tracing Cosmic Rays in the Halo of the Milky Way