(V. A. Dogiel, D. O. Chernyshov, D. Malyshev, A. W. Strong, K. S. Cheng)
We analyze the processes governing cosmic-ray (CR) penetration into molecular clouds and the resulting generation of gamma-ray emission. The density of CRs inside a cloud is depleted at lower energies due to the self-excited MHD turbulence. The depletion depends on the effective gas column density of the cloud. For the Central Molecular Zone, the expected range of CR energy depletion is $Elesssim 10$~GeV, leading to the depletion of gamma-ray flux below $E_gammaapprox 2$~GeV. This effect can be important for the interpretation of the GeV gamma-ray excess in the Galactic Center, which has been revealed from the standard model of CR propagation (assuming the CR spectrum inside a cloud to be equal to the interstellar spectrum). Furthermore, recent observations of some local molecular clouds suggest the depletion of the gamma-ray emission, indicating possible self-modulation of the penetrating low-energy CRs.