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Cosmic Rays (CRs) propagating through dense molecular clouds (MCs) producegamma rays which carry direct information about the CR distribution throughoutthe Galaxy. Observations of gamma rays in different energy bands allowexploration of the average CR density in the Galactic Disk, the so-called levelof the "CR Sea". Fermi-LAT observations have demonstrated the method'sfeasibility based on two dozen MCs in our Galaxy. However, the potential ofFermi-LAT is limited by the most massive and relatively nearby MCs; thus, thecurrent observations cover only a tiny fraction of the Milky Way. In thispaper, we study the prospects of expanding the CR measurements to very andultra-high energies and remote parts of the Galaxy with the current andnext-generation detectors. Based on calculations of fluxes expected from MCs,we formulate the requirements to the sensitivity of the post-Fermi-LATdetectors to map GeV-TeV CRs in the Galactic Disk. We also explore thepotential of the current and future air-shower and atmospheric Cherenkovtelescope arrays for the extension of CR studies to multi-TeV and PeV energybands. We demonstrate that the improvement of the Fermi-LAT sensitivity by afactor of a few would allow a dramatic increase in the number of detectable MCscovering almost the entire Galaxy. The recently completed LHAASO should be ableto take the first CR probes at PeV energies in the coming five years or so.

Probing the galactic cosmic-ray density with current and future $\gamma$-ray instruments