Measurements of the low-energy spectrum of Galactic cosmic rays (GCRs) by detectors on or near Earth are affected by solar modulation. To overcome this difficulty, we consider nearby molecular clouds as GCR detectors outside the Solar System. Using $\gamma$-ray observations of the clouds by the Fermi telescope, we derive the spectrum of GCRs in the clouds from the observed $\gamma$-ray emission spectrum. We find that the GCR spectrum has a low-energy break with the spectral slope hardening by $\Delta \Gamma =1.1\pm 0.3$ at an energy of $E=9\pm 3\mathrm{GeV}$. Detection of a low-energy break enables a measurement of GCR energy density in the interstellar space $U=0.9\pm 0.3\mathrm{eV}/{\mathrm{cm}}^3$.

• Received 10 September 2011

DOI:https://doi.org/10.1103/PhysRevLett.108.051105