The solar disk is a bright source of multi-GeV gamma rays, due to the interactions of hadronic cosmic rays with the solar atmosphere. However, the underlying production mechanism is not understood, except that its efficiency must be greatly enhanced by magnetic fields that redirect some cosmic rays from ingoing to outgoing before they interact. To elucidate the nature of this emission, we perform a new analysis of solar atmospheric gamma rays with 9 years of Fermi-LAT data, which spans nearly the full 11-year solar cycle. We detect significant gamma-ray emission from the solar disk from 1 GeV up to $\gtrsim 200\mathrm{GeV}$. The overall gamma-ray spectrum is much harder ($\sim E_{\gamma }^{-2.2}$) than the cosmic-ray spectrum ($\sim E_{\mathrm{CR}}^{-2.7}$). We find a clear anticorrelation between the solar cycle phase and the gamma-ray flux between 1 and 10 GeV. Surprisingly, we observe a spectral dip between $\sim 30$ and 50 GeV in an otherwise power-law spectrum. This was not predicted, is not understood, and may provide crucial clues to the gamma-ray emission mechanism. The flux above 100 GeV, which is brightest during the solar minimum, poses exciting opportunities for HAWC, LHAASO, IceCube, and KM3NeT.

• Received 9 May 2018

DOI:https://doi.org/10.1103/PhysRevD.98.063019