Steady GeV-TeV solar gamma-ray emission reveal surprises about cosmic-ray transport in the heliosphere

The Sun is a mystery source of GeV-TeV gamma-rays. It is likely that the Sun does not shine at these energies on its own power, but because Galactic cosmic rays (CR) interact with its matter and light. By observing and modeling these interactions in and near the Sun, one may learn about CR propagation in the inner heliosphere and in the solar atmosphere. In this talk, we present observations and a set of ongoing calculations that sharpen our understanding of how the Sun interacts with CR. From data taken with Fermi-LAT and ground-based air shower arrays, we show that the Sun is a luminous source of gamma rays up to at least 300 GeV, with time and spatial variability that anti-correlates with the solar activity cycle. We will show how confounding these results are for any plausible model of CR-driven emission. We argue that future gamma-ray (in particular, HAWC), neutrino, and CR facilities can complement existing observations, and indicate a path forward to understanding CR propagation from the heliopause to the photosphere of the Sun.