  The solar disk is a bright gamma-ray source. Surprisingly, its flux is about
one order of magnitude higher than predicted. As a first step toward
understanding the physical origin of this discrepancy, we perform a new
analysis in 1-100 GeV using 6 years of public Fermi-LAT data. Compared to the
previous analysis by the Fermi Collaboration, who analyzed 1.5 years of data
and detected the solar disk in 0.1-10 GeV, we find two new and significant
results: 1. In the 1-10 GeV flux (detected at $>5\sigma$), we discover a
significant time variation that anticorrelates with solar activity. 2. We
detect gamma rays in 10-30 GeV at $>5\sigma$, and in 30-100 GeV at $> 2\sigma$.
The time variation strongly indicates that solar-disk gamma rays are induced by
cosmic rays and that solar atmospheric magnetic fields play an important role.
Our results provide essential clues for understanding the underlying gamma-ray
production processes, which may allow new probes of solar atmospheric magnetic
fields, cosmic rays in the solar system, and possible new physics. Finally, we
show that the Sun is a promising new target for ground-based TeV gamma-ray
telescopes such as HAWC and LHAASO.
