Unexpected Dip in the Solar Gamma-Ray Spectrum

TL;DR

This study analyzes 9 years of Fermi-LAT data revealing unexpected features in solar gamma-ray emission, including a spectral dip and cycle-dependent flux variations, challenging existing models of solar gamma-ray production.

Contribution

It provides the first detailed analysis of solar gamma-ray spectrum over nearly a full solar cycle, uncovering a spectral dip and flux anticorrelation with solar activity, which are not predicted by current theories.

Findings

Detection of gamma rays from 1 GeV to >200 GeV from the solar disk.

Observation of a spectral dip between 30-50 GeV.

Anticorrelation between gamma-ray flux and solar cycle phase.

Abstract

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 $\gtrsim200$ GeV. The overall gamma-ray spectrum is much harder ($\sim E_{\gamma}^{-2.2}$) than the cosmic-ray spectrum ($\sim E_{\rm CR}^{-2.7}$). We find a clear anticorrelation between the solar cycle phase and the gamma-ray flux between 1-10 GeV. Surprisingly, we observe a spectral dip between $\sim$30-50 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.