Evidence for a New Component of High-Energy Solar Gamma-Ray Production

The observed multi-GeV $\gamma$-ray emission from the solar disk—sourced by hadronic cosmic rays interacting with gas and affected by complex magnetic fields—is not understood. Utilizing an improved analysis of the Fermi-LAT data that includes the first resolved imaging of the disk, we find strong evidence that this emission is produced by two separate mechanisms. Between 2010 and 2017 (the rise to and fall from solar maximum), the $\gamma$-ray emission was dominated by a polar component. Between 2008 and 2009 (solar minimum) this component remained present, but the total emission was instead dominated by a new equatorial component with a brighter flux and harder spectrum. Most strikingly, although six $\gamma$ rays above 100 GeV were observed during the 1.4 yr of solar minimum, none were observed during the next 7.8 yr. These features, along with a 30–50 GeV spectral dip which will be discussed in a companion paper, were not anticipated by theory. To understand the underlying physics, Fermi-LAT and HAWC observations of the imminent cycle 25 solar minimum are crucial.

Figure 1

(Top panel) The solar disk $\gamma$-ray spectrum during solar minimum (before January 1, 2010; blue circles) and after it (red squares). Small shifts along the $x$ axis improve readability. The gray lines show the SSG model renormalized by a factor of 6 to fit the lowest-energy data point (solid line), and the maximum $\gamma$-ray flux that could be produced by hadronic cosmic rays (dashed line). (Bottom panel) The ratio of the $\gamma$-ray flux observed during and after solar minimum. All upper and lower limits are based on $2\sigma$ Poisson fluctuations in the photon count.

Figure 2

(Top panel) The location and energy of solar $\gamma$ rays in helioprojective coordinates. Data are cut into two temporal and two energy bins. The solid disk indicates the solar circle, and the dashed circle indicates the 0.5° ROI. The average 68% containment region of $\gamma$ rays in each bin is depicted at the top left. The histogram depicts the $T_y$ positions of photons compared to the expectation from isotropic solar emission smeared by the PSF (orange line). Events $>100\mathrm{GeV}$ are marked with triangles rather than circles. We stress that the exposure after solar minimum significantly exceeds the exposure during solar minimum. Thus, the observed number of counts does not indicate the relative flux. In each bin, we report the flux from the modeled polar and equatorial components, as described in the text. (Bottom panel) The energy spectrum of polar and equatorial emission, divided into regions during (left) and after (right) solar minimum. The polar emission is approximately constant, while the equatorial emission decreases drastically after solar minimum.