Solar flare gamma rays do vary with the 11-year solar cycle, but the relationship is complex and influenced by multiple factors related to solar activity and particle acceleration processes.
The Sun undergoes an approximately 11-year cycle known as the solar cycle, during which its magnetic activity waxes and wanes. This cycle is marked by changes in the number and intensity of sunspots, solar flares, and coronal mass ejections. Solar flares are sudden, intense bursts of radiation caused by the release of magnetic energy stored in the Sun’s atmosphere. Among the emissions from solar flares are gamma rays, which are the highest-energy form of electromagnetic radiation.
Gamma rays from solar flares are produced primarily when accelerated protons and ions collide with the solar atmosphere, creating secondary particles like pions that decay into gamma rays. This process differs from the lower-energy emissions dominated by electrons. Observations from instruments such as the Fermi Large Area Telescope (Fermi-LAT) have shown that solar flares can emit gamma rays in the range from tens of MeV (million electron volts) up to several GeV (billion electron volts).
During the solar maximum phase of the cycle, when sunspot numbers and solar magnetic activity peak, the frequency and intensity of solar flares increase. This leads to more frequent and often more energetic gamma-ray flares. For example, data collected during Solar Cycle 24 (roughly 2010 to 2018) revealed a significant number of gamma-ray flares detected by Fermi-LAT, including some with emissions extending beyond the visible solar disk. These flares showed spectral characteristics consistent with proton and ion acceleration, indicating that the processes generating gamma rays are closely tied to the heightened magnetic activity of the solar maximum.
However, the variation of gamma-ray emission with the solar cycle is not simply a matter of more flares producing more gamma rays. The intensity and spectral shape of gamma-ray flares depend on the details of particle acceleration mechanisms, the magnetic field configuration, and the location of the flare on the Sun. Some gamma-ray flares have been observed even during quieter phases of the solar cycle, although they tend to be less frequent and less intense.
Moreover, the detection of gamma rays from solar flares beyond the visible edge of the Sun suggests that accelerated particles can travel along magnetic field lines to regions not directly facing Earth, complicating the interpretation of gamma-ray variability solely based on solar cycle phase.
In summary, solar flare gamma rays do show variation correlated with the 11-year solar cycle, with more and stronger gamma-ray flares occurring near solar maximum due to increased magnetic activity and particle acceleration. Yet, the relationship is nuanced, influenced by the complex interplay of solar magnetic fields, flare locations, and acceleration processes that govern the production and propagation of gamma rays from the Sun.
