A puzzling gamma-ray survey of the Sun

Gamma rays produced via thermonuclear fusion in the Sun’s core are absorbed long before they reach the visible surface, or photosphere. Nonetheless, our neighborhood star glows brightly in the gamma-ray spectrum due to interactions with cosmic rays: Speedy protons passing through the solar system smash into the photosphere and unleash cascades of particles and high-energy radiation.

Theorists have struggled to model how the incoming cosmic rays and the solar magnetic fields that steer them produce the Sun’s observed gamma-ray spectrum. New measurements from the Fermi Gamma-Ray Space Telescope’s Large Area Telescope offer the best-ever look at the Sun’s gamma emissions—and reveal that theorists still have a lot of work to do.

Analyzing data collected between August 2008 and November 2017, Tim Linden of the Ohio State University and his colleagues charted gamma-ray events by their energy and their position on the solar disk. Throughout the observing period, Fermi detected a relatively steady flux of photons, in the tens of GeV, that was concentrated at the Sun’s poles. The flux significantly exceeded that predicted by the one and only model that takes on cosmic-ray interactions with solar gas.

The picture became more complex when the researchers focused on events before 2010, when the Sun was in the minimum phase of its roughly 11-year cycle of magnetic activity. From late 2008 to late 2009, Fermi detected six gamma rays with energies exceeding 100 GeV, the only ones of such high energy it would see during the nine years. Those events and other pre-2010 events exceeding 50 GeV emanated mostly from the Sun’s equatorial region. Linden and colleagues conclude that separate cosmic-ray-triggered mechanisms are responsible for the relatively steady, lower-energy gamma rays at the poles and the equatorial, higher-energy photons that peak at solar minimum.

The new study offers theorists a rich and puzzling data set, one that also includes an unexpected dip at 30–50 GeV in what otherwise resembles a power-law spectrum. The underlying physics may become clearer once Fermi completes a full solar cycle’s worth of observations through the upcoming minimum; in fact, earlier this year, as the Sun continued its slide into quiescence, the telescope spotted its seventh event exceeding 100 GeV. The High-Altitude Water Cherenkov Observatory in Mexico and the IceCube Neutrino Observatory in Antarctica may also provide clues by capturing particles produced in air showers triggered by high-energy gamma rays. (T. Linden et al., Phys. Rev. Lett., in press.)