Title: First Observations of Solar Halo Gamma Rays Over a Full Solar Cycle
Authors: Tim Linden, Jung-Tsung Li, Bei Zhou, Isabelle John, Milena Crnogorčević, Annika H. G. Peter, John F. Beacom
First Author’s Institution: Stockholm University and The Oskar Klein Centre for Cosmoparticle Physics, Alba Nova, 10691 Stockholm, Sweden
Status: Available on Arxiv
The Sun—our familiar daytime companion—shines brightly in visible light, but that’s just a small part of its story. In fact, it radiates across the entire electromagnetic spectrum, from low-energy radio waves to high-energy γ-rays. While the Sun doesn’t emit much γ-ray radiation on its own, it plays an active role in producing γ-rays through interactions with galactic cosmic rays (CRs). When CR protons collide with helium nuclei in the Sun’s dense photosphere, they trigger hadronic reactions that generate a bright gamma-ray signal known as the “solar disk.” Meanwhile, CR electrons traveling through the heliosphere can boost sunlight to gamma-ray energies via inverse-Compton scattering, producing a faint, extended glow called the “solar halo”—a diffuse structure that stretches well beyond the solar disk.
Figure 1. Relative scale of the photosphere and the heliosphere. The photosphere is the visible surface layer of the Sun where sunlight is emitted, while the heliosphere is a vast, bubble-like region of space formed by the solar wind that surrounds and protects the entire solar system from interstellar space. Credit: NASA’s Goddard Space Flight Center
Figure 2. The solar disk and solar halo model used in this study. Credit: The middle two panels in Fig. 2 from the original paper.
Observations of the γ-ray solar halo offer valuable insights into the structure of the heliospheric magnetic field because other key ingredients in the model—such as the galactic CR flux and the solar photon field—are relatively well understood. At high energies, CR electrons are nearly homogeneous and isotropic throughout the heliosphere, leading to a γ-ray signal from IC scattering that falls off with the square of the distance from the Sun. On the contrary, at lower energies, the heliospheric magnetic field deflects and scatters incoming CR electrons, effectively reducing their density near the Sun—a phenomenon known as solar modulation. This modulation suppresses the IC γ-ray flux, especially in the inner heliosphere, making the solar halo a sensitive probe of how CR electrons propagate through the Sun’s magnetic environment.
In today’s paper, the authors analyzed an impressive 15 years of data from the Fermi-LAT telescope to isolate the elusive γ-ray glow of the solar halo. Thanks to this long-term dataset and a carefully constructed model, they detected the solar halo at energies ranging from 31.6 MeV to 100 GeV, extending as far as 45 degrees from the Sun—three times wider than previously observed. For comparison, earlier studies using just 1.5 years of Fermi-LAT data could only detect the halo between 0.1 and 10 GeV, with a maximum angular reach of about 15 degrees.To extract the signal, the team fit the data using a three-component model. This included contributions from known astrophysical backgrounds—like γ-rays from Jupiter and the Moon—a simplified solar disk modeled as a uniformly bright circle with a radius of 0.26 degrees, and a detailed model of the solar halo. In their halo model, they assumed an isotropic distribution of CR electrons, treated solar photons as blackbody radiation, and accounted for the effects of solar modulation, which suppresses low-energy CR electrons near the Sun.
Fig. 3 presents both the energy spectrum and the spatial energy distribution of the solar halo, zooming in on the region between 5 and 10 degrees from the Sun and focusing on γ-rays in the 1–3 GeV range, respectively. At these distances and energies, the solar disk and halo are cleanly separated, making this a sweet spot for detecting the halo signal with minimal contamination. The solid lines represent theoretical predictions based on locally measured CR electron fluxes, with different colors corresponding to different values of the solar modulation potential—a parameter that captures how much energy CR particles lose as they navigate the Sun’s magnetic environment. As expected, stronger modulation (i.e., higher potentials) leads to fewer CR electrons near the Sun, and thus a dimmer γ-ray halo.
Importantly, these models weren’t fit to the data—they were independently computed—yet they align remarkably well with the Fermi-LAT observations. The best match occurs with a solar modulation potential of around 500 MV, which agrees closely with values inferred from CR measurements near Earth, lending strong support to the physical accuracy of the halo model.
Figure 3. Energy spectrum of γ-rays emitted from the region 5–10 degrees away from the Sun, along with the spatial distribution of γ-ray emission in the 1–3 GeV energy range. The solid lines represent theoretical predictions, with different colors corresponding to different values of the solar modulation potential evaluated at 1 AU. These models are not fitted to the data but are calculated independently, showing excellent agreement with the Fermi-LAT observations. Credit: Fig. 1 from the original paper.
Thanks to over a decade of observations, the authors were able to track the yearly evolution of the solar modulation potential. Derived from γ-ray data, the modulation potential serves as a proxy for how strongly the heliospheric magnetic field suppresses incoming cosmic rays. Fig. 4 shows the time evolution of the modulation potential calculated from the solar γ-ray emission, compared with values inferred from CR electron flux measurements by PAMELA and AMS-02. The close match between the two datasets confirms that the solar halo is produced by the same population of CR electrons observed near Earth, and highlights the power of γ-ray observations as a tool to probe heliospheric conditions.
Figure 4. Time evolution of the solar modulation potential for electrons at 1 AU. The black and gray dots represent values derived from different solar modulation models based on γ-ray observations, while the solid lines show modulation potentials inferred from direct CR electron measurements near Earth. The potential peaks around 2014—coinciding with the maximum of the solar cycle—and drops during solar minimum. The strong agreement between the two indicates that the CR electron population responsible for the solar halo is the same as the one detected locally. Credit: Fig. 9 from the original paper.
This study showcases how the Sun, often viewed simply as a source of light and heat, also plays a crucial role in shaping the high-energy γ-ray sky through its interaction with CR. By analyzing 15 years of Fermi-LAT data, the authors not only achieved the most detailed detection of the solar γ-ray halo to date but also demonstrated how this halo can be used as a powerful diagnostic tool for studying cosmic ray propagation and the Sun’s magnetic influence. The close match between the observed γ-ray emission and theoretical models—along with its consistency with direct cosmic ray measurements near Earth—confirms that the same CR electrons shaping the solar halo are passing through our own neighborhood in the heliosphere. In doing so, this work bridges solar physics, cosmic ray astrophysics, and γ-ray astronomy, revealing the Sun’s subtle yet significant impact on the high-energy universe.
Astrobite edited by Kaz Gary
Featured image credit: NASA’s Goddard Space Flight Center
