Challenging the Models: New Fermi-LAT Insights into Solar Gamma Rays and Cosmic Rays

A recent study utilized **15 years of observations** from the **Fermi Large Area Telescope (LAT)** to analyze the gamma-ray emission from the Sun in its quiet state, meaning when it's not flaring. This is the first study to separately analyze the flux variation of the two distinct components of this quiet-state gamma-ray emission over solar cycles.

According to theoretical understanding, the Sun's steady-state gamma-ray emission arises from interactions with Galactic cosmic rays (CRs). There are two main components:

* The hadronic component, which is primarily confined to the **solar disk**. It's thought to be produced by CR cascades in the solar atmosphere. This component's flux is expected to **anticorrelate with solar activity** (like sunspot number, SSN) and **correlate with the flux of cosmic rays**.

* The **leptonic component**, which is spatially **extended** beyond the solar disk. This is theorized to be an Inverse Compton (IC) component, where CR electrons scatter off solar photons. Like the disk component, its intensity was expected to **vary with the solar cycle**, being highest during solar minimum and lowest during solar maximum, thus anticorrelating with SSN and correlating with CR flux (specifically CR electron flux).

Previous Fermi-LAT observations had shown that the overall solar gamma-ray flux varies with solar activity, anticorrelating with SSN and changing by nearly a factor of two between solar maximum and minimum. However, these studies had not separated the contributions of the disk and extended components.

This new work analyzed Fermi-LAT data from August 2008 to June 2023, carefully selecting data and using an "off-source" method to evaluate background contamination. They were able to distinguish the two components and study their flux variations over Solar Cycle 24 and the beginning of Cycle 25.

The key findings from this analysis reveal both confirmation of expectations and **significant surprises**:

* For the **disk component**, the results align well with theoretical predictions. Its flux variation:

* **Anticorrelates strongly with the sunspot number (SSN)**.

* **Correlates strongly with the flux of cosmic-ray protons** measured near Earth.

* Correlates with the gamma-ray flux from the Moon, supporting similar production mechanisms.

* The variation is **independent of energy** above 250 MeV.

This confirms that the hadronic emission mechanism for the disk component has been correctly identified.

* For the **spatially extended component**, the behavior was **unexpectedly complex**.

* It showed the expected anticorrelation with SSN and correlation with the disk component **only until approximately mid-2012**.

* **After 2013, there was no longer any significant correlation or anticorrelation observed** between the extended component's flux variation and either the SSN or the cosmic-ray electron flux. Correlation coefficients over the entire period are below 0.3.

* Like the disk component, the extended component's variation was also found to be independent of energy above 250 MeV.

This **surprising lack of correlation for the extended component after 2013** is a major finding. The change in behavior coincides with the start of the **reversal of the Sun's polar magnetic field**, which began at the end of 2012. This suggests that the transport and modulation of cosmic rays, particularly electrons, in the **inner heliosphere (close to the Sun)** may be **unexpectedly complex** and possibly different for electrons and protons.

Paper: https://arxiv.org/abs/2505.06348

Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: Solar Dynamics Observatory/GSFC/NASA