Using Fermi-LAT Gamma Rays to Understand Solar Physics

The Sun is the most important astronomical source. Since time immemorial humans have observed, charted, and quantitatively measured its properties.

This proposal focuses on a new area of solar studies - solar gamma-ray emission - which can provide unique insights into both solar magnetic fields and the Sun´s photosphere.

Notably, while the photosphere is optically thick at most wavelengths, gamma rays that are produced >500 km below its surface can pass through the dense gas and be detected at Earth, allowing us to directly study signals from regions that are inaccessible at other wavelengths.Solar gamma-ray emission is bright, but its mechanism is unusual.

We focus on high-energy gamma rays that are produced when energetic cosmic rays enter the solar system and interact with the Sun.

Cosmic-ray electrons can inverse-Compton scatter (ICS) ambient sunlight, producing a diffuse halo that extends far (>20 degrees) from the Sun.

Hadronic cosmic rays can reach the Sun itself and hit photospheric gas, producing bright gamma-ray emission via hadronic interactions.The Sun plays an active, but poorly understood, role in these processes: (1) Heliospheric magnetic fields and the solar wind alter cosmic-ray morphology and spectrum.

However, current models do not fit the observed cosmic-ray flux. (2) The bright gamma-ray flux from the solar disk requires that photospheric magnetic fields efficiently turn incoming cosmic rays into outgoing cosmic rays in order to produce outgoing gamma-rays.

However, this requires the average magnetic field in the photosphere to be >100 G, while data indicate fields of ~10 G.Proposal - We will measure solar gamma rays to constrain how cosmic rays interact with solar magnetic fields. First, we will measure the ICS halo, producing a model for cosmic-ray electrons from 1 AU into the Sun.

This will tell us how cosmic rays move through the inner solar system.

Second, we will measure both the large-scale morphology, and small-scale spatio-temporal incidence, of solar disk gamma-rays, providing new insights into charge particle transport through the photosphere.Program - Over the last five years, we have performed field-leading studies of solar gamma-rays, discovering many new and unexplained phenomena.

We propose three extensions to our work, with the joint aim of deepening our understanding of solar gamma rays and connecting our results with both theoretical models and multiwavelength solar data.

WP1: ICS Halo - We will measure the ICS halo, which extends outward >20 degrees from the Sun and is produced by the upscattering of sunlight by cosmic-ray electrons transiting the solar system.

By measuring the intensity and profile of this emission, we can measure how cosmic rays move from 1 AU into the outer corona.WP2: Disk (Morphology) - We will measure the gamma-ray morphology across the solar disk. These gamma-rays are produced when incoming cosmic-ray protons are turned around by solar magnetic fields.

Thus, the gamma-ray morphology can unveil the structure of solar magnetic fields.

We will use our results to constrain theoretical models of cosmic ray transport in the solar atmosphere.WP3: Disk (Spatio-Temporal) - We will correlate the timing and location of solar disk gamma rays with photospheric magnetic field data from the MDI instrument on board SOHO, and HMI on the Solar Dynamics Observatory.

We will determine whether peculiar solar magnetic fields or extreme solar events are correlated with enhanced gamma-ray emission.Impact - Completing these WPs will produce detailed measurements of the high-energy solar system: beginning from 1 AU down to the Sun (WP1), and then across the solar disk in both the large-scale morphology (WP2) and fine-scale spatio-temporal variation (WP3).

The results of this study are complementary to existing solar data.

Connecting gamma-ray observations to multi-wavelength data will significantly strengthen our solar observations and produce richer models of our Sun.