Convection-Driven Multi-Scale Magnetic Fields Determine the Observed Solar-Disk Gamma Rays

TL;DR

This paper presents a new model explaining solar-disk gamma-ray emissions driven by multi-scale magnetic fields and turbulence, aligning well with observations and offering a novel way to study cosmic ray transport in the solar atmosphere.

Contribution

It introduces a theoretical framework linking solar magnetic field structures and turbulence to gamma-ray spectra, improving understanding of cosmic ray interactions in the Sun.

Findings

Model's spectrum slope matches Fermi-LAT and HAWC data from 1 GeV to 1 TeV.

Predicted flux is 2-5 times lower than observed, indicating areas for future research.

Large-scale filamentary fields and Alfvén wave turbulence are key to gamma-ray emission shaping.

Abstract

The solar disk is a continuous source of GeV--TeV gamma rays. The emission is thought to originate from hadronic Galactic cosmic rays (GCRs) interacting with the gas in the photosphere and uppermost convection zone after being reflected by solar magnetic fields. Despite this general understanding, existing theoretical models have yet to match observational data. At the photosphere and the uppermost convection zone, granular convection drives a multi-scale magnetic field, forming a larger-scale filamentary structure while also generating turbulence-scale Alfv\'{e}n wave turbulence. Here, we demonstrate that the larger-scale filamentary field shapes the overall gamma-ray emission spectrum, and the Alfv\'{e}n wave turbulence is critical for further suppressing the gamma-ray emission spectrum below $\sim 100$~GeV. For a standard Alfv\'{e}n wave turbulence level, our model's predicted spectrum slope from 1~GeV to 1~TeV is in excellent agreement with observations from Fermi-LAT and HAWC, an important achievement. The predicted absolute flux is a factor of 2--5 lower than the observed data; we outline future directions to resolve this discrepancy. The key contribution of our work is providing a new theoretical framework for using solar disk gamma-ray observations to probe hadronic GCR transport in the lower solar atmosphere.