Transport of high energy protons from a backside solar eruption to produce gamma-ray emission on the front side of the Sun

A sustained gamma-ray emission (SGRE) event from the Sun was observed on 2024 September 9 by the Large Area Telescope (LAT) on Fermi satellite at energies >100 MeV. SGRE requires the precipitation of >300 MeV protons deep into the photosphere. The SGRE event was associated with a shock-driving coronal mass ejection (CME) that originated ~40 degrees behind the east limb of the Sun. The event was observed by multiple spacecraft such as the Solar and Heliospheric Observatory (SOHO), Solar Terrestrial Relations Observatory (STEREO), Parker Solar Probe (PSP),  Solar Orbiter (SO), Solar Dynamics Observatory (SDO), Wind, and GOES, and by ground-based radio telescopes. Based on observations from SO’s Spectrometer Telescope for Imaging X-rays (STIX), we estimate that the eruption location to be S17E129. GOES observed a large solar energetic particle (SEP) event but only in the >10 MeV energy channel because of poor magnetic connectivity. However,  SO was well-connected to the eruption region and hence observed high-energy particles.  We infer that >300 MeV particles from the extended shock precipitated on the frontside of the Sun to produce the SGRE event. Forward modeling of the CME using SOHO and STEREO observations indicate that the CME flux rope had  high initial acceleration of the CME (`2.5 km s^-2), high speed (2500 km s^-1), and associated with type II bursts in the metric to decameter-hectometric (DH) wavelengths. All these properties are characteristic of frontside CMEs that are associated with SGRE events.  Furthermore, the durations of SGRE and type II burst are similar as in longer duration (>3 hours) SGRE events.