The accuracy of REleASE forecasts for different heliographic longitude distances

Forecast systems and predictions of hazardous Solar Energetic Particle (SEP) events are needed to support space missions, as they can have a major impact on technology and humans. Especially in view of future plans for human exploration of Mars, a radiation protection strategy needs to be implemented with the goal of reliably providing advance warning of sudden radiation hazards.
The Relativistic Electron Alert System for Exploration (REleASE; Posner, 2007) utilises the close correlation between near-relativistic electrons and the slower, but more hazardous, protons to provide, on average, one hour of advance warning of particle events at the location where the measurements are taken. Originally, REleASE used real-time data from SOHO/EPHIN and later ACE/EPAM (HESPERIA/REleASE) to issue short-term warnings before a significant flux increase of ~20-50 MeV protons near Earth at the L1 point. More recently, the method was adapted to work with the HET and SEPT instruments onboard STEREO-A, and an operational STEREO/REleASE system was created.
With two REleASE systems now operational, we have the possibility to directly compare forecasts from different points in the heliosphere. Human explorers following Hohmann trajectories to and from Mars will be up to 22° away in longitudinal magnetic connection distance from the alert system. We used the 2022-2025 passage of STEREO-A by Earth to test whether remote REleASE forecasts can provide timely and sufficiently accurate information for the location of another spacecraft.