The alignment of STEREO-A and Earth: A unique opportunity to investigate Solar Energetic Particle events.

A major impact on human and robotic space exploration activities is the sudden and prompt occurrence of solar energetic ion events. In 2023 and 2024,  {STEREO} is approaching the Earth from a behind position, soon passing Earth inside its orbit and thereafter moving ahead of Earth.  {STEREO} thus offers several unique opportunities during this passage. In the period from June 1 to November 1, 2023, 5 {SEP} events have been measured that cause proton fluxes of above 25~MeV to rise above 0.1 /(cm$^2$\; s\; sr\; MeV). Taking into account systematic and statistical uncertainties of the particle measurements we find a good agreement between both spacecraft.

The SOHO/EPHIN and STEREO/SEPTproject is supported under Grant 50~OC~2102 by the German Bundesministerium für Wirtschaft through the Deutsches Zentrum für Luft- und Raumfahrt (DLR). This study has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 101004159 (SERPENTINE).