Cross calibration of SOHO ~1 GV proton and helium fluxes with PAMELA and AMS-02

Reliable measurements of Galactic Cosmic Rays (GCRs) and Solar Energetic Particles (SEPs) require
well-calibrated spaceborne particle instruments. This study presents a cross-calibration of the
Electron, Proton and Helium Instrument (EPHIN) aboard SOHO with high-precision proton and helium
measurements from PAMELA and AMS-02. The analysis extends the established $\Delta E$--$\Delta E$
technique to later mission phases, accounting for detector aging and changes in instrument response
after 2017.

A GEANT4-based model of EPHIN, including a simplified representation of the SOHO spacecraft, is used
to derive energy response functions for penetrating protons and helium nuclei. Simulated detector
responses based on force-field–modulated GCR spectra reproduce the observed EPHIN energy-loss
distributions within about 30\%. Effective energies and fluxes are obtained using a bow-tie inversion
method and compared with AMS-02 and PAMELA observations during quiet solar conditions. The results
show agreement within the combined systematic uncertainties, demonstrating that SOHO/EPHIN
continues to provide valuable and reliable energetic particle measurements for long-term
heliospheric studies.

The EPHIN is supported under Grant 50~OC~2302 by the German Bundesministerium für Wirtschaft through the Deutsches Zentrum für Luft- und Raumfahrt (DLR). We acknowledge partial support from  the Horizon Europe Program project SPEARHEAD (GA 101135044).