Energy spectra of near-relativistic and relativistic electrons

A new data product from SOHO EPHIN

¹ Institut für Experimentelle und Angewandte Physik, Christian-Albrechts-Universität zu Kiel, Germany
² Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany
³ OHB-Systems AG, Bremen, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 4 November 2025
Accepted: 9 February 2026

Abstract

Context. Accurate measurements of the energy spectra of solar energetic particle events are essential for understanding the processes involved in particle acceleration and transport. However, this is particularly difficult for electrons, as they tend to scatter in the instrument, leading to complex response functions.

Aims. We introduce a new electron data product for the EPHIN instrument on board SOHO. It allows the determination of electron spectra in the energy range of about 300 keV to 1 MeV.

Methods. The data product is based on the onboard histogram data of EPHIN. Using a GEANT-4 model, we derived the instrument’s energy response function for electrons. Based on this, we conducted a bow-tie analysis to determine a response factor and an effective energy for each channel, which are independent of the incoming particle spectrum. We then compared the data product with measurements from other instruments such as SolO/EPT, SolO/HET, WIND/3DP, and ACE/EPAM, as well as with a previously published EPHIN energy spectrum of Jovian electrons.

Results. The comparison of the new data product with the other instruments shows a good agreement with HET and with three sectors of EPT. However, the fluxes measured by EPT’s Sun telescope are approximately three times higher than the derived EPHIN flux, which we attribute to anisotropic features of the event. The comparison with the previously published spectrum of Jovian electrons demonstrates that the spectral break previously reported by these authors can be attributed to an incorrect analysis of the instrument data.

Conclusions. The new SOHO/EPHIN electron data product is a unique dataset that allows the extension of energy spectra from instruments such as EPT, EPAM, and 3DP up to energies of 1 MeV. It provides continuous coverage back to 1998, making it a valuable contribution to long-term studies and a significant asset to the scientific community.