Extending SOHO/ERNE proton measurements beyond 100 MeV

Proton measurements of the Energetic and Relativistic Nuclei and electron Experiment (ERNE) aboard the Solar and Heliospheric Observatory (SOHO) reach up to ∼50 MeV in the currently available scientific data product. However, the instrument has some channels that primarily respond to high-energy protons (hundreds of MeV) that have not yet been calibrated or released. Within the EU Horizon Europe project SPEARHEAD (SPEcification, Analysis & Re-calibration of High Energy pArticle Data), the Geant4 model of the instrument has been reconstructed by scratch, and its response functions have been recalculated.

Penetrating particles in the detector are identified by detecting a signal in the plastic scintillator anti-coincidence (AC) detector at the bottom of the detector stack. The anti-coincidence detector is read out by photodiodes, which introduce some detection inefficiency. As there is no pulse-height data available from the AC scintillator, and the detection threshold was not calibrated prior to the launch, the response of the ERNE AC counters is not well known. Without knowledge of the AC response, the physical quantities cannot be obtained from the ERNE observations. To address this gap, an in-flight calibration of the detection threshold has been attempted. We take advantage of the fact that the Electron Proton Helium INstrument (EPHIN), another detector aboard SOHO, provides reliable observations of protons in a similar energy range. With a subsequent bow-tie analysis, the effective energy (~130 MeV) and differential geometric factor (~878 cm2) of this previously unused instrument channel have been determined. Here, we provide an overview of the work done so far and outline the ongoing efforts expected to yield a new dataset of ~130 MeV proton observations over the entire SOHO mission period of 30 years.

SPEARHEAD has received funding from the European Union’s Horizon Europe programme under grant agreement No 101135044. The work reflects only the authors’ view, and the European Commission is not responsible for any use that may be made of the information it contains.