Solar energetic particle spectra from the SOHO-EPHIN sensor by application of regularization methods

Context. The Electron Proton Helium Instrument (EPHIN) on ESA's Solar and Heliospheric Observatory (SOHO) measures solar energetic electrons, protons, and alpha particles with a stack of six solid-state detectors forming a telescope. The energy deposit in these detectors must be inverted to derive the original energy of the incident particles, thus leading to the original energy spectrum of solar energetic particles. Normal inversion techniques, such as least-squares methods, rely on fitting a known functional behavior of the spectral dependence (normally a power law) to the measured data with some account taken for the instrument response. Such procedures can fail to retrieve accurate particle spectra, e.g., when count rates are low and unphysical negative counts result from the fitting procedure.