Correction of detector noise and recalibration of NOAA/MEPED energetic proton fluxes

We report here on new problems in the NOAA/MEPED instruments and the related energetic proton dataset. These problems are solved, and the implied modifications to the earlier recalibration of the dataset are evaluated and adopted. We show that, besides degrading due to radiation damage, the NOAA‐12 and NOAA‐08 satellites suffer from increased electronic noise in the back detector of the proton instrument. We correct the effects of the noise, and present improved estimates for the effective energy thresholds of the MEPED proton detectors which are now determined more robustly than previously. We show that the cumulative Ap index can be used to produce a refined estimate for the temporal evolution of the effective MEPED energy thresholds. The derived energy thresholds of all MEPED instruments increase systematically with the cumulative particle fluxes, and this increase is similar in all satellites. Using the improved energy thresholds we obtain a uniform series of corrected MEPED energetic proton fluxes above 120 keV from 1979 onwards. We find that, due to the effect of the radiation damage and noise, the uncorrected fluxes at these energies were underestimated in the worst case by more than an order of magnitude, and that the earlier correction method also occasionally led to underestimation of the fluxes by nearly an order of magnitude. Such underestimation becomes severe already 1–2 years after the launch of the satellite.