Quantifying energetic electron precipitation based on POES/MEPED angular response functions

Accurate quantification of energetic electron precipitation is critical for assessing radiation belt loss and its effects on magnetosphere-ionosphere coupling and atmospheric chemistry. The electron fluxes measured by multiple NOAA/POES and MetOp satellites near and inside the loss cone have been extensively used in observational and modeling studies of energetic particle precipitation. However, nominal field-of-view angular response of the SEM-2 MEPED particle telescopes on the POES and MetOp satellites was most commonly used. In this work, we first derive more accurate angular response functions of the electron and proton channels of the MEPED telescopes based on Geant4 simulations previously used to derive the energy response. These newly derived angular response functions are then combined with model electron distributions to show that the 0-degree MEPED telescope, intended to measure precipitating electrons, instead usually measures trapped or quasi-trapped electrons, except during times of fast pitch angle diffusion. A drift-diffusion model for energetic electron distribution near the loss cone as a function of longitude, energy, and pitch angle, that accounts for pitch angle diffusion, azimuthal drift, and atmospheric backscatter of electrons, is used to fit sample electron data from MEPED at L = 4 during times with different levels of pitch diffusion rates. The model is also used to compute precipitating electron flux as function of energy and longitude, which is shown to be lower than that would be estimated by assuming that the 0-degree telescope always measures precipitating electrons.