[1] The low-altitude, polar-orbiting NOAA POES satellites are known to exhibit a spurious response to relativistic electrons in the proton telescope instrument of the Medium Energy Proton and Electron Detector (MEPED). This sensitivity has recently been used to identify relativistic electron precipitation from the radiation belts, but has heretofore remained unquantified. Monte Carlo simulations of the proton and electron telescopes were performed using the Geant4 code, and we derive the characteristic response of each instrument to isotropic proton and electron fluxes in the energy range 10 keV to 10 MeV. Geometric factors were found to agree with the nominal G ∼ 0.01 cm 2 sr response for target species (e.g. proton response of the proton telescope), while both telescope types also admit particles of the opposite species. The electron telescope is shown to respond to protons of energies 210-2600 keV with G ∼ 0.01 cm 2 sr. The proton telescope is similarly shown to exhibit a non-negligible response to electrons, which although confined to the P1, P2, P3, and P6 energy channels, does attain an admittance of G ∼ 10 −3 cm 2 sr near 460 keV and G ∼ 10 −2 cm 2 sr near 1400 keV. These results confirm that the proton telescope can be used as an effective tool for detection of relativistic electron precipitation. Moreover, we suggest a method for combining the electron and proton telescope data to obtain corrected fluxes for both species which will facilitate the use of this instrument for quantitative studies of particle precipitation.
We present observations from the NOAA‐15 MEPED telescopes during a radiation belt depletion event on January 19–20, 2000 to investigate the spatial extent of electron precipitation during this interval. Precipitation mapped to the equatorial plane was confined to radial distances less than ∼6.5 Earth radii, indicating that precipitation was not the direct cause of the decrease in trapped flux observed by GOES. We found an enhanced day–night magnetic field asymmetry during the event, suggesting that magnetopause losses may have been responsible. Precipitation at lower L‐values was observed by POES on the dusk passes (18:30–21:00 MLT), but not on the dawn passes, and was observed in conjugate hemispheres. These observations suggest that both precipitation and magnetopause losses were acting during this flux depletion event.
Angular response functions are derived for four electron channels and six proton channels of the SEM-2 MEPED particle telescopes on the POES and MetOp satellites from Geant4 simulations previously used to derive the energy response. They are combined with model electron distributions in energy and pitch angle to show that the vertical 0 • telescope, intended to measure precipitating electrons, instead usually measures trapped or quasi-trapped electrons, except during times of enhanced pitch angle diffusion. A simplified dynamical model of the radiation belt 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 is fit to sample MEPED electron data at L = 4 during times of differing diffusion rates. It is then used to compute precipitating electron flux, as function of energy and longitude, that is lower than would be estimated by assuming that the 0 • telescope always measures precipitating electrons.
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