Recalibration of the MEPED Proton Detectors Onboard NOAA POES Satellites
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In this thesis we have calibrated energetic proton data from the solid state detectors of the MEPED instrument onboard the satellites NOAA 15, 16, 17, 18, and MetOp 02. All these satellites are part of the NOAA/POES program, they fly in polar orbits in 800-850 km altitude, and they carry a suite of almost identical instrumentation. For over 30 years the NOAA/POES satellites have collected valuable information about the particle environment in the Earth's magnetosphere and ionosphere. However, with time these solid state detectors suffer from radiation damage due to energetic particles, which leads to increasing energy thresholds and increasing underestimation of particle fluxes. Data from different satellites must therefore be inter-calibrated in order to be used for quantitative studies. In this thesis we have developed two statistical methods for calibration of the energetic proton data from MEPED instrument onboard the NOAA satellites. The first method is used for calibration when two spacecraft are in the same magnetic local time sector. This method is applied to data from NOAA 16 in 2005, NOAA 17 in 2007, and NOAA 18 in 2009, when these satellites were fortunate to share the orbit with a newly launched satellite. We compare energy spectra based on daily averaged fluxes and estimate the increased energy thresholds of the MEPED instrument in these years. The second method is used to calibrate satellite data at different magnetic local times. Average maps of the flux of energetic protons as a function of magnetic local time and invariant latitude were constructed for different levels of magnetic activity given by the Kp index. By determining the local time dependence of the fluxes in the isotropic zone, these maps were used to calibrate old satellites that were separated in magnetic local time relative to new satellites. This method allows for a better estimation of the temporal evolution of the energy thresholds of the MEPED instruments.