Instrument response function simulation for CLUSTER RAPID mission via GEANT4

Analysis of satellite observations requires a clear understanding of instrument response functions (IRF) of different instruments. Testing and calibration on the ground can provide only preliminary information about the IRF for different particles. While laboratory facilities are strongly limited and could not provide all information about instrument response and its behavior in the space environment, the GEANT4 tool allows one to simulate the instrument behavior in a user-defined radiation environment. Different radiation environments can be modeled by setting particle types and energy, fluxes (intensities) and incident directions. This tool can be applied to get information about past or existing missions as well as to design a new one. In our work we first report on a simulation of the IRF of the Imaging Electron Spectrometer (IES) of RAPID (Research with Adaptive Particle Imaging Detectors) instrument on board of CLUSTER mission satellites. The IES allows one to detect electrons in the energy range of 20 keV to 400 keV with in-plane angular resolution. We report the IRF of this device, estimations of the artifacts introduced with secondary particles on the one hand and one related to the operation speed of the detectors on another. The results obtained are essential for processing RAPID data and are useful for providing analogous investigations for other space mission's instruments.