A new method to improve particle moments: Effects of cutoffs and spacecraft potential on PEACE measurements

We have developed a new semi-analytic method in order to improve moments derived from low energy particle detectors on board spacecraft. The aim is to tackle the problems introduced both by the energy cutoffs of the instruments, which can miss the dense low energy part of the distribution, and by the spacecraft potential, which shifts the whole distribution function in energy. The measured moment values (density, velocity, temperature) and the spacecraft potential are used to compute the new moments by reference to a Maxwellian distribution function. These new moments are the solutions of a nonlinear system which is solved numerically. We applied this method to the moments obtained by the PEACE instrument on board CLUSTER. The numerical algorithm shows a satisfactory convergence except in regions where the temperature is too small and/or the spacecraft potential is too large. The method works well under solar wind and magnetosheath conditions. The results are then compared to the density calculated from the plasma frequency with the WHISPER instrument and with ion density from the CIS instrument.