Uncertainties in plasma measurements: effects of lower cutoff energy and spacecraft charge

The idea of “perfect” detectors is proposed in order to evaluate the uncertainties in plasma measurements due purely to inherent limitation of an instrument and its standard data processing. As the first step, the effects of the lower cutoff energy of a detector and spacecraft charging are investigated. The relationship between measured moments and the moments of a plasma that can be approximated by multiple convective Maxwellians is analytically derived. This relationship can be used to estimate the uncertainties of a measurement and provides a mathematical tool to derive the real moments of the plasma. In addition to the relationship among moment averages, the relationship among linear perturbations is also derived. This latter relationship provides the estimate of the uncertainty of transport ratios, which have become more and more important in quantitative wave analyses. The results show that, in general, the density is underestimated while the temperature and velocity are overestimated. Errors in one moment propagate as errors in other moments. The error may not be significant for ion measurements in the magnetosheath but is generally important in electron measurements. Correction must be made for electrons if one uses the measurements quantitatively. The error is inversely proportional to the flow velocity and temperature. There is a false temperature anisotropy with respect to the flow direction. This false anisotropy becomes significant for cold plasmas when the Mach number is near one. Because of the lower cutoff energy and spacecraft charging, an Alfvenic perturbation can be observed with a significant density fluctuation and the transport ratios can be either overestimated or underestimated depending on the conditions of the plasma.