A New Approach to Determining Cold Plasma Ion Density and Composition From Particle Measurements.

Abstract:

Plasma moments and the underlying particle distribution function are among the most fundamental descriptors of space plasmas. Plasma particle measurements can be especially challenging for very cold, dense plasma in the plasmasphere. In this case, naturally occurring spacecraft charging limits the access of the coldest ions to the spacecraft, cutting off the peak of the particle distribution function. Traditionally, this problem has been attacked by fitting a Maxwellian to the tail of the measured particle distribution function. In this presentation we discuss a new way to determine the densities and temperatures in a cold, multi-species ion plasma. By leveraging the properties of the Maxwell-Boltzmann distribution function we show that the problem of calculating densities and temperatures for each ion species is not underdetermined. Instead, these moments can be calculated directly from the measured tail of the distribution function using a new two-point method described here. We use a synthetic data set that mimics the Van Allen Probe’s HOPE three-ion species plasma measurements to characterize the differences between traditional two-parameter fitting and our new two-point method. We demonstrate how the two-point method can deliver superior results when the measurements are marginal (i.e., low fluxes and/or high spacecraft charging). We also discuss the limits of cold plasma detectability for the three major ion species (H+, He+, O+) in the case of Van Allen Probes measurements. Our results are easily transferable to measurements from ion composition and total ion instruments on other magnetospheric missions. Our analyses provide an easy-to-create metric that allows us to evaluate the accuracy and precision of cold ion composition measurements in the inner magnetosphere.