Simulation-based interpretation of spacecraft particle sensor measurements

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Particle sensors such as Langmuir probes or particle imagers, are used in many satellites and laboratory plasma experiments to infer plasma parameters such as the density and temperature. In its simplest form a Langmuir probe consists of an electrode which can be biased to variable voltages, and from which the electric current can be measured. The collected current as a function of bias voltage, the so-called probe characteristic, can then be interpreted on the basis of theoretical or computational models, to yield local plasma parameters. Much work has been done on this topic over nearly a century. Many theoretical, and more recently, several computational models have been developed to describe the response of Langmuir probes in a plasma. In practice however, probe measurements are almost universally interpreted in terms of simplified analytic models capable of producing real time answers in operation mode. Unfortunately these interpretations are notoriously uncertain, with error bars that can be of order 100%. The reason for these large uncertainties comes from the use of idealised models in which only a fraction of the relevant physical processes are taken into account. For example, some models only consider a probe in a stationary unmagnetised plasma, others account for plasma flow but neglect ambient magnetic fields, yet others account for a magnetic field, but ignore plasma flow or other effects such as photoelectron or secondary electron emission. In essentially all cases, plasma is assumed to be spatially uniform, and the presence of nearby satellite or experimental objects and their geometry is ignored. A promising solution to this predicament is to interpret particle sensor measurements on the basis of detailed computer simulations capable of accounting for the multiphysics which characterises plasma-material interaction, while also accounting for the geometry in which measurements are made. This is unfortunately not possible in real time operation mode owing to the time and computational resources required to do such simulations. One practical solution to be explored and developed in this proposed research, is to construct a library or data base for each experiment or space mission to be supported, from which plasma parameters could be inferred using a suitable regression technique. Compared to current practice, the new approach would lead to significant improvements in the accuracy of inferred plasma parameters. The proposed research would concentrate on producing solution libraries for selected spacecraft and experiments, developing, and assessing different regression techniques. An expected outcome of this research is a change of paradigm in the interpretation of sensor measurements, which would then rely on detailed kinetic simulation results rather than on idealised analytic models. This proposal is based on many years of experience in satellite-environment modelling.