Analysis of the surface contamination layer effects on the sweep measurements of the Langmuir Probes on Swarm satellites

Langmuir probes can get their surface contaminated during its operational life, developing an external layer that significantly affects their measurements. Thus, the well-known theory of Collectors in Gaseous Discharges is no longer valid and these non-ideal conditions need to be adequately modelled. Contaminated probes show a hysteresis loop in their current characteristics that reduces the accuracy of the plasma parameters derived from them. This suggests the presence of an insulating external layer that brings a capacitive behaviour into the circuit. This layer is, therefore, represented by a capacitor and a leakage resistance. The primary goal of this project is to check if this model is suitable to explain the experimental measurements of the Langmuir probes carried by the three Swarm satellites and improve their plasma parameter estimates. It has been found that the RC model is an adequate tool to remove the hysteresis loop found in the experimental current characteristics. The value of the capacitance of the insulating layer is of a few microfarads, while the resistance turned out to be too high to be relevant in the measurements and it can be removed from the model. However, the RC model does not explain all the features found in the data and some of them could be explained including another surface resistance between the insulating layer and the probe. This report sets the path to the development of an improved method to derive the plasma parameters out of the Langmuir probes of Swarm mission.