Cylindrical Langmuir Probes in Flowing High Density Plasmas

Abstract:

With increasing interest in ionospheric plasmas the accuracy of different measurements becomes more and more critical. To measure electron density one of the more common ways is using a Langmuir probe. However, interpreting the results from Langmuir probes on a moving spacecraft is not trivial; classical Langmuir theory is often not sufficient. In the cold plasma limit (ion thermal speed less than the spacecraft speed) and high density (small Debye length) the probe alters the local plasma environment so much that the measured electron current is larger than that simple classical theory predicts. This can result in an overestimation of the electron density. This study focuses on using 3D particle tracing codes to determine how large this effect is, when it occurs, and what is the main cause. From these simulations it is clear the plasma environment around the probe changes as the flow goes from sub-sonic to super-sonic. The collected electron current increases by almost a factor of 2. An empirical equation has been derived to describe this change based on the parameter regime simulated. The efficiency of this correction is tested on the MAVEN mission which has the right instrumentation for evaluate the accuracy of Langmuir probes in ionospheres.