The Unstructured Particle-In-Cell Method with Applications for Objects in Ionospheric Plasmas

Abstract

Spaceborn instruments such as the Langmuir probe are essential to understand our own ionosphere. In this dissertation we explore the behaviour of such instruments through computer simulations, and find that some of the assumptions about them may not be well justified, causing degraded performance. We also develop new theories to counteract these effects, as well as new methods for simulating such objects in plasmas. One of the ways to measure the electron density in the ionosphere is by applying a positive voltage to a thin wire – a Langmuir probe – and then exposing it to the plasma in the ionosphere. The probe will then attract electrons, and using the so-called OML theory, this current of electrons can be used to calculate the electron density. However, the OML theory relies on several simplifying assumptions that are not always well satisfied, leading to a reduced accuracy. Ionospheric plasmas containing objects can be simulated on a computer using the Particle-In-Cell method, and an unstructured, tetrahedral mesh allows for arbitrary geometries, such as that of Langmuir probes attached to a satellite. Using such simulations, we quantify the electron current collected by probes of short length, or situated in a non-Maxwellian plasma. This is not covered by the usual OML theory, and this knowledge may therefore be used to improve ionospheric measurements. We also revealed by simulations that multineedle Langmuir probes mounted on a small satellite may charge the satellite sufficiently to render the measurements invalid, unless care is taken. Finally, we have also contributed with new numerical methods for simulating objects connected in arbitrary circuits in plasmas.