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To suit a wide variety of space mission profiles, different designs of ion thrusters were developed, such as the High‐Efficiency‐Multistage‐Plasma thrusters (HEMP‐T). In the past, the optimization of ion thrusters was a difficult and time‐consuming process and evolved experimentally. Because the construction of new designs is expensive, cheaper methods for optimization were sought‐after. Computer‐based simulations are a cheap and useful method towards predictive modelling. The physics in HEMP‐T requires a kinetic model. The Particle‐in‐Cell (PIC) method delivers self‐consistent solutions for the plasmas of ion thrusters, but it is limited by the high amount of computing time required to study a specific system. Therefore, it is not suited to explore a wide operational and design space. An approach to decrease computing time is self‐similarity scaling schemes, which can be derived from the kinetic equations. One specific self‐similarity scheme is investigated quantitatively in this work for selected HEMP‐Ts, using PIC simulations. The possible application of the scaling is explained and the limits of this approach are derived.
Comparison of mesospheric sodium profile retrievals from OSIRIS and SCIAMACHY nightglow measurements
(2022)
Sodium airglow is generated when excited sodium atoms emit electromagnetic radiation while they are relaxing from an excited state into a lower energetic state. This electromagnetic radiation, the two sodium D lines at 589.0 and 589.6 nm, can usually be detected from space or from ground. Sodium nightglow occurs at times when the sun is not present and excitation of sodium atoms is a result of chemical reaction with ozone. The detection of sodium nightglow can be a means to determine the amount of sodium in the earth's mesosphere and lower thermosphere (MLT). In this study, we present time series of monthly mean sodium concentration profiles, by utilizing the large spatial and temporal coverage of satellite sodium D-line nightglow measurements. We use the OSIRIS/Odin mesospheric limb measurements to derive sodium concentration profiles and vertical column densities and compare those to measurements from SCIAMACHY/Envisat and GOMOS/Envisat. Here we show that the Na D-line limb emission rate (LER) and volume emission rate (VER) profiles calculated from the OSIRIS and SCIAMACHY measurements, although the OSIRIS LER and VER profiles are around 25 % lower, agree very well in shape and overall seasonal variation. The sodium concentration profiles also agree in shape and magnitude, although those do not show the clear semi-annual cycle which is present in the LER and VER profiles. The comparison to the GOMOS sodium vertical column densities (VCDs) shows that the OSIRIS VCDs are of the same order of magnitude although again the semi-annual cycle is not as clear. We attribute the differences in the LER, VER and sodium profiles to the differences in spatial coverage between the OSIRIS and SCIAMACHY measurements, the lower signal-to-noise ratio (SNR) of the SCIAMACHY measurements and differences in local time between the measurements of the two satellites.