Charging Effects and Detection of Mesospheric Dust with the Instrument SPID on the G-Chaser Rocket

Abstract

Smoke Particle Impact Detector (SPID) is a faraday cup impact probe designed and built by the University of Tromso (UIT). Its main purpose is to measure nanometer sized smoke particles (in-situ) in the atmosphere, and to do that it needs to be launched on a sounding rocket. Its design is an open faraday cup with grids to shield out ambient plasma and a larger slanted impact grid to measure the incoming smoke. The particles we are interested in measuring are called Meteoric Smoke Particles (MSPs). They are believed to be condensed material from meteoric ablation and thought to reside in layers in the altitude range of approximately 50-100 km with sizes of around 0.2-3 nm on average. There are many unknowns regarding the smoke particles, particularly their altitude distribution, size, charge and composition. By gaining more knowledge about them we can start to understand better their involvement in atmospheric processes including their possible impact on chemical reactions and formation of ice particles in the mesosphere and the possible connection to Polar Mesospheric Winter Echoes(PMWE). SPID was launched for the first time on the student rocket G-Chaser in January 2019. The launch was successful apart from some minor issues regarding amplification on the shielding grids. The main grid designed to measure the smoke showed a positive current during the entire flight with some interesting areas that might indicate detection of smoke particles. This thesis focuses on estimating the charging of the payload by ambient plasma and induced photocurrent from UV solar photons, as well as the possibility of solar induced currents on the grids and their possible contribution to the measured currents. We find that the payload is primarily negatively charged with an estimated floating potential of maximum 0.46 V up towards apogee of around 184 km with charging due to ambient electrons dominating the examined charging sources. Calculations also determined that there it is possible for the induced photocurrent on the grids to be the cause of the magnitude difference seen in the measured signals due to the spin of the rocket and its coning motion. Another part of the thesis is to examine and determine the charging of the smoke inside the probe, to compare the measured current to theoretical values and examine the various error factors associated with this. Based on theoretical considerations it was explored how the work function of the particles, the dependence of particle properties, their size as well as their initial velocity depended on the their ability to generate charge. Investigations carried out within this thesis showed that there are considerable differences between available charging models, as well as altitude distribution discrepancies between the models and the measured probe current. Possible causes of discrepancies can be due to models used are from and Further investigation is needed to determine the accurate altitude distribution of smoke particles. Since the distribution model we used is from a different time of year, this might explain the possible difference. We conclude the charging mechanism as the grains collide with the measuring plate are not well defined, a task for future dedicated laboratory experiments to describe how small grains of nanometer size can gain charge in these kind of collisions. Which would help to better define and choose an accurate charging model for very small particles in such high velocity collisions.