Photoemission and surface contamination on Rosetta

Project work (15 c)
Student: Ava Gramin, Uppsala University
Supervisors: Fredrik Johansson, Elias Odelstad, Anders Eriksson
Period: Winter 2016/2017

Background

The ESA (European Space Agency) spacecraft Rosetta orbited comet 67P/Churyumov-Gerasiminko for more than two years, 2014-2016. The Swedish Institute of Space Physics in Uppsala designed, built and operated one of the instruments on-board, the dual Langmuir probe LAP, to study the ionized gas (plasma) close to the comet. The principal operational mode of LAP is to measure the current to any of two 50 mm spheres mounted on booms protruding several meters from the spacecraft. The current is carried by charged particles, so the local properties (e.g. density and temperature) of the space plasma can be deduced from this measurement. However, other sources of current also contribute, particularly photoelectrons emitted from the probe itself and surrounding parts on the spacecraft. While this partly contaminates the plasma measurements, it also gives an opportunity to improve the knowledge of photoemission in space, which is crucial not only for Langmuir probe data interpretation but also for the broader topic of spacecraft-plasma interaction and spacecraft charging studies. Another potential problem is contamination of the probe surfaces, due to condensation of gaseous matter from the spacecraft or the comet itself.

Abstract

Comet hunter Rosetta has been on a mission to study comet 67P/Churyumov-Gerasimenko since 2004. When Rosetta reached comet 67P and joined it in its orbit, the spacecraft began its study of, among other aspects, the plasma near the comet. In this report, we will take a closer look on technical aspects of the Langmuir probe instrument on-board Rosetta, focusing on means to improve calibration of data obtained from the spacecraft. The Langmuir probe instrument includes two probes, LAP1 and LAP2. A suspected calibration error was confirmed to follow the expected model. A polynomial fit was made of instrument leakage currents measured in flight, which looked similar between the two, with a few variations dependent on positioning and internal characteristics of the different components. Strong temperature dependence is seen, reaching a maximum temperature at perihelion. This report also investigates surface contamination on the probes, possibly caused by, among other things, dust grains from the comet and the exhaust plumes of the spacecraft. An analysis of the photoemission of the LAP instrument has shown indications that LAP2 is the probe mainly subject to contamination, and it will therefore be the main focus of the contamination section of this study. A method to shift LAP2’s contaminated response was applied to a sweep where the probes had an offset of a few volts. The bias shift was investigated for August 30th 2015 and March 15th 2016, between which the shift is seen to have increased. The method may be used on the entire dataset to examine the shift evolution and whether or not the method holds. A transient effect seen on LAP2 when set to floating mode, initially thought to solely be a result of surface contamination, is now thought to be a measure of the contaminating sheath discharging as well as changing physically over time.

Results

Final report

ESA’s comet hunter Rosetta, with the two Langmuir probes from IRF Uppsala at the end of the booms protruding from the spacecraft. Each probe is a sphere of 50 mm diameter. [Background image credit: ESA]