Correcting Galileo's Energetic Particle Detector DataMethodology, Implications and Applications

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Student thesis: Doctoral ThesisDoctor of Philosophy

Original languageEnglish
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Award date2018
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Abstract

Over the course of its 8-year mission the Energetic Particle Detector, launched in
1989 on the Galileo satellite, took data on the Jovian Particle environment. This thesis focuses on the data from the EPD; specifically, from the CMS telescope on the top of the instrument. Comparing data from the beginning of the mission to the final data retrieved and quantifying the clear discrepancy in the loci defining the elements.
These element loci reveal that the detector is decaying in sensitivity. The amount of energy drop corresponding to each element such that it is clear a dead layer had built up on the front of the detector. The larger element particles lose more energy passing through this dead layer than lighter elements, causing a systematic energy drop as it thickens throughout the mission. This thickening of the dead layer is caused by the radiation impacting onto the detector denaturing the sensitive volume; affecting both the High Rate data and the RealTime count rate data.
The High rate data is only available in short sections of the mission, mainly during the flybys and periods of interest. Chapter 2 aims to correct this data is based on the nature of the detector, and the build-up of the dead layer. Through the chapter the dead layer thickness is estimated throughout the mission using calibration masses with a simulated depth of dead layer; giving an evolution of the dead layer as the mission progresses.
Knowing the thickness of the dead layer allows a correction to be made using a
selection of masses and the known energy lost for the dead layer present. The final correction is made by working backwards; starting with all the possible test masses, and calculating the energy loss of the particle passing through the established dead layer. This loss is the applied to the possible starting energies and a final comparison to real readings is made. This highlights the closest value of the original particle.
Chapter 3 focuses on the Correction of the real time count rate channels, this
requires a different approach; the dead layer thickness can only be used in assessing the progress of the correction. Instead, this correction method uses comparative count rates from relative locations in the Jovian system at different times. These are then used to calculate a value of the decay in terms of number of counts hitting the detector. By systematically applying this value to the counts registered by the detector brings the values closer to the true values. This intensive iterative process encompasses the changing values of counts over the mission and the effects of efficiency dropping over the mission. Each
individual count rate channel is processed in this manner.
Chapter 4 is focused on using the results from the correction of the EPD data and using it to evaluate the sputtering on the Surface of the Jovian moons. Looking in detail at flyby data and the sputtering yields available from literature, the erosion effect on the surface can be calculated. By using the sputtering rate, the real effects of the correction can be felt, the misallocation of elements in the original data means the higher numbers of heavier ions greatly impacted the overall erosion.
The final research Chapter, pulls together the previous three to focus on the surface of the Icy moons, specifically Europa. The first half focuses on imaging the surface and the ability to draw more information from the available images. Using digital elevation techniques to develop a height profile and evaluate key features from a different perspective.
With the addition of a digital elevation model the impact of surface erosion is visible.
By using geological techniques and combining it with feature formation theory an estimation can be made of the age of the surface features, in a small scale proof-of-concept study.
From this work, much can be done towards research for future Jupiter and high
radiation environment instruments; from a perspective of having a greater understanding of the environment as a whole and improving radiation shielding, to also being able to better post collection process new data. The proof-of-concept investigations in Chapter 5 will allow large scale studies of the Europan surface with the arrival of Europa Clipper, and it can easily be adapted to the other moons for use with imaging from the JUICE mission.
Overall the aim is to improve upon current data and then better evaluate the Jovian system and its interaction of the radiation environment with the surfaces of the icy moons.