The High Energy Telescope on Solar Orbiter : Development and Validation of the Onboard Data Processing

Elftmann, Robert

The Solar Orbiter spacecraft of the European Space Agency ESA was successfully launched on the 10th of February 2020 from Cape Canaveral, USA. During the seven-year nominal mission phase, it will travel as close as 0.27 astronomical units to the Sun to study it from up-close. Solar Orbiter will help to address open scientific questions concerning the physical processes on the surface and within the Sun. For this purpose, the spacecraft is equipped with several scientific instruments, divided into remote sensing and in-situ instrumentation. One of these instruments is the Energetic-Particle-Detector (EPD), consisting of four individual telescopes: the Suprathermal Electron and Proton - telescope (STEP), the Electron and Proton Telescope (EPT), the Suprathermal Ion Spectrograph (SIS), and the High Energy Telescope (HET). HET measures electrons with energies between 0.45 up to 18 MeV, protons with energies between 7 to 105 MeV and heavy ions in the range of 7 to several hundred MeV/nuc. It also identifies individual particle and ion species and achieves a separation for 3He and 4He isotopes of up to a 1% ratio. With that, HET measures the energy-dependent particle composition of solar energetic particle events in the given energy ranges and provides insight into their acceleration and propagation processes. In this work, the onboard data processing of HET is developed. It defines the available scientific data for the characterization of the measured particle environment. As part of the pre-development studies, the influence of the instrument's temperature on the measurement of heavy ions with HET's scintillation crystal is investigated. The analysis has shown, that the non-linearity in light output, called ionization quenching, is independent of temperature in case of the scintillation material used in HET. Ionization quenching originates from high ionization densities in the scintillation material when detecting heavy ions. The results are published in form of a peer-reviewed paper in Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms", Volume 451 (2019). Based on these results, existing ionization quenching prediction models can be used for the development of the onboard data processing. Due to the limited available telemetry bandwidth of Solar Orbiter, it is necessary to categorize and evaluate measured data prior to transmission to Earth. Since this is a sophisticated compression process, a detailed description of the onboard data processing, developed in the scope of this thesis, is provided. The performed simulations, describing the interaction of energetic charged particles with HET, which are essential for the development of HET's onboard data processing are presented in detail. Based on the simulation data, the onboard data processing of HET is developed. The resulting procedures analyze measured particles and assign them to pre-defined histograms based on the particle's energy and species. Based on these histograms, data products are defined, which provide selected parts of these histograms in pre-defined cadences. The necessary conversion factors for inferring the measured particle environment based on the defined data products are calculated and provided as part of this thesis. According to the developed data products and their calculated conversion factors, the capabilities of HET for the detection of solar energetic particle events are investigated and presented using simulations of the expected spectral properties and particle compositions, as well as using measurements of instruments of previous missions. The onboard data processing of HET and the defined data products are validated, based on experiments at a heavy ion acceleration facility and the first mission data. From the first data measured by HET during the mission in a real isotropic particle environment, it can be concluded, that HET delivers the expected data and performs as expected in most aspects. Based on this data, optimizations of the onboard data processing are developed in the scope of this thesis and are installed on the HET instrument. In addition to the primary scientific results, e.g. the temperature independence of ionization quenching, this work provides a complete documentation of HET, containing essential information for scientists to understand and correctly interpret the data measured by HET and to develop further optimizations of the onboard data processing if necessary .

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Elftmann, Robert: The High Energy Telescope on Solar Orbiter. Development and Validation of the Onboard Data Processing. 2020.

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