Quantifying Radiation Belt Electron Precipitation Through Bremsstrahlung X-Ray Spectral Imaging

Berland, Grant

Graduate Thesis Or Dissertation

Quantifying Radiation Belt Electron Precipitation Through Bremsstrahlung X-Ray Spectral Imaging Public Deposited

Analytics

Citations

Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/t435gf669

Abstract

Energetic particle precipitation (EPP) is a phenomenon that couples planetary magnetospheres and atmospheres through high-energy charged particle transport into the atmosphere. Due to the large spatial scales over which this process occurs, it is difficult to observe directly and therefore the spatial and temporal scales of EPP are poorly constrained. Open questions of magnetosphere-atmosphere coupling relate to the driving mechanisms of EPP: how does EPP vary seasonally, temporally, and with magnetospheric conditions; and what are the spatial scales over which this process occurs? The study of EPP will improve current understanding of the behavior of the radiation belts and the atmosphere’s response to EPP, and will lend a deeper understanding of the dynamic interactions of planetary magnetospheres and atmospheres. This work specifically focuses on electron precipitation involving the highest energy particles in the magnetosphere: the radiation belts.

Obtaining global measurements of EPP is difficult due to the spatial scale and height above Earth’s surface at which this process occurs; the drivers interact with charged particles in the heart of the radiation belts at 4 – 7 Earth radii, and the effects at Earth cover the entire globe, from mid-latitudes to the poles, and at altitudes too high for in-situ balloon measurements, and too low for radar remote sensing. Measuring EPP’s effects through remote sensing in the ionosphere is difficult due to high atmospheric density driving fast recombination such that the atmospheric effects of EPP dissipate quickly. Perturbations in atmospheric chemistry can be measured as a proxy to precipitation inputs, but the complicated reactions and transport dynamics makes the inversion to precipitation characteristics uncertain. On the other hand, direct in-situ measurements of charged particles from spacecraft cannot easily obtain the spatial and temporal coverage necessary to quantify EPP due to the limited nature of a single (or a few) spacecraft in orbit. Additionally, charged particle instruments are often angular resolution-limited and are unable to resolve the loss cone at various points in the orbit, which is necessary to provide a global image of precipitation.

EPP can instead be inferred through remote measurements of X- and gamma-ray photons, which are a byproduct of relativistic electron precipitation. Once an electron has entered the atmosphere it interacts with neutrals and can spontaneously generate a high energy photon via the bremsstrahlung (“braking radiation”) interaction. The resulting photon energy is correlated with the precipitating electron energy, such that statistical relationships can be formed between the two quantities. By measuring X- and gamma-ray photons that escape the atmosphere with a spacecraft-based imaging spectrometer, a global perspective of EPP can be obtained: the spatial scales of EPP can be directly measured by photon imaging of the upper atmosphere from low-Earth orbit, and by measuring in-situ electron spectra along with photon measurements, the photon flux and spectra can be inverted to estimate precipitating electron flux and spectra.

This thesis is split into three parts. First, the design, development, and testing of the Atmospheric X-ray Imaging Spectrometer (AXIS) instrument onboard the upcoming Atmospheric Effects of Precipitation through Energetic X-rays (AEPEX) CubeSat mission is described. The detectors, shielding design, and signal-to-noise ratio calculations are described in detail. Second, the novel X-ray optics and algorithmic reconstruction technique of the AXIS instrument are developed and described. The wide field-of-view imager implements a coded aperture mask in order to obtain spatial resolution of X-ray photons. Finally, EPP simulations are performed with a kinetic model of precipitation built using GEANT4 to determine the X-ray signal and atmospheric effects of EPP at Earth and Jupiter in support of the AEPEX mission and other studies of planetary magnetospheres. The development of the AXIS instrument and tools developed herein are extended for a NASAfunded concept study of a spacecraft mission to Jupiter, which aims to address open questions of Jupiter’s radiation belts.

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