Solar-Wind Charge Exchange in Comets: Bayesian Diagnostics of Ion Composition and Freeze-In Temperature

Abstract

Cometary X-rays from solar-wind charge exchange (SWCX) probe solar-wind composition and coronal freeze-in conditions, yet most spectra are low- to moderate-resolution and strongly blended. Conventional fits often add more free parameters than the data support, obscuring interpretation. This dissertation develops, validates, and applies the Bayesian Color Model (BCM), which is a framework for low-resolution cometary SWCX that yields stable, physically meaningful results without overfitting.

The model groups lines by ion, sets widths from the instrument response, and encodes state-selective, velocity-dependent physics through priors while controlling complexity via the Bayesian Information Criterion (BIC). The work also establishes NICER as a cometary SWCX observatory by defining an observing/reduction protocol with pre-background, on-comet, and post-background pointings, strict NICER screening, epoch segmentation to preserve short-timescale variability, and, when available, GOES and ENLIL context. Together these elements convert blended NICER spectra into reproducible ion-flux ratios and freeze-in temperature diagnostics.

Applied to an eight-comet, epoch-resolved NICER survey, the framework recovers consistent multi-ion behavior across targets and epochs. Carbon-based diagnostics cluster at ~1.4-1.7 MK, while nitrogen/oxygen diagnostics are higher at ~2.0-2.3 MK. Day-to-week changes in line ratios and inferred temperatures are common and are best explained by ordinary solar-wind structure rather than changes in the comae. Stacked spectra confirm that O VII dominates the 0.37-1.0 keV band under typical conditions, with enhanced O VIII during hotter or disturbed intervals.

Absolute abundances and single-pair temperatures are limited chiefly by NICER’s blending of key diagnostics and by sparse state-selective, velocity-dependent cross sections for heavy ions on H2O and CO2/CO at cometary velocities, including incompletely benchmarked double-electron-capture channels. Within these limits, relative trends across species and epochs are robust.

The BCM is the best choice for low-resolution cometary SWCX spectroscopy and that NICER, used with a dedicated protocol, is a practical platform. Laboratory measurements at cometary velocities and high-resolution comet spectra will sharpen priors, yield empirical ratios, and provide a transfer function to reinterpret archival data, advancing cometary SWCX from qualitative tracer to quantitative probe of heliophysical plasma conditions.