Solar-driven variation in the atmosphere of Uranus

TL;DR

This study analyzes long-term reflectivity data of Uranus, revealing variability linked to the solar cycle, and suggests that cosmic rays and UV radiation influence atmospheric changes, with implications for understanding planetary atmospheres.

Contribution

It demonstrates that both GCR-induced nucleation and UV effects significantly influence Uranus's atmospheric reflectivity, providing new insights into planetary atmospheric variability mechanisms.

Findings

24% of reflectivity variance explained by GCR nucleation

22% of reflectivity variance explained by UV effects

Similar GCR-related variability observed in Neptune

Abstract

Long-term measurements (1972-2015) of the reflectivity of Uranus at 472 and 551 nm display variability that is incompletely explained by seasonal effects. Spectral analysis shows this non-seasonal variability tracks the 11-year solar cycle. Two mechanisms could cause solar modulation, (a) nucleation onto ions or electrons created by galactic cosmic rays (GCR), or (b) UV-induced aerosol colour changes. Ion-aerosol theory is used to identify expected relationships between reflectivity fluctuations and GCR flux, tested with multiple regression and compared to the linear response predicted between reflectivity and solar UV flux. The statistics show that 24%of the variance in reflectivity fluctuations at 472 nm is explained by GCR ion-induced nucleation, compared to 22% for a UV-only mechanism. Similar GCR-related variability exists in Neptune's atmosphere, hence the effects found at Uranus provide the first example of common variability in two planetary atmospheres driven through energetic particle modulation by their host star.