  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.
