Temporal Variability of Saturn's H ₂ Dayglow and Northern Aurora Observed by Hisaki and Cassini

Ultraviolet (UV) emissions from molecular hydrogen in Saturn's atmosphere consist of bright auroral emission over the poles and disk‐wide airglow. The dayside disk emits substantial intensities (dayglow) previously detected by various instruments. Exploration of dayside disk emissions provides insight into the interaction between the solar flux and planetary atmosphere. The respective contributions from the dayglow and the aurorae has not yet been evaluated, and the daily variability of the dayglow over timescales of weeks remains somewhat unexplored. For three weeks in 2014, the low‐Earth orbiting telescope, Hisaki, monitored Saturn while Cassini concurrently observed Saturn's northern hemisphere from within the magnetosphere. The total emitted power varies by a factor of 2.26 over 23 days as observed by Hisaki, and 1.29 over 17 days by Cassini. Cassini data allow separation of the auroral power; the total power is dominated by that from the dayglow emission, with the northern aurora contributing ∼10%–26%. The disk‐wide dayglow component displays strong correlation with solar activity, with Pearson correlation coefficients >0.8. The Cassini data further show that the global power at longer wavelengths (155–162 nm) depends more strongly on the solar spectrum than shorter wavelengths (70–148 nm). The auroral contribution to the total emission (70–170 nm) is ∼2%–14%. It is concluded that the total power is dominated by the dayglow from the disk, with solar photons controlling the dayglow emission.