Temporal Variability of Saturn's H2 Dayglow and Northern Aurora Observed by Hisaki and Cassini
By Leah Clare (Lancaster University)
The ultraviolet (UV) emissions from Saturn are composed of the dayglow from the sunlit atmosphere and the aurorae at the poles. Investigation into the daily variability of the dayglow remains somewhat unconstrained, particularly on timescales of weeks. Utilising coincident Hisaki and Cassini observations across ~3 weeks in 2014, we determine the temporal variability of the UV emitted power, assess the response of the dayglow to solar activity, and constrain the contribution from the northern aurora to the total emitted power. We find that the power varies by a factor of 2.26 over 23 days with Hisaki, and 1.29 over 17 days with Cassini. Upon separation of the northern auroral contribution with Cassini, the contribution is found to be between 10% - 26%. Additionally, the dayglow component displays a strong correlation with solar activity, confirming that the dayglow is controlled by the UV solar flux as shown by previous studies (Gustin et al., 2010; Liu & Dalgarno 1996). This study demonstrates the first analysis of the Saturn campaigns by Hisaki, allowing an assessment of the robustness of such a mission in observing outer planet targets. The multi-mission analysis confirmed that Hisaki was able to track the variability of the UV emissions from Saturn, with comparative trends to the Cassini data.
References:
Gustin, J., Stewart, I., Gérard, J. C., & Esposito, L. (2010). Characteristics of Saturn’s FUV airglow from limb-viewing spectra obtained with Cassini-UVIS. Icarus, 210(1), 270–283. https://doi.org/10.1016/j.icarus.2010.06.031
Liu, W., & Dalgarno, A. (1996). The Ultraviolet Spectrum of the Jovian Dayglow. The Astrophysical Journal, (462), 502–518.
See publication for more details:
https://doi.org/10.1029/2026JA035194

(a) The total emitted UV power obtained from Hisaki/EXCEED. The points are daily average H2 powers from 70 to 148 nm. (b) The total emitted UV power determined with Cassini UVIS; each point is the daily average H2 power for the wavelength range 70–148 nm. (c) The daily average solar F10.7 radio index, a proxy for EUV radiation, scaled to Saturn. Data from Space Weather Canada. (d) The solar EUV power into Saturn's thermosphere. Solar spectral irradiance data are obtained from LISIRD, which uses the Flare Solar Irradiance Model (Chamberlin et al., 2008) and Earth irradiance measurements. The calculation is from Gershman and DiBraccio (2024). (e) The solar H‐Lyman β irradiance at Saturn; data are obtained from LISIRD, which uses the Flare Solar Irradiance Model (Chamberlin et al., 2008) and Earth irradiance measurements.
Solar Activity References:
Chamberlin, P. C., Woods, T. N., & Eparvier, F. G. (2008). Flare irradiance spectral model (fism): Flare component algorithms and results. Space Weather, 6(5). https://doi.org/10.1029/2007SW000372
Gershman, D. J., & DiBraccio, G. A. (2024). Quantifying External Energy Inputs for Giant Planet Magnetospheres. Geophysical Research Letters, 51(15). https://doi.org/10.1029/2024GL109660