Irradiance refers to the amount of light energy or power that falls on a particular surface per unit area. It basically measures how much light is hitting a specific location at a given moment. Full definition
Krivova, N. A., Solanki, S. K., Fligge, A. & Unruh, Y. C. Reconstruction of solar irradiance variations in cycle 23: is solar surface magnetism the cause? (nature.com)
Harder, J. W., Fontenla, J. M., Pilewskie, P., Richard, E. C. & Woods, T. N. Trends in solar spectral irradiance variability in the visible and infrared. (nature.com)
The solar UV irradiance from the thermosphere of Saturn and the solar wind are the most probable sources to account for the long - term variability of the electron radiation belts (Roussos et al. 2014), suggesting that external drivers play indeed an important role in Saturn's magnetospheric dynamics. (swsc-journal.org)