Relationship between Total Solar Irradiance and Magnetic Flux during Solar Minima

Abstract

What drives the small total solar irradiance (TSI) changes of ∼50–100 parts per million (compared with >1000 ppm solar-cycle amplitudes) during a deep solar minimum, i.e., in the practical absence of detectable sunspots and long-lasting active regions? We consider the epoch 2008 June–October and investigate multiple data sets (TSI; various Mg ii line-activity indices, extreme ultraviolet fluxes, and full-disk magnetograms) to show that variations in TSI closely follow changes in total magnetic flux from sources with ∣B∣ > 80 G (up to ∼600 G) that persist even during extended periods with no detectable sunspots. These sources comprise the populations of (a) short-lived (<20 minutes), small-scale (predominantly a single 2″ MDI pixel), ∼evenly distributed regions, and (b) on average, more extended (a few MDI pixels) and longer-lived (140–260 minutes median lifetimes) magnetic areas. We ascribe the latter to ephemeral regions, finding them clustering on ∼200 Mm scales. We speculate that the short-lived MDI sources are linked to the ubiquitous magnetic bright points. Our analysis of magnetic flux variations during solar cycle 23 shows that the magnetic regions present during this deep solar minimum elevate the total magnetic flux above the total flux in just the Gaussian “cores,” fitted to histogram distributions of the full-disk flux. This suggests that solar irradiance during more extended, even deeper minima, such as the Maunder Minimum, may be lower than in 2008.