On the contribution of quiet Sun magnetism to solar irradiance variations: Constraints on quiet Sun variability and grand minimum scenarios

Abstract

While the quiet Sun magnetic field shows only little variation with the solar cycle, long-term variations cannot be completely ruled out from first principles. We investigate the potential effect of quiet Sun magnetism on spectral solar irradiance through a series of small-scale dynamo simulations with zero vertical flux imbalance ($⟨B_z⟩=0$) and varying levels of small-scale magnetic field strength, and one weak network case with an additional flux imbalance corresponding to a flux density of $⟨B_z⟩=100$ G. From these setups we compute the dependence of the outgoing radiative energy flux on the mean vertical magnetic field strength in the photosphere at continuum optical depth $\tau =1$ ($⟨|B_z|{⟩}_{\tau =1}$). We find that a quiet Sun setup with a mean vertical field strength of $⟨|B_z|{⟩}_{\tau =1}=69$ G is about $0.6\%$ brighter than a non-magnetic reference case. We find a linear dependence of the outgoing radiative energy flux on the mean field strength $⟨|B_z|{⟩}_{\tau =1}$ with a relative slope of $1.4\cdot 10^{-4}$ G${}^{-1}$. With this sensitivity, only a moderate change of the quiet Sun field strength by $10\%$ would lead to a total solar irradiance variation comparable to the observed solar cycle variation. While this does provide strong indirect constraints on possible quiet Sun variations during a regular solar cycle, it also emphasizes that potential variability over longer time scales could make a significant contribution to longer-term solar irradiance variations.

15 pages, 10 figures, accepted for publication in ApJ

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