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

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Friday, May 8, 2020

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 (⟨Bz ⟩ = 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 ⟨Bz⟩ = 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 τ = 1 (⟨|Bz|⟩τ=1). We find that a quiet Sun setup with a mean vertical field strength of ⟨|Bz|⟩τ=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 ⟨|Bz|⟩τ=1 with a relative slope of 1.4 · 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.

Graphic chart depicting mean vertical magnetic field strength
Panel (a): Time evolution of the mean vertical magnetic field strength ⟨|Bz|⟩τ=1 for the setups SSD_WEAK (dark blue), SSD_QS (light blue), SSD_Strong (orange), and N100 (magenta). All cases were started at t = 0 hours from the same HD snapshot (with different magnetic initial states). Panel (b): Time evolution of the outgoing radiative energy flux for the setups (1) - (5). The HD case is shown in black. We consider 24 hour averages as indicated in the figure. In the right panel we apply a 2 hour running boxcar average for better readability, since short-term TSI fluctuations have amplitudes of 1 − 2%. These simulations provide a TSI sensitivity of 0.14% for each 10 G of average vertical magnetic field strength in the photosphere at an optical depth of 1.

Publication Name: ApJ
First HAO Author's Name: M. Rempel

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