Solar Photospheric Network Properties and Their Cycle Variation

Abstract

We present a numerical simulation of the formation and evolution of the solar photospheric magnetic network over a full solar cycle. The model exhibits realistic behavior as it produces large, unipolar concentrations of flux in the polar caps, a power-law flux distribution with index −1.69, a flux replacement timescale of 19.3 hr, and supergranule diameters of 20 Mm. The polar behavior is especially telling of model accuracy, as it results from lower-latitude activity, and accumulates the residues of any potential modeling inaccuracy and oversimplification. In this case, the main oversimplification is the absence of a polar sink for the flux, causing an amount of polar cap unsigned flux larger than expected by almost one order of magnitude. Nonetheless, our simulated polar caps carry the proper signed flux and dipole moment, and also show a spatial distribution of flux in good qualitative agreement with recent high-latitude magnetographic observations by Hinode. After the last cycle emergence, the simulation is extended until the network has recovered its quiet Sun initial condition. This permits an estimate of the network relaxation time toward the baseline state characterizing extended periods of suppressed activity, such as the Maunder Grand Minimum. Our simulation results indicate a network relaxation time of 2.9 yr, setting 2011 October as the soonest the time after which the last solar activity minimum could have qualified as a Maunder-type Minimum. This suggests that photospheric magnetism did not reach its baseline state during the recent extended minimum between cycles 23 and 24.

Long-term trends of magnetic bright points

“…Karak & Brandenburg 2015), as well as of the evolution of the magnetic network during a solar cycle (e.g. Thibault et al 2014), agree with these observational findings to a more and more satisfying degree but there are still major open questions to be addressed (e.g. the final chapter of Charbonneau 2010).…”

“…Thibault et al (2014) found, for example, that when an injection of new flux into the magnetic network is stopped, the filling factor of the magnetic network only decreases exponentially with an e-folding time of about 1.9 yr. Thus, they concluded that the magnetic network was unable to fully relax to its ground state even during the extended period of the last solar cycle around 2009.…”

Temporal relations between magnetic bright points and the solar sunspot cycle

“…However, it is a first strong indication for such processes. Moreover, from the viewpoint of physics, such a linkage between magnetic network activity and sunspot activity is very plausible and also used in theoretical solar cycle modeling (e.g., Thibault et al 2012Thibault et al , 2014.…”

A comparison of stratospheric photochemical response to different reconstructions of solar ultraviolet radiative variability

“…7 in Bolduc et al (2014). However, this component is not dependent on the cycle phase and is basically constant, on average, for the whole time period considered, which is inconsistent with the conclusions of Thibault et al (2014). These authors find that after the last cycle emergence, the magnetic network relaxation time towards the baseline state characterizing periods of suppressed activity is about 2.9 years.…”