Characteristics of magnetic solar-like cycles in a 3D MHD simulation of solar convection

Abstract

We analyse the statistical properties of the stable magnetic cycle unfolding in an extended 3D magnetohydrodynamic simulation of solar convection produced with the EULAG-MHD code. The millennium simulation spans over 1650 years, in the course of which forty polarity reversals take place on a regular ∼40 yr cadence, remaining well-synchronized across solar hemispheres. In order to characterize this cycle and facilitate its comparison with measures typically used to represent solar activity, we build two proxies for the magnetic field in the simulation mimicking the solar toroidal field and the polar radial field. Several quantities that characterize the cycle are measured (period, amplitudes, etc.) and correlations between them are computed. These are then compared with their observational analogs. From the typical Gnevyshev-Ohl pattern, to hints of Gleissberg modulation, the simulated cycles share many of the characteristics of their observational analogs even though the simulation lacks poloidal field regeneration through active region decay, a mechanism nowadays often considered an essential component of the solar dynamo. Some significant discrepancies are also identified, most notably the in-phase variation of the simulated poloidal and toroidal large-scale magnetic components, and the low degree of hemispheric coupling at the level of hemispheric cycle amplitudes. Possible causes underlying these discrepancies are discussed.

The Maunder minimum (1645–1715) was indeed a grand minimum: A reassessment of multiple datasets

“…Charbonneau 2010). Although modern solar-type dynamo models reproduce the "regular" part of the behaviour of the large-scale solar magnetic field (Ghizaru et al 2010;Brown et al 2011;Schrinner et al 2012;Gastine et al 2012;Käpylä et al 2012), it is still challenging to extend the integration time over several magnetic cycles as needed to reproduce a Maunder-like minimum (see, however, Passos & Charbonneau 2014;Augustson et al 2015). Thus, while the MM is identified as a special mode of the solar dynamo, we are not yet able to precisely model it.…”

Hemispheric Coupling: Comparing Dynamo Simulations and Observations

“…The presence of a stably stratified fluid layer underlying the convectively unstable layers appears to be an important agent favoring self-organization of the magnetic field on large spatial scales in these simulations (on this point see also Browning et al 2006). The magnetic cycle developing in this "millennium simulation" is analyzed in detail in Passos & Charbonneau (2014), to whom we refer the interested reader for a full description of cycle characteristics, period-amplitude relationships, etc. Figure 9A,B shows two meridional cuts of the zonally-averaged toroidal magnetic field, at epochs of (A) cycle maximum and (B) minimum.…”

“…9 by the black box. Similarly, a polar field proxy is constructed by integrating the radial magnetic field at high latitudes in the subsurface layers, indicated here by the dashed box (see also Passos & Charbonneau 2014). Figure 10 shows time series of these two proxies, calculated independently for the northern (red and orange) and southern (blue and green) hemispheres.…”

“…The time sequence in black on Figure 11 shows the time sequence of hemispheric lag values for successive cycles defined here as the time of cycle minimum in Fig. 10 A 500 yr time series segments of the hemispheric toroidal (top) and poloidal (bottom) proxies in the northern and southern hemispheres, in the EULAG-MHD "millennium simulation" analyzed by Passos & Charbonneau (2014). The simulated cycles numbers are shown for reference.…”

Supermodulation of the Sun's magnetic activity: the effects of symmetry changes

“…At high latitudes there is an equatorward flow at the tachocline, with poleward motion in most of the convection zone, giving way to equatorward flow above 0.9R⊙ and rapid poleward velocities nearer to the surface (Passos, Charbonneau & Miesch 2015;Hanasoge et al 2015). The corresponding azimuthal fields accumulate near the tachocline and display dipole symmetry; they reverse cyclically and have been followed for 40 activity cycles, with an average period of 40 years (Passos & Charbonneau 2014;Passos, Charbonneau & Miesch 2015). The activity cycles vary in a manner consistent with weak chaotic modulation but so far without any indication of grand minima.…”