Mean-field electrodynamics is a subject well worth its own full-length review, so the foregoing discussion will be limited to the bare essentials. Detailed discussion of the topic can be found in Krause and Rädler (1980); Moffatt (1978), and in the recent review article by Hoyng (2003).
The task at hand is to calculate the components of the and tensor in terms of the statistical properties of the underlying turbulence. A particularly simple case is that of homogeneous, weakly isotropic turbulence, which reduces the and tensor to simple scalars, so that the mean electromotive force becomeset al., 2001, for further discussion). At any rate, difficulties in computing and from first principle (whether as scalars or tensors) have led to these quantities often being treated as adjustable parameters of mean-field dynamo models, to be adjusted (within reasonable bounds) to yield the best possible fit to observed solar cycle characteristics, most importantly the cycle period. One finds in the literature numerical values in the approximate ranges for and for .
The cyclonic character of the -effect also indicates that it is equatorially antisymmetric and positive in the Northern solar hemisphere, except perhaps at the base of the convective envelope, where the rapid variation of the turbulent velocity with depth can lead to sign change. These expectations have been confirmed in a general sense by theory and numerical simulations (see, e.g., Rüdiger and Kitchatinov, 1993; Brandenburg et al., 1990; Ossendrijver et al., 2001).
Leaving the kinematic regime, it is expected that both and should depend on the strength of the magnetic field, since magnetic tension will resist deformation by the small-scale turbulent fluid motions. The groundbreaking numerical MHD simulations of Pouquet et al. (1976) suggested that Equation (21) should be replaced by something likeet al., 1993; Blackman and Brandenburg, 2002). This is rarely used in solar cycle modelling, since the whole point of the mean-field approach is to avoid dealing explicitly with the small-scale, fluctuating components. On the other hand, something is bound to happen when the growing dynamo-generated mean magnetic field reaches a magnitude such that its energy per unit volume is comparable to the kinetic energy of the underlying turbulent fluid motions. Denoting this equipartition field strength by , one often introduces an ad hoc nonlinear dependency of (and sometimes as well) directly on the mean-field by writing:
Adding this contribution to the MHD induction equation leads to the following form for the axisymmetric mean-field dynamo equations:5.
With the large-scale flows, turbulent diffusivity and -effect considered given, Equations (25, 26) become truly linear in and . It becomes possible to seek eigensolutions in the formproduct exceeds a certain critical value beyond which , corresponding to a growing solutions. Such solutions are said to be supercritical, while the solution with is critical.
Clearly exponential growth of the dynamo-generated magnetic field must cease at some point, once the field starts to backreact on the flow through the Lorentz force. This is the general idea embodied in -quenching. If -quenching - or some other nonlinearity - is included, then the dynamo equations are usually solved as an initial-value problem, with some arbitrary low-amplitude seed field used as initial condition. Equations (25, 26) are then integrated forward in time using some appropriate time-stepping scheme. A useful quantity to monitor in order to ascertain saturation is the magnetic energy within the computational domain:
One of the most remarkable property of the (linear) dynamo equations is that they support travelling wave solutions. This was first demonstrated in Cartesian geometry by Parker (1955), who proposed that a latitudinally-travelling “dynamo wave” was at the origin of the observed equatorward drift of sunspot emergences in the course of the cycle. This finding was subsequently shown to hold in spherical geometry, as well as for non-linear models (Yoshimura, 1975; Stix, 1976). Dynamo waves6 travel in a direction given bynegative -effect in the low latitudes of the Northern solar hemisphere.
We first consider models without meridional circulation ( in Equations (25, 26)), with the -term omitted in Equation (26), and using the diffusivity profile and angular velocity profile of Figure 5. We will investigate the behavior of models with the -effect operating throughout the bulk of the convective envelope (red line in Figure 6), as well as with an -effect concentrated just above the core-envelope interface (green line in Figure 6). We also consider two latitudinal dependencies, namely , which is the “minimal” possible latitudinal dependency compatible with the required equatorial antisymmetry of the Coriolis force, and an -effect concentrated towards the equator7 via an assumed latitudinal dependency .
It is noteworthy that co-existing dynamo branches, as in Panel B of Figure 7, can have distinct dynamo periods, which in nonlinearly saturated solutions leads to long-term amplitude modulation. This is typically not expected in dynamo models where the only nonlinearity present is a simple algebraic quenching formula such as Equation (24). A portion of the magnetic energy time-series for that solution is shown in Panel A of Figure 8 to illustrate the effect. Note that this does not occur for the solution (Panel B of Figure 8), where both branches propagate away from each other, but share a common latitude of origin and so are phased-locked at the onset (cf. Panel D of Figure 7).
Vector magnetograms of sunspots active regions make it possible to estimate the current helicity which is closely related to the usual magnetic helicity , and the amount of twist in the sunspot-forming toroidal flux ropes (see, e.g., Hagyard and Pevtsov, 1999, and references therein). Upon assuming that this current helicity reflects that of the diffuse, dynamo-generated magnetic field from which the flux ropes formed, one obtains another useful constraint on dynamo models. In the context of classical mean-field models, predominantly negative current helicity in the N-hemisphere, in agreement with observations, is usually obtained for models with negative -effect relying primarily on positive radial shear at the equator (see Gilman and Charbonneau, 1999, and discussion therein).
The models discussed above are based on rather minimalistics and partly ad hoc assumptions on the form of the -effect. More elaborate models have been proposed, relying on calculations of the full -tensor based on some underlying turbulence models. While this approach usually displaces the ad hoc assumptions away from the -effect and into the turbulence model, it has the definite advantage of offering an internally consistent approach to the calculation of turbulent diffusivities and large-scale flows. Rüdiger and Brandenburg (1995) remain a good example of the current state-of-the-art in this area; see also Rüdiger and Arlt (2003), and references therein.
From a practical point of view, the outstanding success of the mean-field model remains its robust explanation of the observed equatorward drift of toroidal field-tracing sunspots in the course of the cycle in terms of a dynamo-wave. On the theoretical front, the model is also buttressed by mean-field electrodynamics which, in principle, offers a physically sound theory from which to compute the (critical) -effect and magnetic diffusivity. The models’ primary uncertainties turn out to lie at that level, in that the application of the theory to the Sun in a tractable manner requires additional assumptions that are most certainly not met under solar interior conditions. Those uncertainties are exponentiated when taking the theory into the nonlinear regime, to calculate the dependence of the -effect and diffusivity on the magnetic field strength. This latter problem remains very much open at this writing.
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