4.5 The solar dynamo

There are many recent reviews on the solar dynamo so there is no need for a detailed discussion here. For a comprehensive overview of solar dynamo theory as a whole an excellent place to start is the recent article by (Ossendrijver, 2003). Mean-field models of the solar activity cycle are reviewed in these volumes by Charbonneau (2005). Tobias (2004) focuses on the role of the solar tachocline in particular. Further details and perspectives on solar and stellar dynamos are provided by Weiss (1994), Mestel (1999), Schrijver and Zwaan (2000), and Rüdiger and Hollerbach (2004). Dynamo theory from a more general astrophysical perspective has been reviewed comprehensively by Moffatt (1978), Parker (1979), Childress and Gilbert (1995), and most recently by Brandenburg and Subramanian (2004).

Some insight into the nature of solar dynamo processes may be obtained from the evolution equation for the mean field, which is just the longitudinal average of Equation (42):

The term involving represents field generation by turbulent convection or other processes, such as shear instabilities (see Section 8.2). Note that our derivation of Equation (20) involves no additional approximations beyond the standard anelastic (or compressible) MHD equations. However, this equation is the starting point for mean-field dynamo theory in which additional approximations are made in order to make the system more tractable. In many mean-field models the rotation profile and the meridional circulation are specified and the Lorentz force is neglected, making the approach kinematic. Some type of parameterization is then introduced for the turbulent emf and Equation (20) is solved for .

The simplest and most common parameterization may be derived by exploiting the linearity of the induction equation in (neglecting Lorentz force feedback on ) and by assuming scale separation between the mean and fluctuating fields. The problem can be further simplified by assuming that the fluctuations are pseudo-isotropic, meaning that their statistics are invariant under rotation of the coordinate system but not necessarily invariant under reflection. In this case the turbulent emf may be represented in terms of the mean field as:

In analogy with Equation (22), we will in this paper loosely refer to the -effect in the general sense of field generation via the turbulent emf term in Equation (20). This does not necessarily imply that the parameterization in Equation (22) is an accurate one. In practice, solar dynamo processes may be much more subtle than this simple expression suggests (see Section 6.5). Still, the classical -effect is a useful concept and remains an important ingredient of dynamo theory.

Unlike the -effect, the -effect can work both ways: it may convert toroidal field energy to poloidal field energy or vice versa. The field conversion and amplification process is often associated with vorticity and shear as in the classical scenario, first described by Parker (1955), in which field lines are lifted and twisted by helical eddies. In the special case of homogeneous, pseudo-isotropic turbulence, the parameter is directly proportional to the mean kinetic helicity of the flow, (Moffatt, 1978; Ossendrijver, 2003). Rotation induces vorticity and breaks the reflection symmetry of the fluid equations, so rotating flows are generally helical and tend to be efficient dynamos, although rotation is not required for sustained dynamo action (Cattaneo et al., 2003).

Although Equation (20) only strictly applies to the mean (longitudinally-averaged) field (or some other suitable spatial or ensemble average), similar processes also operate on fluctuating (non-axisymmetric) fields. All toroidal field structures are amplified to some extent by rotational shear and processes akin to the (generalized) -effect generate magnetic energy on a wide range of spatial scales. Most solar dynamo models focus on the axisymmetric component of the field but observations indicate that the magnetic field structure in the solar photosphere and corona is quite complex, with a large non-axisymmetric component (see Section 3.8 and Figure 4). Solar variability is dominated not by mean fields but by localized structures such as active regions, filaments, and coronal loops.

Our current paradigm for how the solar dynamo operates is illustrated in Figure 8. The density stratification tends to make solar convection highly anisotropic, characterized by relatively weak, broad upflows amid a complex, evolving network of strong downflow lanes and plumes (0). Turbulent downflow plumes possess substantial vorticity and helicity which may amplify fields through the -effect (1). These fields are then pumped downward by the anisotropic convection and accumulate in the overshoot region and tachocline (2). Intermittent plumes may dredge up some of this flux and return it to the convection zone where it may be further amplified and again pumped down. Differential rotation in the tachocline stretches and amplifies this disorganized field into strong, coherent toroidal flux tubes and sheets (3). As the field becomes stronger, it eventually becomes buoyantly unstable and rises toward the surface (4). The Coriolis force acting on these rising structures twists them in a systematic way which depends on latitude (5). Weaker structures may be shredded by turbulent convection in the envelope and the flux is then recycled (6). Stronger fields and configurations (e.g., twisted tubes) remain coherent throughout the convection zone and emerge from the surface as bipolar active regions (7). Large-scale poloidal fields may be generated by the -effect (1) or by the turbulent diffusion of surface flux after the tubes have emerged (7). Due to the manner in which field is amplified by the -effect (3) and to the tilts induced in surface active regions due to the Coriolis force (5), surface diffusion would tend to build large-scale poloidal fields opposite in sign to the prevailing field, eventually producing a global polarity reversal.

Figure 8: Schematic illustration of the solar dynamo. Numbers indicate particular processes as described in the text (courtesy N. Brummell).

"Large-Scale Dynamics of the Convection Zone and Tachocline" Mark S. Miesch http://www.livingreviews.org/lrsp-2005-1

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