There exist various mechanisms capable of limiting the magnitude of the (presumably) dynamo-generated, large-scale solar magnetic field. One such mechanism is the so-called ``alpha -quenching''. The underlying idea is that the Lorentz force associated with the dynamo-generated magnetic fields impedes the small scale, turbulent fluid motions giving rise to the so-called ``alpha -effect'' (the production of poloidal from toroidal fields in the framework of mean-field dynamo theory). In mean-field models, a popular ---yet essentially ad hoc--- prescription for alpha -quenching consists in replacing the coefficient (alpha ) of the alpha -effect source term in the dynamo equations by an expression of the form alpha -> alpha (B) =alpha_0 /(1+(|B|/B_eq)(2)) , where alpha_0 is a measure of the strength of the alpha -effect in the linear regime, and B_eq is the equipartition field strength, based on the kinetic energy of the turbulent, convective fluid motions (B_eq ~ 10(4) G at the base of the solar convection zone). In principle, such ``Weak Quenching'' allows the production of magnetic fields of roughly equipartition strength, as demonstrated by the numerous conventional mean-field dynamo models making use of eq. (1), or some close variant, published to date. Vainshtein & Cattaneo (1992, ApJ 393, 165) and Gruzinov & Diamond (1995, Phys. Plasmas 2, 1941) have argued, however, that alpha -quenching should be described by alpha -> alpha (B) =alpha_0 /(R_m(|B|/B_eq)(2)) where R_m is a magnetic Reynolds number based on the microscopic properties of the flow (R_m>> 1 for solar interior conditions). This now describes a much stronger form of alpha -quenching, and, with R_m>> 1, could be fatal to large-scale dynamo action, in the sense that the dynamo could only produce magnetic fields of strength << B_eq. This is in marked contradiction with the demands set by recent models of bipolar magnetic region emergence, which require field strengths of order 10x B_eq ~ 10(5) G for the observed latitudes and tilt of emergence to be adequately reproduced. In this contribution, we investigate the circumstances under which interface dynamos can avoid alpha -quenching, either in the ``Weak'' or ``Strong'' forms defined above. In interface dynamos the alpha -effect is assumed to operate within the solar convective envelope, while the strongest magnetic fields are generated by shearing below the core-envelope interface (Parker 1993, ApJ 408, 707; Charbonneau & MacGregor, submitted to ApJ). This spatial segregation of the alpha -effect source region is the key to avoiding alpha -quenching. This is illustrated using a few nonlinear, kinematic interface dynamo solutions applicable to the Sun.
American Astronomical Society Meeting Abstracts #188
- Pub Date:
- May 1996