Welcome to the Interface Dynamo Homepage. The solar dynamo
refers to the process whereby the Sun's large-scale magnetic
is cyclically regenerated every 22 years (for the full magnetic
cycle). This dynamo-generated magnetic field is ultimately
what powers all manifestations of solar activity. The study
of the dynamo problem is then not only interesting from the
standpoint of magnetohydrodynamics in and of
itelf, but also represents the first step in the
causal chain of physical mechanisms that links to Sun's
magnetic activity to its terrestrial influences. Interface
dynamos are one class of dynamo models that appear particularly promising
in explaining many observed properties of the solar magnetic field.
They are but one aspect of the work carried out in the
Solar
Interior Section
of the
High Altitude Observatory,
a scientific division of the
National Center for Atmospheric Research
in Boulder, Colorado.
Keeping with the bicycle example, imagine now a situation whereby the current produced by a bicycle's mechanical dynamo is used to power a small electric motor, itself used to propel the bicycle. Would the bicycle continue to move indefinitely once set in motion ? Of course not. This would be a perpetual motion machine, a known physical impossibility as per the Second Law of Thermodynamics. Energy must be continuously provided to the system, to offset losses due to ohmic dissipation (heat generated from electric current flowing in a medium having a high yet finite electrical conductivity), and, in the case of the bicycle example, air drag and mechanical friction in the wheel's axles. Yet the general notion that a dynamo-generated current can be fed back into the dynamo so as to further its growth, provided that an external energy source be available, forms the basis of magnetic field amplification in electrically conducting fluids.
There are no permanent magnets or wire loops inside the Sun. The solar interior is in a state called plasma, essentially a fluid made of highly ionized constituents, so that there is an abundance of free flowing electrical charges (although the fluid is globally neutral). In such a situation any magnetic field is effectively ``frozen'' into the plasma, in the sense that any bulk motion of the plasma must carry along the magnetic fieldlines. This opens the possibility of using fluid motions to distort a pre-existing magnetic field in such a way as to produce electric currents (as in the hydroelectric generator) which will themselves induce a secondary magnetic field (just as with electromagnets) which is then itself distorted by fluid motions to produce new electric currents, and so on. Remember, no perpetual motion machines ! This self-excited dynamo process requires an input of energy, here in the form of the mechanical (kinetic) energy of fluid motions. As it turns out, there is plenty of mechanical energy available in the solar interior. Part of it is in the form of rotational kinetic energy, another part in the form of small-scale, turbulent fluid motions, pervading the outer 30% in radius of the solar interior (the convection zone).
The solar dynamo problem consists in demonstrating that observed and inferred fluid motions in the solar interior can indeed cyclically regenerate the large-scale solar magnetic field in a manner compatible with what is inferred from the observational manifestations of the solar cycle (see slides 17 through 20 of the HAO Slide Set).
Analysis of helioseismic observations (as produced for example by the LOWL instrument) have shown that the observed surface latitudinal differential rotation, characterized by equatorial acceleration, persists to the base of the convection zone, below which the angular velocity rapidly converges to solid-body rotation at a rate equal to the surface mid-latitude. This has the interesting consequence that three distinct dynamo modes can exists in association with such internal angular velocity profiles:
Which of the three dynamo modes is preferentially excited at a given dynamo number depends on the ratio of magnetic diffusivities across the core-envelope interface, shear layer thickness, assumed angular dependency for the alpha-effect, etc.
What operationally distinguishes interface dynamos from other
mean-field alpha-omega dynamos is (1) the fact that the shear
and alpha-effect regions are spatially segregated, and (2)
they operate in regions having markedly different magnetic
diffusivities.
The following diagram shows the time history of the peak
(green) and r.m.s (yellow) toroidal magnetic field
above (dotted lines) and below (solid lines) the core-envelope
interface, for a supercritical equatorial interface mode (left)
and a supercritical hybrid mode (right). These solutions were
obtained using a specific form of growth limiting parametrization
compatible with the aforementioned numerical experiments.
Yet the toroidal magnetic field within the shear layer
(solid lines) manages to reach equipartition. This occurs because
those strong fields are spatially localized away from the region where
the most easily perturbed part of the dynamo cycle is operating.
Click here to view full size diagram
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Return to Solar Interior Home Page.
Last revised July 10, 1996 - P. Charbonneau
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