In physics, the dynamo theory proposes a mechanism by which a celestial body such as Earth or a star generates a magnetic field. The dynamo theory describes the process through which a rotating, convecting, and electrically conducting fluid can maintain a magnetic field over astronomical time scales. A dynamo is thought to be the source of the Earth's magnetic field and the magnetic fields of Mercury and the Jovian planets.

History of theory

When William Gilbert published de Magnete in 1600, he concluded that the Earth is magnetic and proposed the first hypothesis for the origin of this magnetism: permanent magnetism such as that found in lodestone. In 1822, André-Marie Ampère proposed that internal currents are responsible for Earth magnetism ^[2] . In 1919, Joseph Larmor proposed that a dynamo might be generating the field.^[3][4] However, even after he advanced his hypothesis, some prominent scientists advanced alternative explanations. The Nobel Prize winner Patrick Blackett did a series of experiments looking for a fundamental relation between angular momentum and magnetic moment, but found none.^[5][6]

Walter M. Elsasser, considered a "father" of the presently accepted dynamo theory as an explanation of the Earth's magnetism, proposed that this magnetic field resulted from electric currents induced in the fluid outer core of the Earth. He revealed the history of the Earth's magnetic field through pioneering the study of the magnetic orientation of minerals in rocks.

In order to maintain the magnetic field against ohmic decay (which would occur for the dipole field in 20,000 years), the outer core must be convecting. The convection is likely some combination of thermal and compositional convection. The mantle controls the rate at which heat is extracted from the core. Heat sources include gravitational energy released by the compression of the core, gravitational energy released by the rejection of light elements (probably sulfur, oxygen, or silicon) at the inner core boundary as it grows, latent heat of crystallization at the inner core boundary, and radioactivity of potassium, uranium and thorium.^[7]

At the dawn of the 21st century, numerical modeling of the Earth's magnetic field has not been successfully demonstrated. Initial models are focused on field generation by convection in the planet's fluid outer core. It was possible to show the generation of a strong, Earth-like field when the model assumed a uniform core-surface temperature and exceptionally high viscosities for the core fluid. Computations which incorporated more realistic parameter values yielded magnetic fields that were less Earth-like, but indicated that model refinements ^[which?] may ultimately lead to an accurate analytic model. Slight variations in the core-surface temperature, in the range of a few millikelvins, result in significant increases in convective flow and produce more realistic magnetic fields.^[8][9]

Formal definition

Dynamo theory describes the process through which a rotating, convecting, and electrically conducting fluid acts to maintain a magnetic field. This theory is used to explain the presence of anomalously long-lived magnetic fields in astrophysical bodies. The conductive fluid in the geodynamo is liquid iron in the outer core, and in the solar dynamo is ionized gas at the tachocline. Dynamo theory of astrophysical bodies uses magnetohydrodynamic equations to investigate how the fluid can continuously regenerate the magnetic field.^[10]

It was once believed that the dipole, which comprises much of the Earth's magnetic field and is misaligned along the rotation axis by 11.3 degrees, was caused by permanent magnetization of the materials in the earth. This means that dynamo theory was originally used to explain the Sun's magnetic field in its relationship with that of the Earth. However, this hypothesis, which was initially proposed by Joseph Larmor in 1919, has been modified due to extensive studies of magnetic secular variation, paleomagnetism (including polarity reversals), seismology, and the solar system's abundance of elements. Also, the application of the theories of Carl Friedrich Gauss to magnetic observations showed that Earth's magnetic field had an internal, rather than external, origin.

There are three requisites for a dynamo to operate:

• An electrically conductive fluid medium
• Kinetic energy provided by planetary rotation
• An internal energy source to drive convective motions within the fluid.^[11]

In the case of the Earth, the magnetic field is induced and constantly maintained by the convection of liquid iron in the outer core. A requirement for the induction of field is a rotating fluid. Rotation in the outer core is supplied by the Coriolis effect caused by the rotation of the Earth. The Coriolis force tends to organize fluid motions and electric currents into columns (also see Taylor columns) aligned with the rotation axis. Induction or generation of magnetic field is described by the induction equation:

where u is velocity, B is magnetic field, t is time, and ${\displaystyle \eta =1/(\sigma \mu )}$ is the magnetic diffusivity with ${\displaystyle \sigma }$ electrical conductivity and ${\displaystyle \mu }$ permeability. The ratio of the second term on the right hand side to the first term gives the magnetic Reynolds number, a dimensionless ratio of advection of magnetic field to diffusion.

Tidal heating supporting a dynamo

Tidal forces between celestial orbiting bodies cause friction that heats up their interiors. This is known as tidal heating, and it helps keep the interior in a liquid state. A liquid interior that can conduct electricity is required to produce a dynamo. Saturn's Enceladus and Jupiter's Io have enough tidal heating to liquify their inner cores, but they may not create a dynamo because they cannot conduct electricity.^[12][13] Mercury, despite its small size, has a magnetic field, because it has a conductive liquid core created by its iron composition and friction resulting from its highly elliptical orbit.^[14] It is theorized that the Moon once had a magnetic field, based on evidence from magnetized lunar rocks, due to its short-lived closer distance to Earth creating tidal heating.^[15] An orbit and rotation of a planet helps provide a liquid core, and supplements kinetic energy that supports a dynamo action.

Kinematic dynamo theory

In kinematic dynamo theory the velocity field is prescribed, instead of being a dynamic variable: The model makes no provision for the flow distorting in response to the magnetic field. This method cannot provide the time variable behaviour of a fully nonlinear chaotic dynamo, but can be used to study how magnetic field strength varies with the flow structure and speed.

Using Maxwell's equations simultaneously with the curl of Ohm's law, one can derive what is basically a linear eigenvalue equation for magnetic fields (B), which can be done when assuming that the magnetic field is independent from the velocity field. One arrives at a critical magnetic Reynolds number, above which the flow strength is sufficient to amplify the imposed magnetic field, and below which the magnetic field dissipates.

Practical measure of possible dynamos

The most functional feature of kinematic dynamo theory is that it can be used to test whether a velocity field is or is not capable of dynamo action. By experimentally applying a certain velocity field to a small magnetic field, one can observe whether the magnetic field tends to grow (or not) in response to the applied flow. If the magnetic field does grow, then the system is either capable of dynamo action or is a dynamo, but if the magnetic field does not grow, then it is simply referred to as “not a dynamo”.

An analogous method called the membrane paradigm is a way of looking at black holes that allows for the material near their surfaces to be expressed in the language of dynamo theory.

Spontaneous breakdown of a topological supersymmetry

Kinematic dynamo can be also viewed as the phenomenon of the spontaneous breakdown of the topological supersymmetry of the associated stochastic differential equation related to the flow of the background matter.^[16] Within stochastic supersymmetric theory, this supersymmetry is an intrinsic property of all stochastic differential equations, its interpretation is that the model's phase space preserves continuity via continuous time flows. When the continuity of that flow spontaneously breaks down, the system is in the stochastic state of deterministic chaos.^[17] In other words, kinematic dynamo arises because of chaotic flow in the underlying background matter.

Nonlinear dynamo theory

The kinematic approximation becomes invalid when the magnetic field becomes strong enough to affect the fluid motions. In that case the velocity field becomes affected by the Lorentz force, and so the induction equation is no longer linear in the magnetic field. In most cases this leads to a quenching of the amplitude of the dynamo. Such dynamos are sometimes also referred to as hydromagnetic dynamos.^[18] Virtually all dynamos in astrophysics and geophysics are hydromagnetic dynamos.

The main idea of the theory is that any small magnetic field existing in the outer core creates currents in the moving fluid there due to Lorentz force. These currents create further magnetic field due to Ampere's law. With the fluid motion, the currents are carried in a way that the magnetic field gets stronger (as long as ${\displaystyle \;\mathbf {u} \cdot (\mathbf {J} \times \mathbf {B} )\;}$ is negative^[19]). Thus a "seed" magnetic field can get stronger and stronger until it reaches some value that is related to existing non-magnetic forces.

Numerical models are used to simulate fully nonlinear dynamos. The following equations are used:

• The induction equation, presented above.
• Maxwell's equations for negligible electric field:
  $${\displaystyle {\begin{aligned}&\nabla \cdot \mathbf {B} =0\\&\nabla \times \mathbf {B} =\mu _{0}\mathbf {J} \end{aligned}}}$$

  
• The continuity equation for conservation of mass, for which the Boussinesq approximation is often used:
  $${\displaystyle \nabla \cdot \mathbf {u} =0,}$$

  
• The Navier-Stokes equation for conservation of momentum, again in the same approximation, with the magnetic force and gravitation force as the external forces:
  $${\displaystyle {\frac {D\mathbf {u} }{Dt}}=-{\frac {1}{\rho _{0}}}\nabla p+\nu \nabla ^{2}\mathbf {u} +\rho '\mathbf {g} +2{\boldsymbol {\Omega }}\times \mathbf {u} +{\boldsymbol {\Omega }}\times {\boldsymbol {\Omega }}\times \mathbf {R} +{\frac {1}{\rho _{0}}}\mathbf {J} \times \mathbf {B} ~,}$$

  
  where ${\displaystyle \,\nu \,}$ is the kinematic viscosity, ${\displaystyle \,\rho _{0}\,}$is the mean density and ${\displaystyle \rho '}$ is the relative density perturbation that provides buoyancy (for thermal convection ${\displaystyle \;\rho '=\alpha \Delta T\;}$ where ${\displaystyle \,\alpha \,}$ is coefficient of thermal expansion), ${\displaystyle \,\Omega \,}$ is the rotation rate of the Earth, and ${\displaystyle \,\mathbf {J} \,}$ is the electric current density.
• A transport equation, usually of heat (sometimes of light element concentration):
  $${\displaystyle {\frac {\,\partial T\,}{\partial t}}=\kappa \nabla ^{2}T+\varepsilon }$$

  
  where T is temperature, ${\displaystyle \;\kappa =k/\rho c_{p}\;}$ is the thermal diffusivity with k thermal conductivity, ${\displaystyle \,c_{p}\,}$ heat capacity, and ${\displaystyle \rho }$ density, and ${\displaystyle \varepsilon }$ is an optional heat source. Often the pressure is the dynamic pressure, with the hydrostatic pressure and centripetal potential removed.

These equations are then non-dimensionalized, introducing the non-dimensional parameters,

where R_a is the Rayleigh number, E the Ekman number, P_r and P_m the Prandtl and magnetic Prandtl number. Magnetic field scaling is often in Elsasser number units ${\displaystyle B=(\rho \Omega /\sigma )^{1/2}.}$

Energy conversion between magnetic and kinematic energy

The scalar product of the above form of Navier-Stokes equation with ${\displaystyle \;\rho _{0}\mathbf {u} \;}$ gives the rate of increase of kinetic energy density, ${\displaystyle \;{\tfrac {1}{2}}\rho _{0}u^{2}c\;}$, on the left-hand side. The last term on the right-hand side is then ${\displaystyle \;\mathbf {u} \cdot (\mathbf {J} \times \mathbf {B} )\;}$, the local contribution to the kinetic energy due to Lorentz force.

The scalar product of the induction equation with $\frac{1}{{\mathrm{\mu <!--\; \mu \; -->}}_{}}$Zdroj:https://en.wikipedia.org?pojem=Geodynamo
Text je dostupný za podmienok Creative Commons Attribution/Share-Alike License 3.0 Unported; prípadne za ďalších podmienok. Podrobnejšie informácie nájdete na stránke Podmienky použitia.

---