Solar dynamo
Encyclopedia
The solar dynamo is the physical process that generates the Sun
's magnetic field
. The Sun is permeated by an overall dipole
magnetic field, as are many other celestial bodies such as the Earth
. The dipole field is produced by a circular electric current
flowing deep within the star, following Ampère's law
. The current is produced by shear (stretching of material) between different parts of the Sun that rotate at different rates, and the fact that the Sun itself is a very good electrical conductor (and therefore governed by the laws of magnetohydrodynamics
).
Any electrically conducting fluid can form a dynamo simply by shear within the fluid itself, because of a consequence of Lenz's law
of induction: moving the fluid through a pre-existing magnetic field will induce electrical currents in the fluid that distort the pre-existing magnetic field. The direction of the distortion is such that the existing field line
s tend to be dragged along with the fluid, like threads of dye embedded in taffy or syrup. If the flow has a strong shear component then the individual field lines are stretched by the flow, amplifying the existing magnetic field. Such systems are called MHD dynamos. Depending on the structure of the flow, the dynamo may be self-exciting and stable, self-exciting and chaotic, or decaying.
The Sun's dynamo is self-exciting: the direction of the field reverses itself about every 11 years, causing the sunspot cycle as ropes of magnetic field lines rise to the surface of the Sun and manifest as sunspots on the surface. The solar dynamo is thought to be located at the tachocline
, a region inside the Sun that exhibits a very large shear profile.
The detailed mechanism of the solar dynamo is not known and is the subject of current research.
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
's magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
. The Sun is permeated by an overall dipole
Dipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...
magnetic field, as are many other celestial bodies such as the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
. The dipole field is produced by a circular electric current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
flowing deep within the star, following Ampère's law
Ampère's law
In classical electromagnetism, Ampère's circuital law, discovered by André-Marie Ampère in 1826, relates the integrated magnetic field around a closed loop to the electric current passing through the loop...
. The current is produced by shear (stretching of material) between different parts of the Sun that rotate at different rates, and the fact that the Sun itself is a very good electrical conductor (and therefore governed by the laws of magnetohydrodynamics
Magnetohydrodynamics
Magnetohydrodynamics is an academic discipline which studies the dynamics of electrically conducting fluids. Examples of such fluids include plasmas, liquid metals, and salt water or electrolytes...
).
Any electrically conducting fluid can form a dynamo simply by shear within the fluid itself, because of a consequence of Lenz's law
Lenz's law
Lenz's law is a common way of understanding how electromagnetic circuits must always obey Newton's third law and The Law of Conservation of Energy...
of induction: moving the fluid through a pre-existing magnetic field will induce electrical currents in the fluid that distort the pre-existing magnetic field. The direction of the distortion is such that the existing field line
Field line
A field line is a locus that is defined by a vector field and a starting location within the field. Field lines are useful for visualizing vector fields, which are otherwise hard to depict...
s tend to be dragged along with the fluid, like threads of dye embedded in taffy or syrup. If the flow has a strong shear component then the individual field lines are stretched by the flow, amplifying the existing magnetic field. Such systems are called MHD dynamos. Depending on the structure of the flow, the dynamo may be self-exciting and stable, self-exciting and chaotic, or decaying.
The Sun's dynamo is self-exciting: the direction of the field reverses itself about every 11 years, causing the sunspot cycle as ropes of magnetic field lines rise to the surface of the Sun and manifest as sunspots on the surface. The solar dynamo is thought to be located at the tachocline
Tachocline
The tachocline is the transition region of the Sun between the radiative interior and the differentially rotating outer convective zone. It is in the outer third of the sun . This causes the region to have a very large shear as the rotation rate changes very rapidly...
, a region inside the Sun that exhibits a very large shear profile.
The detailed mechanism of the solar dynamo is not known and is the subject of current research.