Magnetohydrodynamic Dynamo
A magnetohydrodynamic dynamo (or MHD dynamo) generates a magnetic field through a flow of electrically conductive material. The dynamo theory provides a theoretical description . The magnetic fields of the earth, the sun and other astronomical objects are generated by dynamo processes.
principle
The principle of a dynamo is that a flow of electrically conductive material (e.g. liquid metal or plasma ) induces electrical currents in an initially existing magnetic field. These currents are in turn accompanied by a magnetic field which - under favorable conditions - can strengthen the original magnetic field. If this amplification compensates for dissipative losses in the field, a dynamo process is created. Here, kinetic energy is transferred from the flow into magnetic energy.
Magnetohydrodynamic dynamos, also called homogeneous dynamos, differ from generators (technical dynamos) in that in the latter the current conduction is predetermined by insulated lines and electrical power is emitted, while in MHD dynamos a more or less homogeneous electrically conductive material is present and only the energy contained in the static field is generated. However, the generated static field rotates in such a way (see cowling theorem ) that it can in turn induce currents in a conductive environment; however, the power required for this comes from the rotary motion.
mechanism
The basic mechanism now seems well understood. As a rule, two physical causes intervene to strengthen the magnetic field. To speak of magnetic field lines in the picture:
The transverse to the flow field lines are 1. (added at a spherical object by a differential rotation) through an existing shear flow in the direction of the flow drawn . This effect can be seen from the fact that magnetic field lines in highly conductive matter can be thought of as frozen . The energy density of the magnetic field is thereby increased, the energy comes from the kinetic energy. This effect is called the Ω effect after the common symbol for angular velocity.
2. A second mechanism must now ensure that the magnetic field component generated in this way, which points in the direction of flow, is again bent across the flow . This prevents the initial magnetic field from weakening. In the sun this is given by the so-called α-effect: This describes the ascent of magnetic flux tubes as a result of the buoyancy with simultaneous twisting as a result of the Coriolis force .