magnetohydrodynamic waves in the Earth (MHD waves) Magnetohydrodynamic waves (
Alfvén waves) are produced by coupling forces between the magnetic field and highly conductive fluids. To understand magnetohydrodynamic waves, let us first consider purely magnetic waves. If we replace the usual strings of a guitar with strings made up of a permanently magnetized material and pluck one of the strings, we can observe the physical vibration of the string. Because the string is magnetized, the magnetic field surrounding it will also vibrate with the string. A magnetometer placed near the guitar will be able to detect the changes in the intensity of the magnetic field with time that are created by the oscillations of the string. If we use a continuously recording, fluxgate-type magnetometer (see
geomagnetic measurement), the measurements of the magnetic field intensity will vary systematically in time, with a period corresponding to the mechanical vibration frequency of the string. We can thus show that magnetic waves can be generated by the vibration of a magnetized string.
Let us imagine that we could further replace the strings with the magnetic lines of force (the field lines) themselves, but could retain their elastic properties. Also, to pluck a string, let us squirt jets either of charged particles or of highly conductive fluid across the string. The permanently magnetized string that we first considered vibrates when plucked, and so would the imaginary elastic field line when the fluid was squirted across it. Once again, we could deduce with fluxgate magnetometers that waves were generated within our imaginary string (the field line) and that they travelled most efficiently along the string. These are Alfvén waves.
The outer core of the Earth is not permanently magnetized because it is at a temperature above the
Curie temperature (the temperature below which a material can become magnetic) of its constituents, a molten alloy of iron and nickel. The electrical conductivity of this liquid is high enough to conduct the electric currents that generate the Earth's magnetic field (see
magnetic field). Physical motion of the fluids (i.e. hydrodynamics) can ‘carry’ the magnetic field along. This ability of a highly conducting material to carry the magnetic field along with it is known as the frozen-in flux model, the subject of H. Alfvén's Nobel-prize-winning theorem. The complicated oscillatory motions of the conducting liquid are influenced by the Earth's rotation and the thermo-compositional convection in the outer core (leading to more complex Magnetic– Archimedean–Coriolis (MAC) waves, so named by S. I. Braginsky in the 1960s). In addition to MAC waves, there are also other kinds of MHD waves. In the guitar-string experiment we needed continuously recorded magnetic data to determine the frequency of vibration of the string. Similarly, scientists who study magnetic processes in the core need at least a semi-continuous record of variations in the Earth's magnetic field. Such data come mainly from magnetic observations and from certain palaeomagnetic investigations. Cyclic variations in the observed values can help in evaluating physical models of the fluid motion in the outer core; with sufficient data, it may perhaps be possible to elucidate the origin of the Earth's main field.
Dhananjay Ravat
