ELF/VLF Research at Ryvingen
Man has been aware of the earth’s magnetic field since ancient times, through its effect on the lodestone. Until about thirty years ago it was assumed from observations near the earth’s surface, that this held extended indefinitely out into space as if there were a powerful dipole magnet ( similar to the common bar magnet) at the centre of the earth. It was also thought that the earth’s atmosphere became steadily less and less dense with altitude until it blended with the cold emptiness of interplanetary space. But with the coming of the space age, satellite-borne experiments revealed a much more complicated picture.
The earth’s field is confined within a space around the earth known as the magnetosphere. It is greatly distorted from a simple dipole field by the solar wind, a stream of electrons and ions which blows from the sun continuously. The magnetosphere is compressed on the sun’s side of the earth and extends only to about 10 times the earth’s radius, but on the down-wind side it is dragged out to very great distances. It is in a state of constant agitation caused by fluctuations in solar radiation, and, far from being cold and empty, is filled with a rich mixture of various kinds of waves and charged particles interacting with each other – a plasma.
The properties of this plasma are largely fixed by the magnetic field; some of the particles are, indeed, trapped by the field to form the so-called radiation belts and may be accelerated to energies of up to millions of electron-volts, corresponding to very high plasma temperatures. Solar-wind energy flowing towards earth partially penetrates the magnetosphere and becomes stored in the plasma. From there it constantly leaks to the lower atmosphere and gives rise to such phenomena as the polar aurora, ionospheric disturbances and even, according to recent evidence, effects on the weather. This behaviour is much more obvious at times of high solar activity and whenever there is a sub-storm, a kind of explosive oscillation of the whole magnetosphere which happens suddenly; sub-storms occur at intervals ranging from a few hours to days.
In short, the magnetosphere is a very complicated region which is still imperfectly understood, in spite of intensive research since it was discovered.
Perhaps one of the most interesting of the waves found in the magnetosphere is the whistler, a natural radio signal of very low frequency, about 1—10 kHz. It takes its name from the characteristic tone of descending pitch that we hear when it is converted into sound waves. The theory of how whistlers are produced was first put forward by L. R. 0. Storev, working in Cambridge in 1953, and is now well established. They originate in the pulses of electromagnetic radiation that thunderstorms generate through lightning discharges. Some of the energy in the very-low-frequency part of the spectrum they emit penetrates upwards through the ionosphere and becomes guided along lines of force of the earth’s magnetic field, in field-aligned ducts of intense ionisation. The magnetospheric plasma through which the signals travel is a dispersive medium; that is, it spreads out the pulse in such a way that the higher frequencies in the signal travel faster than the lower frequencies and therefore arrive sooner at the other end of the field line, in the region magnetically conjugate to the lightning source. So we hear the higher frequencies first.
Apart from being fascinating in their own right, whistlers can help to unravel complex structure and motions in the magnetosphere. How much a whistler is dispersed is related to the density of electrons and of the magnetic field in the plasma through which it has passed; so, if we measure the dispersion, we can deduce the maximum altitude reached by the whistler signal and the electron density at that point. Because of the geometry of the earth’s field, whistlers can penetrate far into the magnetosphere. For example, a lightning discharge occurring at a magnetic latitude of 6o degrees may produce whistlers travelling out to, perhaps, four earth’s radii, about 25,000 km from earth. They can be received on the ground with fairly simple equipment; basically a loop aerial and a low-noise audio amplifier are all we need to probe a large region of the magnetosphere, without resorting to an expensive satellite experiment. The technique is analogous to the use of pulsed high-frequency radio waves for sounding the ionosphere, but in this case the probing signals are supplied by nature.
There were great advantages in being able to use Transglobe’s base at Ryvingen during the winter of 1980. It is situated at an ideal magnetic latitude about 600 for studying the plasmapause, a field-aligned boundary in the magnetosphere across which the plasma density changes suddenly by a factor of about 100. This important feature of the magnetosphere was discovered from whistler data by D. L. Carpenter of Stanford University. In spite of its medium magnetic latitude, Ryvingen has a high geographic ‘atitude and, therefore, long hours of darkness in winter, so the lower layers of the ionosphere usually produced by ultra-violet radiation from the sun are thin and relatively transparent to whistlers. This, combined with a number of good thunderstorms in the conjugate area in the North Atlantic south of Greenland, means that large numbers of intense whistlers can be received at Ryvingen. Furthermore, an important advantage of Antarctica for very-low-frequency observations is the extremely small amount of local interference. In more accessible regions, electrical noise from thunderstorms and from industrial plant and power distribution grids poses severe problems.
The receiver used at Ryvingen was developed at Sheffield and is a goniometer, or direction-finding receiver. Measuring the whistlers’ direction of arrival makes the whistler technique more powerful because it provides longitudinal information about the field-aligned ducts that cannot be deduced from the whistler dispersion. Basically, the equipment consists of two large. vertical loop aerials (enclosing an area of 6o m2) set at right angles to each other and each connected to an amplifier; the direction from which a signal arrives is found by comparing the signals picked up by the two aerials. The data are recorded on magnetic tape.