Submitted by admin on April 22, 2009 - 21:09
The first definite record of the appearance of the aurora australis was made by Captain Cook in his southern voyages in 1773. Apart from general reports on exceptional displays, no serious study of the phenomenon was carried out in New Zealand until the late 1920s when I. L. Thomsen and M. Geddes, working as amateurs, followed a plan of research. This activity was accepted as an official programme of the newly established Carter Observatory in 1939, and a detailed record was secured up to 1958. Due to the enthusiasm of some hundreds of volunteers, a coverage was secured over the whole of New Zealand, Tasmania, and parts of Australia. Since 1958 the Department of Scientific and Industrial Research has organised intensive research with modern instruments such as spectrographs, all-sky cameras, and radar, both in New Zealand and in the Antarctic. This work was also stimulated by the requirements of the International Geophysical Year (IGY), during 1957–58.
Although the full detailed mechanism of the cause of the aurora is not yet known, it seems quite certain that atomic particles coming from the sun and entering our atmosphere cause the characteristic “lights”. As these atomic particles are charged their paths are deflected by the earth's magnetic field; hence the aurorae occur most frequently near the polar regions. At times of intense activity, however, they can make an appearance even in the tropics. General auroral frequency is similar to that of sunspot activity fluctuations in a period of 11 years, and many individual instances are on record of the occurrence of brilliant aurorae at the time of the appearance of large active sunspots. These factors clearly show the connection with solar events.
Two principal appearances are noted with aurorae–the arc and ray types. Each may appear separately or in combination. The arc is a diffuse wide band of light extending across the southern horizon, while rays have an appearance analogous to searchlight beams. In general, all types are common both to the aurora borealis (or “northern lights” in the northern hemisphere) and to the aurora australis. Arcs occur in a region of the atmosphere 100 km above the earth, while rays can extend from this level to 500 km.
During intense auroral displays, long distance radio reception can be adversely affected, as may be inferred from the fact that aurorae inhabit the same region as the E-layer of the ionosphere (100 km above the earth). The E-layer is one of the reflecting regions for radio waves of certain frequencies, making possible long distance communications.
Foreknowledge of probable sudden ionospheric disturbances of this nature is of value to radio communication engineers, and for some years the Carter Observatory has supplied such a prediction service with some measure of success both to the New Zealand Post Office and to the New Zealand Broadcasting Corporation.
by Ivan Leslie Thomsen, F.R.A.S.(LOND.), Director, Carter Observatory, Wellington.