Aurorae and the Zodiacal Light

It is impossible to separate once science from another. Even astronomy is no longer "on its own", as it used to be. It is bound up closely with chemistry and physics, and it is also linked with weather study, or meteorology, by the phenomenon known as the Aurora Polaris, or Polar Light.

Aurorae have been known from very early times, and are so common in high latitudes that a night in North Norway or Antarctica would seem drab without them. In England they are less frequent, though displays are seen on an average at least ten times a year, while in the tropics they are rare. They are not unknown; there is a famous story of how the cohorts of the Roman emperor Tiberius once rushed northwards to the help of the people of Ostia because of a red glow in the sky that they took for a tremendous fire, but which proved to be merely an aurora. However, there can be no doubt that observers in the far north and south have the best views.

Aurorae occur in the upper atmosphere, at heights ranging from over 600 miles down to as low as 60. Sometimes the lights take the form of regular patterns, while at others they shift and change rapidly, often showing brilliant colours and providing a spectacle that is second only to the glory of a total solar eclipse. One of the greatest displays of modern times took place on January 25,1938, when all Britain witnessed the spectacle. From Cornwall "the whole of the western sky was lit with a vivid red glow like a huge neon sign; gradually shafts of white light were intermingled with the redness, changing quickly to an uncanny grey light and then to a brilliant silver, while green patches appeared here and there". From Sussex the dominating colour was red, though during the course of my own observations I recorded many other hues as well. The aurora was brilliant and widespread enough to cause interest and even alarm over the whole country, and it was seen from places as far south as Vienna (Fig. 8.1).

So far as England is concerned, there have been few comparable displays since then, but there were brilliant aurorae on 13 March 1989 and 8-9 November 1991; there were vivid hues, and from much of England the aurora cast obvious shadows. There was another good display on 6 April 2000, when again the predominant colour was red. But of course England is not in the "auroral zone". On average, aurorae are seen on 240 nights per year in North Alaska, Iceland and North Norway; 25 nights per year in central Scotland and along the Canada-USA border,

Fig. 8.1. Aurora Borealis, seen by the Yukon River-Circle, Alaska , in August 2000.

(Photograph by Dirk Obudzinski.)

Fig. 8.1. Aurora Borealis, seen by the Yukon River-Circle, Alaska , in August 2000.

(Photograph by Dirk Obudzinski.)

and only one night per year in Central France, while Greece and Italy may have no more than one or two good displays per century. The Southern Lights are of course on display from Antarctic, to be enjoyed mainly by the local penguins, but are sometimes seen from the South Island of New Zealand and even the southernmost tip of Africa.

Since meteorology is the science of the atmosphere, and aurorae are atmospheric phenomena, it might seem logical to assume that they are not the concern of astronomers. Yet the cause of aurorae is to be found not on the Earth, but in the Sun. Active regions on the Sun send out electrically charged particles, which stream across space to the Earth and enter the upper air, making it glow. Because the particles are electrified, they spiral down toward the magnetic poles, though the process is not quite so straightforward as might be thought. Auroral activity is more or less permanent at high latitudes (both north and south) along the so-called auroral ovals, which are "rings" symmetrically displaced around the magnetic poles. Thus from Tromso in Norway, latitude 69 degrees north, aurorae are commoner than they are as seen from the North Pole itself. When the streams of particles are particularly intense, the ovals broaden and expand, bringing auroral displays further north and south of the main regions.

Aurorae are high-altitude phenomena; generally they lie about 60 miles from ground level, and extend up to around 180 miles, though in extreme cases they may reach 300 miles. They always lie far above normal clouds, so that with an overcast sky there is no chance of seeing an auroral display of any kind. Active regions on the Sun are often associated with spots, so that aurorae are most frequent at or near spot maximum. Moreover, a major flare occurring near the centre of the solar disk is often followed a day later by a bright aurora. Since the particles must therefore cover the 93-million-mile gap in about 24 hours, this delay indicates a speed of about 1,000 miles per second.

One interesting and puzzling problem is that of "auroral noise". Hissing sounds, rustling and whistling have been reported on many occasions, and are extremely difficult to explain. Odours have also been reported, but smelly aurorae seem to be even more baffling than noisy aurorae.

Scientifically, aurorae are important not only because of their link with the Sun, but because they provide information about the upper air. It is therefore useful to observe them whenever possible, and to make estimates of their positions against the starry background, so that their heights may be worked out. The main work here has been done by Norwegian scientists, led by Professor Carl Stormer of Oslo; but amateurs can play a major role; a full-scale survey has been organized by the British Astronomical Associations's Aurora Section, which has members all over the world. Observers taking part are asked to fill in forms telling of the presence or absence of aurora, coupled with notes of any displays that may be seen. Negative reports are not without value, and may in fact be of great help.

The lights are so varied that to describe all the forms would need more pages. One never knows quite what an aurora is going to do next, but a great display often begins as a glow on the horizon, rising slowly to become an arc. After a while, the bottom of the arc brightens, sending forth streamers, after which the arc itself loses its regular shape and develops folds like those of a radiant curtain. If the streamers extend beyond the zenith, or overhead point of the sky, they converge in a patch to form a corona (not, of course, to be confused with the corona which surrounds the Sun). Finally the display sends waves of light flaming up from the horizon towards the zenith, after which the light dies gradually away. The whole phenomenon may extend over hours.

For observing aurorae, by far the best instrument is the naked eye, coupled with a red torch and a reliable watch. Binoculars are of little help, and telescopes absolutely useless. Points to note are the bearing of the centre of the display, reckoned in degrees (0 to 360) from north round by east; the type and prominence of the aurora; the various forms seen, such as arcs, curtains, draperies and flaming surges; colours, and duration. Times should be taken at least to the nearest minute. There is obvious scope for the photographer, and spectroscopic work is of great interest, but simple naked-eye observation is not to be despised.

Though aurorae are so spectacular, they are not the only lights seen in the heavens. The sky itself seems to shine with a feeble radiance known as the airglow, and sometimes a cone of light can be seen after dusk or before dawn, extending upwards from the hidden Sun and tapering towards the zenith. Since it extends along the Zodiac, this cone is known as the Zodiacal Light. It can be quite prominent when seen from countries where the air is clear and dust-free, but from Britain it is always hard to see. The Zodiacal Band, a faint, parallel-sided extension of the cone, may extend right across the sky to the far horizon, though it is so dim that it is seldom to be observed at all except from the tropics.

Unlike the aurora, the Zodiacal Light originates well beyond the top of the air. It is due to light reflected from a layer of thinly-spread matter extending from the Sun out beyond the orbit of the Earth, rather like a tremendous plate. The layer cannot be broad, as is shown by the fact that the Light is never seen except close to the ecliptic. The best times for observations are late evenings in March and early mornings in September, because at these times the ecliptic is most nearly perpendicular to the horizon, and the Light is thus higher in the sky (Fig. 8.2).

Since the Zodiacal Light is faint, so its intensity is not easy to estimate. The best way to measure it is to compare it with a definite area of the Milky Way, and the width of the base, in degrees, should also be noted. Though the Light is predominately white, a pinkish or at least warmish glow has been reported in the lower parts, and should be looked for.

Last and most elusive of these glows is the Gegenschein, which is a faint hazy patch of light always seen exactly opposite the Sun in the sky. It appears at its most conspicuous in September, when it looks like a round luminous patch about forty times the apparent width of the Moon, but it is extremely hard to see, and even a distant lamp is enough to hide it. From England I have looked for it frequently, but have seen it only once in 1942, when the whole country was blacked out because of German air-raids. It is excessively difficult to photograph, though with the latest equipment some good pictures have been obtained. The German name is generally used; the English equivalent is the "Counterglow".

For all these observations, one thing should be borne in mind: Never begin work before you have made your eyes thoroughly accustomed to the dark. To come outdoors from a brilliantly-lit room and expect to see an auroral glow or the Zodiacal Light straight away is fruitless, and it is usually necessary to walk about

for at least a quarter of an hour before starting your programme, though the exact period is bound to vary with different people. For recording observations, a torch with a red bulb is the ideal, since an ordinary white light will dazzle you sufficiently to ruin the sensitivity of your eyes for some minutes afterwards.

Here again, then, the amateur has a part to play. There is no need to wait years for a great aurora; studying the fainter lights and glows is a fascinating hobby, and it is a pity that city dwellers never have a chance to see the ghostly beauty of the Zodiacal Light.

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