OBSERVATION OF AURORAE

Southland Times, Issue 23651, 28 October 1938, Page 4

OBSERVATION OF AURORAE

INVESTIGATIONS IN SOUTHLAND

DISPLAYS FREQUENTLY VISIBLE In a period of maximum activity such as the present year, displays of the aurora australis visible in Southland occur as often as 100 times a year. The frequency of displays coincides with periods of maximum sunspot activity which existed in a cycle of 11 years. These points were emphasized by Mr Murray Geddes, of South Hillend, director of auroral investigations in Southland, in lantern lecture at a meeting of the Southland branch of the Royal Society of New Zealand last night. Mr Geddes said that a great deal was known about the aurora borealis because it could be seen and studied from many countries. On the other hand very little was known about the aurora australis because it could be seen and studied from only two countries —New Zealand and New Zealand offered the better opportunities for observation, particularly in the most southerly localities. It was a duty, therefore, for persons living in the south of New Zealand to assist scientific investigation by studying occurrences of the aurora. It was thought by many people that aurora occurred only occasionally, he said, but in fact during periods of maximum activity they occurred as frequently as 100 times a year. TYPES OF DISPLAY An aurora was not a matter purely of chance, assuming various forms in a haphazard fashion. It fell actually into well-definied types. There was the horizon glow which was just a glow on the southern horizon. It had no definite boundaries and faded slowly from the horizon into the dark sky above. It might be the top of a display below the horizon or the reflection of an aurora far to the south and was an uninteresting form which usually escaped general notice. Another type was the homogeneous > quiet arc which was very common in Southland. It was like a bow of light across the southern sky and was the fundamental form from which the majority of displays developed. It might exist for a few minutes or for hours before changing into some more complex tvne and it might be double. The third type was seen in rays like the beams from a searchlight. These seldom existed on their own. They usually appeared in bundles which moved laterally towards the east or the west and were sharply defined on the side towards which they were moving and diffused and hazy on the other border. They might be long or short, white, green or red. . The rayed arc was the form into which the ordinary quiet arc invariably developed. The quiet arc gradually rose in the sky, becoming narrower and sharp along the lower border. Then suddenly it dropped of a hazy arc developed below it; rays shot up at one end and spread rapidly along the arc, until its whole length was surmounted by long or short rays. If the arc had a fold in it the rays followed the fold.

When the rays became very long they took on the appearance of a curtain or drapery, particularly when the arc had a fold in it to start with. When the rays became extra long they might meet at a point almost overhead, forming a crown of light, or a corona, a very rare fofm observed only three times in New Zealand. Diffuse surfaces and pulsating forms might appear at various stages during a display. They were like clouds of light without any well-defined form. Occasionally they were pulsating, fading in and out. When an aurora was at its height it might start to flame. Slowly at first and then more rapidly waves of light seemed to flow up from the horizon like the ripples when a stone was thrown into a pool. This took place across the tops of the ordinary forms.

TYPICAL DEVELOPMENT The development of a typical aurora was in the following order:—(a) Horizon glow: (b) homogeneous quiet arc, low at first, then rising; (c) various changes in the arc as it rose; became double and then joins up again; (d) arc became very thin and narrow, the lower border being sharp; (e) the arc suddenly dropped and became diffuse; (f) rays developed and the whole rayed arc grew from one end; (g) the rays lengthened and drapery developed; (h) everything became very confused with spasmodic ray activity, luminous patches, pulsating forms, with little trace of the original arc; (i) slowly the main arc built up again and the whole process was repeated. The whole ' thing was a definitely ordered process. For the origin of the aurora it was necessary to study the sun which was a gaseous body at a tremendously high temperature. It possessed an outer atmosphere known as the chromosphere. This could not be seen but the denser regions below this known as the photosphere could be seen. Both the chromosphere and the photosphere were subject to tremendous storms. One type of storm was seen as a sunspot, which was something in the nature of a cyclone on a vast scale. There were various other types of storm. The most noteworthy of these were prominences —eruptions of hydrogen or calcium gas which had tremendous power. There was a definite tendency for aurorae to occur when there was considerable sunspot or other activity on the sun. It was difficult to say what type of storm caused the effect, but it emitted activity, probably in the form of electrified particles which passed out in long streams into space. Some of this reached the earth. Here the particles entered the atmosphere causing ionization or the stripping of an electron from the gas atoms in the atmosphere. The stripped electron wandered at will for a fraction of a second before joining up again with another atom. This produced the auroral glow. The earth had magnetic properties, attracting the activity from the sun towards the two magnetic poles; hence the frequency of aurorae in and about the polar regions. (Electrified particles streaming down along lines of magnetic force gave rise to rays. EFFECT ON WIRELESS The height of the aurora varied from 60 miles to about 700 miles. The average was from 60 to 100 miles. There was an 11 years period between maximum sunspot activity and the aurora followed this also. The last minimum was in 1933 and the present was the maximum. Activity would decline from this year onwards until 1944. The occurrence of the aurora affected wireless transmission. A wireless wave from the transmitter to the receiver passed up into the air and was reflected

by one of the ionized layers, usually the Kennelly heaviside layer. This reflecting region was the same as that in which the aurora occurred. This caused distortion of wireless waves during the progress of an aurora. Intensive studies on the aurora for the past eight years had been carried out in New Zealand. In recent years photographic stations and a base-line had been established in Southland. Tht object of the base-line work was to measure the height and position of aurorae and to obtain a photographic record of various forms. The Southland aurorae mainly occurred in the vicinity of the Auckland and Camnbell Islands. Studies of height gave results in close agreement with the northern hemisphere results. Those obtained so far varied from 80 kilometres to 500 kilometres. Unusual forms seen in Southland included sunlit aurorae, red aurorae, pulsating arcs, high arcs, rayed arcs, isolated from the main displays. MEETING OF COUNCIL

At a meeting of the council of the branch attention was drawn to a report that spotted shags had been shot at Curi'- Bay. It was stated that a complaint had been forwarded to the Department of Internal Affairs about tht shooting of these protected birds and it was decided to support the protest. An offer by Dr C. C. Anderson to give ai address on radiology for the November meeting was accepted. There were present Dr G. H. ttley (nresident), Dr C. C. Anderson, Messrs Murray Geddes, A. S. Hogrr, G. Jaquiery and J. H. Sorenson (secretary).