A Proposed Auroral Index Figure.
The search for a simple index figure by means of which the intensities of individual auroral displays may be compared, as well as for the purposes of statistics and correlation has been made by many observers. In New Zealand the problem is somewhat accentuated by having to deal with the reports from numerous observers of varied training and experience. Many observers have used an arbitrary scale which is usually defined as “faint,” “moderate,” “bright,” or “brilliant.” Such a scheme, without further definition does not take into account auroral forms visible, movement or duration, and depends a good deal on personal experience of witnessing aurorae. In Canada, Currie and Jones developed a method of giving an hourly index which took these factors into account, but it indicates the need of fairly continuous observations by trained observers at a single station, as well as neglecting the fairly rapid changes that may occur even within a few minutes. In New Zealand the whole story of a display is made up from a large number of individual reports made at various times of the night hours depending on each observer's circumstances.
The scale proposed by la Cour and published in the Supplement to The Photographic Atlas of Auroral Forms, indicates that he considered intensity to be related to auroral form. In his sequence of auroral forms during a typical display, Geddes implies a rise and fall in intensity with auroral form. From these and other considerations the assumption is made that the intensity of any auroral display at any instant, is directly proportional to the auroral form in evidence. Where several auroral forms are visible in the sky at the same time, the intensity is taken as the sum for all the forms.
The following scheme for relating auroral intensity to auroral form is suggested. In the original paper, symbols are given for the different forms, which brings out the scheme mu more clearly. In the table given here the usual auroral abbreviations are used.
|la Cour's Scale||Proposed Scale||Auroral Forms.|
|3||G||(bright), HA (single, faint),|
|1–2||4||HA||(single, moderate), PA (whole arc),|
|HA||(multiple, faint), R, DS.|
|6||HA||(single, bright), HA (multiple, moderate).|
|6||HA||(multiple, bright), RA (1 or 2 rays).|
|HB||(single, moderate), RA (few rays).|
|8||HA||(folded, bright), HB (complex, bright),|
|10||RA||(whole arc raying).|
|11||RB,||F, RA (whole arc raying, considerable movement).|
Examples, using this scheme, were studied for observations made at the single stations, Timaru and Campbell Island for the display of 1946, April 23. Two interesting results were obtained after plotting intensity against time.
The greater general auroral intensity for the station nearer the auroral zone was immediately obvious.
An examination of the discrepancies between maxima and minima of activity between the two stations led to the better interpretation of the results by showing that the forms were moving in geomagnetic latitude during the course of the display.
The above scheme, as with all others, refers in each case to observations made at a single station, whereas considering New Zealand observations alone, reports on displays have to be considered from Auckland to Invercargill. A description was given of the method by means of which all these reports can be quickly analysed for grosser features by use of the hieroglyph auroral symbols, and the whole display reduced as if seen from some point halfway between Dunedin and Invercargill. Examples of results obtained for the displays of 1946, February 7–8, 8, March 24–25, 25–26, 28–29, April 23–24 (N.Z.S.T.) were given. The intensities were shown in tabulated form for cach 0.1 hour during the display.
From such tables of observational results it was suggested thot the following advantages were obtained:—
The variation of auroral activity during small intervals of time, 6 minutes, were available for study. Rate of change of intensity was available within this limit of time if necessary.
By means of the simple sum of intensities during the hour an hourly auroral number becomes available.
The simple sum of the hourly numbers gives the complete auroral number for the whole display. This is in effect a rough integration which takes into account all the variations of intensity and the duration of the display.
Most auroral data are concerned with observations from single stations, whereas it would be highly desirable to know by means of some index the auroral condition right from the auroral zone to the lowest geomagnetic latitude at which the display is seen, so as to obtain a complete evaluation of the auroral activity. No suggestion can be made at this stage, and in the meantime New Zealand, Campbell Island, Tasmanian and Australian observations are to be dealt with separately. Brief mention was made of the apparently great expansion and contraction of the auroral zone in the Southern Hemisphere in sympathy with the solar cycle.