FORECASTING

Sun (Auckland), Volume IV, Issue 999, 16 June 1930, Page 8

FORECASTING

Sir,— We must pass over most of the interesting sun-spot features which laborious observation lias revealed, and s^ mo others which we now learn from theory. The time and place at which the first inrush of a sun-spot period will occur—indeed, the whole course of a sun-spot period—depends upon tne condition in which surface levels were left by the inflows of the preceding period and, consequently, the development of the succeeding protuberance?. There can be no exact repetition of the cycles. Yet there must be some approximation to a fixed series of cycles. Usually the first inrush of a period in either hemisphere occurs in latitude somewhat higher than 30 degrees. And it is almost inevitably followed by a second, and then a third, in almost the same latitude, upon the same crest. Each inflow proceeds to lower the surface level in its immediate vicinity and. when sufficiently vigorous, it sucks in the covering photosphere, thus producing a sun-spot, which passes through definite phases of development. maturity and decadence. 13eoause of the course which the inflow must take beneath the surface {curving to the westward), and because each inflow is fed more particularly by the faster moving surface material from the westward, other vortices usual develop behind the primary one and there is produced a train or group of ."■pots which are all connected with the same sub-surface stream, and in which the eastern vortices naturally grow weaker, the western stronger. These spots or groups of spots will Inst just so long as the surface levels about them remain high enough to f eed the inflows to such an extent as to draw in the photosphere. Thus, the life of such inflows or spots may be computed, from the relative rates of movement of sun-spot and surface flow, to be from four to seven months. By the end of that time there is, in place of the previous crest, a depression or valley all round the sun, dividing the protuberance into two parts, each of which continues to develop and move Equatorward. The disturbance caused by nnv inflow facilitates the development of others. By considering the changes that are going on one may. at any stage of the sun-spot period, judge where the next spot will make its appearance.

By the time that the earliest inflows have completed their work in their particular latitude, other inflows wil. start at intermediate distances upon the crest on either side: and thereafter two systems of spots (inflows) will c arry on the work. The ore. working upon the oncoming crest for several years, produces spots of diminishing vigour in successively higher latitudes. and finally dies out at about latitude 40 degrees, the last inflows ©*• this “drift” being too weak and covered by too great a thickness of photosphere to cause spots. The other sys--em follows its protuberance into lower latitudes: it usually continues several years longer and it finishes its work near or at the Equator—it cannot go beyond that. , The stage of “maximum” display sun-spots is usually reached about fou and a-half years after the cycle at a time when there are spots In systems or “drifts.” And. during dine to minimum, there is from aooi one and a-half to three years later * period during which the display r®* mains about the same, or even creases again slightly, before contin«ing the decline for about three an a-half years more to minimum. Tnu® the sun-spots and the all-importan prominences attendant upon occur in a variety of latitude, at various distances from the solar axis, : consequently with various degrees * energy. Nearly always they P reS r! combinations and always their prominences are the prime factors of terre-" trial disturbances. • F. R FIELD. (To Ee Continued.)