On the Sudden Commencement of Geomagnetic Storms
Yumura, T.
Abstract
In order to clarify some points of the subject, a statistical investigation on the sudden commencement of geomagnetic storm is made in this paper, though similar investigations have been carred out morphologically or theoretically by many authors.
The results obtained are as follows;
(1) The geomagnetic latitudinal distribution of the direction angles of horizontal vector of the sudden commencement, reckoned from the magnetic meridian, is approximately symmetrical with respect to the magnetic equator (Fig. 2).
(2) The hourly distribution of the direction angles depend upon the local time of each observatory and the values between 12h and 24h L.T. are concentrated around nearly 0°, that is to say, around the direction of the magnetic north, but during 6h to 12h L.T, they fluctuate with a large amplitude on both sides of their mean values in high latitudes (Fig. 3).
On the other hand, the inverted SC's are appearing more frequently in the same interval, 6h to 12h L.T., in high magnetic latitudes.
(3) A conventional conception that the horizontal vectors of SC. are nearly pointing to the geomagnetic north is roughly right in middle or low magnetic latitudes, but not so in high latitudes.
(4) The distribution of the magnitudes of horizontal vectors for the geomagnetic latitude, which at expressed by the ratios of the values at various observatories to those observed simultaneously at Huancayo, shows a result similar to that already knows, namely the W-type distribution with symmetry to the magnetic equator. The curve has a minimum value of about 0.4 near the magnetic latitude 30°and increases from the minimum point monotonously with the increasing magnetic latitude (Fig. 4).
By this method, however, the values of each observatory fluctuate in so wide a range around the mean value that the above result is not so accurate.
(5) The magnitude ratios of horizontal vectors at various observatories those at Huancayo are distributed on a regular curve with the period of one day, where the minimum point of the curve appears around noon at Huancayo (Fig. 5). This result may be due to the large amplitude of SC. in the daytime at Huancayo compared with those at other observatories.
How can we interpret the fact the variation of the sudden commencement, which has been considered to be originated from without the earth's atmosphere, shows the above-mentioned daily variation due to the sun's altitude?
(6) Next, it is tried to subtract the apparent influence of the sun's ultra violet light from the result given in (4). The result gives no essential difference, but will be useful for more reasonable interpretation of the geomagnetic latitudinal distribution of SC; the magnitudes obtained in this way are just half of that at土30°and 1.5 times as large as that at土60°(Fig. 7).
However, we must in future determine the distribution for each season by using more data.
(7) The ratio of the amplitude of the vertical component to that of the horizontal vector shows a regular distribution in respect to the geomagnetic latitude (Fig. 8) This distribution curve is approximately symmetrical to the magnetic equator， with the maxima at about土20°-30°(positive regions) and the minima at about +15°and -35° (negative regions).
It may be difficult to interpret such distribution by Chapman's theory of the magnetic storm.
Corrigenda is attached to this text.
