Geomagnetic Disturbance Index Research Articles

The variance (power) spectra ofCi, Kp, andAp

Time series of the geomagnetic disturbance indices Ci, Kp, and Ap, are subjected to variance (power) spectrum analysis. Application of this technique yields the distribution of the statistical variance of a time series as a function of the frequency or period of the variations. Furthermore, its use overcomes certain theoretical and computational difficulties inherent in standard Fourier analysis. The spectra of the three indices are identical in almost all respects. They demonstrate the mathematical existence of the 6-month period, and periods of 27, 14, 9, 7, 5½, and 4½ (27/N, N = 1,2, 3, 4, 5, and 6) days. No interpretation of the overtones of the 27-day period is given. The spectra show that, over intervals of some months, there are considerable changes in the distribution of the variance among these periods. However, there are few systematic differences in the spectra between times of sunspot maxima and minima.

On the aurora of 19 August 1950, photographed from Greece

Further details are given concerning an auroral photograph already reproduced and discussed (Abbott, 1951). The association between the occurrence of auroræ in low latitudes and the 3 hr geomagnetic disturbance index K p is considered. The aurora was reported from places over a wide area (Europe, the Atlantic and North America); its reported appearances ranged from 18 August at 2130 hours to 19 August, 0530 hours UT (at Nantucket); and from New Zealand it was seen after sunset, from about 0730 UT on 19 August.

Variations of Intensity of the Aurora at Macquarie Island

Observations of intensity of the aurora at Macquarie Island (geomagnetic co� ordinates 61 oS., 243�E.) are examined. The intensity is found to be dependent on geomagnetic planetary disturbance index Kp but the form of this dependence is different for different auroral forms. After eliminating the effects of variation of K p the residual diurnal and annual variations of intensity are small and irregular.

The Southern Auroral Zone as Defined by the Position of Homogeneous Arcs

Observations on homogeneous arcs (H.A.) made at Macquarie Island (geomagnetic coordinates 60�7 �S., 243�1 �E.) are examined. The latitude of H.A. is found to be significantly dependent on the geomagnetic planetary disturbance index Kp and the time of day. For given values of Kp and time the probability distribution of latitude of H.A. has a sharply defined mode about 0.5� north of the mean.