Time variations of geomagnetic activity indices Kp and Ap: an update

Kp and Ap indices covering the period 1932 to 1995 are analysed in a fashion similar to that attempted by Bartels for the 1932±1961 epoch to examine the time variations in their characteristics. Modern analysis techniques on the extended data base are used for further insight. The relative frequencies of occurrence of Kp with di€erent magnitudes and the seasonal and solar cycle dependences are seen to be remarkably consistent despite the addition of 35 years of observations. Many of the earlier features seen in the indices and special intervals are shown to be replicated in the present analysis. Time variations in the occurrence of prolonged periods of geomagnetic calm or of enhanced activity are presented and their relation to solar activity highlighted. It is shown that in the declining phase the occurrence frequencies of Kp = 4±5 (consecutively over 4 intervals) can be used as a precursor for the maximum sunspot number to be expected in the next cycle. The semiannual variation in geomagnetic activity is reexamined utilising not only the Ap index but also the occurrence frequencies of Kp index with di€erent magnitudes. Lack of dependence of the amplitude of semiannual variation on sunspot number is emphasised. Singular spectrum analysis of the mean monthly Ap index shows some distinct periodic components. The temporal evolution of 44 month, 21 month and 16 month oscillations are examined and it is postulated that while QBO and the 16 month oscillations could be attributed to solar wind and IMF oscillations with analogous periodicity, the 44 month variation is associated with a similar periodicity in recurrent high speed stream caused by sector boundary passage. It is recon®rmed that there could have been only one epoch around 1940 when solar wind speed could have exhibited a 1.3-year periodicity comparable to that seen during the post-1986 period. Introduction Bartels (1963), the initiator of the widely used Kp and Ap indices of geomagnetic activity, attempted a study of the systematic temporal changes in these indices covering the interval 1932±1961. The basic data had just been published as an IAGA Bulletin (Bartels, 1962). In his work he described, in detail, the occurrence characteristics of di€erent magnitudes of the Kp index and their solar cycle dependence as well as placing a special emphasis on the semiannual oscillation of the geomagnetic activity. Since his pioneering work, these indices and other geomagnetic indices such as Aa and Dst (see Mayaud, 1980; Rangarajan, 1989 for details) have also been examined in terms of their cyclic components related to solar activity and other parameters (Delouis and Mayaud, 1975; Gonzalez et al., 1993; and references therein). IAGA Bulletins regularly give statistics on quiet and disturbed intervals and occurrence frequencies of individual magnitudes of Kp index from 0° to 9°. However, as far as we are aware, there has been no e€ort to update the analysis of Bartels (1963). When Bartels' work was published in 1963, the ®rst in situ observations from satellites and spacecraft were just beginning to be made. In the next three decades, the solar causes of geomagnetic storms, the distinctive characteristics of geomagnetic activity in di€erent phases of the solar cycle, the role of solar wind velocity, southward component of IMF and the magnetospheric electric ®elds in the control of geomagnetic activity and the di€erences in ground signatures in di€erent latitude zones were well studied (see Tsurutani and Gonzalez, 1992; Eselevich and Fainshtein, 1993; Bravo and Rivera, 1994 and Taylor, et al. 1994 and references therein). With the availability of indices for an additional 34 y, it is considered worthwhile to extendBartels' analysis ofKp indices to cover the entire period 1932±1995 thus providing a summary of information signi®cantly enhancing the content in terms of di€erent levels of geomagnetic Correspondence to: G. K. Rangarajan email: [email protected] Ann. Geophysicae 15, 1271±1290 (1997) Ó EGS ± Springer-Verlag 1997