Geomagnetic and solar wind cycles, 1900–1975

Abstract

Recent research has established the reality of the 80 to 100‐year cycle (long cycle) in solar‐terrestrial phenomena that for many years has been suspected to occur. This paper investigates some of the systematic changes that took place during the most recent long cycle. The variations of one hundred years of data on the aa index of geomagnetic activity are reinvestigated. Since the solar wind drives geomagnetic activity the results of the study are interpreted in terms of the solar wind. It is shown that the 11‐year solar cycle as expressed by the number of sunspots (the sunspot cycle) is very different from the 11‐year solar cycle as expressed by the solar wind and geomagnetics (the solar wind cycle). With an objective technique, using only sunspot numbers and not involving any judgement as to the transient or recurrent nature of any geomagnetic activity, the solar wind cycle is decomposed into the sun of two equally strong periodic variations, each having the period of the sunspot cycle, but differing in phase. One term in the decomposition, the R component, is chosen to have the phase and relative amplitude of the sunspot cycle. The other term in the decomposition, the I component, is found to be almost, but not quite, 180° out of the phase with R and to have very closely related amplitudes. The source of the R component is shown to be sporadic or short‐lived solar events and that of the I component is shown to be long lived solar features such as coronal holes. The amplitudes are interpreted as being due to solar wind parameters, probably 1BZ1V². The solar wind observed at earth oscillates rhythmically between the two sources. The amplitude of both the oscillations increased smoothly and steadily from 1900 to 1960, corresponding to the ascending branch of the long cycle of solar activity which has been established elsewhere as occuring in the solar wind with an average period since the Middle Ages of 87 years. From these results it is concluded that there is a very strong relationship between coronal conditions at the sites of origin of long lived streams of solar wind and the corresponding sites of sporadic and/or short lived events 5 or 6 years later. Moreover, there is a remarkable and orderly evolution of these conditions over at least 60 years. The interruption of the smooth evolution in 1960 suggests we are now in the declining phase of the long cycle, which is to be expected since the average cycle period is 87 years and the last minimum was in the first decade of this century.