Institute of Geophysics and Planetary Physics
University of California, Los Angeles, California 90024
Originally published in: Geophys. Res. Lett., Vol. 1, No. 1, 1974.
Abstract.The geomagnetic activity index, Ci, exhibits a double sunspot cycle in which 11-year periods alternately have more, then less, than average activity. The amplitude of the cycle-to-cycle variation has varied markedly since 1885, being large from 1885 to 1907, small from 1908 to 1948 and large again from 1949 to 1970. If we assume that the 22-year variation is caused by reversals every sunspot cycle of the heliographic dependence of the dominant polarity of the interplanetary magnetic field, the behavior of the Ci index implies that the amplitude of the heliographic latitude dependence varies with time. If, as has been proposed, the heliographic latitude dependence is associated with the magnetic field at the solar poles, then the Ci index variation also implies that the polar magnetic field strength has undergone long-term changes.
Rosenberg and Coleman (1969) have shown that during the period from 1964 there was an annual modulation of the relative amount of inwardly and outwardly directed interplanetary magnetic field, such that during September the interplanetary field was directed predominantly (68% of the time) towards the sun and during March predominantly away from the sun. The modulation of the inward flux was found to be in phase with the latitude of the earth relative to the solar equator, which maximizes around September 5. Rosenberg and Coleman termed this phenomenon the heliographic latitude dependence of the dominant polarity of the interplanetary magnetic field. While the modulation of inwardly directed flux was in phase with the heliographic latitude of the earth during the early part of solar cycle 20, Rosenberg and Coleman predicted a reversal of the phase of the modulation each sunspot cycle beginning shortly after solar maximum in accord with Babcock's model of the solar dipole magnetic field (Babcock, 1961).
Later Wilcox and Scherrer (1972) tested this prediction for solar cycles 17 through 20 using an index of interplanetary magnetic polarity inferred from the signature of polar cap magnetograms (Svalgaard, 1972). While the prediction was clearly confirmed for the years 1949 to 1970, the heliographic latitude dependence was not nearly as evident during the years 1927 to 1948. This can be seen by visually comparing the phase of the inferred polarity with the test function of Wilcox and Scherrer, or by Fourier analysis of the inferred polarity (Russell et al., 1974). Since some properties of the Svalgaard index have been found to have changed with time (Fougere, 1974; Russell et al., 1974), it is possible that this lack of correspondence with the expected variation during the early years signals a further change in the index. It is also possible that the quality of the index remained constant from 1927-1948 while the amplitude of the heliographic latitude dependence of the interplanetary magnetic polarity decreased to a level undetectable with the index. This note proposes an independent measure of the heliographic latitude dependence of the polarity of the interplanetary magnetic field which can be used to investigate the annual modulation of the interplanetary polarity from 1885 to the present, thus permitting a choice between the two possible causes of the apparent absence of the modulation from 1927-1948.
The proposed measure is the difference in geomagnetic activity during successive solar cycles. A double-sunspot cycle in geomagnetic activity was first discussed by Chernosky (1966). During Zurich even-numbered cycles, beginning about two years after solar maximum, geomagnetic activity is greater than normal, while during odd-numbered solar cycles geomagnetic activity is less than normal during the corresponding period. Russell and McPherron (1973) suggested a mechanism explaining this effect. These authors showed that, expressed in magnetospheric coordinates, southward fields were more probably when the interplanetary field was inward in March and outward in September. Combining this observation with the apparent minimal response of the magnetosphere to northward magnetic fields, would create a semiannual variation in geomagnetic activity, as observed. The annual modulation of the relative amounts of inward and outward magnetic flux either enhances or diminishes the strength of the interaction, and the reversal every solar cycle of the phase of the modulation thus produces a double-sunspot cycle in geomagnetic activity.
If, as Russell and McPherron suggest, the 22-year cycle is due to the heliographic latitude dependence of the interplanetary magnetic field, then the 22-year cycle can test for the presence of the heliographic latitude dependence. Figure 1 is a plot of the annual variation of the Ci index averaged over modified cycles from 1885 to 1970, where a modified solar cycle begins with the third year after solar maximum and ends on the second year after the next solar maximum. These curves show that every second solar cycle has a lower yearly mean Ci index, as expected. However, the magnitude of this effect varies significantly from solar cycle to solar cycle. It is quite strong from 1885-1907, and from 1949-1970, weak from 1928-1948 and almost disappears from 1908-1927. Thus, the data suggest that the amplitude of the heliographic latitude dependence of the dominant polarity of the interplanetary magnetic field does change with time.
|Figure 1. Annual variation of Ci index averaged over modified solar cycles from 1885 to 1970. Dashed line shows annual variation averaged over entire period.|
In view of the apparent temporal variation of the amplitude of the heliographic latitude dependence and its small amplitude from 1927 to 1948, we interpret the difference between the expected and measured phase of the annual wave in the Svalgaard index in this period to be due to the decrease of the amplitude of the modulation to a value below that detectable with the index rather than a degradation of the index prior to 1948. While the amplitude during the two most recent solar cycles appears to be larger than average, the data from the years 1885-1907 indicate that such an amplitude may not be exceptional. Finally, if the heliographic latitude dependence is determined by strength and polarity of the sun's polar magnetic fields, then these results indicate that the polar fields not only reverse in sign but also change gradually in strength from solar cycle to solar cycle.
Acknowledgments. Discussions of this paper with R. L. Rosenberg are much appreciated. This work was supported by the National Science Foundation under NSF grant GA 34148-X.
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