The solar wind observations have revealed a 1.3-1.4 year periodicity in the Sun (Richardson et al, 1994). The reason for it is not yet known, but as any variability in the solar wind, it is reflected in the geomagnetic activity. According to one of these ground-based studies (Silverman and Shapiro, 1983), the importance of this new variability fluctuates with a roughly 65-year period, with maximums in about 1948 and 2013. This agrees with the fact that the period has not been observed in solar wind before 1987.
The 1.3-1.4-year variability has been observed in the geomagnetic or auroral data by, e.g., Shapiro (1967), Silverman and Shapiro (1983), and Paularena et al. (1995).
- Paularena, K. I., A. Szabo, and J. D. Richardson, Coincident 1.3-year periodicities in the ap geomagnetic index and the solar wind, Geophys. Res. Lett., 22, 3001, 1995.
- Richardson, J. D. and K. I. Paularena, J. W. Belcher, and A. J. Lazarus, Solar wind oscillations with a 1.3 year period, Geophys. Res. Lett., 21, 1559-1962, 1994.
- Shapiro, R., Interpretation of the subsidiary peaks at periods near 27 days in power spectra of geomagnetic disturbance indices, J. Geophys. Res., 72, 4945, 1967.
- Silverman, S. M. and R. Shapiro, Power spectral analysis of auroral occurrence frequency, J. Geophys. Res., 88, 6310-6316, 1983.