Spectral analysis of sunspot number and geomagnetic indices (1868-2001)

Abstract The multiple-taper spectral analysis was applied to a a , A p and R z annual and monthly average series (1868–2001). The significant periods at 95% confidence level for annual averages are 11.1 and 10.2 years for R z and 11.1, 5.3, 4.3, 2.7 and 2.1 years for a a . For monthly averages, the significant periods were 133 (11.1 years) and 17.7 months for R z and 133 (11.1 years), 63.7 (5.3 years), 17.5, 12.7 and 6 months for a a . The periods associated with the solar cycle are observed in both series. The a a spectrum showed in addition a peak near 4.3 years, which is not present in the R z spectrum. Other works have also shown this peak in geomagnetic activity. This peak is believed to be caused by the dual-peak structure in the a a . The 22-year Hale cycle was not detected either in R z or a a spectrum in the period 1868–2001, but it was seen in R z data for 1700–2000 annual averages. The explanation of this 22-year cycle has been questioned and while in some spectral analysis this periodicity has been found, others have failed in its detection. The a a spectrum showed in addition a peak of 6 months, associated to the semi-annual variation of geomagnetic activity. Three hypotheses have been considered to explain the seasonal activity variation. These are the Russell–McPherron effect, the equinoctial effect and the axial effect.

[1]  B. Tsurutani,et al.  Periodic variation in the geomagnetic activity : a study based on the Ap index , 1993 .

[2]  R. Cùrrie.  Geomagnetic line spectra-2 to 70 years , 1973 .

[3]  D. Thomson,et al.  Spectrum estimation and harmonic analysis , 1982, Proceedings of the IEEE.

[4]  D. Thomson.  Quadratic-inverse spectrum estimates: applications to palaeoclimatology , 1990, Philosophical Transactions of the Royal Society of London. Series A: Physical and Engineering Sciences.

[5]  R. P. Kane.  Quasi-biennial and quasi-triennial oscillations in geomagnetic activity indices , 1997 .

[6]  Robert L. McPherron,et al.  Semiannual variation of geomagnetic activity , 1973 .

[7]  W. Gonzalez,et al.  Long-term correlation between solar and geomagnetic activity , 2004 .

[8]  P. Mayaud,et al.  Spectral analysis of the geomagnetic activity index aa over a 103‐year interval , 1975 .

[9]  M. Lockwood,et al.  Solar causes of the long-term increase in geomagnetic activity , 1999 .

[10]  E. Cliver,et al.  The 22‐year cycle of geomagnetic and solar wind activity , 1996 .

[11]  Antony C. Fraser-Smith,et al.  Spectrum of the geomagnetic activity index Ap , 1972 .

[12]  G. Rangarajan,et al.  Time variations of geomagnetic activity indices Kp and Ap: an update , 1997 .

[13]  John R. Herman,et al.  Sun, weather, and climate , 1985 .

[14]  R. Cùrrie.  Long period magnetic activity −2 to 100 years , 1976 .

[15]  D. J. Gorney,et al.  Solar cycle effects on the near‐Earth space environment , 1990 .

[16]  B. Tsurutani,et al.  Dual-peak solar cycle distribution of intense geomagnetic storms , 1990 .

[17]  H. Rishbeth,et al.  Increased magnetic storm activity from 1868 to 1995 , 1998 .

[18]  David A. Juckett,et al.  Period and phase comparisons of near‐decadal oscillations in solar, geomagnetic, and cosmic ray time series , 2001 .

[19]  M. Menvielle,et al.  The K-derived planetary indices - Description and availability , 1991 .

[20]  Ian G. Richardson,et al.  Sources of geomagnetic activity over the solar cycle: Relative importance of coronal mass ejections, high‐speed streams, and slow solar wind , 2000 .

[21]  Y. Kamide,et al.  Mountains versus valleys: Semiannual variation of geomagnetic activity , 2000 .