Figure 2

Power spectral density (log10, in nT ² /yr ⁻¹ ) obtained from OMM series. For the three components of the magnetic field at the Kakioka observatory, superimposed with −4 and −1 slopes (black). The series have been corrected for referenced jumps (Chulliat and Telali 2007). A multitaper analysis has been performed. For each sub-series, we removed the end-to-end linear trend before applying a Hanning window (see De Santis et al. 2003). Generating many realizations of 100-year synthetic AR-2 series (for which we know the theoretical −4 slope of the spectrum), we retrieve with the above method a slope of −4.2±0.3 (i.e., a typical error of about 5% to 10%) in the period band from 2 to 25 years.