Spectral analysis of palaeomagnetic time series and the geomagnetic spectrum

There are now sufficient archaeomagnetic data from rapidly deposited sediments and baked clays to start bridging the gap in the geomagnetic spectrum between the frequency ranges covered by observatory records and polarity reversals. The form of the continuum spectrum of internal origin can be only loosely constrained but is broadly consistent with earlier speculations. The power spectral density function appears to increase rapidly with period up to periods of about 60 years, then more slowly up to a plateau in the region of 10 4 to 10 5 years, and thereafter starts to fall. There is somewhat inconclusive evidence for a drop in power density at periods around 10 2 years. Prospects for refining the spectrum are excellent.