Harmonics of the geomagnetic annual variation.

The maximum entropy method (MEM) of BURG (1968, 1970, 1972) has been applied to 2300 years of mean daily D, H, and Z data. Composite spectra and line counts, aside from the fundamental annual (mainly P20 mode) and semiannual (P10 mode) lines, show harmonics at periods 365/(n+1), n=1, 2, …, 6 days. In this range the geomagnetic spectrum consists of a P20 mode line structure superposed on a background P10 mode continuum. Mode mixing may have adversely affected previous Earth response estimates (BANKS, 1969) and must be considered in future studies. Finally, the ‘27-day peak’ is shown to be a broad band signal from 24 to 31 days, confirming results of RASSSACH et al. (1966).