Nearly 300 years have passed since the watchmaker
G. Graham constructed a more sensitive compass and showed
that the variations in geomagnetic direction varied with an
irregular daily pattern.
More recently, but still 75 years ago, Loeve showed
that a proper description of such nonstationary
processes required at least two time or frequency variables.
Forty years of experience studying geomagnetic and
related problems, however, have shown few, if any,
proper computations of such spectra in the literature
including my own. The purpose of this talk
is to suggest a way to remedy this.
The solution to these problems, the multitaper method of
spectrum analysis, was introduced in 1982 but its implications
for geomagnetism not recognized until recently.
Taking long (> 4 months) blocks of geomagnetic
data, and concentrating on frequencies below
1000 uHz to avoid ultraviolet effects, one
- the cross-spectra are dominated by many offset frequencies including +/- 1 and +/- 2 cycles/day;
- the coherence at these offset frequencies is often stronger than at zero offset;
- there are strong couplings from the "quasi two-day" cycle;
- frequencies are usually not symmetric;
- the spectra are dominated by the normal modes of the Sun.