Harmonic content of geomagnetic indices.

Zaitsev at al. (1993) applied fast Fourier analysis of magnetic field measured by a particular magnetic observatory. They reported that some patterns can be recognized in the contour plots c_n =(a_n²+b_n²)^1/2, where a_n and b_n are coefficients at sin and cos functions, versus time for different phases of sub-storms evolution. Similar techniques can be applied for the planetary geomagnetic indices Ap and Dst. The spectral content changes prior to the growth phase of the sub-storm. The higher-frequency coefficients increase, and the maximum is shifted upwards. This feature can serve as a precursor, if spotted. We will present the analysis of 2001-2003 data.