Zajtsev et al. (1993) applied fast Fourier transform (FFT) (Cooley et al., 1969) of the magnetic field components measured by a particular magnetic observatory. They reported that some patterns could be recognized in the contour plots of Fourier coefficients versus time for different phases of sub-storm evolution. Similar techniques can be applied to the planetary geomagnetic indices <em>A<sub>p</sub></em> and <em>D<sub>st</sub></em>. The spectral content changes prior to the growth phase of the sub-storm. The higher frequency coefficients, the larger the power spectrum is shifted upwards. This feature can serve as a precursor, if spotted. We present an analysis of the February 2001, 2003, and 2017 time intervals. The technique can be potentially used as a forecast tool for predicting geomagnetic activity.