An example of steps for detecting a wave on 29 November 2011. (a) Variations of magnetic field for three axes of the magnetometer. (b) A power spectral density (PSD) graph expressed with a logarithmic value of the signal ratio. The ratio is calculated by Rⁱ(f) = Cⁱ(f)/M(f) in step 3. (c) A PSD graph after end of the steps. Savgol (Savitzky-Golay) filter is applied to the PSD along the frequency domain, and non-wave signals are removed. (d) A PSD graph only with a valid wave event. The upper, bottom and peak frequency of the identified wave is automatically recorded.