Induction coils permit the measurement of small and very rapid changes of themagnetic field. A new set of induction coils in the UK (at = 3.2) recordmagnetic field changes over an effective frequency range of 0.1–40 Hz,encompassing phenomena such as the Schumann resonances, magnetosphericpulsations and ionospheric Alfvén resonances (IARs). The IARs typicallymanifest themselves as a series of spectral resonance structures (SRSs) withinthe 1–10 Hz frequency range, usually appearing as fine bands or fringes inspectrogram plots and occurring almost daily during local night-time,disappearing during the daylight hours. The behaviour of the occurrence infrequency () and the difference in frequency between fringes (Δ)varies throughout the year. In order to quantify the daily, seasonal andannual changes of the SRSs, we developed a new method based on signal andimage processing techniques to identify the fringes and to quantify thevalues of , Δ and other relevant parameters in the data set. Thetechnique is relatively robust to noise though requires tuning of thresholdparameters. We analyse 18 months of induction coil data to demonstratethe utility of the method.
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