Automatic detection of ionospheric Alfvén resonances using signal and image processing techniques

C. D. Beggan

doi:10.5194/angeo-32-951-2014

Abstract

Abstract. Induction coils permit the measurement of small and very rapid changes of the magnetic field. A new set of induction coils in the UK (at L = 3.2) record magnetic field changes over an effective frequency range of 0.1–40 Hz, encompassing phenomena such as the Schumann resonances, magnetospheric pulsations and ionospheric Alfvén resonances (IARs). The IARs typically manifest themselves as a series of spectral resonance structures (SRSs) within the 1–10 Hz frequency range, usually appearing as fine bands or fringes in spectrogram plots and occurring almost daily during local night-time, disappearing during the daylight hours. The behaviour of the occurrence in frequency (f) and the difference in frequency between fringes (Δf) varies throughout the year. In order to quantify the daily, seasonal and annual changes of the SRSs, we developed a new method based on signal and image processing techniques to identify the fringes and to quantify the values of f, Δf and other relevant parameters in the data set. The technique is relatively robust to noise though requires tuning of threshold parameters. We analyse 18 months of induction coil data to demonstrate the utility of the method.

Highlights

Spectral resonance structures (SRSs) are a type of extremely low frequency magnetic field phenomena detectable in the 0.5–10 Hz region of the magnetic field spectrum
ionospheric Alfvén resonances (IARs) are thought to arise from the partial reflection of magnetohydrodynamic Alfvén waves in the ionosphere, excited along magnetic field lines by the leakage of electric fields associated with terrestrial lightning activity (Greifinger and Greifinger, 1968; Fukunishi et al, 1996; Füllekrug et al, 1998)
We examine the use of a combination of signal and image processing techniques to identify the occurrence and properties of spectral resonance structures (SRSs) fringes in spectral data

Summary

Introduction

Spectral resonance structures (SRSs) are a type of extremely low frequency magnetic field phenomena detectable in the 0.5–10 Hz region of the magnetic field spectrum. Their origin is attributed to the occurrence of ionospheric Alfvén resonances (IARs) between the E and F2 region layers of the upper atmosphere. The waves reflect off the “walls” of the E region and F2 region where the gradients of electron density reach a maximum. This partial reflection sets up a series of resonant frequencies along the field lines, giving rise to between 2 and 12 harmonics (the so-called SRS). The SRSs are visible as a series of fringes in dynamic spectrograms of the magnetic field rate of change

Methods

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Discussion

Conclusion