In June 2012, the BGS Geomagnetism team installed two 100 Hz sampling rate induction coil magnetometers in the Scottish Borders, which measure very rapid changes of the geomagnetic field. Figure 1 shows a spectrogram (power at each frequency versus time) from one of the coils in the frequency band of 0.1-40 Hz. The vertical bands of peak power at 8 Hz and harmonics are the Schumann resonances, while the weaker signals at 1-25 Hz between 18:00 and 06:00 UT are due to the so-called Ionospheric Alfvén Resonances (IAR).
The Schumann Resonances (SR) are generated by the emission of broadband lightning strikes typically in the equatorial regions. Around 100 strikes per second ‘echoing’ around the earth-ionosphere cavity creates a fundamental frequency around 7.8 Hz and higher harmonics, although these frequencies vary over time.
The IAR are generated by the vibration of magnetic field lines passing through the ionosphere up to 1000 km altitude in space. The IAR structures typically arise with a frequency around 1Hz and fan out into discrete lines, increasing in frequency from late evening to midnight and then decreasing during early morning, as the ionospheric electron density (and hence the Alfvén speed) varies.
The SR are sensitive to season, peaking in northern hemisphere summer, at a minimum in winter and are supposedly further modified by other quasi-periodic atmospheric oscillations such as the Madden-Julian Oscillation, though robust correlation with such atmospheric phenomena remains to be definitively proven.
The IAR show a strong solar cycle modulation becoming more prominent at the (present-day) solar minimum. They show unexplained behaviours which are presently not understood. For example, a debate is still on-going as to their excitation source, and the frequency and number of the observed individual fringes extending out to 30 Hz remain unexplained. The changes to the IAR signatures are strongly modulated by space weather. Of particular importance is the total electron content (TEC) of the cavity in which they resonate. Such TEC changes are a major cause of space weather disruption to global navigation satellite systems, and understanding and predicting these changes is a high priority for space weather research.
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