- The BGS have been recording high frequency induction coil data since Sep 2012 at Eskdalemuir, Scottish Borders
- Magnetic field variations in the 1-40 Hz band allow us to probe the geophysics of troposphere, upper atmosphere and magnetosphere
- This is a rich dataset with which to study pulsations, Schumann and Ionospheric Alfvén Resonances
In June 2012, the BGS Geomagnetism team installed two high frequency (100 Hz sampling rate) induction coil magnetometers at Eskdalemuir, in the Scottish Borders, which permit us to measure the very rapid changes of the magnetic field. These system are ideal for exploring the Ionospheric Alfvén Resonator (IAR) and atmospheric Schumann Resonances (SR). These signatures contain important information on the Earth’s magnetosphere, ionosphere and ionosphere-atmosphere coupling. They are sensitive to atmospheric wave propagation and space weather, and have the potential to be an important diagnostic of such events.
Figure 1 shows a spectrogram (power at each frequency versus time) from one of the coils for 16th January 2013 in the frequency band of 0.1-40 Hz. The diffuse (vertical) band of peak power at 8, 14, 20 and 26 Hz are the Schumann resonances, while the weaker signals at 1-25 Hz between 18:00 and 06:00 UT are due to the Ionospheric Alfvén Resonances. The (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, with additional higher harmonics.
The IAR are generated by the vibration of magnetic field lines passing through the ionosphere up to 1000km in space and are found only at magnetically quiet periods. 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 decrease during early morning. A thin line at 25 Hz is a sub-harmonic of the UK power grid
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 disappear during strong geomagnetic activity and show unexplained behaviours and frequency changes which are presently not theoretically understood.
The candidate will examine the two main resonances in the induction coil data, looking firstly at the longer term changes of the Schumann Resonances and their relation to solar, seasonal and atmospheric phenomenon. This includes examining the polarisation variations and comparison to other induction coil data from the northern hemisphere (Canada, Japan).
For the IAR, we wish to investigate the source excitation (lightning?) and the theoretical generation and propagation mechanisms from the literature and compare to the observed IAR at Eskdalemuir. The candidate will extract the relevant IAR parameters (frequency, number of fringes, bandwidth, Q-factor) and use these to develop a theoretical framework for the influences and factors governing the development and evolution of IAR on any given day.
From the new understanding of how the ionosphere and magnetosphere parameters influence the SR and IAR, we will seek to invert for these parameters using the ground-based magnetic field data.
Training and Skills
The project will build upon 20 years of experience within the Radio and Space Plasma Physics (RSPP) and BGS geomagnetism groups in the analysis of ground-based magnetometer data. Training in magnetometer instrumentation techniques, relevant plasma and atmospheric physics will be provided as well as training in computer programming, simulations and the data analysis required. The student will gain a great deal of expertise in research methods, data management, analytical thinking and computer programming. All students contribute regularly to group meetings through presentation of their work.
Year 1: Investigate SR relation to solar cycle, seasonal and atmospheric oscillations
Year 2: Investigate IAR and theoretical versus observational models
Year 3: Investigate ionosphere magnetosphere parameter inversion for parameter retrieval
Partners and collaboration (including CASE)
The Leicester supervisors, Prof Yeoman and Dr Wright, have over 20 years experience working on various magnetometer systems, including IAR investigations with induction coils. The external partners for this project are Dr Ciaran Beggan, who has worked on the induction coil programme for the past seven years and Prof Alan Thomson, geomagnetism team leader at BGS. This project includes additional funding from the British Geological Survey to support visits and collaboration with BGS Edinburgh. The BGS will supply the data co-supervise the project and provide the opportunity of a three-month placement in the Edinburgh office with the Geomagnetism team.
Prof. Tim Yeoman
Radio and Space Plasma Physics Group
Department of Physics and Astronomy
University of Leicester
University Road, Leicester
Tel: +44 116 2523564