These are natural ultra-low frequency oscillations known to researchers as "pulsations continuous" (Pc). The physics is familiar to anyone who has studied bells or resonant cavities. Earth's magnetic field carves out a cavity in the surrounding solar wind. Gusts of solar wind can make the cavity "ring" akin to a bell (references: #1, #2, #3). Human ears cannot hear this ringing; it is electromagnetic rather than acoustic. The physical effect is felt beneath our feet. As the cavity vibrates, magnetic fields swing back and forth, causing electrical currents to flow through the ground below.
The Pc waves Stammes detected are a variety known as Pc4, which oscillate in the frequency range 6.7–22 mHz. Such waves are good at energizing particles trapped in Earth's magnetic field and often cause local outbursts of bright auroras. www.spaceweather.com
Similar occurrences were also reported on 12 September and 23 October 2016:
http://www.ascensionnow.co.uk/quick-info/-earths-magnetic-field-rings-like-a-bell - 12 Sept. 2016
http://www.ascensionnow.co.uk/quick-info/sinusoidal-ground-currents-in-norway - 23 Oct. 2016