What's happening here? Ground currents are a sign of changing magnetic fields. Earth's magnetic field around the Lofoten Islands was swinging back and forth, inducing a sinusoidal amperage in the soil beneath Stamme's observatory.
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 extends out into space and carves out a cavity in the surrounding solar wind. Pressure fluctuations in the solar wind can excite wave modes in the cavity--usually in a noisy cacophany of many frequencies, but sometimes with almost-monochromatic purity. In such cases, Earth's magnetic field "rings like a bell" with slow tones that reach all the way down to the ground. That's what happened on Oct. 23rd. References: #1, #2, #3.