Telluric currents are electrical currents flowing within the Earth or ocean water. I first stumbled upon the phenomena in the late 70’s when I experimented with something known as a ghost line. This was a method of sending telegraphy without wires by using the Earth itself as a conductive medium and two sets of parallel electrodes like so:
At the sending station, a battery was keyed into two set of ground rods that were separated as far as practical and were perpendicular to the direction that it was desired to send the signal. At the receiving station, a similar pair of ground stakes was connected to a sensitive relay or other sensitive detecting device. The earth being partially conductive then acted like a voltage divider and a portion of the battery voltage keyed into the first set of ground rods, 1-2, would appear at the second set 3-4.
This was used during WWI to telegraph messages to a location inside of enemy lines. It wasn’t practical to run wires across enemy lines because they could either be cut or followed to your location.
I was experimenting with sending audio this way rather than just keyed DC voltages, but one thing I discovered early on was that I could measure a voltage between the two ground rods I had places in our back yard. This couldn’t have been due to battery action because both ground rods were copper so there were no dissimilar metals.
The voltages and currents involved where small but still I found it fascinating that current would come out of the Earth. Also, the current would vary from day to night and also in seemingly random manner.
Today it is thought these currents are induced into the Earth as the result of short-term changes in the magnetosphere, conductive ocean water moving across magnetic lines of force, and leakage currents from the Earth’s internal dynamo.
Atmosphere to Earth currents are not considered because it is thought that they have a low value of only 1000-2000 A. I believe this to be incorrect and I believe there is evidence to support this. In spite of mainstream science insisting that Auroras are 50+ miles up, there are many many accounts on the part of Alaskans of the Auroras coming down to Earth, including crackling sounds and the smell of ozone. I have read accounts of a writhing electrical snake taking lives. I have read an account by a chemist of such coming through his laboratory and doing a great deal of damage as well as leaving him with sunburn like burns. I believe these things do happen and that this also contributes to telluric currents.
At any rate; the phenomena is still not entirely understood today although major contributing factors are understood, there are still rapid changes that are not well understood.