all manner of things can cause a build up of charge in what is essentially a massive network of thousand KM+ antennas: radio waves, the earth's magnetic field,

all manner of things can cause a build up of charge in what is essentially a massive network of thousand KM+ antennas: radio waves, the earth's magnetic field, cosmic rays, wind interacting with dust particles causing static charges, etc. A bit of an interesting side note: it is possible to build a radio set passively powered by radio waves, requiring no external power: http://en.wikipedia.org/wiki/Crystal_radio http://en.wikipedia.org/wiki/Crystal_radio