High voltage power lines produce corona ions when the electric field gradient at the cable surface is high enough to ionise the air. This occurs when the field is concentrated around imperfections in the wire (e.g. dirt or water droplets). Most of the corona ions are attracted back to the wire as the polarity of the cable changes half a phase later but some ions can escape and are carried away from the line by the wind, these ions have been detected as far as 7 km from a line [1]. The ions remain in largely uni-polar clouds of space charge, with a predominance of negative ions in poor weather and positive ions in fair weather (a cloudless day with no precipitation), travelling horizontally in discrete clouds of varying sizes above the ground. They attach to aerosols, including air borne pollutants.

The Earth has a natural potential gradient between the ground and the ionosphere which is created and sustained by ionisation caused by cosmic rays and global thunderstorm activity. The strength of the field, as measured from the ground, is around 100 V m^-1 in fair weather. Corona ions produced by power lines cause perturbations in the field measured from the ground as they are carried overhead by the wind.

Previous measurements of the perturbations of the Earths electric field by Fews et al [1], [2] have shown a characteristic changing electric field downwind of high voltage electric power lines when compared to upwind. However, such measurements have been limited to field determination at sampling intervals of a few seconds. It is conceivable that there could be short term fluctuations in the electric field caused by uneven distributions of ions and smaller clouds of space charge. This work investigates the fine structure of the electric field as recorded downwind of power lines in a crosswind to investigate whether any smaller fluctuations are present when recorded at 5 ms intervals. This should lead to a greater understanding of corona loss and propagation.