Corona ions are the air ions produced when the electric field very close to the surface of the conductors of a high-voltage power line is large enough to strip the electrons away from the air molecules.
Elsewhere on this site we deal with the question of whether the corona ions produced by power lines have health effects.
Here we give some basic information about what quantity or concentration of ions are produced.
Why do power lines produce corona ions?
Most power lines are designed not to produce significant quantities of corona, or certainly not in dry weather (all power lines are more likely to produce corona in wet weather when the drops of water on the conductor surface concentrate the electric field). If the conductor bundle (see here for an explanation of the parts of a power line) has three or four conductors, or if it has two conductors but the voltage is 275 kV or below, there will normally be little corona and low concentrations of ions. But if the bundle has only two conductors and is operating at 400 kV, the electric field can get close to the corona threshold. If the line is in poor condition, or the fittings were designed for 275 kV rather than 400 kV, it can produce corona even in dry weather. This is recognisable by the characteristic crackling sound.
What do we know about concentrations?
Measuring ion concentrations is difficult. Sometimes they are detected by their effect on the earth's DC electric field, but we prefer to try to measure them directly. Here are three examples of ion concentrations measured upwind and downwind over a short period of time for three different power lines.
The first example is a line that produces little corona because it operates only at 275 kV. The concentrations found are the normal ambient concentrations, not affected by the power line:
The second example is a 400 kV power line, but it still doesn't produce corona because it has "quad" bundles (we would expect "triple" bundles to behave similarly). The concentration downwind is no higher than upwind:
The third example is 400 kV, but with twin conductors, and fittings designed for 275 kV as well. This line does produce marked corona, and the concentrations downwind are higher than upwind. They are also quite variable as the wind blows in gusts:
What do we know about distance?
Piecing together the above measurements and other information, we think that the following graph is the best simplified model we can produce on present knowledge for how the concentration of ions varies with distance from a power line when the wind is blowing across the line. But the actual concentration is more complicated, depending a lot on specific meteorological conditions.
Note that, following the publication of the CCRG "corona ions" paper which used this model, there was some discussion whether a better model should extend to slightly greater distances. That may be true in some situations but we still think that, given all the unknowns and variables, this is not too bad a starting point!