EFFECTIVE AREA OF A HORIZONTAL LONG-WIRE ANTENNA COLLECTING THE ATMOSPHERIC ELECTRIC VERTICAL CURRENT

The effective area of an antenna collecting the vertical air-Earth cur rent is a coefficient of proportionality between the collected current and the air-Earth current density. The effective area can be correctl y defined only if the model behind the proportionality relation is ade quate. The current collected by a horizontal long-wire antenna is not exactly proportional to the vertical air-Earth Maxwell current density because of the different behavior of the displacement and conduction components of the current. Thus, two different effective areas are sep arately defined for the displacement and nondisplacement components. F irst, the dynamic effective area characterizes rapid variations of the displacement current. It is calculated assuming that the air does not contain any space charges and that the electric current flow lines ma tch the electric field lines. Second, the static effective area charac terizes the nondisplacement current, and it is calculated by taking in to account the facts that the conductivity close to the wire surface i s unipolar, that the wind-determined horizontal ion trajectories do no t match the electric field lines, and that there are space charges due to the electrode effect of the wire and of the ground. Traditionally, the atmospheric electric vertical current density measurements have b een interpreted by using the dynamic effective area as a calibration c oefficient. This turns out to be a satisfactory approximation in the c ase of strong turbulence when the near ground space charge layer is hi gh and the static effective area approaches the dynamic effective area . In the limit of low turbulence the traditional interpretation result s in errors of several tens percent. A reduced height of the antenna h elps to keep the static effective area close to the dynamic effective area and to suppress the errors.