Measurements of current density in the fair weather atmosphere

Abstract

Simultaneous measurements of potential gradient and conductivity have been made from an airplane over an altitude range of from 15 m to 6 km. Within the exchange layer over the oceans, the conduction current density decreases with altitude, requiring a convection current which is probably caused by the upward diffusion of positive space charge. The conduction current is a maximum at 15 m above the earth's surface, where it is found to be on the average some 40 per cent, and on some soundings 200 per cent, higher than that above the exchange layer. Overland, space charge convection produces a conduction current within the exchange layer which is randomly variable, and on the average some 20 per cent higher than that above the layer. A convection current of some 50 per cent of the total current is sometimes observed at the top of the exchange layer. Above it, the current density is constant with altitude to within 10 per cent, and is found to vary from 1.1×10−12 amp/m2 over Maryland to 4.1×10−12 amp/m2 over Greenland, approximately inverse to the variation in columnar resistance. The average value of current density over the oceans is 2.7×10−12 amp/m2, giving an estimated world-wide conduction current of 1400 amp. Over Greenland, the total potential and average current density aloft on individual soundings both vary together with time and are in phase with the universal variation in thunderstorm activity. A comparison of simultaneous measurements at the surface and aloft in Greenland and near Key West, Florida, show that surface measurements of current density at a single station cannot be used to provide an index of world-wide effects on a day-to-day basis.