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.
The simultaneous measurement of both polar conductivities from a P4Y airplane using two Gerdien‐type condensers is described. An altitude distribution of conductivity representative of clear arctic air is presented. From this distribution, a columnar resistance of 6.0×1016 ohm‐m2 is obtained by integration from the earth's surface to 6 km. The average ratio of negative to positive conductivity was found to be 1.1. The charge on the aircraft was found to have negligible effect on the conductivity measurement for fair weather conditions. Triboelectric charging of the conductivity chamber central electrodes by aerosols was observed on most fair weather flights. Concurrent observations at the earth's surface occasionally exhibited the same effect. Even in its presence, techniques were used which permitted valid measurements of conductivity to be made. It is suggested that this effect may have been responsible for unusually high positive‐to‐negative conductivity ratios reported by others.
An experimental investigation of the variations of atmospheric electric quantities in space and time was conducted. A review of past measurements reveals several inaccuracies and inconsistencies which stimulated the investigation. The simultaneous measurement of both polar conductivities, of potential gradient, and of several meteorological quantities from a multi‐engine airplane, and the derivation of other atmospheric electric variables, are described. Altitude ‘profiles’ of conductivity and potential gradient up to 6 km obtained on a representative sounding over the Chesapeake Bay show a secondary exchange layer above the more common surface exchange layer. The space‐charge density profile exhibits positive charge concentrations at the tops of these layers, near the bases of the associated temperature inversions. Upward convection of positive space charge causes the computed conduction‐current density within the exchange layers to be some 40 pct higher than its relatively constant value above the layers. The product of columnar resistance and current density, employed by previous workers to obtain atmospheric potential, provides erroneous values whether the conduction current near the Earth's surface or the average value over the entire altitude range is used. On the basis of a 6‐km atmospheric potential of 327 kv obtained from the integrated potential gradient, the true extrapolated total potential of the high atmosphere is 341 kv at a time near the diurnal maximum.
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