Atmospheric conduction current: observations with a new method of measurement
Burke, Hsiao-hua Kuo
Few, Arthur A.
Master of Science
Atmospheric electric field intensity, air conductivity, air-earth current density and air space charge density are the four closely related parameters of atmospheric electricity. The normal field intensity, about I3 volts/m near ground, decreases with altitude and also varies with cloud conditions and space charge concentrations. The conductivity of air, approximately 3 x 1 ohm m near ground, increases with altitude and change*with aerosol content, humidity and other local conditions. The average conduction current density, on the other hand, stays more stable and is about 2 pA/m for all altitudes. During fair weather conditions, the directions of both the field and the current are downward from air to the ground ( traditionally referred to as the positive direction). Prior to our new method of measuring the atmospheric conduction current, there has been no direct measurement above ground. The instrument consists of an aluminum hemisphere pair suspended above the ground with the measuring electronics and the transmitter enclosed in the spherical structure. The upper hemisphere receives the positive component of the air-earth conduction current and the lower hemisphere the negative. The sum of the two is measured and the data are transmitted to a recorder. This is a direct and, therefore, more accurate method for measuring the air-earth current above the earth's surface. This method also avoids possible electrode effects and convection currents, which could not be distinguished from the conduction current, that are problems in the case of direct ground measurement. The electronic design employed here also includes compensation for the displacement current and temperature drifts. Experiments including testing Ohm's Law, the enhancement factor and effects of a mercury lamp light (UV-visible) were performed in the laboratory before detecting the air-earth conduction current. The instrument reads current densities from -5 pA/m to +5 pA/m with sensitivity up to .2 pA/m. Data for 13 weeks were collected. The results are divided into two categories: fair weather and disturbed weather conditions. Diurnal variations, seasonal trends, sunrise and fog effects are extensively discussed under fair weather conditions. During disturbed weather, the effects of low rain and thunder clouds are discussed. Observations indicate that charge separation occurs inside these clouds even when lightning activity is not observed. The charge concentrations within overcast clouds sufficient to reverse the normal atmospheric electric field direction are calculated. Negative current readings after rain and thunder storms are observed and discussed. From these results the air-earth charge balance theory is also modified. This experiment shows that simultaneous measurements of field intensity, current density, conductivity and space charge density at different locations are also required in order to draw more quantitative and accurate conclusions.