A study of enhanced fair-weather electric fields occurring soon after sunrise

In this paper we describe several series of electric field soundings made i n the lowest few hundred meters above the ground on 6 days at Kennedy Space Center in Florida. These soundings are used to determine the charge densit y and thickness of the charged electrode layer just above the surface of th e Earth both before and after sunrise during fair weather. On most of the d ays considered, there was an anomalous enhancement in the ground-level elec tric field that was probably associated with the "sunrise effect" previousl y described by others. At our tether site we found that the electrode-layer charge density began increasing at about the same time as the local enhanc ement in the electric field magnitude at the ground. Shortly before the pea k in the local E enhancement, the electrode-layer charge density decreased while the charge thickness increased; these changes were coincident with a decrease in relative humidity, a shift in the average wind direction, and i ncreases in the fluctuations in relative humidity, wind speed, and wind dir ection. The typical decrease in charge density was from 0.2 to 0.05 nC m(-3 ), while the charge layer thickness increased from less than 20 m to almost 200 m. Our measurements suggest that enhanced positive electrode layers ac cumulate before sunrise very close to the surface because there is relative ly little radioactivity in the soil or air. Local, upward mixing of the den ser, low-lying, electrode-layer charge may account for the observed sunrise enhancement in electric field. The larger enhancements observed at some si tes may indicate that upward convection is supplemented by advection of den ser charge from above water surfaces a few tens of meters (or less) from th e measurement sites.