Airborne atmospheric electrical measurements may be useful in studies of air pollution, turbulence, and aerosol distributions. A short summary of the electrical nature and behavior of the atmosphere is given relative to the atmospheric electrical detection of aerosol plumes. An example of the efficacy of this approach is illustrated through an experiment in which an aircraft equipped with atmospheric electrical instruments flew side by side with another aircraft equipped to measure particles and gases while tracking a plume out to 90 km. The approach described would be equally viable for the detection of exhaust tracks from ships and submarines and for detecting and monitoring radioactive plumes.

A fundamental problem in atmospheric electric research has been measurement of the intensity and variation of Earth's electric field, which is proportional to ionospheric potential (Vi). To obtain its magnitude an electric field sounding through the atmosphere is required. Data from the three programs that have measured Vi during the last half century were combined to determine the secular variation of global circuit intensity. The average geoelectric potential (Vi) magnitude has remained relatively constant at about 240 kV, except for a temporary increase of as much as 40% following a period of intense atmospheric nuclear testing in the early 1960s. Experiments were conducted to investigate the affect of temperature on Vi. Continental-scale hourly ground-level air temperature variation modulates the intensity of Vi and there is negative feedback involving cloud development that would stabilize temperature on the diurnal time scale. However, increased temperature-driven convection will increase water vapor in the troposphere and thus provide positive feedback enhancing global warming on longer time scales. Significant differences exist between the intensity of the DC global circuit and the diurnal and annual variations of global lightning frequency. The previously unknown minimum-to-maximum seasonal variation of Earth's electric field is about 15% of the mean with a maximum in late Northern Hemisphere summer and a minimum in winter.

The perturbation of the natural atmospheric electric field by growing trees is considered with both measurements and electrostatic calculations. It is concluded that tree growth alone can account for the decline in measured field strength over four decades at Nagycenk, Hungary, with no need for a decrease in intensity in the global electrical circuit.

Since the global circuit is maintained by currents from thunderstorms and electrified clouds, which are controlled by temperature, we are investigating the use of ionospheric potential (V-I) as a measure of the variation of global temperature. We report positive correlation between V-I and global temperature obtained from three different data sets. V-I is also positively correlated with an inferred global lightning/deep cloud index which is positively correlated with global temperature. Thus, there is a consistent picture of warmer temperatures leading to more deep convection and higher V-I. Since a series of single V-I soundings at any appropriate location may provide a globally representative measure of temperature variation in real time, it is suggested that routine monitoring of this parameter could provide considerable cost and operational advantages compared to current methodology involving observations at thousands of ground stations and satellite radiation measurements. (C) 1999 Elsevier Science B.V. All rights reserved.

A unique series of simultaneous ionospheric potential (VI) balloon soundings were obtained every 3 h over 2 full days at Weston, MA and Darwin, Australia, on the other side of the earth. These comparisons were to test the assumption that the ionosphere at sub-auroral latitudes is an equipotential and that a single measurement can provide a globally representative number. Another objective was to evaluate meteorological conditions affecting the measurements in a clean dry continental atmosphere (Darwin) compared with a more variable moist less clean atmosphere (Weston). The results indicate that for the Darwin data the VI measurements were within 10% of the classic Carnegie curve diurnal variation while the Weston data were more variable and often too large. The major source of error appears to be due to hydrated aerosol at Weston causing high electric fields in the exchange layer that were not fully compensated by electric fields above the inversion. The prototype instrumentation also contributed some error. The major finding of this experiment is that a layer of low conductivity air near the ground can have an unexpectedly large effect on electric field sounding data leading to error in the estimates of VI magnitude. This finding would not have been possible if simultaneous measurements in different airmasses had not been made since the individual electric field profiles appeared normal.

The first RF pulse from total lightning' discharges (cloud and ground flashes) has been used in different ways to locate the origin of flashes in two new types of lightning detection systems. The multisensor LASI time-of-arrival (TOA) system uses GPS timing of the first pulse. The ATLAS single sensor system uses the amplitude of the first pulse, which is invariant in magnitude and polarization for all lightning discharges, to determine distance from the sensor. It is significantly more accurate than past single sensor lightning mapping systems. The polarity of the first pulse generally identifies lightning type (IC or CG). Both systems utilize only the first pulse which makes signal processing much simpler than with previous lightning locating systems. Knowing the position where lightning begins (maximum electric fields, mixed phase hydrometeors and updrafts) is valuable for identifying convective cells producing the hazardous meteorological conditions caused by thunderstorms. It is also important for research studying thunderstorm electrification and associated microphysical problems.

A novel coronal current-determining instrument is being used at NASA-KSC which overcomes previous difficulties with wind sensitivity and a voltage-threshold 'deadband'. The mounting of the corona needle at an elevated location reduces coronal and electrode layer space-charge influences on electric fields, rendering the measurement of space charge density possible. In conjunction with a space-charge compensation model, these features allow a more realistic estimation of cloud base electric fields and the potential for lightning strike than has previously been possible with ground-based sensors.