Sven Israelsson

The global atmospheric electric circuit, solar activity and climate change

Abstract The study of the global atmospheric electric circuit has advanced dramatically in the past 50 years. Large advances have been made in the areas of lightning and thunderstorm research, as related to the global circuit. We now have satellites looking down on the Earth continuously, supplying information on the temporal and spatial variability of lightning and thunderstorms. Thunderstorms are electric current generators, which drive electric currents up through the conducting atmosphere. They maintain the ionosphere at a potential of ∼+250 kV with respect to the Earths surface. The global electric circuit is completed by currents ∼2 pA / m 2 flowing through the fair weather atmosphere, remote from thunderstorms, and by transient currents due to negative cloud-to-ground lightning discharges. The time constant of the circuit, ∼>2 min , demonstrates that thunderstorms must occur continually to maintain the fair weather electric field. New discoveries have been made in the field of sprites, elves and blue jets, which may have a direct impact on the global circuit. Our knowledge of the global electric circuit modulated by solar effects has improved. Changes to the global circuit are associated with changes of conductivity linked with the time-varying presence of energetic charged particles, and the solar wind may influence the global electric circuit by inferred effects on cloud microphysics, temperature, and dynamics in the troposphere. We now have a better understanding of how the conductivity of the atmosphere is influenced by aerosols, and how this impacts our measurements of the fair-weather global circuit. The global atmospheric electric circuit is also beginning to be recognised by some climate researchers as a useful tool with which to study and monitor the Earths changing climate.

Variation of fair weather atmospheric electricity at Marsta Observatory, Sweden, 1993-1998

Abstract A modified atmospheric electrical station of the Kasemir–Dolezalek construction is continuously operating in the Marsta Observatory (59°56′N,17°35′W) located in rural area 10 km north of Uppsala, Sweden. The routinely recorded parameters are the electric field, positive and negative polar conductivities of air, and space charge density. The effect of possible local anthropogenic air pollution on the fair weather atmospheric electric measurements at Marsta is estimated according to Sheftel et al., 1994a (J. Geophys. Res. 99, 10,793) by comparing the Sunday and weekday values of air conductivity. The effect of local air pollution appears essentially less than at other evaluated continental atmospheric electric stations. The natural periodic variations of fair weather electric field and vertical air-earth current averaged over many years at Marsta are compared with the periodic variations of electric field measured during the Carnegie expeditions over the oceans where the global component of variations dominates over the local component. The diurnal variations of electric field and vertical current at Marsta are well correlated with the Carnegie curve during winter and ill correlated during summer. The correlation coefficient reaches 98% for the winter measurements of vertical air-earth current. In addition, a test has been carried out for a hypothesis that numerical reduction of the data according to the local temperature and wind variation could suppress the local component of fair weather atmospheric electric variations and thus help to study the global component of variation. The hypothesis proved to be inadequate. The reduction suppresses the annual variation, but the shape of the diurnal variation remains the same and the correlation with the Carnegie curve is even worse than in the case of unreduced measurements. The Marsta Observatory is recommended as a basis station for long-term routine atmospheric electric measurements to gather data for the study of climate variation because of the large weight of global component in the variation of fair weather air-earth vertical current and electric field.

Vertical profiles of electrical conductivity in the lowermost part of the turbulent boundary layer over flat ground

Abstract Results of experimental investigations and a theoretical estimation of the polar conductivity profiles in the turbulent surface layer over flat ground are discussed. The results presented are evidence for the occurrence of the steady electrode effect over land. By reducing the number of factors which influence the vertical conductivity profile and applying a special measuring technique, it has been possible to obtain steady profiles. The experimental results are in good agreement with the hypothesis of a turbulent electrode effect in the boundary of strong turbulent mixing. The agreement implies the confirmation of the relation between the surface wind, the electrical conductivity and the thickness of the electrode-effect layer.

Effective area of a horizontal long‐wire antenna collecting the atmospheric electric vertical current

The effective area of an antenna collecting the vertical air-Earth current is a coefficient of proportionality between the collected current and the air-Earth current density. The effective area can be correctly defined only if the model behind the proportionality relation is adequate. The current collected by a horizontal long-wire antenna is not exactly proportional to the vertical air-Earth Maxwell current density because of the different behavior of the displacement and conduction components of the current. Thus, two different effective areas are separately defined for the displacement and nondisplacement components. First, the dynamic effective area characterizes rapid variations of the displacement current. It is calculated assuming that the air does not contain any space charges and that the electric current flow lines match the electric field lines. Second, the static effective area characterizes the nondisplacement current, and it is calculated by taking into account the facts that the conductivity close to the wire surface is unipolar, that the wind-determined horizontal ion trajectories do not match the electric field lines, and that there are space charges due to the electrode effect of the wire and of the ground. Traditionally, the atmospheric electric vertical current density measurements have been interpreted by using the dynamic effective area as a calibration coefficient. This turns out to be a satisfactory approximation in the case of strong turbulence when the near-ground space charge layer is high and the static effective area approaches the dynamic effective area. In the limit of low turbulence the traditional interpretation results in errors of several tens percent. A reduced height of the antenna helps to keep the static effective area close to the dynamic effective area and to suppress the errors.

Numerical ray tracing in the atmospheric surface layer

A numerical ray‐tracing model, including calculation of the sound pressure, was developed. It is valid for propagation from a point source in a moving, stratified atmosphere. Numerical integration of the ray equations was performed, and all rays reaching a specific point were found. Several expressions for the height dependency of the wind speed and the temperature were used, e.g., Monin–Obukhov similarity theory functions, the parameters of which were determined by use of a least‐squares method. Measurements of sound propagation from a point source over finite impedance ground were made. Meteorological parameters were monitored simultaneously, wind direction and relative humidity at a single height, wind speed and temperature at five elevations. Comparison with the model was made out to a distance of 150 m. The agreement between the model values and those measured was good. The influence of the directional characteristics of the source was studied, and found to be very important.

Effects of meteorological conditions and source height on sound propagation near the ground

Abstract Simultaneous measurements of sound propagation from a loudspeaker and meteorological variables were carried out for various ground covers and meteorological conditions. The wind and temperature gradients cause refraction of the sound rays, and hence influence the sound level. The curvature of a nearly horizontal sound ray can be calculated by using measurements of wind and temperature. The curvature is found to be closely connected to the sound level, when distinctions between octave band, ground, distance, source and receiver heights are made. A new variable, the sound propagation parameter, W, including refraction, distance, source and receiver heights, is introduced. The parameter is used to examine if a specific set-up of source-receiver can be sensitive to meteorological effects. The sound level for different octave bands and ground cover is given for negative and positive values of W, outside an interval close to W = 0 , where large scattering of the sound level is found.

Measurements of the electrode effect over flat, snow-covered ground

Abstract Space charge density profiles up to 3 m above ground were recorded with the Obolensky-filter method. A measuring technique with two instruments was used, where one reference instrument was placed on a fixed level and the other was moved up and down. The space charge difference between the reference and the actual level, and the correlation coefficient were calculated. From the analysis of the recordings, made under very similar atmospheric and ground conditions (snow-covered ground), we found strong evidence for the existence of the electrode effect. Very good agreement between the experimental results and theoretical calculations were observed.

Long-term audible noise and radio noise performance from an operating 400 kV transmission line

Long-term measurements of audible noise, radio interference and meteorological variables were carried out close to a 400 kV transmission line in the southern part of Sweden. The aim was to determine the dependence of noise levels on weather conditions. A data processing procedure was developed to exclude sound measurements influenced by sources other than the line. The results are presented and discussed. >

Measurements of meteorological effects on long‐range sound propagation using m‐sequence correlation

An m‐sequence correlation method was used for measuring sound levels 1‐km distant from a point source 1.25 m above the ground, a crop field and a snow‐covered ground, respectively. Meteorological parameters were monitored simultaneously. Wind speed and temperature were measured at several elevations, together with wind direction, relative humidity, and atmospheric pressure at a single height. The effect of refraction on the sound level variation is interpreted in terms of the curvature for near‐horizontal sound rays. It is found that the sound level increases as the curvature goes from negative to positive values. Comparisons with other investigations were made, and qualitative agreement is found. Calculations with acoustic theory were made, the results of which show large discrepancies with those measured.

On the conception ‘Fair weather condition’ in atmospheric electricity

Measurements of atmospheric electrical and meteorological parameters during different meteorological conditions indicate that the use of the conception ‘Fair weather condition’ in atmospheric electricity is discussable. Fair weather contains a very broad stability range, from very unstable to strong stable stability of the atmosphere. For turbulent fluctuations of the electric parameters (the most local variations) the variations are determined by the micrometeorological processes for all stability conditions.These fluctuations represent frequencies greater than one period per four minutes. For lower frequencies (less local variations), however, the stability dependency increases. During stable conditions the electric field and vertical current density were nearly wholly influenced by the charges and their transfer in the nearest layer. During near-neutral and unstable conditions the electrical parameters were influenced by more separated sources. Measurements of how well Ohms law was fulfilled also indicate the difficulties by using the conception fair weather. The measurements also indicate the importance of taking the convection current density into consideration in studies of the electric charge transfer in the atmosphere.In the more large scale of variations measurements of the electric field by radiosoundings show that 88% of the ionospheric potential is derived from the troposphere, where the meteorological processes are of fundamental character for the atmospheric electrical phenomena. The relatively great stability of the diurnal variation of the large scale or global electric field is also valid for the meteorological processes in this scale of variations.