Abstract
Abstract. Long-term decreases found recently in both the atmospheric electrical potential gradient (PG) and the air-Earth current density (Jz), using observation series from the UK and Hungary, have motivated studies of other European data. Two surface data series somewhat longer than a decade were available: PG data obtained at Serra do Pilar (Portugal), and PG, Jz and positive air conductivity measurements at Athens (Greece). Selecting data to minimise local effects, the 1960–1971 Serra do Pilar PG values decrease at dawn and in the evening. Dawn data obtained at Athens (1967–1977) indicate a reduction in Jz, while the simultaneous PG values there increase (coincident air conductivity values decrease) for the periods investigated. The Athens PG increase is attributed to local aerosol influences, typical of urban environments. Despite the urban influence, the Athens Jz shows similarities with soundings of the ionospheric potential. The decline in Jz at Athens occurs simultaneously with a decrease reported previously in Jz at Kew (UK), indicating that, at least, a regional decrease in the global atmospheric electrical circuit occurred during part of the twentieth century. Similar surface changes occur in European atmospheric electrical parameters, with a decrease of about 0.5% to 0.7% per year between 1920 and 1970 (possibly extending back to 1898), an annual decrease of between 2.7 and 3.4%, between 1959 and 1971 and a continued decrease of about ~1% per year between 1967 and 1984, possibly still continuing. Keywords. Meteorology and atmospheric dynamics (Atmospheric electricity) – Geomagnetism and paleomagnetism (Time variations, secular and long term) – Atmospheric composition and structure (Aerosols and particles)
Highlights
Few sets of continuous measurements of atmospheric electrical parametersexist for long periods at the same observation site
The potential gradient (PG) measured at the surface depends on global and local effects in a complicated fashion: the air-earth current density arises from the global atmospheric electrical circuit, the surface PG depends strongly on the local air conductivity, which is modulated by atmospheric aerosol (e.g. Harrison and Carslaw, 2003)
Israel originally suggested that the PG reduction was probably a local effect, and, as air conductivity is inversely proportional to aerosol number concentration, that the PG reduction could be attributed to a reduction of the mean aerosol content in the Davos valley
Summary
Few sets of continuous measurements of atmospheric electrical parameters (such as the potential gradient, PG, vertical air-Earth current density, Jz or ionospheric potential, VI ). Harrison (2002) suggested that such a PG decline was consistent with the twentieth century decrease in galactic cosmic rays (GCR) (Carslaw et al, 2002) causing a reduction in the ionospheric potential (VI ) and local increase in the columnar resistance. Together these determine the air-Earth current density, to which the PG is proportional. In addition to the previous findings showing a long-term PG decline at Eskdalemuir and at Lerwick (Harrison, 2002) as well as Nagycenk (Marcz and Harrison, 2003), the Bavarian Alps measurements have a decrease in the 1970s and early 1980s
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