Cosmic ray and air conductivity profiles retrieved from early twentieth century balloon soundings of the lower troposphere

doi:10.1016/j.jastp.2006.09.008

Journal of Atmospheric and Solar-Terrestrial Physics

Volume 69, Issues 4–5, April 2007, Pages 515-527

https://doi.org/10.1016/j.jastp.2006.09.008 Get rights and content

Abstract

Manned balloons were used in Europe to make atmospheric electricity research flights from the late nineteenth century. Subsequent ascents provide unique early twentieth century atmospheric electrical data above the continental boundary layer. Air conductivity data from three balloon ascents between 1905 and 1913 have been used to constrain a model of the atmospheric conductivity profile. At 1 m resolution, the model has been used to calculate the columnar resistance up to 5 km. The average columnar resistance to 5 km was found to be (163±16) PΩm². The ionisation rate profile from cosmic rays was found to have reduced by approximately 8% per kilometre increase in altitude (between the surface and 5 km altitude) during the 50 yr between the mean date of the three ascents (∼1909) and measurements made near the middle of the century (∼1959). These results indicate a decrease in cosmic ray ion production in the lower troposphere during the first half of the twentieth century, due to decreased penetration into the troposphere. The associated increase in the cosmic ray component of the local columnar resistance was 0.3 PΩm² yr⁻¹ (0.2% yr⁻¹).

Introduction

The finite electrical conductivity of atmospheric air permits a vertical current to flow between the positively charged ionosphere and the surface, which in turn produces a vertical gradient in electric potential close to the surface. The combination of charge generation in disturbed weather regions and current flow in fair weather regions constitutes the global atmospheric electrical circuit (Rycroft et al., 2000; MacGorman and Rust, 1998). All three global circuit properties of the potential gradient (PG), the air-earth conduction current density (J_c) and the ionospheric potential (V_I) have been measured at different times and locations during the twentieth century (Harrison, 2004a). Mülheisen (1971) showed that V_I changes at distant locations vary together, supporting the idea that V_I was a global equipotential. Consequently, for studies of the global circuit, V_I is the primary quantity of interest, followed, because of the complicating effects of local influences, by J_c and finally the PG (Märcz and Harrison, 2005). Despite the sensitivity to local effects in polluted air, many more measurements of the PG are available than for the other two quantities: V_I in particular is considerably under-sampled spatially and temporally. The first experimental determinations of V_I were made indirectly using columnar resistance data from the first stratospheric balloon flight (Explorer II, in 1935) (Gish, 1944). Clark (1958) measured profiles of PG up to 6 km and found the total potential to be approximately 300 kV. Markson (1986) calculated a yearly average V_I of 240–260 kV.

In European surface PG measurements at many sites, a long-term reduction is apparent in atmospheric electrical parameters across the twentieth century (Märcz and Harrison, 2003, Märcz and Harrison, 2005). There could be several possible causes including: (1) a change in the global atmospheric electrical circuit, (2) a change in the local columnar electrical properties or (3) a change in surface air conductivity modifying the PG. These changes may have occurred independently or in combination. Possibility (3) alone was suggested by Williams (2003) as an explanation for the 70-yr PG reduction at Eskdalemuir, Scotland (Harrison, 2002, Harrison, 2004b). However, any associated visibility changes, which are closely related to the surface air conductivity (Brazenor and Harrison, 2005) are not evident in 1920–1950 data from Eskdalemuir (Harrison, 2003). Galactic cosmic ray (GCR) ion production is the primary source of air's conductivity away from the continental boundary layer, and the known twentieth century decrease in GCR (Carslaw et al., 2002) is the basis on which (2) is expected to have occurred.

The detection of long-term changes in atmospheric electrical parameters is necessarily limited by the data available. Because of the possibility of local aerosol changes affecting surface PG data, measurements made away from the continental planetary boundary layer (PBL) are particularly useful, as aerosol effects are relatively small. Comparison between PG measurements made in marine air between 1929 and 1968 showed a change of −0.6% per year (Harrison, 2004b). An alternative to marine air is free tropospheric air, as the free troposphere's aerosol content is generally low and less variable compared to the PBL (Pruppacher and Klett, 1997). Measurements of atmospheric parameters within the free troposphere (and away from local sources of anomalous aerosol or charge separation, such as clouds) may consequently be more globally representative. Because of this, data from early twentieth century balloon ascents are analysed here to investigate changes in the atmospheric electrical properties away from the boundary layer, and the hypothesis of an increase in columnar resistance.

Section snippets

Atmospheric electricity measurements from European balloon ascents

Hydrogen-filled balloons provided a unique platform for early atmospheric electricity investigations. Exner and Lecher, working in Vienna, had shown in 1885 that the PG reduced with increasing altitude, a result which was confirmed by the first European atmospheric electricity balloon ascents of Tuma in 1892, and Le Cadet in 1893 (Chauveau, 1925). Such free balloon ascents carried observers to operate the instruments and record the observations, which, as no oxygen was carried, limited the

Columnar resistance theory

If measurements of total conductivity σ and PG are simultaneously recorded at the same height z, the vertical conduction current density J_c can be calculated using Ohm's Law: $J_{c} = PG σ .$ The conduction current density is independent of height up to the ionosphere, if there are no local sources of charge separation producing an additional local electric field. Deviations from this assumption are likely to occur where the atmosphere is not in electrical steady-state, typically because of rapid local

Calculations of global circuit parameters

The validation of the model assumptions for ion production rate and aerosol profile with the balloon data allows it to be used for further calculations, such as the columnar resistance R_c up to the height of available measurements (5 km), denoted R_c5. As Fig. 2 shows, this is usually a substantial part (∼80%) of the total R_c, but there is a strong sensitivity to stratospheric aerosol concentrations not represented in the measurements, which are from the lower atmosphere. Table 3 shows the values

Discussion

The good agreement between the balloon air conductivity observations and the modelled air conductivity during fair-weather indicates that the columnar resistance model fairly represents the steady-state atmospheric electrical conditions between the surface and (at least) 5 km. This provides confidence in the accuracy of the early conductivity profile measurements, and their usefulness in determining geophysical changes. The measurements of the air-earth current density, however, depend on the

Acknowledgement

One of the authors (AJB) acknowledges a studentship from the Natural Environmental Research Council (NERC).

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Cosmic ray and air conductivity profiles retrieved from early twentieth century balloon soundings of the lower troposphere

Abstract

Introduction

Section snippets

Atmospheric electricity measurements from European balloon ascents

Columnar resistance theory

Calculations of global circuit parameters

Discussion

Acknowledgement

Atmospheric Environment

Atmospheric Research

Journal of Atmospheric and Solar–Terrestrial Physics

Atmospheric Environment

Atmospheric Environment

Atmospheric Research

Advances in Space Research

Journal of Atmospheric and Solar-Terrestrial Physics

Journal of Atmospheric and Solar-Terrestrial Physics

Measurement of worldwide diurnal atmospheric electricity variations

Monthly Weather Review

Sur les variations de l’état électrique des hautes regions atmosphériques, par beau temps

Comptes Rendus

Surface determination of the air-earth electrical current density using co-located sensors of different geometry

Review of Scientific Instruments

Action of radioactivity and of pollution upon parameters of atmospheric electricity

Cosmic rays, clouds and climate

Science

Atmospheric Electricity

The fair-weather atmospheric electric potential and its gradient

Spannungsgefälle und vertikaler Leitungsstrom in der freien Atmosphäre, nach Messungen bei Hochfarhten im Freiballon

Annalen der Physik

Messungen der elektrischen Leitfähigkeit der freien Atmosphäre bei 4 Ballonfahrten

Nachrichten von der Gesellschaft der Wissenschaften zu Göttingen

Luftelektrische Messungen bei 2 Ballonfahrten

Nachrichten von der Gesellschaft der Wissenschaften zu Göttingen

Messungen der dichte des vertikalen elektrischen Leitungsstromes in der freien atmosphäre bei der ballonfahrt am 11 V 1905

Nachrichten von der Gesellschaft der Wissenschaften zu Göttingen

Messungen der Dichte des vertikalen elektrischen Leitungsstromes in der freien Atmosphäre bei der Ballonfahrt am 30 VIII 1905

Nachrichten von der Gesellschaft der Wissenschaften zu Göttingen

Ein neuer Apparat zur Messung der elektrischen Leitfähigkeit der Luft

Nachrichten von der Gesellschaft der Wissenschaften zu Göttingen

Actual and potential results from electrical exploration of the atmosphere

Transactions—American Geophysical Union