International Space Science Institute - Europlanet Workshop on “Planetary ” Bern, Switzerland, 23 – 27 July 2007

MEASUREMENTS OF NON- TERRESTRIAL ATMOSPHERIC ELECTRICITY R. Giles Harrison. Department of , , Reading, Berks, RG6 6BB UK. ([email protected])

The vertical gradient in the electric potential of the measurements are required, with compensation for terrestrial atmosphere was discovered through pioneer- temperature changes 4. ing measurements in the eighteenth century, which con- tributed substantially to the early understanding of at- The global circuit current can be measured directly at mospheric electricity. Apparatus for continuously re- the surface, using horizontal collecting electrodes. The cording of the vertical Potential Gradient (PG) was fair-weather current density is typically ~2pA.m -2, developed in the nineteenth century, using the water which necessarily requires a collector of reasonable dropper sensor invented by Lord Kelvin with a me- area to provide sufficient signal resolution. A compli- chanical electrometer. Kelvin’s water dropper sensor cation associated with finite area collectors is that dis- was implemented rapidly internationally, on early bal- placement currents are induced from PG changes. loon research flights 1, on the Eiffel Tower, and almost These currents can be compensated by using a local PG continuously at Kew Observatory, London from 1864- measurement, or by using a combination of collectors 1932. Additional instrument technologies were devel- with different geometry 5. Long-term measurements oped around 1905 to determine air conductivity (the require careful attention to insulator maintenance, to Gerdien condenser), and the air-earth current density prevent water degradation or insect damage. Manual (Wilson plate). Many modern atmospheric electrical measurements at Kew using the Wilson plate span instruments are still based on similar principles. 1909-1979 6.

The Potential Gradient is positive in fair weather con- Above the surface, balloon, aircraft or airship plat- ditions (typically ~+150V.m -1), but becomes more forms are required, although early experiments used variable in magnitude and polarity in disturbed condi- kites. Routine releases of radiosondes for meteorologi- tions such as rainfall. A wide dynamic range PG sensor cal observations provide cost-effective opportunities for measurements in varied meteorological conditions for atmospheric electrical measurements, using dispos- is the field mill, which determines the charge induced able equipment, typically Geiger counters, charge sen- in alternately exposed and shuttered electrodes. Be- sors or conductivity sensors using the Gerdien con- cause of electric field distortions around the instrument denser principle. and its support mast, an absolute calibration is re- quired. This can be undertaken by electrostatic simula- References tion calculations, or using a low distortion potential [1] R.G. Harrison and A.J. Bennett, (2007). Cosmic ray and sensor, such as the long horizontal wire antenna. Such air conductivity profiles retrieved from early twentieth cen- wire antennas require fair weather, but turbulent 2 con- tury balloon soundings of the lower troposphere J Atmos Sol- Terr Phys 69 , 4-5, 515-527. ditions, ultra high integrity insulation and a very high 3 [2] J.F. Barlow and R.G. Harrison, (1999) .Turbulent transfer impedance electrometer . of space charge in the atmospheric surface layer. In H.J. Christian (ed) Proc 11 th International Conference on Atmos- Vertical current flow in fair weather conditions results pheric Electricity , NASA/CP-1999-209261, 575-578. from charge separation elsewhere in disturbed weather [3] R.G. Harrison, (1997). An antenna electrometer system regions, which is coupled through the global atmos- for atmospheric electrical measurements Rev. Sci. Instrum pheric electric circuit by the finite electrical conductiv- 68 , 3 1599-1603. ity of atmospheric air. The air conductivity almost ex- [4] K.L.Aplin and R.G. Harrison, (2001). A self-calibrating programmable mobility spectrometer for atmospheric ion clusively results from the “small ions” (cluster ions) measurements Rev Sci Instrum , 72 , 8 3467-3469. present in air, produced by natural background radioac- [5] A.J.Bennett and R.G. Harrison, (2006). Surface determi- tivity and cosmic rays. Direct atmospheric electrical nation of the air-earth electrical current density using co- measurements of cluster ions are generally obtained located sensors of different geometry Rev Sci Instrum 77, using Gerdien’s cylindrical sampling approach, which 066104 527.doi:10.1016/j.jastp.2006.09.008 draws ion-containing air axially through a radial de- [6] R.G. Harrison and W.J. Ingram, (2005). Air-earth current 76 flecting field. The balance between radial and axial measurements at Kew, London, 1909-1979 Atmos Res , (1-4), 49-64, doi:10.1016/j.atmosres.2004.11.022 motion allows the ion mobility to be determined as well as the number concentration. Sensitive current