Experimental measurements of the small-ion density and electrical conductivity of the stratosphere

Abstract

Simultaneous measurements of the small-ion density and electrical conductivity of the stratosphere over southern Victoria with the usual short balloon-borne Gerdien condensers have shown that such measurements are subject to serious errors. The errors arise from the aerodynamic properties of instruments which depend on the vertical ascent of the balloon to aspirate the condensers. Without a flow rate correction which must be calibrated as a function of height for an individual instrument, the recorded ion density may be in error by more than a factor of 2 at the higher altitudes. It is shown also that conductivity measurements are invalid above the altitude where the air velocity profile at the exit of the condenser becomes different from that at the entrance. For the present instrument this altitude is approximately 22 km. Application of an experimental correction factor to the ion density results has led to mobility-height profiles consistent with the inverse density rule up to this limiting altitude. Experiments have been conducted (a) to measure the polar ratios of ion density and conductivity with small vertical sounding balloons and (b) to measure the positive ion density and conductivity from a plastic balloon floating at 30.5 km, where aspiration of the condenser was achieved by means of a pump. Both experiments have yielded results consistent with the previously indicated high stratospheric dust concentration over Melbourne. The ratios of positive to negative ion density and of positive to negative conductivity for the altitude range 4 to 24 km were 1.43±.14 and 1.04±.09, respectively. The positive-ion density and conductivity at 30.5 km were approximately 1000 ions/cm3 and 300 esu, respectively. These last two results are again consistent with the inverse density rule for small-ion average mobility. The floating balloon experiment indicated a considerable time variation in ion density and conductivity.