Global scale comparison of simultaneous ionospheric potential measurements

A unique series of simultaneous ionospheric potential (V-I) balloon soundin gs were obtained every 3 h over 2 full days at Weston, MA and Darwin, Austr alia, on the other side of the earth. These comparisons were to test the as sumption that the ionosphere at sub-auroral latitudes is an equipotential a nd that a single measurement can provide a globally representative number. Another objective was to evaluate meteorological conditions affecting the m easurements in a clean dry continental atmosphere (Darwin) compared with a more variable moist less clean atmosphere (Weston). The results indicate th at for the Darcyin data the V-I measurements were within 10% of the classic Camegie curve diurnal variation while the Weston data were more variable a nd often too large. The major source of error appears to be due to hydrated aerosol at Weston causing high electric fields in the exchange layer that were not fully compensated by electric fields above the inversion. The prot otype instrumentation also contributed some error. The major finding of thi s experiment is that a layer of low conductivity air near the ground can ha ve an unexpectedly large effect on electric field sounding data leading to error in the estimates of V-I magnitude. This finding would not have been p ossible if simultaneous measurements in different airmasses had not been ma de since the individual electric field profiles appeared normal. (C) 1999 E lsevier Science B.V. All rights reserved.