THE ELECTRIC CURRENT DENSITIES BENEATH THUNDERSTORMS (CONDUCTIVITY).

Abstract

The Maxwell current density (J(s)), electric field (E), and positive and negative air conductivities were measured simultaneously under several thunderstorms at the NASA Kennedy Space Center (KSC), Florida, during the summer of 1981. The components of J(m) include displacement current as well as field-dependent (J(s)) and convection (J(c)) currents. The measurements under active storms show that: (a) J(m) is usually dominated by displacement currents when E is close to zero, (b) J(m) is steady with time in the intervals between lightning discharges, (c) J(m) is usually not altered significantly by lightning, and (d) the average values of J(m) change slowly over time scales that are comparable to those required for storm development. Field-mill data have been used to derive estimates of the time-average J(m), ‘J(m), under a number of storms at KSC in the years 1976-1978 and 1981. Maps of ‘J(m) are consistent with the locations of radar echoes and lightning charges, and the patterns of ‘J(m) develop and change shape slowly with time. Maximum values of ‘J(m) for large storms are typically on the order of 10 to 15 na/m², and those for small storms are 2 to 4 na/m². Since J(m) is a solenoidal vector, area-integrals of the ‘J(m) maps (‘I) on the ground provided at least a lower limit to the total storm current aloft. Maximum values of ‘I for small convective storms are on the order of 0.1 to 0.5 A, and the maximum values for large storms are at least 3 to 5 times larger. Attempts to infer the location, magnitude, and geometry of the current sources aloft from the field-derived estimates of ‘J(m) have been hampered by a 10-20% variance in the values of ‘J(m). These errors prevent a unique characterization of the current sources aloft unless other data can be included in the analysis. Polar conductivities have been found to be highly variable in a thunderstorm environment, but the total conductivity often remains comparable to that found in fair weather. Conductivities derived from Maxwell current estimates of Jₑ vs. E are about an order of magnitude larger than the direct measurements; therefore, the Jₑ vs. E method of estimating σ may not be valued.