Formation of decimeter-scale, long-lived elevated ionic conductivity regions in thunderclouds

Abstract

We propose a scenario in which elevated ionic conductivity regions (EICRs) with dimensions of the order of 0.1–1 m are formed in the turbulent thundercloud environment. The starting point in this scenario is the occurrence of electron avalanches in the vicinity of colliding hydrometeors, leading to the formation of ion production centers. Their dimensions are of the order of 1 0 − 3− 1 0 − 2 m, and their lifetime is of the order of 1 0 − 4− 1 0 − 3 s. When a new ion production center is created inside the decimeter-scale residual ion concentration spot left behind by a previously established center, the local ion concentration steadily increases, which leads to the formation of decimeter-scale EICRs whose lifetime is measured in seconds. The relatively high conductivity of EICRs (up to 1 0 − 9 S/m or so) relative to the background conductivity (1 0 − 14 S/m or less) ensures their polarization in external electric field within a few milliseconds or so. The EICR formation mechanism requires only one condition: the rate of occurrence of ion production centers per unit time in a unit volume should exceed the percolation-theory-based critical level of 1 0 − 1 m− 3 s− 1. Hydrometeor collision rates three and even four orders of magnitude higher than this value have been reported from observations. Presence of EICRs in the cloud provides local electric field enhancements and pre-ionization levels that will lead to the formation of additional ion production centers and may be sufficient for the initiation and development of streamers and, eventually, lightning.