The interaction of a dipolar thunderstorm with its global electrical environment

Abstract

A two‐dimensional numerical model is used to calculate the electric field and current that flow from a dipolar thundercloud into the global electric circuit. The model domain includes a hemisphere in which the thundercloud acts, an equalization layer between 65 and 100 km, and a magnetic conjugate hemisphere. To maintain the measured fair‐weather electric field and ionospheric potential, the output current from the thunderstorm is assumed to flow into the finite volume of the atmosphere extending from the ground to 100 km. Our calculations show different patterns of current flow for equatorial and high‐latitude thunderclouds. When the geomagnetic field lines are vertical, the vertical current in the equalization layer above a thundercloud is more confined to flow in a relatively narrow current tube rather than spreading horizontally, as in the case of equatorial horizontal magnetic field lines. The total upward current above the model thunderstorm is 0.7 A. This vertical current is constant up to about 65 km, and then it decays by about 15%equalization layer, where it is redirected horizontally and then downward toward the ground. The altitude of this transition depends on the day and night ionospheric conditions and on the direction of the magnetic field lines. Above 65 km, in the magnetic conjugate hemisphere, there is a further horizontal spreading of the thunderstorm current. Below this altitude the spreading stops, and only vertical currents flow to the ground. Our results show that it is important to include a realistic model of the equalization layer with current flow to the magnetic conjugate hemisphere to evaluate the role of thunderstorm charging of the global circuit.