Transient tropospheric electric fields resulting from sudden changes in ionospheric conductivity

Abstract

The transient electric fields that would be generated by sudden changes in the atmospheric conductivity profile are estimated using a theoretical model. The model assumes the existence of a horizontal electric field with a spatial extent of ≥20‐km at 150‐km altitude and involves the solution of the boundary value problem in two dimensions. The geomagnetic field lines below 150‐km altitude are assumed to be vertical, but the anisotropy of the medium above ∼70‐km altitude is fully accounted for. The computed electric field strength depends sensitively on the size of the source field as well as the conductivitity profile of the atmosphere. In the steady state, a small‐scale (∼20‐km) source field produces relatively weak dc electric fields in the troposphere, whereas a large scale (∼100‐km) source produces stronger observed fields. On the other hand, during a transient the peak electric fields produced in the case of a small‐scale source field can be considerably larger than those due to large‐scale source distributions. The more rapid the increase in the slope of the conductivity profile with altitude, the more efficient should be the downward mapping of both transient and steady state fields. Since the transient electric fields are oscillatory and die out at a rate depending on the local conductivity, the ac electric fields at the ground level are found to last longer than those at higher altitudes.