Lower ionospheric modification by lightning-EMP: Simulation of the night ionosphere over the United States

It has recently been suggested that successive intense lightning-electromag netic pulse (EMP) events could cause significant large-scale changes to the properties of the nighttime lower ionosphere. In order to examine this qua ntitatively, data on lightning detected over the United States are combined with the output from a simulation code. During the course of a night stron g lightning-EMP events can lead to significant (similar to 100% or even gre ater) increases in the electron density of the lower ionosphere, with the l argest increases at similar to 90 km altitude. Regions with significant dec reases in electron density are also possible. It is shown that changes in t he electron temperature of the lower ionosphere are unlikely to be signific ant. The time required to produce large-scale changes of ionospheric electr on density above an active thunderstorm may explain the observation of a th understorm "modification time" before red sprite activity is initiated.