A simplified D-region and its application to magnetic storm after-effects.

Energetic electron precipitation from the Earth's radiation belts can provide the dominant D-region ionization source at middle latitudes during the recovery phase of geomagnetic storms. To study the ionospheric response, a simplified D-region ionic model is developed with emphasis placed on obtaining a reasonably accurate evaluation of the free electron concentration. The precise distribution amongst the multiply hydrated water cluster ions and high affinity negative ions is not considered. Instead the complicated heavy ion chemistry is replaced by a scheme which simply applies an average rate of clustering and subsequent recombination. The model provides an adequate fit with observed quiet-time electron concentration in the D-region. It is shown that typical poststorm electron precipitation can increase mesospheric electron densities by a factor of three during the day and by more than an order of magnitude at night. Such enhancements are consistent with VLF and LF radio wave disturbances observed during the recovery phase of major geomagnetic storm. The disturbances are expected to exhibit considerable short term variability and should persist for over a week consistent with the observed decay rate for the enhanced geomagnetically trapped electron flux.