The purpose of the present report is to present further work on the influence of precipitating energetic particles on ELF propagation. The effort principally involves the effects of solar particle events in enhancing the conductivities of the earth's ionosphere wave guide. Calculations have been made of the sensitivity of the ELF signal strength to changes in the conductivities over the transmitter that may occur during SPE conditions. In order to be able to calculate more accurately the changes in ELF signal strength associated with solar particle events and other disturbed conditions at twilight, new electron density data measured in the 4 August 1972 event have been used to provide increased knowledge of twilight behavior which will lead to improvements in ion chemistry modelling. The effects of the charge deposition in the polar caps on the earth's electric fields and current systems have been modeled. More realistic ambient electron and ion density daytime profiles have been proposed to provide accurate baseline signal strength calculations.

A study has been made of extremely low frequency (ELF) wave propagation anomalies as related to energetic particle precipitation, principally during solar particle events (SPE). Based on calculation of the predicted signal strengths at Tromso for transmissions from the Wisconsin Test Facility (WTF) a criterion has been selected for possible use in a field test operation. If the ion pair production rates at 40 km are equal to or greater than 1 x 1000/cc/sec then it is probable that a 3 dB or larger reduction in signal strength would occur for such an event. Since this preliminary criterion is based on ELF signal strength computation assuming no local time variations along the propagation path, more detailed calculations of the local time ionospheric effects should be performed. A study was made of the expected effect of local time variations during solar particle events on the ELF propagation over the path from WTF to Tromso. Electron and ion density profiles for the various segments of the test path were calculated with the ion chemistry model, taking into account the local time for each segment. In a comparison of conditions measured and calculated for SPE72 on 4 August 1972 near the peak of the event and conditions measured and calculated for a similar case assuming a season of 21 December, very little difference in signal strength attenuation over the path was found.