Summary of ELF propagation variations at mid and high latitudes during the November/December 1982 and February 1984 solar proton events

This paper reports the results of the first concerted effort to study the effects of solar proton events (SPEs) on ELF propagation using a manmade source (i.e., the Wisconsin Transmitter Facility (WTF)). The major result is the observation of substantially increased effective attenuation rates along high altitude paths during SPEs. The largest values of inferred attenuation rate during SPE disturbed propagation conditions were for the WTF/Gulf of Alaska path (2.51 dB/Mm at night, and 2.66 dB/Mm during the day). In fact, during the 23 November 1982 minimum nighttime field strength period of 0630 to 0830 GMT, the effective attenuation rate for this path exceeded 3 dB/Mm. These attenuation rates are substantially higher than those measured during ambient propagation conditions on WTF/east-and-northeast paths (∼1.0 dB/Mm at night, and ∼ 1.25 dB/Mm during the day). The probable reason for the higher effective attenuation rates observed on the WTF/Gulf of Alaska path is that this path is nearly tangent to the exterior boundary of the disturbed polar cap. That geometry causes lateral refraction of the transverse electromagnetic (TEM) mode and thus shadow zones.