Ionospheric effects of relativistic electron enhancement events

The relativistic electron population as measured both at geosynchronous orb it and at low altitudes at subauroral latitudes exhibits pronounced fluctua tions in association with magnetospheric substorm and solar activity. A gro und-satellite correlative study based on amplitude and phase measurements o f VLF signals propagating in the earth-ionosphere waveguide indicates that the relativistic electron enhancements are accompanied bq similar enhanceme nts in nighttime ionospheric conductivity produced by associated enhanced p recipitation. VLF signal amplitudes are found to exhibit >10 dB changes, sh owing the same 27 day cycle and 2-3 day rise and fall time pattern as relat ivistic electron enhancement events recorded by GOES 7 and SAMPEX, and indi cating that the nighttime lower ionospheric electron density at subauroral latitudes is detectably affected by 27-day periodicity in solar rotation.