Ionospheric effects of relativistic electron enhancement events

The relativistic electron population as measured both at geosynchronous orbit and at low altitudes at subauroral latitudes exhibits pronounced fluctuations in association with magnetospheric substorm and solar activity. A ground‐satellite correlative study based on amplitude and phase measurements of VLF signals propagating in the earth‐ionosphere waveguide indicates that the relativistic electron enhancements are accompanied by similar enhancements in nighttime ionospheric conductivity produced by associated enhanced precipitation. VLF signal amplitudes are found to exhibit >10 dB changes, showing the same 27 day cycle and 2–3 day rise and fall time pattern as relativistic electron enhancement events recorded by GOES 7 and SAMPEX, and indicating that the nighttime lower ionospheric electron density at subauroral latitudes is detect ably affected by 27‐day periodicity in solar rotation.