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 detectably affected by 27-day periodicity in solar rotation. ¿ 1999 American Geophysical Union