Ultraviolet Insolation Drives Seasonal and Diurnal Space Weather Variation

We present several findings which improve understanding of the seasonal and diurnal variation in auroral and magnetospheric activity. The total ionospheric conductivity in the nightside auroral oval from UV insolation (SP) is calculated, and its seasonal and diurnal variation is shown to correlate very highly with that of the AL and Am indices of geomagnetic activity (r=0.89 and r=0.75 respectively). Such excellent correlations with Am have been previously obtained by other researchers using instead the acute angle between the Earth's dipole axis and the Earth-Sun line, y. However the ionospheric conductivity formulation provides a more physical model to explain the equinoctial (McIntosh) effect. Namely, the level of geomagnetic activity is well-ordered by whether the nightside auroral oval is sunlit in one hemisphere or neither. We improve calculations of the expected pattern of seasonal and diurnal variations in the solar wind input. The elliptical nature of the Earth's orbit results in observed interplanetary magnetic field (IMF) strengths about 7% larger in January than June. When the sun's spin axis tilt to the ecliptic plane is considered, the predicted IMF southward component (Bs) maximizes in February, as is observed. We also calculate the seasonal and diurnal variation of a more general solar wind-magnetosphere coupling function, EKL. EKL proves to have very little (0.5%) diurnal variation, and has a seasonal variation of about 14%. For the first time, the seasonal and diurnal variation in the FPC, the polar cap flux (from Polar UVI observations, cross-calibrated to a DMSP-based standard) and in magnetotail stretching (the b2i index) are presented. Magnetotail stretching proves to correlate better (r=-0.57) with EKL than with SP. FPC correlates better with SP, but the correlation (r=0.46) is not nearly so strong as for the indices of geomagnetic activity, Am and AL. Our survey of the seasonal and diurnal variation of the magnetosphere thus shows that some aspects (geomagnetic indices) correlate best with UV insolation, while others (magnetotail stretching) correlate best with solar wind input.