We show that large-scale wave structure (LSWS), in plasma density in the bottomside F layer, is a hitherto unheralded contributor to the long-standing enigma of day-to-day variability in equatorial spread F (ESF). Little is known about LSWS; it seems to appear in altitude near a vertical shear in zonal plasma drift, during the post-sunset rise of the F layer, and its growth via an interchange instability appears to predispose quasi-periodically spaced regions to development of plasma bubbles. First indications are that LSWS development is necessary and sufficient for ESF occurrence. We suggest that variability in LSWS development, perhaps together with the shear in zonal drift, may contribute to day-to-day ESF variability. A need revealed by this study is that a cluster of distributed sensors, not isolated ones, is necessary to pursue the problem of day-to-day variability.