Day-to-Day Variability of the Thermosphere and Ionosphere

Day-to-day variability is an intricate aspect of the ionosphere and thermosphere (IT) system. Its ubiquitous but random presence makes it difficult to track down and hard to predict, presenting a serious challenge for both theoretical understanding and space weather forecasting. We currently understand the day-to-day variability as being externally driven by three different forces: (1) solar radiation, (2) solar wind/geomagnetic storms, and (3) upward propagating atmospheric waves generated in the meteorological regime. This chapter reviews the characteristics of day-to-day variability revealed in recent years. We note that our knowledge on the day-to-day variability is highly limited, and a systematic picture of such variability across an array of major IT parameters (e.g., plasma density and drift, thermosphere density, wind, and temperature) is yet to be achieved.