Revelations in the Study of Geospace Variability

Recent investigations reveal an important and previously inconspicuous set of drivers of quiescent thermosphere-ionosphere variability, and confirm that the Earth's atmosphere behaves as a complex component of the solar-terrestrial system, rather than a series of independent layers. Lower atmospheric drivers produce manifest thermosphere-ionosphere variations that characterize quiescent space weather conditions and underlie any geospace response to solar geomagnetic disturbances. Diurnal heating of the Earth's lower and middle atmosphere through the absorption of solar infrared and ultraviolet radiation, as well as latent heating of evaporation associated with raindrop formation in deep convective tropical clouds, excite a spectrum of global-scale waves known as atmospheric tides. Some of these tides propagate upward, grow exponentially in the increasingly rarified atmosphere, and penetrate geospace. Therein, they impact the electro-dynamo process in the lower thermosphere-ionosphere or directly modulate the diurnal variations generated by in-situ absorption of extreme ultraviolet radiation aloft. The lower atmospheric tidal drivers are in turn affected by meteorological disturbances and carry those impacts into geospace as well. They also evolve from month-to-month and inter-annually, further complicating our ability to fully anticipate their impacts. This presentation traces the evolution in our understanding of the drivers of thermosphere-ionosphere variability, underscores the challenges associated with unraveling the inherent complexity of this coupled subsystem that is also affected by the geomagnetic field, and identifies some unresolved questions for geospace research.