The Role of Atmospheric Planetary Waves in the Day-to-Day Variability of the Earth's Thermosphere and Ionosphere

Abstract

Planetary‐scale waves originating from the lower atmosphere continuously and dramatically imprint their dynamical state on the thermosphere and ionosphere, where they can cause fluctuations in density and composition. These waves are understood to be a prominent driver of the coupling mechanism by which lower atmospheric forcing induce variability in both regions of the upper atmosphere. This dissertation presents studies of the role of different mechanisms responsible for the planetary wave induced day-to-day variability in these regions. The ubiquitous and random presence of day-to-day variability makes it difficult to detect and challenging to predict, presenting a daunting task for both theoretical understanding and space weather modelling of these regions. Previous work on the detection of these waves has been limited in part to challenges in examining them with either short‐term datasets or episodic cases. To address this, the impact of the periodic signatures that upward propagating atmospheric planetary-scale wave induced on the ionosphere is first investigated. This is facilitated by the development of a novel algorithm that tracks and quantifies the activities of these waves in the mesosphere lower-thermosphere (MLT) region. This novel algorithm has the advantage over prior work in that it quantifies the effects of the activities of these waves in the day‐to‐day variability of the ionosphere over a long-term, estimates the efficiency with which these waves generate a corresponding response in the ionosphere, and account for external contributions (such as Solar EUV and magnetospheric forcing). Next, the response of the thermosphere and ionosphere to quasi-6 day planetary-scale wave events in the middle atmosphere are examined. Intial focus is on altitude dependence of planetary wave – induced variability of the neutrals in the MLT region and the ions in the F-region ionosphere using a combination of satellite missions between 80 and 600 km altitude such as ICON, GOLD, and COSMIC-2 observations. This is followed by comparing these observations with physics-based SD-WACCM-X and GLOW models, providing an insight into the different mechanisms driving the coupling between the ionosphere-thermosphere system and the rest of the Earth's atmosphere. First direct evidence of Q6DW in visible airglow emission rates, and GOLD TDisk and ΣO/N2, and Q6DW effects on neutrals above 200 km altitude in the thermosphere are presented. Plus, evidence for coupling from Q6DW to ionosphere via a mix of E-region dynamo, secondary waves and composition variation is presented. These observational findings confirm multiple pathways by which Q6DW induces short-term variability in the thermosphere and ionosphere via changes in thermospheric ΣO/N2, E-region dynamo electric field and modified F-region winds. A comparison of these observations with SD-WACCM-X physics-based model shows the model reproduces most of the observed Q6DW signatures, but does not capture the composition related variations well. Through comparison with GLOW (driven with Q6DW perturbations from SD-WACCM-X simulations), it is shown that the GLOW model can reproduce the observed Q6DW visible airglow signatures in the lower and upper thermosphere but cannot account for those in the middle thermosphere near 130 km altitude. The consequences of the observed coupling mechanisms and efficiencies would help to improve models that predict uncertainties in satellite reentry.