Resolving thermospheric vertical wind ambiguities and energy processes

This study applies a generalized vertical coordinate system approach alongside thermodynamic control volume analysis to explore the nuanced interpretations of energy transfer processes associated with vertical motion in the thermosphere. Using simulations from the thermosphere-ionosphere-electrodynamics general circulation model (TIEGCM) V3.0 model, a key finding reveals that transforming vertical winds in height coordinates onto constant-pressure surfaces contains a substantial lifting component, which is an aspect often overlooked in previous research. This distinction is critical for internal energy assessments, as vertical winds defined in pressure coordinates directly contribute to the adiabatic heating/cooling rate, while only a portion of the vertical wind in height coordinates contributes to these energy changes. These differences are demonstrated through schematic representations of thermodynamic control volumes that model a column of thermospheric gas undergoing various energy transfer processes. Accurately determining the portion of observed vertical winds that affect internal energy requires a clear understanding of height and pressure variations, which is a nontrivial challenge in observational contexts. Furthermore, applying the generalized vertical coordinate framework to airglow emissions in the upper thermosphere uncovers an inherent ambiguity: vertical winds inferred from airglow observations may not align with those defined on pressure or height surfaces. This insight suggests that discrepancies in the behavior and magnitude of vertical winds derived from Fabry–Perot interferometer (FPI) observations of red-line emissions in the upper thermosphere are due to poorly known relationships between the emission structure relative to pressure or height surfaces.