Resolving Thermospheric Vertical Wind Ambiguities and Energy Processes

TL;DR

This paper investigates the interpretation of vertical wind measurements in the thermosphere, highlighting the importance of coordinate choice and revealing ambiguities in airglow-based wind observations affecting energy transfer analysis.

Contribution

It introduces a generalized vertical coordinate approach and demonstrates how coordinate choice impacts energy transfer assessments in thermospheric vertical wind studies.

Findings

Vertical winds in pressure coordinates significantly influence adiabatic heating/cooling.

Transforming height coordinate winds onto pressure surfaces reveals a substantial lifting component.

Airglow emission-based vertical winds may not correspond directly to pressure or altitude surface winds.

Abstract

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 TIEGCM V3.0 model, a key finding reveals that transforming vertical winds in height coordinates onto constant pressure surfaces contain a substantial lifting component; 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 or cooling rates 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 clear understanding between height and pressure surfaces; 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 altitude surfaces. This insight suggests that discrepancies in the behavior and magnitude of vertical winds derived from FPI observations of red-line emissions in the upper thermosphere are due to poorly known relations between the emission structure relative to pressure or height surfaces.