Elucidating the Role that Diabatic and Adiabatic Effects Play in Anomalous Thermospheric Features

Low-Earth orbit (LEO) is highly coveted by modern-day satellite missions but is immersed within a complex and dynamic thermosphere that forms, even in quiet-times, a corrugated environment of mass density “pits and bumps” that plague satellites by producing a variable drag force. Furthermore, the dynamical equations that govern the thermosphere rate of change of mass, momentum, and energy are coupled, requiring simultaneous solving of these equations to discern the formation, evolution, and possibly the sustainability of mass density features encountered by LEO satellites. One such feature that has held scientists’ attention for many years is the anomalous cusp enhancement found on the dayside in the high-latitude regions. It is classified as anomalous due to the lack of explanation for feature formation, but to add to the mystery, current state-of-the-art models fail to properly reproduce the cusp enhancement observed in satellite missions (e.g., CHAMP’s observations). The physics responsible for the cusp, and other anomalous mass density structures, still elude scientists, and the most popular explanation of direct energy injections forming/sustaining the cusp fails to sufficiently describe the density peaks captured by CHAMP’s measurements alone. Results from Hsu et al., 2016 suggest that indirect mechanical effects may instead be the cause.
A form of the first law of thermodynamics enables a unique way to study direct energy effects, in addition to both indirect and/or mechanical effects in tandem, by expressing heat transfer and work in terms of diabatic and adiabatic processes. Expressed using potential temperature, theta, and other thermosphere properties, the first law of thermodynamics can be expressed as
DT/Dt=(P/P_0 )^(R_n/c_p ) Dθ/Dt+ω/(ρc_p )
where DT/Dt is the total neutral temperature rate of change due to (1) diabatic, or “direct energy”, rate of change, (P/P_0 )^(R_n/c_p ) Dθ/Dt, and (2) adiabatic, or “indirect and/or mechanical”, energy rate of change, ω/(ρc_p ). Each of these terms can be computed externally using the thermosphere field properties output by the NCAR-TIEGCM. This approach allows for model-produced mass density structures to be investigated as to whether a diabatic process, such as Joule heating, an adiabatic process, or combination of both are responsible for the mass density structure. Adiabatic processes involve vertical winds across pressure surfaces that may be generated by heat sources or by mechanical processes. Evaluating the model’s energy and force terms elucidates the process responsible for causing diabatic and adiabatic heating/cooling and connecting them to mass density structures. Presently, thermosphere-ionosphere models do not reproduce anomalous mass density structures, like the cusp. However, by investigating how the model-produced mass density structures are associated with diabatic and adiabatic processes, advances can be made to understand cusp formation. All in all, this study provides the unique opportunity to finally discern whether diabatic (“direct”) or adiabatic (“indirect and/or mechanical”) methods are more central in the story of the anomalous cusp enhancement.