Spectral energetics of the lower thermosphere
A spectral energetics analysis of the lower thermosphere is carried out using simulated data from the NCAR Thermosphere-Ionosphere General Circulation Model (TIGCM). The results clarify the physical processes through which upwardly propagating semidiurnal tides dissipate and release their energy into the lower thermosphere. Energy residing within the study region is partitioned into reservoirs of available potential energy, irrotational kinetic energy, and nondivergent kinetic energy at four vertical levels. A definition of available potential energy is used that is appropriate for regions of variable mean molecular weight. The reservoirs are further subdivided by vector spherical harmonic wave numbers, and an energy budget is computed for each mode. The source, sink, and transformation terms are obtained using a post-processor that reproduces the contribution of each term in the momentum and thermodynamic equations. The loss terms for the zonal wave number two modes represent the dissipating forces for the semidiurnal tides. Viscosity, heat conduction, and ion drag represent the primary dissipative forces. Numerical smoothing within the TIGCM, representing the subgrid-scale diffusion, is found to have a non-negligible contribution to the tidal dissipation. A small terdiurnal tide that is excited by ion drag is also observed in the model. A sensitivity analysis is carried out to ascertain the effects of the seasonal cycle, solar cycle, UT, and geomagnetic activity. At solar maximum, solar heating at the trough of the tide is an important dissipative force; the altitude of tidal dissipation is correspondingly lower. At high values of geomagnetic forcing, the propagating semidiurnal tide is completely dissipated within the study region.
- Research Organization:
- Michigan Univ., Ann Arbor, MI (United States)
- OSTI ID:
- 7164197
- Resource Relation:
- Other Information: Thesis (Ph.D.)
- Country of Publication:
- United States
- Language:
- English
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