Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmsa24a..04e&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #SA24A-04
Physics
0350 Pressure, Density, And Temperature, 0355 Thermosphere: Composition And Chemistry, 7853 Spacecraft/Atmosphere Interactions
Scientific paper
Solar extreme ultraviolet (EUV) irradiance is the primary source of energy in the thermosphere, and EUV irradiance variations are the dominant source of thermospheric density variability. Irradiance variations are driven by solar oscillations (e.g., due to the 11-year solar cycle and 27-day rotational modulation) and by the Earth's rotation and revolution (e.g., diurnal and annual cycles). The annual irradiance variation consists of a local component arising from the inclination of Earth's axis to its orbital plane (i.e., seasonal effects), and a global component arising from the eccentricity of Earth's orbit, which causes the Earth-sun distance to vary by +/-1.7 percent. Empirical density models typically use the EUV flux incident at the Earth (rather than at 1 AU) as an input argument, thereby combining the effects of solar-driven EUV irradiance variations with those of Earth-orbit-driven global annual variations. However, there is a significant global annual variation in thermospheric density that is not accounted for with this approach. In this presentation, we examine the annual variation in global average thermospheric density (derived from the orbits of many near-Earth objects) and its dependence on the phase of the solar cycle. We consider the relative merits of empirically representing this oscillation via the EUV irradiance or via harmonic terms. We conclude that it is more advantageous to use the EUV irradiance at 1 AU to represent purely solar-driven EUV variations, while employing separate annual Fourier terms to represent the global annual density oscillation.
Emmert John T.
Lean Judith L.
Picone Michael J.
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