Stan Solomon and Liying Qian High Altitude Observatory National Center for Atmospheric Research

1. Annual/Semiannual Seasonal Variations in Thermospheric Density: Evidence for Lower Atmosphere Effects Stan Solomon and Liying Qian High Altitude Observatory National Center for Atmospheric Research with thanks to Bruce Bowman Air Force Space Command NADIR MURI Meeting • CU/LASP • 21 October 2008

2. Model & Measurement of Thermosphere Solar Cycle Variation Qian, L., S. C. Solomon, and T. J. Kane, J. Geophys. Res., submitted, 2008.

3. Model-Data Comparison for 2003

4. Model-Data Comparison for 2003 07337 08744 12138 12388 14483

5. Issues with the Annual/Semiannual Seasonal Variation In order to obtain this good agreement between model and measurement of thermospheric neutral density, we apply an ad-hoc correction to the seasonal variation.

6. Observed Seasonal Variation is Larger than Standard Model

7. Multi-Satellite, Multi-Year Comparison with Standard Model

8. Issues with the Annual/Semiannual Seasonal Variation • In order to obtain this good agreement between model and measurement of thermospheric neutral density, we apply an ad-hoc correction to the seasonal variation. • What could be the physical mechanism behind such variation? • Inadequate model description of thermospheric general circulation? • Effect of Sun-Earth distance? • Russell-McPherron effect? • Magnetic field asymmetry? • Variation in momentum deposition at mesopause? • Variation in eddy mixing at mesopause?

9. Effect of Thermospheric Circulation and Sun-Earth Distance z=400km ln(p0/p)=2 Global mean, solar maximum, geomagnetic quiet, constant eddy diffusivity.

10. Issues with the Annual/Semiannual Seasonal Variation • In order to obtain this good agreement between model and measurement of thermospheric neutral density, we apply an ad-hoc correction to the seasonal variation. • What could be the physical mechanism behind such variation? • Inadequate model description of thermospheric general circulation? • Effect of Sun-Earth distance? • Russell-McPherron effect? • Magnetic field asymmetry? • Variation in momentum deposition at mesopause? • Variation in eddy mixing at mesopause? • The way to change the density of the upper thermosphere is to change its scale height (kT/Mg) • This could take the form of a temperature correction or a composition correction. • Increase in temperature leads to increase in density • Decrease in mean molecular mass leads to increase in density • (We are pretty much stuck with k and g) • We have chosen to change M through the mechanism of changing the eddy diffusion coefficient at the lower boundary, which changes the O/N2 ratio. • (Increase in Kzz reduces O by increasing downward transport.)

11. Imposed Variation of Eddy Diffusion Coefficient at ~97 km This solves the seasonal density problem through imposing compositional variation But, is there evidence for this effect in actual composition measurements?

12. Comparison with Density and Composition Data TIEGCM with/without seasonal variation of eddy diffusivity at lower boundary

13. What are the Implications of this Hypothesis? • It is entirely reasonable that there should be systematic seasonal changes in turbulent mixing in the mesopause region. Breaking gravity waves are a likely source of turbulence. Wave generation, stratospheric and mesospheric jets, and atmospheric tides, are all known to have significant seasonal variation. • But what could cause the “hemispherical asymmetry?” • I.e., higher eddy diffusivity during northern hemisphere summer than southern hemisphere summer • We have ruled out Earth-orbit and geomagnetic asymmetry, i.e., the external drivers. • That leaves the lower-middle atmospheric system, which is known to be significantly hemispherically asymmetric. • The ultimate cause of this is the land-mass distribution. • So, thermospheric variation is fundamentally a product of planetary geology.

14. A Local Result on Eddy Diffusion Coefficient Starfire results from Alan Liu and Chet Gardner, COSPAR 2008