1

Annual/Semiannual Seasonal Variations in

Thermospheric Density: Evidence for Lower

Atmosphere Effects

Stan Solomon and Liying QianHigh Altitude Observatory

National Center for Atmospheric Research

with thanks to

Bruce BowmanAir 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 VariationIn 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

)/(log

loglog)log(

10

1010

tiegcmsatellite

tiegcmsatelliteD

ρρ

ρρρ

=

−=

7

Multi-Satellite, Multi-Year Comparison with Standard Model

8

Issues with the Annual/Semiannual Seasonal VariationIn 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

Global mean, solar maximum, geomagnetic quiet, constant eddy diffusivity.

ln(p0/p)=2 z=400km

10

Issues with the Annual/Semiannual Seasonal VariationIn 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 densityDecrease 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