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Wave Coupling Between the Lower Atmosphere and Thermosphere: Solar Cycle Influences

Jeffrey M. Forbes1, Sean L. Bruinsma2, Maura E. Hagan3, and Xiaoli Zhang1

1Department of Aerospace Engineering Sciences, University of ColoradoBoulder, Colorado, USA

2Department of Terrestrial and Planetary Geodesy, CNES, Toulouse, France

3National Center for Atmospheric Research, Boulder, Colorado, USA

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Χ = 2πλ z

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μ 0

ρT ~ 1

Longer vertical wavelength waves more easily penetrate into the thermosphere

(2,2)

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λ z ~ 100 km

(2,3)

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λ z ~ 75 km

(2,4)

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λ z ~ 43 km

F10.7 = 110

Normalized to 1

Exponential growth of a vertically-propagating tide ceases (peak occurs) when the time scale for molecular dissipation is of order the wave period:

GSWM Simulations, No Winds

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* is the ratio of the scale height for increase in dissipation (density) to the vertical scale of the wave:

The decrease in amplitude above the peak increases with

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β = 2πHD λ z

F10.7 = 60

F10.7 = 170

at a higher altitude during SSMAX

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Χ ~ 1

(2,2)

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λ z ~ 100 km

Waves often affect the upper thermosphere more during solar minimum

GSWM Simulations, No Winds

Large means large reflection (Yanowitch, 1967; Lindzen, 1968; 1970)

DE3 = Eastward-Propagating Diurnal Tide with Zonal Wavenumber = 3SE2 = Eastward-Propagating Semidiurnal Tide with Zonal Wavenumber = 2

Appears as wave-4 longitude structure in a LST = constant reference frame.

DE2 = Eastward-Propagating Diurnal Tide with Zonal Wavenumber = 2SE1 = Eastward-Propagating Semidiurnal Tide with Zonal Wavenumber = 1

Appears as wave-3 longitude structure in a LST = constant reference frame.

Reminder!

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Eastward-Propagating Diurnal Tide with Zonal Wavenumber = 3 (DE3)

F10.7 = 60

F10.7 = 170€

lz ~ 45 km

Factor of 4-5 difference in relative density amplitude at 400 km

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1.0

1.9

3.2

2.7

DE3Texo

DE3density

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DE3, similar to (2,4)

DE2,similar to (2,3)

DE2 Penetrates Into the Thermosphere Much More Efficiently Than DE3

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Not obvious 180-deg phase differences between day and night, which would occur if the structures were predominantly associated with diurnal tides

1-Week-Mean CHAMP Densities Imply Strong Influence of Semidiurnal Tides

Wave-3 character suggests DE2 and SE1

Wave-4 character suggests DE3 and SE2 ( )

Not obvious symmetry about the equator, which would be consistent with DE3; more antisymmetric behavior (SE2)

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λ z ~ 90 km

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CONCLUSIONS• Long vertical wavelength waves experience exponential growth to a higher altitude in the thermosphere than waves with shorter vertical wavelengths

• Waves of a given wavelength achieve higher peak amplitudes (and at a higher altitude) during SSMAX than SSMIN

• However, the profile shape above the peak varies with solar cycle, such that (for a wave of a given wavelength) higher amplitudes are achieved in the upper thermosphere (ca. > 300 km) during solar minimum

• Solar cycle effects at, e.g., 400 km are much more pronounced for density as opposed to temperature, since the former reflects a height-integrated response to the former.

• The behavior of the DE3 tide derived from CHAMP measurements is consistent with the above

• Density observations during 2008 suggest the strong presence of SE2 and SE1 in addition to DE3 and DE2