Solar Cycle Variability of the Global Diurnal and Semidiurnal

Report
Wave Coupling Between the Lower Atmosphere
and Thermosphere: Solar Cycle Influences
Jeffrey M. Forbes1, Sean L. Bruinsma2, Maura E. Hagan3, and Xiaoli Zhang1
1Department
2Department
3National
of Aerospace Engineering Sciences, University of Colorado
Boulder, Colorado, USA
of Terrestrial and Planetary Geodesy, CNES, Toulouse, France
Center for Atmospheric Research, Boulder, Colorado, USA
1
Longer vertical wavelength waves more easily penetrate into the thermosphere
(2,2)
GSWM Simulations, No Winds
 z ~ 100 km
(2,3)
 z ~ 75 km

F10.7 = 110
(2,4)
 z ~ 43 km

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:
 
2  0
 
2
z
T ~1
2
The decrease in amplitude above the peak increases with
Waves often affect the upper thermosphere more during solar minimum
GSWM Simulations, No Winds
F10.7 = 170
F10.7 = 60
(2,2)
 z ~ 100 km
 ~ 1


ata
highe
atitude
duing
SSMAX
* is the ratio of the scale height for increase in
dissipation (density) to the vertical scale of the wave:   2HD  z
Large
means large reflection (Yanowitch, 1967; Lindzen, 1968; 1970)
3
Reminder!
DE3 = Eastward-Propagating Diurnal Tide with Zonal Wavenumber = 3
SE2 = 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 = 2
SE1 = Eastward-Propagating Semidiurnal Tide with Zonal Wavenumber = 1
Appears as wave-3 longitude structure in a
LST = constant reference frame.
4
Eastward-Propagating Diurnal Tide with Zonal Wavenumber = 3 (DE3) z ~ 45 km
F10.7 = 170

F10.7 = 60
Factor of 4-5
difference in
relative density
amplitude at
400 km
5
DE3
Texo
2.7
1.9
DE3
density
3.2
1.0
6
DE2 Penetrates Into the Thermosphere Much More Efficiently Than DE3
DE3,
similar
to
(2,4)
DE2,
similar
to
(2,3)
7
1-Week-Mean CHAMP Densities Imply Strong Influence of Semidiurnal Tides
Not obvious
symmetry
about the
equator, which
would be
consistent with
DE3; more
antisymmetric
behavior (SE2)
Not obvious
180-deg phase
differences
between day
and night,
which would
occur if the
structures
were
predominantly
associated
with diurnal
tides
Wave-3
character
suggests DE2
and SE1
Wave-4
character
suggests DE3
and SE2
( z ~ 90 km )

8
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
9

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