Coupling to the Lower Atmosphere, an
Observation-Based Perspective
Jeff Forbes (CU), Xiaoli Zhang (CU), Sean Bruinsma (CNES),
Jens Oberheide (Clemson U), Jason Leonard (CU)
Thermosphere density variability mainly due to:
1.
Solar radiation forcing
2.
Magnetospheric forcing
3.
Meteorological influences from below
The importance of 3. has only been realized very recently, and much of the
variability is in the form of temporal and longitudinal variability imposed
by tides propagating upwards from the lower atmosphere.
The NADIR MURI has been instrumental in achieving the
realization that satellite drag variability is linked to troposphere
variability was well as to solar and magnetospheric variability.
1
Solar and Lunar Tidal Coupling
1.
Atmospheric tides excited in the lower atmosphere grow exponentially with
height until they reach the thermosphere where exponential growth is
terminated due to molecular dissipation.
1.
We have discovered that the waves with the longest vertical wavelengths can
penetrate up to at least 400-500 km.
1.
The TIMED satellite has provided observations that define the tidal spectrum
entering the thermosphere at 110 km, and densities measured by the CHAMP
and GRACE satellites provide information on those waves that penetrate to 300500 km – tidal theory can “fill in the gap” to some degree.
1.
Solar tides generated by latent heating due to deep tropical convection carry the
longitude and temporal variability of this source into the thermosphere.
1.
The CHAMP and GRACE data enable investigation of lunar tide variability in
the thermosphere for the first time, which is shown here to be significant.
1.
The 2007-2010 solar minimum period has enabled separation of lower
atmosphere variability from solar and geomagnetic variability in satellite drag.
Tides Manifested as Longitude Structures
Relative density variations about the zonal mean observed with (top) coplanar CHAMP at 332 km and (bottom) GRACE at 476 km in December
2008, for (left) evening and (right) morning. [Bruinsma and Forbes, 2010].
How does the global wave spectrum evolve
temporally and spatially in the thermosphere?
The longitude variability of
diurnal temperature
amplitude evolves with
altitude due to:
• Filtering of verticallypropagating tides by
molecular dissipation
• In-situ EUV source in the
thermosphere
• In-situ source due to
longitude-dependent ion
drag
Development of an Empirical Specification of
Longitude-Dependent Tidal Structures
Methodology: Hough Mode Extensions as Basis Functions
Methodology: A fitting scheme using “Hough Mode Extensions (HMEs)” is
applied to TIMED/SABER and TIMED/TIDI measurements of temperatures and
winds over 80-110 km and -50o to +50o latitude during 2002-2008.
1.
SABER and TIDI are analyzed for diurnal and semidiurnal tidal components
of various zonal (longitudinal) wavenumbers (i.e., SW2, SW1, SE2; DW1,
DE3, etc.).
1.
Each tidal component is fit with several HMEs; winds and temperatures are
fit simultaneously; or, one parameter is fit and the other is used as
validation.
1.
Internal consistency of the HMEs yields the corresponding tidal density
perturbations.
1.
The HME methodology and the above internal consistency between winds,
temperatures and densities has been tested using GCM output.
5.
Validate using independent data sets, e.g., CHAMP data at 400 km.
5
Sample HME: Eastward-Propagating Diurnal Tide
with Zonal Wavenumber = 3 (DE3)
6
Sample Fit and Density Prediction
SABER temperature at 110 km,
September, UT = 0000, 20022008 average
Density perturbations at 110 km predicted by HMEs,
DW2, DW1, D0, DE1, DE2, DE3,
SW4, SW3, SW2, SW1, S0, SE1, SE2, SE3
Temperature at 110 km, UT = 0000,
2002-2008 average, based on fit to
SABER temperatures and TIDI winds.
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Validation-CHAMP densities at 390 km
vs. solar cycle
CHAMP-DE3
HME-DE3
fitted to 20022008 mean TIMED
data;
solar flux
dependence from
HMEs
CHAMP-SE2
HME-SE2
8
Validation-CHAMP zonal winds at 400 km
vs. solar cycle
CHAMP DE3
zonal wind
Oberheide et al. (2009)
CHAMP
HME
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Sample Density Perturbations
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Scatter in surface impact predictions due to
presence of longitude-dependent tides
Differences in impact latitude
depend on longitude and local
time of reentry. Current
empirical models do not include
these longitude-local time
variations
Illustration of impact trajectories for a single local time
and for multiple longitudes superimposed on sample
density perturbation distribution for a single longitude.
Scatter in satellite predicted orbital position
due to presence of longitude-dependent tides
Lunar Tidal Variations in Satellite Drag
Lunar Tide from GRACE orbit has a period of 13.56 days
2007-2010 Averages
Lunar Tide from CHAMP orbit has a period of 13.28 days
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Conclusions from the MURI Effort
1. Atmospheric solar and lunar tides originating in the lower
atmosphere add significant longitudinal, local time and
temporal variability to the satellite drag and reentry
environments.
1.
These variations are not taken into account in existing
empirical models, and are not adequately taken into account at
the lower boundaries of physics-based models of the
ionosphere-thermosphere system.
2. There exist distinct seasonal-latitudinal patterns and interannual consistency in the tidal amplitudes, and therefore the
mean patterns are predictable, and can be included in existing
empirical models, and/or at the lower thermosphere boundaries
of physics-based models.
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