Wind and Temperature Spectrometry of the Upper Atmosphere in Low-Earth Orbit
- Wednesday, 01 June 2011
Multi-point measurements can enhance the capabilities of the GPS network, as well as other communication applications.
Wind and Temperature Spectrometry (WATS) is a new approach to measure the full wind vector, temperature, and relative densities of major neutral species in the Earth’s thermosphere. The method uses an energy-angle spectrometer moving through the tenuous upper atmosphere to measure directly the angular and energy distributions of the air stream that enters the spectrometer. The angular distribution gives the direction of the total velocity of the air entering the spectrometer, and the energy distribution gives the magnitude of the total velocity. The wind velocity vector is uniquely determined since the measured total velocity depends on the wind vector and the orbiting velocity vector.
The orbiting spectrometer moves supersonically, Mach 8 or greater, through the air and must point within a few degrees of its orbital velocity vector (the ram direction). Pointing knowledge is critical; for example, pointing errors 0.1° lead to errors of about 10 m/s in the wind. The WATS method may also be applied without modification to measure the ion-drift vector, ion temperature, and relative ion densities of major ionic species in the ionosphere. In such an application it may be called IDTS: Ion-Drift Temperature Spectrometry.
A spectrometer-based coordinate system with one axis instantaneously pointing along the ram direction makes it possible to transform the Maxwellian velocity distribution of the air molecules to a Maxwellian energy-angle distribution for the molecular flux entering the spectrometer. This implementation of WATS is called the gas kinetic method (GKM) because it is applied to the case of the Maxwellian distribution.
The WATS method can be easily applied to measure the ion-drift, ion temperature, and ion densities simultaneously in the same satellite, thus providing an effective tool to study ion-neutral coupling in the upper atmosphere. The WATS method lends itself to miniaturization, and it is possible to design WATS instruments that consume very little power in a small volume, compatible with the new CubeSats. Therefore, it may soon prove realistic to operate many WATS instruments to carry out simultaneous multi-point measurements over a broad range of altitudes (120 to 600 km) and all altitudes and longitudes.
The WATS method follows from the recognition that in a supersonic platform moving at 8,000 m/s, the measurement of small wind velocities in the air on the order of a few 100 m/s and less requires precise knowledge of the angle of incidence of the neutral atoms and molecules. The same is true for the case of ion-drift measurements. WATS also provides a general approach that can obtain non-equilibrium distributions as may exist in the upper regions of the thermosphere, above 500 km and into the exosphere. Finally, WATS serves as a mass spectrometer, with very low mass resolution of roughly 1 part in 3, but easily separating atomic oxygen from molecular nitrogen.
This work was done by Federico Herrero of Goddard Space Flight Center. GSC-15753-1
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