Measurements of the Thermospheric Wind Vector, Temperature and Relative Densities on the ANDE mission

A satellite in low Earth orbit moves at 8 km/s, much faster than the average molecular RMS speed - around 1 km/s, depending on molecular mass. A spectrometer whose aperture points into the ram detects a molecular flux with angular distribution that depends on the air temperature. We describe a new method to obtain the neutral wind vector, temperature and relative densities of major species (O and N2) in the thermosphere; it requires measurement of the angular distribution of the molecular flux and its energy distribution at an angle near the angle of maximum flux. The angle of the flux peak gives the direction of the total velocity vector in the satellite frame of reference. The energy distribution gives the magnitude of the total velocity vector. Then the full wind vector follows from the total velocity vector, the satellite velocity vector, and the spectrometer pointing angle. We have analyzed the fluxes produced by a Maxwellian gas drifting into a spectrometer aperture at a known satellite velocity to determine the required level of precision of the energy and angle measurements. Current technologies enable the energy-angle measurements, and the amount of data required to obtain the wind, temperature and relative densities is small enough that it can be transmitted to the ground for analysis. To achieve temperature/wind errors of 1K/10m/s we need to sample the angular distribution over 3 full-widths (25 deg) with angular resolution of 2 deg, and sample the energy spectrum with a resolution of 1 part in 20. The proposed measurement scheme can detect Maxwellian as well as non-Maxwellian distributions in the gas because it must measure these distributions explicitly - revealing any departure from Maxwellian immediately. We will discuss our implementation of this method in the Atmospheric Neutral Density Experiment (ANDE) mission of the U.S. Naval Research Laboratory.