Comparing High-Latitude Thermospheric Winds From FPI and CHAMP Accelerometer Measurements

It is generally assumed that horizontal wind velocities are independent of height above the F1 -region (> 300 km) due to the large viscosity of the upper thermosphere. This assumption is used to compare two completely different methods of thermospheric neutral wind observation, using two distinct locations in the highlatitude Northern Hemisphere. The measurements are from ground-based FabryPerot Interferometers (FPI), and from in-situ accelerometer measurements onboard the CHAMP satellite, which was in a near polar orbit. The UCL KEOPS FPI is located in the vicinity of the auroral oval at the ESRANGE site near Kiruna, Sweden (67.8°N, 20.4°E). The UCL Longyearbyen FPI is a polar cap site. It is located at the Kjell Henriksen Observatory on Svalbard (78.1°N, 16.0°E). The comparison is done in a statistical sense, comparing a longer time series obtained during nighttime hours in the winter months (November to January); with overflights of the CHAMP satellite between 2001 and 2008 over the observational sites, within ±2° (±220 km horizontal range). The FPI is assumed to measure the line-of-sight winds at ~240 km height. This is the peak emission height of the atomic oxygen 630.0 nm (red line) emission. The cross-track winds are derived from state-of-the-art precision accelerometer measurements at altitudes between 450 km (in 2001) to 330 km (in 2008); i.e. 100- 200 km above the FPI wind observations. In addition to testing the consistency of the different measurement approaches, the study aims to clarify the effects of viscosity on the height dependence of thermospheric winds.