Comparing high-latitude thermospheric winds from Fabry-Perot interferometer (FPI) and challenging mini-satellite payload (CHAMP) accelerometer measurements

It is generally assumed that horizontal wind velocities are independent of height above the F<span classCombining double low line"inline-formula">1</span> region (&gt;300km) due to the large molecular 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 high-latitude Northern Hemisphere. The measurements are from ground-based Fabry-Perot interferometers (FPI) and from in situ accelerometer measurements onboard the challenging mini-satellite payload (CHAMP) satellite, which was in a near-polar orbit. The University College London (UCL) Kiruna Esrange Optical Platform Site (KEOPS) FPI is located in the vicinity of the auroral oval at the ESRANGE site near Kiruna, Sweden (67.8<span classCombining double low line"inline-formula">ĝ</span>N, 20.4<span classCombining double low line"inline-formula">ĝ</span>E). The UCL Longyearbyen FPI is a polar cap site, located at the Kjell Henriksen Observatory on Svalbard (78.1<span classCombining double low line"inline-formula">ĝ</span>N, 16.0<span classCombining double low line"inline-formula">ĝ</span>E). The comparison is carried out in a statistical sense, comparing a longer time series obtained during night-time hours in the winter months (DOY 300-65) with overflights of the CHAMP satellite between 2001 and 2007 over the observational sites, within <span classCombining double low line"inline-formula">±2</span><span classCombining double low line"inline-formula">ĝ</span> latitude (<span classCombining double low line"inline-formula">±230</span>km horizontal range). The FPI is assumed to measure the line-of-sight winds at a height of <span classCombining double low line"inline-formula">ĝ1/4240</span>km, i.e. the peak emission height of the atomic oxygen 630.0nm emission. The cross-track winds are derived from state-of-the-art precision accelerometer measurements at altitudes between <span classCombining double low line"inline-formula">ĝ1/4450</span>km (in 2001) and <span classCombining double low line"inline-formula">ĝ1/4350</span>km (in 2007), i.e. 100-200km above the FPI wind observations. We show that CHAMP wind values at high latitudes are typically 1.5 to 2 times larger than FPI winds. 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.