HL­TWiM Empirical Model of High­Latitude Upper Thermospheric Winds

Abstract

We present an empirical model of thermospheric winds (HL-TWiM - High-latitude Thermospheric Wind Model) which speci es F region high-latitude horizontal neutral winds as a function of dayday-of-year, latitude, longitude, local time, and geomagnetic activity. HL-TWiM represents the large-scale neutral wind circulation, in geomagnetic coordinates, for the given inputconditions. The model synthesizes the most extensive collection to date of historical high-latitude wind measurements; it is based on statistical analyses of several decades of F region thermospheric wind measurements from 21 ground-based stations (Fabry-Perot Interferometers (FPIs) and Scanning Doppler Imaging FPIs (SDIs)) located at various northern and southern high latitudes and 2 space-based instruments (UARS WINDII and GOCE). The geomagnetic latitude and local time dependences in HL-TWiM are represented using Vector Spherical Harmonics (VSH), day-of-year and longitude variations are represented using simple harmonic functions, and the geomagnetic activity dependence is represented using quadratic B-splines. In this paper, we describe the HLTWiM formulation and tting procedures, and we verify the model against the neutral wind databases used in its formulation. HL-TWiM provides a necessary benchmark for validating new wind observations and tuning our physical understanding of complex wind behaviors. Results show stronger Universal Time (UT) variation in winds at southern than northern-high latitudes. Model-data intra-annual comparisons in this study show semi-annual oscillation (SAO) like behavior of GOCE winds, rarely observed before in wind data.