5 research outputs found
HLâTWiM Empirical Model of HighâLatitude Upper Thermospheric Winds
We present an empirical model of thermospheric winds (Highâlatitude Thermospheric Wind Model [HLâTWiM]) that specifies F region highâlatitude horizontal neutral winds as a function of day of year, latitude, longitude, local time, and geomagnetic activity. HLâTWiM represents the largeâscale neutral wind circulation, in geomagnetic coordinates, for the given input conditions. The model synthesizes the most extensive collection to date of historical highâlatitude wind measurements; it is based on statistical analyses of several decades of F region thermospheric wind measurements from 21 groundâbased stations (FabryâPerot Interferometers and Scanning Doppler Imaging FabryâPerot Interferometers) located at various northern and southern high latitudes and two spaceâbased instruments (UARS WINDII and GOCE). The geomagnetic latitude and local time dependences in HLâTWiM are represented using vector spherical harmonics, 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 HLâTWiM formulation and fitting procedures, and we verify the model against the neutral wind databases used in its formulation. HLâTWiM provides a necessary benchmark for validating new wind observations and tuning our physical understanding of complex wind behaviors. Results show stronger Universal Time variation in winds at southern than northern high latitudes. Modelâdata intraâannual comparisons in this study show semiannual oscillationâlike behavior of GOCE winds, rarely observed before in wind data