G.6 HWM93 additional information

The HWM is an empirical model of the horizontal neutral wind in the upper thermosphere. It is based on wind data obtained from the AE-E and DE 2 satellites. A limited set of vector spherical harmonics is used to describe the zonal and meridional wind components. The first edition of the model released in 1987 (HWM87) [RD.42] was intended for winds above 220 km. With the inclusion of wind data from ground-based incoherent scatter radar and Fabry-Perot optical interferometers, HWM90 was extended down to 100 km and using MF/Meteor data HWM93 was extended down to the ground [RN.6]. Solar cycle variations are included (since HWM90), but they are found to be small and not always very clearly delineated be the current data. Variations with magnetic activity (A_p) are included. Mid- and low-latitude data are reproduced quite well by the model. The polar vortices are present, but not in full detail. The model describes the transition from predominately diurnal variations in the upper thermosphere to semidiurnal variations in the lower thermosphere and a transition from summer to winter flow above 140 km to winter to summer flow below. Significant altitude gradients in the wind extend up to 300 km at some local times. The model software provides zonal and meridional winds for specified latitude, longitude, time, and A_p index. A comparison of the HWM values with winds derived from IRI parameters and from ionosonde measurements have shown in general good agreement.