| Summary: | Daytime thermospheric winds observed by the balloon-borne instrument HIWIND (High-altitude Interferometer WIND experiment) during two flights in June 2011 and 2018 from Kiruna (68 degrees N, 20 degrees E), along with simultaneous European Incoherent SCATter radar ion drift data, are analyzed. National Center for Atmospheric Research TIEGCM (Thermosphere Ionosphere Electrodynamics General Circulation Model) simulations for both flights are compared with observations. The observed thermospheric winds from the two flights have many similarities. HIWIND-observed thermospheric winds tend to be equatorward during the morning hours before noon and close to zero in the afternoon. In contrast, TIEGCM predicts poleward winds before noon and near zero in the afternoon. Southward interplanetary magnetic field B-z, occurring as the balloon passed through morning, was associated with greater equatorward meridional winds. The TIEGCM-simulated zonal winds have large differences with observations under more active conditions. The second HIWIND flight confirms some important results from the first flight and further shows thermospheric wind variations under different interplanetary magnetic field conditions. HIWIND observations in general provide invaluable data for model validation and highlight deficiencies in current high-latitude simulations. 80NSSC17K0013 NNX14AE06G NNX15AK75G NNX17AG69G NNX17AI42G
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