Comparisons Between the Global Ionosphere Thermosphere Model and HIWIND Measurements

Neutral winds in the daytime middle thermosphere are difficult to measure. The High altitude Interferometer WIND Observation instrument made measurements of these winds from a balloon in the summers of 2011 and 2018 during both daytime and nighttime at high latitudes. This paper presents a study comparing HIWIND neutral winds to global ionosphere thermosphere model (GITM) results. The meridional winds from GITM matched the trends and magnitude of the measurements of HIWIND, while the zonal winds showed much more disagreement in general. However, there existed a strong shear in the GITM results. When results from ±5° latitude were considered, they mostly bound the zonal HIWIND measurements. This study showed that: (a) HIWIND was on the edge of the high-latitude convection region where strong shears in the zonal flow occurred. Placement of the ion drifts and the aurora are important in determining the neutral winds in this region. GITM simulations using idealized drivers showed very different results. (b) On three of the 4 days, GITM's dawn region winds didn't match the HIWIND or GOCE wind measurements. These indicate that the placement of the aurora and ion drifts may be incorrect, or GITM's balance between acceleration terms could be off. (c) Viscosity and gradient in pressure accelerations were dominant, but mostly opposite. Ion drag and Coriolis were smaller, but are important, especially when the other two terms cancel each other. Specification of high-latitude drivers (i.e., the electric potential and auroral precipitation) are critically important to modeling the thermospheric winds.