CELLULAR STRUCTURES IN THE HIGH-LATITUDE THERMOSPHERE

An organized density (and pressure) structure was recently discovered in the neutral thermosphere at high-latitudes. The structure consists of two to four high- and low-density regions having diameters of 1000 to 2000 km. The density in each region is enhanced or depleted from th e hemispheric average by up to 30%. The structure is thus a significan t feature of the near-Earth space environment at high-latitudes. We re fer to each distinct region of enhanced or depleted density as a densi ty ''cell.'' The cells extend upward from about 120 km into the upper thermosphere, and once formed they remain approximately fixed with res pect to the geomagnetic pole. A parametric study of the density cell m orphology for different magnetic activity levels is described for equi nox solar minimum using the National Center for Atmospheric Research t hermosphere ionosphere general circulation model (NCAR-TIGCM). Density data were sought to verify the existence of the structures first pred icted by the NCAR model. The TIGCM simulations were used to predict th e large density perturbations observed by the S85-1 satellite in a cir cular sun-synchronous orbit near 200 km altitudes. The most obvious ma nifestation of the cells was the presence of density peaks located nea r 70 degrees Lambda on the dayside and nightside, and a density minimu m near the magnetic pole. Since high-latitude densities are generally expected to increase during magnetic activity, the low densities over the pole are perhaps the most interesting feature of the cell structur e discussed here. The satellite data confirm the existence of the cell ular structure over a range of magnetic activity levels. The discovery of the cells is important because the structure provides a unifying f ramework for the analysis and interpretation of high-latitude data fro m both past and future experiments. The cells result from various form s of coupling between the ionosphere and thermosphere. The cell format ion is quantitatively consistent with concepts from dynamic meteorolog y.