NEUTRAL DENSITY CELLS IN THE HIGH-LATITUDE THERMOSPHERE .2. MECHANISMS

NCAR-TIGCM simulations predict mesoscale cellular structures in the hi gh Latitude neutral density at altitudes from 120-350 km. During magne tically active conditions, the density structure at 200 km consists of low-density cells near dawn and dusk and high-density cells near noon and midnight. Mechanisms causing the structured density cells are a r esult of thermosphere-ionosphere coupling and can be explained in term s of dynamic meteorology. For example, at high latitudes ion drag caus es the neutral circulation to flow cyclonically in the dawn sector and anticyclonically in the dusk sector. Low densities are contained with in the cyclonic circulation at all altitudes. Below about 170 km, the densities inside the anticyclonic flow are high, while above that alti tude densities within the anticyclonic flow are low. While typical dyn amic meteorology explains low densities in the centre of cyclonic circ ulation and high densities inside anticyclonic circulation, the dusk l ow-density cell in the centre of anticyclonic how is unexpected. The a nticyclonic dusk low-density cell is explained by anomalous antibaric flow due to high-speed winds. 120 km and 200 km altitudes are used to demonstrate the relationship between the high latitude densities and w inds as well as the effect of joule heating and auroral particle preci pitation on the density structures. Copyright (C) 1996 Elsevier Scienc e Ltd