The Global Ionosphere-Thermosphere Model and the Nonhydrostatic Processes

The recently developed global ionosphere-thermosphere model (GITM) uses a 3-D spherical grid that can be stretched in both latitude and altitude. GITM is nontraditional in that it uses an altitude-based grid and does not assume a hydrostatic solution. Using GITM, the primary characteristics of nonhydrostatic effects on the upper atmosphere are investigated. Our results show that after a sudden intense enhancement of high-latitude Joule heating, the vertical pressure gradient force can locally be 25% larger than the gravity force, resulting in a significant disturbance away from hydrostatic equilibrium. This disturbance is transported from the lower-altitude source region to high altitudes through an acoustic wave. The magnitude of the vertical wind perturbation increases with altitude and reaches 150 (250) m s(-1) at 300 (430) km, which is not typically reproduced by hydrostatic models. The source of nonhydrostatic effects and the sensitivity of the vertical wind and neutral density at low satellite orbits to the energy deposited at low and high thermosphere have been investigated as well. The simulations show that the atmosphere at all altitudes is out of hydrostatic equilibrium after the sudden energy enhancement. But the maximum of the nonhydrostatic effects at high altitudes (300 km) arises from sources below 150 km and propagates vertically through the acoustic wave. The heating above 150 km is responsible for a large increase of the average vertical velocity (40 m s(-1)) and neutral density (50%) at 300 km and higher altitudes.