[1] A unique conjunction of the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) and the Challenging Minisatellite Payload (CHAMP) satellites provided simultaneous columnar neutral composition, SO/N 2 , and thermosphere density observations, enabling a novel study of thermospheric response to the 7-9 November 2004 geomagnetic storm. Both SO/N 2 and mass density showed profound response to this severe geomagnetic storm, but their latitudinal and temporal structures differed markedly. In particular, high-latitude depletion and low-latitude enhancement in SO/N 2 were observed throughout the storm period, especially during the main phase. In contrast, neutral density at 400 km altitude increased from pole to pole shortly after the storm, with strongest enhancement of order 200%-400% during the main phase. Comparisons of observed thermosphere response with simulations from the National Center for Atmospheric Research Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIEGCM) were carried out to interpret the observed contrasting characteristics of thermosphere composition and mass density in response to this geomagnetic storm. The TIEGCM simulations show that the contrasting characteristics occur not only in SO/N 2 and mass density at a constant altitude at 400 km, but also in O/N 2 and mass density on a constant-pressure surface. At an altitude of 400 km (CHAMP altitude), storm-time mass densities significantly increase due to an increase in scale height throughout the vertical column between the heat source and satellite altitude. For a given increase in scale height, the more scale height increments separating the heat source from the satellite altitude, the greater is the mass density response. It is shown that scale height change is caused partly by storm-time neutral temperature enhancements due to heating and partly by changes in mean molecular weight due to winds. These findings indicate that wind effects can cause significant deviations from a mass density pattern resulting solely from neutral temperature changes by altering the mean molecular weight, particularly at high latitudes.
Under hydrostatic equilibrium, a typical assumption used in global thermosphere ionosphere models, the pressure gradient in the vertical direction is exactly balanced by the gravity force. Using the non‐hydrostatic Global Ionosphere Thermosphere Model (GITM), which solves the complete vertical momentum equation, the primary characteristics of non‐hydrostatic 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, which has been simulated in a global circulation model for the first time. Due to the conservation of perturbation energy, the magnitude of the vertical wind perturbation increases with altitude and reaches 150 (250) m/s at 300 (430) km during the disturbance. The upward neutral wind lifts the atmosphere and raises the neutral density at high altitudes by more than 100%. These large vertical winds are not typically reproduced by hydrostatic models of the thermosphere and ionosphere. Our results give an explanation of the cause of such strong vertical winds reported in many observations.
[1] It is important to understand Joule heating because it can significantly change the temperature structure, atmosphere composition, and electron density and hence influences satellite drag. It is thought that many coupled ionosphere-thermosphere models underestimate Joule heating because the spatial and temporal variability of the ionospheric electric field is not totally captured within global models. Using the Global Ionosphere Thermosphere Model (GITM), we explore the effect of the electric field temporal variability, model resolution, and vertical velocity differences between ion and neutral flows on Joule heating in a self-consistent thermosphere/ionosphere system. First, the response of Joule heating to a step change in the externally driven electric field has been studied. While Joule heating is strongly affected by the convection electric field, both neutral winds and electron densities can significantly alter the spatial distribution of the Joule heating. Owing to the ramping up of neutral winds, there is a temporal variation of the Joule heating energy deposition rate when the electric field is constant. Second, we compare the calculated neutral gas heating rates when GITM is run with three different temporal variations of the electric fields, having the same temporally averaged electric field ( E) but different standard deviations (s E ). The neutral gas heating rate increases with the electric field temporal variability, and due to the feedback of the neutral winds and electron densities, the percentage increase is different from s E 2 / E 2 , which is normally used to describe the effect of electric field temporal variability on the Joule heating. Third, comparison of the neutral gas heating rate with different model resolutions shows that at 200 km altitude, the polar average neutral gas heating rate increases by 20% when the latitudinal resolution increases from 5°to 1.25°. This is due to the model's ability to better capture small-scale features in the electric field and particle precipitation. Last, inclusion of the vertical velocity difference (which is neglected in many models) is less significant than the other two factors and appears to be negligible at high latitudes. While the magnitude of the neutral gas heating rate at middle and low latitudes is smaller than that at high latitudes, the relative importance of the vertical velocity difference is larger, and the contribution can reach 15% of the averaged Joule heating at middle and low latitudes.Citation: Deng, Y., and A. J. Ridley (2007), Possible reasons for underestimating Joule heating in global models: E field variability, spatial resolution, and vertical velocity,
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