Yue Deng - Investigation of the thermosphere and ionosphere coupling under different magnetospheric conditions.
Following a space weather disturbance, intense currents flowing through the thermosphere (upper atmosphere) dramatically increase the Joule heating and expansion of atmosphere, which exerts an increased drag on satellites in orbit around the Earth. After that, thousands of objects in space alter their predicted orbits and must be relocated by satellite tracking stations. Furthermore, these currents create GICs (ground induced currents) on the Earth's surface, which could cause serious problems to pipelines, transoceanic cables, and power lines. It is vital to understand the variability within this critical region of our atmosphere so scientists can predict its effects on satellite tracking and power grids.
The thermosphere and ionosphere are two overlapping regions of the atmosphere that are tightly coupled in many ways, including chemistry, momentum transference, and heating due to velocity differences. An understanding of both the thermosphere and ionosphere is important for a number of space research and space weather applications. To examine the coupling between thermosphere and ionosphere, Yue helped to develop the new 3-Dimensional Global Ionosphere-Thermosphere Model (GITM), which couples neutral and ion chemical, dynamical and energetic processes self-consistently. GITM is different than other similar models mainly in that it relaxes the hydrostatic equilibrium condition on the thermosphere and has flexible resolution.
During her Ph.D. period, Yue has used GITM to explore responses to magnetospheric and solar forcing in the upper atmosphere and ionosphere. First, it has been examined the effect of the vertical ion convection on the electron density at high-latitudes, which has been ignored in most previous studies. A vertical ion circulation in the noon-midnight meridional plane has been proposed based on our findings to explain the source of the tongue of ionization. Secondly, the impact of solar activity and geomagnetic activity on the neutral wind has been thoroughly quantified. The neutral wind dependences on solar illumination, cross polar cap potential and aurora precipitation has been separated to extract a better physical understanding of the coupled system. Finally, the thermosphere-ionosphere coupling is studied by quantifying the effects of oscillating electric fields on the Joule heating, the primary driver of global ionospheric storms. From a simple steady-state case to a multi-frequency case, a series of numerical experiments is conducted to quantify the importance of both spatial and temporal electric field variability on the Joule heating.
After joining in NCAR, Yue has investigated the primary characteristics of non-hydrostatic effects on the upper atmosphere through using GITM. The results show that after a sudden intense enhancement of Joule heating, the pressure gradient force can locally be 25% larger than the gravity force, resulting a large disturbance of buoyancy force, the sum of the pressure gradient force and the gravity force. This disturbance can be transported from the source region to the high altitude through an acoustic wave, which has been simulated in a global circulation model for the first time.
Coupling a new empirical Poynting flux model with NCAR-TIEGCM, the influence of the high-latitude energy inputs and energy distributions on the global thermospheric temperature, density, and composition has also been investigated. First, the Joule heating calculated with the average electric field which is called "simple Joule heating" and the Poynting flux from the empirical model are compared to show the contribution of electric field and magnetic field variability to the Joule heating. Secondly, an inter-comparison among three different methods to distribute the Poynting flux in altitude has been conducted. The difference of the thermosphere response suggests that not only the total amount of energy input, but the way to distribute the energy are significant for the impact of magnetosphere on the thermosphere and ionosphere.
Figure 1: Earth thermosphere/ionosphere process. The major sources of thermosphere/ionosphere system include solar irradiance, gravity waves and turbulence from low atmosphere, and particle precipitation and electric field from magnetosphere. Thermosphere and ionosphere are coupled with each other through density, momentum and energy exchange.
Figure 2: The time vs. altitude distribution of buoyancy acceleration (m/s2) ,
, at (22.50, -77.50) 15 minutes after a sudden intense enhancement of the Joule heating in the aurora region.
Figure 3: Altitude profile of the polar average (poleward 400 for the northern hemisphere and -400 for the southern hemisphere) neutral temperature (K) for the case without Joule heating in the polar region (blue lines), the simple Joule heating case (black lines) and Poynting flux case (red lines). The solid lines represent the northern (summer) hemisphere and the dash lines represent the southern (winter) hemisphere.
ASP Spotlight September 2007
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