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**Chapter 2** **Description of the TIEGCM**

This chapter describes the NCAR TIEGCM in further detail – Inputs, boundary conditions,and equations solved by the model are presented.The TIEGCM is a three dimensional numerical model of the earth’s upper atmosphere, with a lower boundary of 97 km and an upper boundary that varies between 500 and 700 km. It solves a self consistent aeronomic scheme of the coupled thermosphere and the ionosphere at every time step, producing a representation of the upper atmosphere’s structure in terms of temperatures, winds and densities. Solar EUV flux forms the primary input for the model, while indices such as Kp and F10.7 characterize the auroral energies and EUV variability at different wavelengths. The Thermosphere General Circulation Model (TGCM) and Thermosphere/Ionosphere General Circulation Model (TIGCM) [6], [7] are two general circulation models that were also developed at NCAR, that form the origins of the present day TIEGCM. The TGCM modeled the thermosphere in terms of temperature perturbations and wind structure, primarily by solving the thermodynamic equation and the continuity equation using the assumption of hydrostatic equilibrium. The TIGCM expanded upon this by including a self-consistent aeronomic scheme to compute total temperatures, densities of neutral species such as N(4S),N(2D) and NO, and calculates a global ionosphere in terms of various ion densities. Building upon this, the TIEGCM included electrodynamic interactions between the thermosphere and the ionsphere and a more realistic non-dipolar geomagnetic field model, allowing calculations of the dynamo effects of thermospheric winds and neutral/plasma dynamics. At present, it is widely used in the atmospheric science community, and represents our best understanding of many aspects of the atmosphere, and specifically, the thermosphere-ionosphere interactions.

**Equations**

The equations driving the physics of the aforementioned models are presented below. The primary independent variables in these equations are time (t), latitude (φ), longitude (λ) and the model vertical coordinate z, calculated as loge(p0/p), where p0 is the reference pressure and p is pressure. These equations are solved using a finite differencing technique at every timestep.is the ion drag tensor, r is the distance from the center of the earth and Fλ and Fφ are the zonal and meridional momentum sources. The two equations relate the change in velocities with time (∂u/∂t, ∂v/∂t) with the vertical viscosity term, the coriolis, momentum and ion drag force, horizontal and vertical advection, and the pressure gradient force.

**Inputs**

A more comprehensive document covering many aspects of the model not presented here is the TIEGCM model description provided by the HAO. However, presented in this section are a few of the inputs, boundary conditions etc. relevant to the work presented here.

**Solar Input**

The TIEGCM uses the EUVAC (EUV flux model for Aeronomical Calculations) as the default solar input for the spectral range of 5-105 nm. EUVAC is an empirical representation of the solar irradiance, generated by using a reference spectrum at solar minimum and a wavelength dependent variability depending on solar activity. The variability is characterized by solar indices, most frequently the F10.7 index.where fref is the spectrum at solar minimum, A is the wavelength dependent variability factor, and P = (F10.7 + F10.7A)/2. F10.7A is the 81 day average of F10.7. Instead of using a solar proxy model, one can also use measured solar irradiance spectra as an input. An example of this is the TIMED/SEE instrument that captures the solar spectra between 0.1 and 195nm. The data can be processed into a binning scheme compatible with the model and specified in a netCDF file format. If such an input file is provided, the EUVAC model values are overridden.Example of inputs that are not controlled by the user include the solar flux, ionization branching ratios and cross sections and absorption coefficients. Though these cannot be changed from the input parameter list that the user specifies for a model run, they can be changed from the relevant sub-routines.

**1 Introduction **

**1.1 Nitric Oxide in the lower thermosphere **

**1.2 Thermospheric Modelling of NO **

**1.3 Overview of Thesis**

**2 Description of the TIEGCM **

**2.1 Equations**

**2.2 Inputs**

2.2.1 Solar Input

2.2.2 Magnetospheric Inputs

2.2.3 Boundary Conditions

**2.3 Outputs**

**3 Modifying the TIEGCM **

**3.1 Modified branching ratios **

3.1.1 NO+ + e → 0.95N(2D) + 0.05N(4S) + O

3.1.2 N+2 + e → 1.52N(2D) + 0.48N(4S)

3.1.3 N2 + hv/e∗ → 0.5N(2D) + 0.5N(4S)

**3.2 Modified rate coefficients**

3.2.1 N(4S) + NO → N2 + O

3.2.2 N(2D) + O2 → NO + O

3.2.3 N(2D) + O → N(4S) + O

**3.3 Other changes**

3.3.1 N+ + O2 → N(2D) + O+2

3.3.2 N(2D) + N2 → N(4S) + N2

**3.4 N2(A) Chemistry **

**3.5 Summary **

**4 Results **

**4.1 SNOE **

**4.2 Overview of the chemistry **

**4.3 Model-Data Comparisons **

4.3.1 NO+ + e → 0.95N(2D) + 0.05N(4S) + O

4.3.2 N+2 + e → 1.52N(2D) + 0.48N(4S)

4.3.3 N2 + hv/e∗ → 0.5N(2D) + 0.5N(4S)

4.3.4 N(4S) + NO → N2 + O

4.3.5 N(2D) + O2 → NO + O

4.3.6 N(2D) + O → N(4S) + O

4.3.7 N+ + O2 → N(2D) + O+2

4.3.8 N(2D) + N2 → N(4S) + N2

4.3.9 N2(A) Chemistry, N2(A) + O → NO + N(2D)

4.3.10 Overall effect of updated chemistry

**5 Conclusions **