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The GCMs represented a significant advance in physics‐based MIT modeling, capturing the essential physics in the IT system. They include the chemical reactions dominant at low altitudes, and the coupled equations of momentum, energy, and continuity for ion, electrons, and neutrals (Rees et al., 1980; Fuller‐Rowell et al., 1988, 2000; Dickinson et al., 1981, 1984; Roble et al., 1987; Ridley et al., 2006). They have been used to simulate the ionospheric response to energy deposition and dissipation at high latitudes in many publications. We focus on three of the most widely used models: the Thermosphere Ionosphere Electrodynamics Global Circulation Model (TIEGCM), the Coupled Thermosphere Ionosphere Plasmasphere Electrodynamics (CTIPe) model, and the Global Ionosphere‐Thermosphere Model (GITM), all of which have been run for many years. Model runs on demand of all three are available at the Community Coordinated Modeling Center (CCMC) (http://ccmc.gsfc.nasa.gov/).

All three require specification of the high‐latitude electric field or high‐latitude Poynting flux and conductivity in order to simulate the effect of energy deposition and dissipation. The electric field or Poynting flux can be specified by W05, Cosgrove14, or AMIE as described in section 1.2. The conductivity can be obtained from empirical models by Roble and Ridley (1987) or Fuller‐Rowell and Evans (1987).

The CTIPe model is a nonlinear, coupled thermosphere‐ionosphere‐plasmasphere physics‐based code that includes a self‐consistent electrodynamics scheme for the computation of dynamo electric fields. There are four distinct components that run simultaneously and are coupled: a global thermosphere, a high‐latitude ionosphere, a mid‐and low‐latitude ionosphere/plasmasphere, and an electrodynamical calculation of the global dynamo electric field (Fuller‐Rowell & Rees, 1980; Codrescu et al., 2012). The thermospheric component is divided into a geographic latitude x longitude grid with resolution 2° x 18°. The vertical resolution is defined in terms of the logarithm of pressure from a lower boundary at 80 km altitude to altitudes above 500 km. The primary inputs to the model are the high‐latitude electric field, usually provided by the Weimer (2005) empirical model, auroral electron precipitation from the empirical model by Fuller‐Rowell and Evans (1987), and EUV radiation (Solomon & Qian, 2005). Full descriptions of the model with examples of output can be found in Codrescu et al. (2012). CTIPe is run in near‐real time at the Space Weather Prediction Center (SWPC), and model output is available for comparison with observations at http://helios.swpc.noaa.gov/ctipe/index.html.

TIEGCM provides a three‐dimensional, nonlinear representation of the coupled IT system. A self‐consistent calculation of ionospheric wind dynamic effects (Richmond et al., 1992) is included. The primary external drivers of the model are solar irradiance, magnetospheric energy, and tidal perturbations at the lower boundary of the model. Magnetospheric energy inputs include particle precipitation and high‐latitude electric fields, which drive convection. The current version of TIEGCM (v2) uses a fixed grid with spatial resolution of 2.5° x 2.5° x half scale height spatial resolution (longitude x latitude x altitude) with 30 second temporal resolution. As with CTIPe, the vertical extent of the model is defined by pressure with typical range of 97 km to 500 km. The model can be downloaded directly from the NCAR website at http://www.hao.ucar.edu/modeling/tgcm/.

GITM is the most recent of the widely used GCMs. It differs from CTIPe and TIEGCM as follows: (1) spatial resolution is flexible; (2) the assumption of hydrostatic equilibrium is dropped allowing for vertical acceleration; (3) the adoption of an altitude grid instead of scaling by pressure; (4) advection is solved explicitly; (5) chemistry is solved explicitly without the assumption of chemical equilibrium. As a result, the time step in GITM is 2–4 s. The model uses the same magnetospheric drivers as CTIPe and TIEGCM, viz. high‐latitude electric fields and energetic particle precipitation.

As described in section 1.2 (energy entering the IT system), the primary coupling to magnetospheric energy input is via the empirical models of the high‐latitude electric field provided by the W05 or Cosgrove14 models, or an inversion of assimilated high‐latitude data from the AMIE model. More rarely, MHD models such as the Block‐Adaptive‐Tree‐Solarwind‐Roe‐Upwind‐Scheme (BATS‐R‐US) (Powell et. al., 1999), Open Geospace General Circulation Model (OpenGGCM) (Raeder et al., 1998), or Lyon‐Fedder‐Mobarry (LFM) model (Lyon et al., 2004) can be coupled directly to the IT models (CTIPe, TIEGCM, GITM, or others), which removes the need for separate high‐latitude electric field specification.