The concept of ionospheric disturbance dynamo, as introduced by Blanc and Richmond [1980],, means that the main criterion set forth by Mayaud [1965, 1967] to discriminate the irregular variations from the regular ones would be no longer valid. Indeed it asserted that the basic difference between these two species of the transient geomagnetic variations is as follows: the latter are synchronous in local time from one longitude to another, while the former are such in universal time. Now the flow of significant ionospheric current vortices driven by auroral heating during daytime after the occurrence of auroral disturbances for a few hours (see Figure # of their paper), or a few days, would induce magnetic perturbations that are synchronous in local time, and furthermore rather smooth (see the lower diagram of their Figure 9). Consequently, the discrimination between the regular and irregular variations would become more difficult. We wish to comment on this new concept, and on the consequences in the K indices scalings or in the derivation of the Dst index, all the more that we believe we can afford an experimental fact in fayour of the theoretical finding of these authors. Figure I is an example of the eastward electrojet effects discussed in Mayaud [1965]. It illustrates a case when this phenomenon is rather small in amplitude and induces an almost smooth variation at auroral latitudes.

The equations of ionospheric electrodynamics are developed for a geomagnetic field of general configuration, with specific application to coordinate systems based on Magnetic Apex Coordinates. Two related coordinate systems are proposed: Modified Apex Coordinates, appropriate for calculations involving electric fields and magnetic-field-aligned currents; and Quasi-Dipole Coordinates, appropriate for calculations involving height-integrated ionospheric currents. Distortions of the geomagnetic field from a dipole cause modifications to the equations of electrodynamics, with distortion factors exceeding 50% at some geographical locations. Under the assumption of equipotential geomagnetic-field lines, it is shown how the field-line-integrated electrodynamic equations can be expressed in two dimensions in magnetic latitude and longitude, and how the height-integrated and field-aligned current densities can be calculated. Expressions are derived for the simplified calculation of magnetic perturbations above and below the ionosphere associated with the three-dimensional current system. It is shown how the base vectors for the Modified Apex coordinate system can be applied to map electric fields, plasma-drift velocities, magnetic perturbations, and Poynting fluxes along the geomagnetic field to other altitudes, automatically taking into account changes in magnitude and direction of these vector quantities along the field line. Similarly, it is shown how Quasi-Dipole coordinates are useful for expressing horizontal ionospheric currents, equivalent currents, and ground-level magnetic perturbations. A computer code is made available for efficient calculation of the various coordinates, base vectors, and related quantities described in this article.

The NCAR thermospheric general circulation model (TGCM) is extended to include a self‐consistent aeronomic scheme of the thermosphere and ionosphere. The model now calculates total temperature, instead of perturbation temperature about some specified global mean, global distributions of N(²D), N(4S) and NO, and a global ionosphere with distributions of O+,NO+, O2+, N2+, N+, electron density, and ion temperature as well as the usual fields of winds, temperature and major composition. Mutual couplings between the thermospheric neutral gas and ionospheric plasma occur at each model time step and at each point of the geographic grid. Steady state results for this first Eulerian model of the ionosphere, are presented for solar minimum equinox conditions. The calculated thermosphere and ionosphere global structure agrees reasonably well with the structure of these regions obtained from empirical models. This suggests that the major physical and chemical processes that describe the large‐scale structure of the thermosphere and ionosphere have been identified and a self‐consistent aeronomic scheme, based on first principles, can be used to calculate thermospheric and ionospheric structure considering only external sources.

[1] Large ionospheric variability is found at low to middle latitudes when a quasi-stationary planetary wave is specified in the winter stratosphere in the National Center for Atmospheric Research thermosphere-ionosphere-mesosphere electrodynamics general circulation model for solar minimum conditions. The variability includes change of electric field/ion drift, F2 peak density and height, and the total electron content. The electric field/ion drift change is the largest near dawn in the numerical experiments. Analysis of model results suggests that, although the quasi-stationary planetary wave does not propagate deep into the ionosphere or to low latitudes due to the presence of critical layers and strong molecular dissipation, the planetary wave and tidal interaction leads to large changes in tides, which can strongly impact the ionosphere at low and middle latitudes through the E region wind dynamo. Large zonal gradients of zonal and meridional winds from the tidal components and the zonal gradient of electric conductivities at dawn can produce large convergence/ divergence of Hall and Pedersen currents, which in turn produces a polarization electric field. The ionospheric changes are dependent on both the longitude and local time, and are determined by the amplitudes and phases of the superposing wave components. The model results are consistent with observed ionospheric changes at low and middle latitudes during stratospheric sudden warming events, when quasi-stationary planetary waves become large.

Key developments have been made to the NCAR Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM‐X). Among them, the most important are the self‐consistent solution of global electrodynamics, and transport of O+ in the F‐region. Other ionosphere developments include time‐dependent solution of electron/ion temperatures, metastable O+ chemistry, and high‐cadence solar EUV capability. Additional developments of the thermospheric components are improvements to the momentum and energy equation solvers to account for variable mean molecular mass and specific heat, a new divergence damping scheme, and cooling by O(3P) fine structure. Simulations using this new version of WACCM‐X (2.0) have been carried out for solar maximum and minimum conditions. Thermospheric composition, density, and temperatures are in general agreement with measurements and empirical models, including the equatorial mass density anomaly and the midnight density maximum. The amplitudes and seasonal variations of atmospheric tides in the mesosphere and lower thermosphere are in good agreement with observations. Although global mean thermospheric densities are comparable with observations of the annual variation, they lack a clear semiannual variation. In the ionosphere, the low‐latitude E × B drifts agree well with observations in their magnitudes, local time dependence, seasonal, and solar activity variations. The prereversal enhancement in the equatorial region, which is associated with ionospheric irregularities, displays patterns of longitudinal and seasonal variation that are similar to observations. Ionospheric density from the model simulations reproduces the equatorial ionosphere anomaly structures and is in general agreement with observations. The model simulations also capture important ionospheric features during storms.

The Magnetosphere‐Thermosphere‐Ionosphere‐Electrodynamics General Circulation model of Peymirat et al. [1998] is used to investigate ionospheric‐wind‐dynamo influences on low‐latitude ionospheric electric fields during and after a magnetic storm. Simulations are performed with time‐varying polar cap electric potentials and an expanding and contracting polar cap boundary. Three influences on equatorial electric fields can be of comparable importance: (1) global winds driven by solar heating; (2) direct penetration of polar cap electric fields to the equator that are partially shielded by the effects of Region‐2 field‐aligned currents; and (3) disturbance winds driven by high‐latitude heating and ion‐drag acceleration. The first two influences tend to have similar magnetic local time (MLT) variations in a steady state, while the disturbance‐wind influence tends to have the opposite MLT variations. The nighttime disturbance winds at upper midlatitudes that affect the global ionospheric wind dynamo are predominantly westward after the simulated magnetic storm. The nighttime winds drive an equatorward dynamo current that tends to charge the low‐latitude ionosphere positively around midnight, which can lead to reductions or reversals of the normal equatorial night‐side east‐west electric fields. The simulations partly support the theories of the so‐called “disturbance dynamo” [Blanc and Richmond, 1980] and “fossil wind” [Spiro et al., 1988], both of which predict long‐lasting disturbances in the equatorial eastward electric field associated with magnetic storms. However, the simulations do not support the element of fossil wind theory that links the disturbance‐wind influence on equatorial electric fields to polar cap contraction following the storm. The simulations show a stronger wind‐produced enhancement of steady state shielding than predicted by the model of Forbes and Harel [1989], due to the fact that the disturbance winds extend well equatorward of the Region‐2 currents.

[1] During magnetic storms the ionospheric total electron content (TEC) at low-and midlatitudes often shows great enhancements, which may be associated with mechanisms producing midlatitude storm-enhanced density (SED). The TEC enhancements may result from different ionospheric drivers such as electric fields, neutral winds, and neutral composition effects. To study the importance of the ionospheric drivers in producing the TEC enhancement, we perform numerical simulations for the 29-30 October 2003 superstorm period in the American longitude sector ($ À70°W) using the Sheffield University Plasmasphere Ionosphere Model (SUPIM) with values for the neutral wind, temperature, and composition provided by the National Center for Atmospheric Research (NCAR) Thermosphere Ionosphere General Circulation Model (TIEGCM). Various numerical experiments were run to identify the relative importance of the storm-time ionospheric drivers. For carrying out the storm-time SUPIM simulation, the storm-time upward/poleward E Â B drifts are derived from ROCSAT-1 satellite measurements at low and equatorial latitudes and input to SUPIM, while the storm-time neutral wind and composition disturbances are obtained from TIEGCM run. The simulation results presented in this paper, mainly during the evening period, show that the enhanced upward E Â B drifts due to storm-time eastward penetration electric field can expand the low-latitude equatorial ionization anomaly (EIA) to higher latitudes and produce the TEC enhancement. However, by the effect of penetration electric fields alone, the TEC enhancement is less than by combining the storm-generated equatorward neutral winds and the penetration electric fields. Disturbance neutral composition effects decrease the plasma density at higher latitudes and increase it at low and equatorial latitudes. However, the composition effects do not produce a density increase as large as that produced by the neutral-wind and electric-field effects. Our simulations suggest that the storm-generated equatorward neutral winds play an important role in producing the TEC enhancement at low-and midlatitudes, in addition to the eastward penetration electric field.

[1] We report on a series of simulations with the National Center for Atmospheric Research (NCAR) thermosphereionosphere-mesosphere-electrodynamics general circulation model (TIME-GCM) which were designed to replicate and facilitate the interpretation of the longitudinal structure discovered in IMAGE satellite airglow observations of the equatorial ionization anomaly (EIA) at the far-ultraviolet (FUV) 135.6-nm wavelength during March -April 2002 equinox. Our TIME-GCM results indicate that the fourpeaked longitudinal variation in the EIA observed by IMAGE-FUV near 20:00 local solar time can be explained by the effects of an eastward propagating zonal wavenumber-3 diurnal tide (DE3) that is excited by latent heat release associated with raindrop formation in the tropical troposphere.