A numerical simulation study of the thermospheric winds produced by auroral heating during magnetic storms, and of their global dynamo effects, establishes the main features of the ionospheric disturbance dynamo. Driven by auroral heating, a Hadley cell is created with equatorward winds blowing above about 120 km at mid-latitudes. The transport of angular momentum by these winds produces a subrotation of the mid-latitude thermosphere or westward motion with respect to the earth. The westward winds in turn drive equatorward Pedersen currents which accumulate charge toward the equator, resulting in the generation of a poleward electric field, a westward E × B drift, and an eastward current. When realistic local time conductivity variations are simulated, the eastward mid-latitude current is found to close partly via lower latitudes, resulting in an ‘anti-Sq’ type of current vortex. Both electric field and current at low latitudes thus vary in opposition to their normal quiet-day behavior. This total pattern of disturbance winds, electric fields, and currents is superimposed upon the background quiet-day pattern. When the neutral winds are artificially confined on the nightside, the basic pattern of predominantly westward E × B plasma drifts still prevails on the nightside but no longer extends into the dayside. Considerable observational evidence exists, suggesting that the ionospheric disturbance dynamo has an appreciable influence on storm-time ionospheric electric fields at middle and low latitudes.

The ionospheric disturbance dynamo

A review of theoretical and observational results describing atmospheric gravity wave (AGW)/traveling ionospheric disturbance (TID) phenomena at high latitudes is presented. Some recent experimental studies of AGW's using the Chatanika incoherent scatter radar and other geophysical sensors are reported. Specifically, the following features are described in detail: (1) cause/effect relations between aurorally generated AGW's and TID's detected at mid-latitudes, including probable ‘source signature’ identification, (2) AGW source phenomenology, particularly a semiquantitative assessment of the relative importance of Joule heating, Lorentz forces, intense particle precipitation, and other mechanisms in generating AGW's, and (3) detection of TID's in the auroral ionosphere. Several instances of F region electron density, temperature, and plasma periodicities accompanied by horizontal plasma velocities which were consistent with theoretical AGW/TID models are documented.

Atmospheric gravity waves generated in the high‐latitude ionosphere: A review

The global neutral gas temperature distribution and circulation of the thermosphere are calculated for equinox and solstice conditions by using NCAR's thermospheric general circulation model (TGCM) The variables are determined on a 5° grid in latitude and longitude at 24 constant pressure surface layers in the vertical from about 90 to 500 km Global empirical models of electron density and neutral composition are used to specify ion drag and the neutral gas background properties The energy sources that drive the thermosphere include heating caused by the absorption of solar EUV and UV radiation and a high-latitude heat source associated with auroral processes Starting from a global average state, the TGCM reached a diurnal reproducible pattern in approximately 5 days and showed no apparent instabilities in the basic flow The calculated motion patterns indicate that the observed day-to-day variability in the thermosphere is likely because of variations in the heat and momentum sources such as the result of neutral plasma interactions and waves coming from the lower atmosphere The calculated horizontal structure of wind and temperature is qualitatively different between the upper and lower thermosphere This difference occurs because the diurnal and semidiurnal forcing are in phase in the optically thick lower thermosphere but out of phase in the optically thin upper thermosphere Using the latest calculated heating efficiencies and measured fluxes of EUV, the calculated diurnal temperature amplitude is 250 K, compared to 320 K predicted by the MSIS empirical model, indicating that solar heating is primarily responsible for maintaining the structure of the thermosphere The zonally averaged properties of the TGCM essentially agree with previous results obtained from a two-dimensional model of the zonally symmetric thermosphere However, for reasons discussed, the latitudinal variations of temperatures is somewhat larger than previously calculated Our previously estimated auroral heating terms need to be reduced by 30%–40% to get best agreement with observations

A three-dimensional general circulation model of the thermosphere

Major phenomena of the equatorial ionosphere-thermosphere system under disturbed conditions

A unifying description is provided of some important dynamic properties of the upper atmosphere in composition and temperature characteristic of a variety of phenomena, including diurnal and seasonal tides, magnetic storms, and momentum coupling with the magnetosphere. A theoretical multiconstituent model is used which can link the large-scale variations of composition and temperature to the dynamics and energetics of the thermosphere. Global mean properties of the thermosphere are reviewed, and an attempt is made to convey some understanding of the dynamic properties of energy and diffusive mass transport in the thermosphere. Attention is given to sources of energy for the thermosphere, the transport processes involved in the solar diurnal tide of the thermosphere, energy and particle sources for the annual tide, feedback from composition changes to wind-field and temperature variations, energy deposition in the thermosphere during magnetic storms and substorms, and momentum source signatures in the thermosphere.

Some properties of upper atmosphere dynamics.

A high-resolution Fabry-Perot spectrometer has been used at Fritz Peak Observatory, Colorado (39.9°N, 105.5°W), to determine the nighttime thermospheric temperatures and winds during geomagnetic storm periods from the line profiles and Doppler shifts of the (O I) 15867-K (6300 A) line emission. Data obtained during four geomagnetic storm periods in 1974 are presented: March 20–22, October 15–20, June 29 to July 8, and April 16–27. The data are compared to temperatures determined from the Ogo 6 global empirical model of neutral composition and temperature and to winds calculated from a semiempirical dynamic model of the neutral thermosphere. The results show considerable variations in the dynamic character of the thermosphere which are also related to the intensity of geomagnetic activity. During geomagnetic storms the nighttime equatorward winds are greatly enhanced from their quiet time values with a maximum measured velocity of 640 m s−1 during a Kp = 9 storm. The zonal winds generally develop a westward component relative to the geomagnetic quiet zonal winds. During intense geomagnetic activity the zonal winds are westward at 100–200 m s−1 in the early evening hours, flowing in the direction of magnetospheric convection and opposite of model predictions. The meridional winds measured south of the site are generally smaller than those measured to the north. Sometimes the meridional winds measured north and south of the station flow in opposite directions. The nighttime neutral gas temperatures are observed to increase from their geomagnetic quiet values during the storm and are in general concurrence with the Ogo 6 model predictions.

R. G. Roble

Papers

Direct measurements of nighttime thermospheric winds and temperatures, 2. Geomagnetic storms