Thermosphere-ionosphere General Circulation Model Research Articles

Thermosphere and ionosphere dynamics during 20–30 March 1979 time period: Comparison of TIGCM calculated densities with observations

The Thermosphere Ionosphere General Circulation Model developed at the National Center for Atmospheric Research, (NCAR TIGCM), has been used to simulate the time-dependent variations of global thermosphere and ionosphere structure and dynamics during the 20–30 March 1979 time period. Thermospheric density variations predicted by the TIGCM during this period are statistically compared to satellite electrostatic triaxial accelerometer neutral density measurements obtained between 170 and 240 km altitude and to predictions made by the Mass Spectrometer Incoherent Scatter empirical model (MSIS-86). In its present state of development, the TIGCM has attained about the same accuracy (standard deviation), as MSIS-86. Incorporation of improved representation of ion drag, resulting from the downward flow of magnetospheric plasma on the nightside, has contributed to the TIGCM model accuracy.

The flywheel effect: Ionospheric currents after a geomagnetic storm

In the period following a geomagnetic storm the high‐latitude, magnetospheric‐driven convection pattern is normally weak. However, the neutral circulation, set up by ion‐neutral momentum coupling during the main phase of the storm, may continue for several hours after the storm has ended. This persistent neutral circulation has the potential to drive Hall currents for some hours. In this paper we investigate these “flywheel” currents by simulating a storm which occurred on the 23rd of November 1982 using the National Center for Atmospheric Research Thermosphere Ionosphere General Circulation Model (NCAR‐TIGCM). The resulting high‐latitude, height‐integrated Hall currents are dominated by the neutral‐wind‐driven component for several hours after the end of main phase of the storm. The direction of these currents is reversed from normal. Analysis of the neutral and ion components of this current system indicates that the neutral component may drive as much as 80% of the high‐latitude current system immediately after the storm has ended, and may continue to dominate this system for 4 to 5 hours.

Auroral effects on midlatitude semidiurnal tides

The effect of auroral activity on mid‐latitude semidiurnal tides was investigated using simulations from the NCAR Thermosphere/Ionosphere General Circulation Model (TIGCM). Model runs were made for solar cycle minimum equinox conditions for four levels of geomagnetic activity parameterized by the total hemispheric power index and the cross polar cap potential drop. Simulations at 42.5°N (gg) predicted that the upper thermosphere semidiurnal winds and temperatures generally increase with increasing geomagnetic activity, while the lower thermosphere fields were relatively insensitive to the level of auroral forcing in the model. The modelled semidiurnal mid‐latitude tidal response was determined by the magnitude and phasing of the waves generated by in situ solar forcing and the auroral momentum and energy sources, in conjunction with those propagating up from the lower atmosphere. The predicted sensitivity of the model tides to the level of geomagnetic activity may contribute to the observed tidal variability at mid latitudes. Successful modelling of observations will require careful specification of the high‐latitude energy and momentum sources.

Simulations of the September 1987 lower thermospheric tides with the National Center for Atmospheric Research Thermosphere‐Ionosphere General Circulation Model

The National Center for Atmospheric Research thermosphere‐ionosphere general circulation model (TIGCM) was used to simulate incoherent scatter radar observations of the lower thermosphere tides during the first Lower Thermosphere Coupling Study (LTCS) campaign, September 21–26, 1987. The TIGCM utilized time‐varying histories of the model input fields obtained from the World Data Center for the LTCS period. These model inputs included solar flux, total hemispheric power, solar wind data from which the cross‐polar‐cap potential was derived, and geomagnetic Kp index. Calculations were made for the semidiurnal ion temperatures and horizontal neutral winds at locations representative of Arecibo, Millstone Hill, and Sondrestrom. The diurnal tides at Sondrestrom were also simulated. Tidal inputs to the TIGCM lower boundary were obtained from the middle atmosphere model of Forbes and Vial (1989). The TIGCM tidal structures are in fair general agreement with the observations. The amplitudes tended to be better simulated than the phases, and the mid‐ and high‐latitude locations are simulated better than the low‐latitude thermosphere. This may indicate a need to incorporate coupling of the neutral atmosphere and ionosphere with the E region dynamo in the equatorial region to obtain a better representation of low‐latitude thermospheric tides. The model simulations were used to investigate the daily variability of the tides due to the geomagnetic activity occurring during this period. In general, the ion temperatures were predicted to be affected more than the winds, and the diurnal components more than the semidiurnal. The effects are typically largest at high latitudes and higher altitudes, but discernible differences were produced at low latitudes. Additional observational studies are clearly warranted to test and refine the model predictions.

Lower thermospheric neutral densities determined from Søndre Strømfjord Incoherent Scatter Radar during LTCS 1

Ion‐neutral collision frequencies determined from measurements obtained by the incoherent scatter radar located at Søndre Strømfjord, Greenland, have been used to derive lower thermospheric neutral densities during the first Lower Thermosphere Coupling Study (LTCS 1), September 21–26, 1987. Periods of Joule and particle heating which might disturb the E region thermal equilibrium were systematically eliminated. The mean profile of neutral density for the period is in good agreement with the mass spectrometer incoherent scatter 1986 (MSIS‐86) model between 92 and 104 km. A tendency to overestimate collision frequencies above 105 km may arise from range‐smearing effects. The results of a tidal analysis performed on the neutral density between 92 and 109 km show that the amplitudes of the diurnal and semidiurnal components of the tides are approximately equivalent. The observations are generally in better agreement with the MSIS‐86 predictions than with the thermosphere‐ionosphere general circulation model (TIGCM) simulation of the LTCS 1 interval. The observed phase of the diurnal component is approximately constant with height above 98 km and is in close agreement with the MSIS‐86 model phases; however, the TIGCM diurnal phases are shifted by 6–8 hours to later local times. The phase of the semidiurnal tide is in good agreement with predictions of the MSIS‐86 model and the TIGCM simulation of this interval, except near 98 km. The observed semidiurnal phase is also consistent with previous high‐latitude results (Kirkwood, 1986). The relative amplitude of the observed semidiurnal oscillation is up to 15% larger than that previously observed at the European Incoherent Scatter facility but is consistent with the amplitudes presented in an earlier study of Millstone Hill measurements (Salah, 1974).

High‐latitude lower thermospheric neutral winds at EISCAT and Sondrestrom during LTCS 1

The incoherent scatter radar located at Søndre Strømfjord, Greenland (67°N, 51°W, 74.5°Λ) and the EISCAT incoherent scatter facility located in northern Scandinavia (69.5°N, 19°E, 66.3°Λ) both obtained E and F region measurements during the first campaign of the Lower Thermosphere Coupling Study (LTCS 1, September 21–25, 1987). Neutral winds deduced from these measurements have been analyzed for their mean flow and tidal components. A number of the altitude profiles for the mean winds and the diurnal and semidiurnal wave components at the two radar locations show similar variations with height, indicating that latitudinal rather than longitudinal effects are dominant in determining the observed wind field. Diurnal tidal amplitudes and phases are reasonably well represented by theoretical model results (Forbes, 1982). The semidiurnal amplitudes and phases, although somewhat consistent between the two radars, are not well represented in equinox tidal model results (Forbes and Vial, this issue). Results from both radars indicate a vertical wavelength for the zonal semidiurnal oscillation of approximately 60 km. During a period of impulsive magnetospheric forcing (September 22–23), winds deduced from measurements at both radars show enhanced eastward flows near midnight accompanied by equatorward winds at Sondrestrom. Comparison with the results of a National Center for Atmospheric Research thermosphere‐ionosphere general circulation model (TIGCM) simulation of the LTCS 1 interval shows generally better agreement with the observations at EISCAT than at Sondrestrom. During the period of activity on September 22–23 the TIGCM is reasonably successful at simulating the eastward surge near midnight in the EISCAT (but not in the Sondrestrom) observations and the equatorward flow after midnight in the Sondrestrom results. The observed winds in magnetic latitude and magnetic local time coordinates indicate the presence of anticyclonic divergent flow near dusk and cyclonic converging flow near dawn.

Solar tides in the equatorial upper thermosphere: A comparison between AE‐E data and the TIGCM for solstice, solar minimum conditions

Equatorial thermospheric tidal temperatures and densities inferred from Atmosphere Explorer E (AE‐E) mass spectrometer data are compared with theoretical predictions from the National Center for Atmospheric Research Thermosphere/Ionosphere General Circulation Model (TIGCM) for solar minimum, solstice conditions. The thermospheric diurnal and semidiurnal tides are excited in situ by solar heating and by ion‐neutral momentum coupling. Semidiurnal tides are also generated by upward propagating waves excited by heating in the lower atmosphere. The model calculations include all of these sources. The TIGCM reproduces the gross tidal features observed by the satellite, including the midnight temperature anomaly, and the diurnal phases are in good agreement for the densities of atomic oxygen and molecular nitrogen. However, for the neutral temperature, the predicted phases are 1–2 hours earlier than observed. In addition, the diurnal temperature and density amplitudes predicted by the model are considerably weaker than indicated by the AE‐E measurements. The semidiurnal variations found in the observations agree well with the model for December solstice but not for June. The present results indicate that upward propagating tides from the lower atmosphere are responsible for at least half of the amplitude of the semidiurnal tide in the upper thermosphere.

Thermospheric dynamics at the South Pole

A self‐aligning Fabry‐Perot spectrometer has been installed at Amundsen‐Scott Station, Antarctica (Geographic South Pole) and has been used to determine neutral upper thermosphere winds and temperatures, obtained from the measurement of the Doppler shift and Doppler width of the line profiles of the [OI] 15,867 K (630 nm) line emission, during the austral winter of 1989 (April 1989 to September 1989). These first ground‐based measurements of F‐region neutral dynamics at the South Pole, show a rich variety of dynamic phenomena and strong couplings with the ionospheric plasma. Data for two contrasting days in April 1989 are presented here. The data of April 23 UT illustrate the diurnal variations of winds and temperatures during geomagnetic quiet‐to‐moderate conditions, while the data of April 27 UT illustrate diurnal variations during geomagnetically disturbed periods. These data are compared with the average pattern obtained at a similar geomagnetic latitude and conditions, but in the Northern Hemisphere (Longyearbyen, Spitsbergen) and with predictions of the average dynamics in the South Pole region made by the NCAR thermosphere‐ionosphere general circulation model (TIGCM). The measured winds at the South Pole have a stronger prevailing westward component in its diurnal cycle than is either observed at Longyearbyen (in the Northern Hemisphere) or predicted at South Pole by the TIGCM.