Thermospheric Circulation Research Articles

Thermospheric temperatures and meridional winds measured at Albany, New York during geomagnetically disturbed periods

High quality optical data, obtained by using a high-resolution Fabry-Perot interferometer, were used to derive the thermospheric neutral temperatures and meridional wind velocities at a mid-latitude station, Albany (42.7°N, 73.8°W), New York, during the period 1978–1982. A total of 24 geomagnetically active nights, when the K p index was more than 3, were selected during the observation period. The neutral temperatures varied between about 900 and 1600 K, with a mean value of 1276 ± 54 K. Thermospheric temperatures were also estimated for these selected nights, using the 83 and 86 versions of the MSIS thermospheric model to compare with the experimental values. The experimentally determined temperature variations at the F-region heights were found to be in qualitative accord with the predicted variations based on the thermospheric models. However, the MSIS-83 model gave temperatures smaller than the measured values by about 250 K, while the difference was less than 200 K with the MSIS-86 estimates. In summary, the results of this study show a fair agreement between the observed temperature variations and the estimated variations using the MSIS models for geomagnetically active periods, and there is an increasing need to resolve the quantitative differences. The meridional wind velocities are generally equatorward and they increase from about 125 m s −1 shortly after sunset, to a maximum value of around 200 m s −1 before local midnight, and then decrease to less than about 100 m s −1 toward the early morning hours and become poleward by morning twilight. The mean meridional wind is essentially equatorward in agreement with the thermospheric circulation theory and also in general accord with other mid-latitude optical wind observations.

Geomagnetic quiet-time behavior of the thermospheric meridional winds and temperature at Albany, New York

High resolution Fabry-Perot interferometer data were used to deduce the thermospheric temperatures and meridional winds at a mid-latitude station, Albany (42.7°N, 73.8°W), New York, during 6 geomagnetically quiet nights in the year 1981. The meridional wind velocities are generally equatorward and they increase from less than 100ms −1 shortly after sunset, to a maximum value of around 170ms −1 before local midnight, and then decrease to small values toward early morning hours and become poleward by morning twilight. The neutral temperature varied between 890 and 1450 K and a detailed comparison was made with the predictions from the empirical models of MSIS-83 and MSIS-86. The observed variations in the thermospheric neutral temperatures are in qualitative accord with the computed structure based on the MSIS models. In general, the MSIS-86 model gave temperatures smaller than the observed values by nearly 42 K compared to the 106 K difference obtained between the MSIS-83 model values and the experimental temperatures. In summary, the results of this study show a good agreement between the observed temperature structure and the MSIS model values for quite-time geomagnetic conditions. The mean meridional wind is essentially equatorward in agreement with the thermospheric circulation theory and is also in general accord with other mid-latitude optical wind observations.

A three-dimensional model of ionospheric F-region

The three-dimensional stationary in solar-terrestrial system numerical model of the global ionosphere at F-region altitudes is developed. The input parameters are: the structural parameters of empirical thermospheric models (temperature, composition), electric fields, solar UV-radiation spectrum, corpuscular flows at high latitudes. The model includes the calculations of thermospheric circulation, electron density, electron and ion temperatures. The model reproduces the main morphological peculiarities of the distribution of thermospheric circulation and ionospheric plasma parameters. A comparison analysis of the results of the model calculation corresponding to different thermospheric models (DTM, MSIS, Jacchia-77, MSIS-83) is carried out. It is shown, that thermospheric circulation systems are the effective indicator of faithfulness of thermospheric models.

Variations of thermospheric winds in northern scandinavia between 1980 and 1986: A study of geomagnetic activity effects during the last solar cycle

Since late 1980, a ground-based Fabry-Perot interferometer has been in operation near Kiruna, Sweden (20.4°E, 67.8°N). This instrument has recorded thermospheric neutral winds under a wide range of geophysical conditions which have occurred during six observing winters. These data cover the period from the time of maximum geomagnetic activity during the last solar cycle, 1980/82, to the 1985/86 period of generally quiet geomagnetic activity near sunspot minimum. A statistical analysis of the data from about 400 nights of observation provides a graphic description of the generally rapid time-dependence of thermospheric winds in the vicinity of the mean auroral oval to individual geomagnetic disturbances. The data also provide an excellent data base describing the auroral oval. The data describe ion convection patterns as they respond to variable geomagnetic activity, and also the mean distribution of O I 630 nm emission as a function of local time, latitude and geomagnetic activity. These results can be used to examine the geomagnetic input parameters to a global thermospheric model (for example the semi-empirical global models of magnetospheric convection) as required to bring the simulations of thermospheric circulation into overall improved agreement with the observations.

Observations of the variations of thermospheric winds in Northern Scandinavia between 1980 and 1986: A study of geomagnetic activity effects during the last solar cycle

Since late 1980, a ground-based Fabry-Perot interferometer has been in operation near Kiruna, Sweden (20.4° E, 67.8° N). This instrument has recorded upper thermospheric neutral winds under a wide range of geophysical conditions which have occurred during six observing winters. These data cover the period from the time of maximum geomagnetic activity during the last solar cycle, 1980/82, to the 1985/86 period of generally quiet geomagnetic activity near sunspot minimum. A statistical analysis of the data from about 400 nights of observation provides a graphic description of the generally rapid time-dependence of upper thermospheric winds in the vicinity of the mean auroral oval to individual geomagnetic disturbances. The data also provide an excellent data base describing the auroral oval. The data describe ion convection patterns as they respond to variable geomagnetic activity, and also the mean distribution of OI 630 nm emission as a function of local time, latitude and geomagnetic activity. These results can be used to examine the geomagnetic input parameters to a global thermospheric model (for example the semi-empirical global models of magnetospheric convection) as required to bring the simulations of thermospheric circulation into overall improved agreement with the observations.

Interactions Between the Polar Ionosphere and Thermosphere

The temperature, composition and circulation of the ionosphere and thermosphere in the polar regions are closely coupled and display a marked variation with altitude, latitude, longitude, universal time, season, solar cycle, and geomagnetic activity. To a large degree, this variation is a consequence of the effect that magnetospheric electric fields, particle precipitation, and heat flows have on the ionosphere-thermosphere system. These magnetospheric processes act to produce ionospheric hot spots, plasma blobs, localized ionization troughs, extended tongues of ionization and ion composition changes. These ionospheric features then affect the thermosphere because of ion-neutral momentum and energy coupling. The resulting interactions act to modify the thermospheric circulation, composition, and temperature, and this, in turn, affects the ionosphere. However, there are significant time delays associated with the various interactions. These and other results are reviewed.

Auroral zone thermospheric dynamics: 2. Individual nights

Ground‐based Fabry‐Perot spectrometer measurements of the O I 15867 K (630.0 nm; 1 K = 1 cm−1) emission line were obtained from College, Alaska, during the winters of 1981 to 1983. Averages of the temperatures and winds, derived from these measurements, show a general flow driven by ion drag due to magnetospheric convection. This hypothesis was supported by results from a global thermospheric circulation model. Individual nights, however, show differences from the average flow pattern. During the recovery phase of an auroral substorm, when the westward electrojet is over the observing site, the equatorward meridional wind decreases in magnitude. The zonal wind is generally westward in the evening and eastward in the morning. However, the crossover from westward to eastward flow is a function of the location of the substorm onset and the arrival of the westward electrojet over College. Zenith winds can be very large (200 m/s) and show no systematic pattern, perhaps due to a superposition of wave modes from the auroral zone source region. An example of a night with little auroral activity highlights the effects of magnetospheric convection and the auroral electrojet on the normal auroral zone wind pattern. Temperature measurements occasionally show large enhancements, usually associated with type‐A red aurora.

Mean neutral circulation in the winter polar F region

Direct measurements of the F region neutral wind field in the northern (winter) hemisphere during late November and December 1981 were obtained by using the Dynamics Explorer (DE 2) satellite and the network of ground‐based Fabry‐Perot interferometers sited in the North American and Scandinavian sectors. All available data for the study period have been collated and averaged according to universal time (UT) to provide a global‐scale experimental determination of the mean or average circulation in the northern hemisphere F region. The data are organized into UT “bins” and are presented as a sequence of 12 “climate maps” depicting the neutral wind system at 2‐hour UT intervals during a “mean” day in December 1981. The data set provides the most comprehensive experimental measure of the global‐scale high‐latitude thermospheric circulation yet reported. The averaged measurements from the different instruments exhibit a satisfactory degree of internal consistency when viewed in terms of the global‐scale neutral flow. A detailed comparison with the predictions of the NCAR thermospheric general circulation model is discussed. The model predictions are in good general agreement with the UT‐dependent mean circulation, reproducing the basic control of the high‐latitude neutral wind system by ion drag forces modulated in UT as a result of the offset between the geomagnetic and geographic poles. Differences in flow directions and velocities between model predictions and experimental results are ascribed to natural variability in the thermosphere as well as to the use in the model of a less than fully realistic, symmetric ion convection geometry and density distribution. The two basic conclusions of this study are that (1) instrumentation deployed around the globe and on satellites can provide powerful, composite data sets that can be used to monitor global‐scale mean thermospheric “climatology” and that (2) the mean thermospheric (F region) neutral circulation for the given geophysical situation (solar maximum, near solstice) can be modeled reasonably well using a three‐dimensional, time‐dependent model with appropriate parameterizations for the energy and momentum inputs.

A theoretical and empirical study of the response of the high latitude thermosphere to the sense of the “ Y” component of the interplanetary magnetic field

The strength and direction of the Interplanetary Magnetic Field (IMF) controls the transfer of solar wind momentum and energy to the high latitude thermosphere in a direct fashion. The sense of “ Y” component of the IMF ( BY) creates a significant asymmetry of the magnetospheric convection pattern as mapped onto the high latitude thermosphere and ionosphere. The resulting response of the polar thermospheric winds during periods when BY is either positive or negative is quite distinct, with pronounced changes in the relative strength of thermospheric winds in the dusk-dawn parts of the polar cap and in the dawn part of the auroral oval. In a study of four periods when there was a clear signature of BY, observed by the ISEE-3 satellite, with observations of polar winds and electric fields from the Dynamics Explorer-2 satellite and with wind observations by a ground-based Fabry-Perot interferometer located in Kiruna, Northern Sweden, it is possible to explain features of the high latitude thermospheric circulation using three dimensional global models including BY dependent, asymmetric, polar convection fields. Ground-based Fabry-Perot interferometers often observe anomalously low zonal wind velocities in the (Northern) dawn auroral oval during periods of extremely high geomagnetic activity when BY is positive. Conversely, for BY negative, there is an early transition from westward to southward and eastward winds in the evening auroral oval (excluding the effects of auroral substorms), and extremely large eastward (sunward) winds may be driven in the auroral oval after magnetic midnight. These observations are matched by the observation of strong anti-sunward polar-cap wind jets from the DE-2 satellite, on the dusk side with BY negative, and on the dawn side with BY positive.

The thermal structure of the dayside upper atmosphere of venus above 125 km

A one-dimensional model of the Venus thermosphere has been constructed which includes computation of the heating efficiency of solar ultraviolet radiation, heat loss by radiation to space of infrared-active species, thermal transport by molecular and eddy conduction, and viscous dissipation. By comparing model predictions with results obtained from the Pioneer Venus Orbiter space-craft, the results indicate that energy transport parameterized by eddy heat conduction plays a dominant role in determining thermospheric temperature T. It is suggested that there exists a feedback mechanism linking heating and thermospheric circulation such that eddy cooling maintains an asymptotic temperature T ∞∼300°K for both solar-maximum and solar-minimum conditions. We also study the variation in thermospheric temperature with solar zenith angle, atomic oxygen-mixing ratio, rate of vibrational excitation of CO 2 by ground-state O atoms, and the assumed transfer of O( 1D) electronic energy to CO 2 vibrational energy.

First measurements of thermospheric winds in Antarctica by an optical ground-based method

The installation of a Fabry–Perot interferometer at Halley (75.5° S, 26.8° W; L = 4.2), Antarctica, has now allowed the first comparison to be made between Southern Hemisphere ground-based thermospheric wind measurements and the predictions of a three-dimensional time-dependent thermospheric global circulation model. Because some terms of the thermospheric momentum equation are more dependent on geomagnetic than geographic latitude, the large separation of these that exists at Halley provides a distinct contrast to the dynamic behaviour observed in the more frequently studied northern polar thermosphere. Although the initial results from the experiment are in general agreement with the model, some consistent and significant differences between the observed wind field and that predicted are evident in the morning sector. These may be related to uncertainties in mapping magnetospheric boundaries to ionospheric heights in the Southern Hemisphere.

High latitude thermospheric circulation during the Energy Budget Campaign

The thermospheric wind data obtained during 16 nights of observation from Esrange during the Energy Budget Campaign (EBC), using a Fabry-Perot interferometer (FPI) to observe the OI 630 emission line, are assessed with the aid of a global theoretical dynamical model of the thermosphere. The systematic behaviour of thermospheric winds, as seen from Kiruna, northern Sweden, is of a poleward day-time wind, which evolves into a nearly ubiquitous equatorward wind between about 1600 UT and 0300 UT. The times of the changes between poleward and equatorward winds are influenced by the nature and intensity of the geomagnetic activity on any given night. The magnitude of the night-time equatorward wind is related to auroral and magnetic activity, but not directly to indices such as K p . Equatorward winds of 400 m s −1 or more occurred at some time on about 50% of the nights reported here, with a highest wind of above 600 m s −1 (11 Nov. 1981). Zonal winds respond more quickly and to a larger extent to magnetic activity than do meridional winds. Under very quiet magnetic conditions, the zonal wind may flow eastward (anti-sunward) until 0200 UT and then reverse to westward until the end of observations at dawn (0700 UT), following the general pattern of winds at middle latitudes due to the solar-driven diurnal pressure gradients. With increasing activity, a period of evening westward winds occurs, usually with only a small meridional component. These westward winds are due to ion drag acceleration in the afternoon/evening sector of the auroral oval, with entrainment of the gas in the region of rapid westward ion motion for periods of many hours due to an approximate balance between Coriolis and inertial acceleration terms. The amplitude of the westward winds increases and their time of onset becomes earlier as the level of geomagnetic activity increases. After about 2000–2100 UT (magnetic midnight at Kiruna) on geomagnetically disturbed nights, there is a sudden onset of strong eastward and equatorward winds, with the onset occurring earlier with increasing activity. For the same ground-level magnetic disturbance at Kiruna, westward (evening) winds exceed eastward (morning) winds by a factor of 2. Westward windsof 500 ms −1 and eastward winds of 300 ms −1 are equally frequent. However, since the eastward winds are usually associated with an approximately equal southward meridional component, the total wind magnitude and energy does not differ too much between the two regimes. Even under very disturbed conditions, the wind usually returns poleward or anti-sunward before 0600 UT. This behaviour has to be interpreted in the context of global circulation, due to an ubiquitous low-latitude solar EUV energy source and a complex high latitude energy and momentum source dependent on geomagnetic activity, but it is also modulated by UT and season, due to the variable solar illumination of the geomagnetic polar cap. At 1200 UT the auroral oval is far poleward of northern Scandinavia (1000 km) except during extremely disturbed conditions, whereas between 1800 and 2200 UT the auroral oval is usually overhead and may be well equatorward under very disturbed conditions. With increasing magnetic activity there is a general equatorward expansion of the auroral oval, so that the time at which the locality comes under the influence of the auroral oval shifts successively to earlier UT and local time. During disturbed periods the cross-polar cap electric potential usually increases, driving ions to higher velocities. The enhancement of ionospheric electron density which occurs virtually throughout the entire auroral oval at the time of geomagnetic disturbances increases both the conductivity, and thus the ionospheric current (auroral electrojet), and also the effectiveness of ion drag in the regions of enhanced convective ion velocities. At magnetic midnight the auroral oval is usually close to Kiruna, however, due to continuity, the thermospheric circulation regime which is observed is dominated by fast anti-sunward winds flowing over the polar cap. This is the reason for the earlier change to eastward and equatorward winds with increasing activity. Intense local heating and ion drag acceleration occurs in the region about magnetic midnight during discrete magnetospheric substorms, which causes marked modulation of the thermospheric wind. There were three occasions during the 16 nights reported here when the local input during a substorm was sufficient to halt or temporarily reverse a pre-existing equatorward wind of 300 ms −1.

Thermospheric circulation, temperature, and compositional structure of the southern hemisphere polar cap during October–November 1981

Thermospheric temperature, composition, and winds at F region heights over the southern hemisphere polar cap have been measured by the Dynamics Explorer 2 (DE 2) satellite during October–November 1981. Perigee of the DE 2 satellite during this period was over the southern hemisphere polar cap, and measurements made along 0900–2100 local time polar passes are used to determine the average universal time dependence of thermospheric properties for this local time slice. The F region wind vector is derived from the meridional wind component measured by the Fabry‐Perot interferometer (FPI) and the zonal wind component measured by the wind and temperature spectrometer (WATS) instruments on board DE 2. The neutral gas temperature is derived from FPI and WATS measurements, and the compositional structure is determined from the neutral atmosphere composition spectrometer instrument. The measured temperature, composition, and winds along the 0900–2100 local time polar passes all display a universal time dependence that is due to the displacement between geomagnetic and geographic poles. The temperature and molecular nitrogen densities are enhanced, and the atomic oxygen density is reduced in the magnetic polar cap, in comparison with lower‐latitude regions. The neutral wind pattern follows the pattern of ion drift associated with magnetospheric convection. The National Center for Atmospheric Research thermospheric general circulation model (TGCM) is used to calculate the temperature, composition, and winds over the southern hemisphere polar cap region for conditions similar to those of October 1981. The TGCM includes displaced geomagnetic poles. The temperature, composition, and winds are determined along the DE 2 satellite track for various universal times. The calculated winds are in general agreement with the observed winds, both having a two‐cell pattern of magnetospheric convection. The calculated and observed composition patterns are also in general agreement with predictions made by the mass spectrometer and incoherent scatter radar 1983 empirical model when models and observations are normalized to the same constant pressure surface. The calculated and observed maximum compositional variations in the magnetic polar cap occur near 0900 UT, when the dawn sector of the auroral oval passes over the south geographic pole. The overall pattern of the calculated temperature variation is similar to that observed except for the period between 1300 and 1500 UT. Better agreement with temperature in this time range is obtained with the inclusion of auroral particle precipitation within the magnetospheric cusp.

Plasma characteristics of polar cap F‐layer arcs

First results are reported of a comprehensive campaign to measure ionospheric structure and dynamics of nighttime polar cap F‐layer or F‐region arcs. Airborne optical and digital ionosonde data were collected simultaneously with ground based incoherent scatter radar data, continuously over many hours in time and 500 to 1000 km in space. Polar cap F‐region sub‐visual auroral arcs, which are commonly observed during quiet magnetic conditions, are found to represent boundaries (or shears) in the polar cap plasma circulation pattern. F‐region electron concentration enhancements are found in these features and can be of significance to polar thermospheric circulation, composition, and thermal structure. The quiet polar ionosphere is so structured that high time resolution intensified optical images of thermospheric emmissions can make a major contribution to interpretation of incoherent scatter radar data.

Comparison of plasma flow and thermospheric circulation over northern Scandinavia using EISCAT and a Fabry-Perot interferometer

The EISCAT incoherent scatter radar, operating in a full tristatic mode, provided data on the ionospheric plasma drift above northern Scandinavia, during the 24 h period, 11 UT 25 November to 11 UT 26 November 1982. For the hours of darkness, 14 UT until 05 UT, observations of thermospheric winds were made by means of a ground-based Fabry-Perot interferometer (FPI) operated at Kiruna Geophysical Institute (21° E, 68° N). During this period, the radar observations describe well the ebbing and flowing of regions of strong convective ion flow associated with the auroral oval. As individual geomagnetic disturbances occur, the overall ion flow pattern intensifies and moves equatorward. The zonal thermospheric wind observed by the FPI responds rapidly to surges of the local ionospheric convection, while the meridional wind response is slower and apparently to much larger-scale features of the geomagnetic input to the high latitude thermosphere. From the data base, periods of strong heating of the ionospheric ions and of the thermospheric gas can be identified, which can be compared with Joule and particle heating rates deduced from the observations of ionospheric drifts, neutral winds, electron densities and auroral emission rates. A three-dimensional, time-dependent global thermospheric model is used to distinguish local and global features of the thermospheric wind field. Meridional and zonal wind components at 312 km may be theoretically derived from the EISCAT data using an appropriate model (MSIS) for neutral temperature. The EISCAT-derived meridional wind is within about 50 m s −1 of the FPI observations throughout the period of joint observations. The EISCAT-derived zonal wind is systematically larger (by about 50%) than the FPI measurement, but the two independent measurements follow closely the same fluctuations in response to geophysical events until 03 UT, when the EISCAT solution is driven away from the FPI measurement by a sharp increase in both neutral and ion temperatures. Between 03 and 05 UT the EISCAT-derived zonal wind is 200–400 m s −1 westward. Allowance for the neutral temperature rise would reduce the EISCAT values towards the very small zonal winds shown by the FPI during this period. We describe the relatively straightforward analysis required to derive the meridional wind from the radar data and the limitations inherent in the derivation of zonal wind, using the ion energy equation, due to the lack of precise knowledge of the background neutral temperature from the EISCAT data alone. For analysis of EISCAT ion drift observations at 312 km, the ground-based FPI temperature measurements do not improve the accuracy of the analysis, since the median altitude of the FPI measurement is probably in the range 180–240 km throughout the observation period. This median altitude and the temperature gradient both fluctuate in response to local geomagnetic events, while the temperature gradient may be considerably greater than that predicted by standard atmospheric models. When the neutral temperature is well known, or when there is a large enhancement of the ion temperature, the EISCAT-derived zonal wind exceeds the FPI measurement, but the consistency with which they correlate and follow ion-drag accelerations suggests that the differences are purely due to the considerable altitude gradients which are predicted by theoretical models.

The effect of realistic conductivities on the high-latitude neutral thermospheric circulation

The dynamics of the high latitude thermosphere are dominated by the ion circulation pattern driven by magnetospheric convection. The reaction of the neutral thermosphere is influenced by both the magnitude of the ion convection velocity and by the conductivity of the thermosphere. Using a threedimensional, time-dependent, thermospheric, neutral model together with different ionospheric models, the effect of changes in conductivity can be assessed. The ion density is described by two models: the first is the empirical model of Chiu (1975) appropriate for very quiet geomagnetic conditions, and the second is a modified version of the theoretical model of Quegan et al. (1982). The differences in the neutral circulation resulting from the use of these two ionospheric models emphasizes the need for realistic high latitude conductivities when attempting to model average or disturbed geomagnetic conditions, and a requirement that models should couple realistically the ionosphere and the neutral thermosphere. An attempt is made to qualitatively interpret some of the features of the neutral circulation produced at high latitudes by magnetospheric processes.

Geomagnetic activity effects on semidiurnal winds in the lower thermosphere

Over the period from July 1976 to November 1977 regular incoherent scatter observations of E and F region ion‐drifts were conducted at Millstone Hill (42.6°N) by using the steerable (L band) and fixed (UHF) radars. Data were collected on a total of 46 days and analyzed to give the ionospheric electric field and daytime neutral winds in the lower thermosphere at 105, 115, 125, and 135 km. The wind measurements characteristically exhibited strong 12‐hour oscillations with the eastward wind component reaching a maximum 3 hours ahead of the southward wind component. These results imply that the neutral winds are being dominated by semidiurnal tides. The geomagnetic activity dependence of the semidiurnal winds was determined by dividing the observations into quiet and disturbed periods on the basis of the Kp index. During disturbed conditions the winds were found to be significantly altered from the quiet time behavior, with the semidiurnal amplitudes reduced by 20 to 50% at the lowest altitudes and the altitude of maximum wind increased to be above 125 km. The vertical wavelength also appeared to decrease under disturbed conditions. Some of these effects might be explained by geomagnetic‐activity induced changes in lower thermospheric temperature and density, which alter the dissipation of the upward propagating tidal component. Changes in the prevailing winds also appear to be associated with Kp increases, implying a modification of the general thermospheric circulation by high‐latitude heating. The incremental winds are small (∼5m/s) and eastward/equatorward at 105 km, becoming stronger ( ∼25m/s) and westward/equatorward at higher altitudes.

Equatorial disturbance dynamo electric fields

F‐region vertical drift data from Jicamarca, Peru show that equatorial east‐west electric fields are sometimes perturbed 16‐24 hours after the onset of geomagnetic storms. These disturbance dynamo electric fields, which must be caused primarily by the action of neutral winds at low and middle latitudes, decrease and sometimes even reverse the quiet time electric field pattern during both daytime and nighttime. The long delay excludes the possibility that gravity waves are responsible and suggests that the thermospheric circulation is disturbed. The data also show that after some storms there are no such delayed disturbances, a fact which may be due to the longitudinal structure of the disturbances at high latitudes and/or that only very strong storms can produce major thermospheric perturbations that extend to middle and low latitudes.

A three-dimensional, time-dependent simulation of the global dynamical response of the thermosphere to a geomagnetic substorm

Some of the major features of the atmospheric circulation predicted to occur following a geomagnetic substorm in the earth's thermosphere are described on the basis of a three-dimensional, time-dependent simulation. The circulation changes and the gravity wave oscillatory phenomena are traced at two levels of the thermosphere, 120 and 240 km, over a period of 4 1 2 h following the substorm onset. For both aspects of the response, and for each height, the characteristics, magnitudes and propagation are quite distinct, and the three-dimensional model shows that very large differences occur between one local time and another. The night-time period, and particularly that between 0000 and 0600 LT, shows, at middle latitudes, the largest wind and wave amplitudes, and the most complex response. In general, the response at any location to an individual substorm can be considered as a series of wave-like disturbances. This is due to the nature of the impulsive disturbance which is concentrated in an annular ring corresponding to the auroral oval, of which each part generates both poleward-travelling as well as equatorward-travelling components, with approximate circular symmetry. Observing the response at 240 km, it appears that focusing occurs of the initial poleward wavefront, and in subsequent poleward wavefronts, the latter arising from the apparent ‘reflection’ of waves initially propagating sunward as they interact with the mean anti-sunward flow on the dayside. Three major waves can therefore be identified in the night-time mid-latitude thermosphere—the initial wave from the nearby auroral oval, a second wave following about 90 min later originating in the dayside auroral oval. This wave is stronger than the first and, being focused and guided by the enhanced mean anti-sunward flow, is capable of generating larger waves. About another 90 min later a third wave arrives due to the dayward propagating wave being reflected, refocused and guided by the mean flow, which can also cause comparable disturbances throughout the thermosphere. There is a strong enhancement of the contra-rotating vortices of the high latitude region—providing sunward flow in the morning and evening auroral ovals with a strong return anti-sunward flow over the polar cap, which extends well into the mid-latitude regions in the midnight and early morning period. In general, amplitudes of the wind and wave disturbances increase continuously with altitude above about 100 km, even allowing for the omission of propagating tidal winds from our model (generated by the lower atmosphere). For the modest disturbance simulated here—a substorm we consider typical of a period when K p ~ 6-winds of 200–250 m s −1 occur at 120 km, increasing to 500 m s −1 at 240 km, and near 400 km the winds exceed 600 m s −1. While at 240 km the response is dominated by oscillations superimposed on an enhancement of the general thermospheric circulation, at 120 km there is strong evidence that two long-lasting vortices may be created by the dynamical and energetic effects of the substorm, while there is yet no direct observational counterpart of such vortices—an anticyclone centred between 2200 and midnight, and a cyclone centred between 1000 and midday both in the auroral oval—they may be quite characteristic features of the slow recovery of the thermosphere from the impulsive effects of the additional energy input during geomagnetic substorms.

Thermosphere circulation modeled

When solar storms force the earth's auroras to lower latitudes, winds in the thermosphere reverse direction and are whipped up to velocities of 2250 km/h. A computer model has now been developed that will describe the circulation of the thermosphere—a 400‐km blanket enveloping the earth, with its bottom boundary at an altitude of 80 km—and its interaction with the auroras.Raymond G. Roble, of the National Center for Atmospheric Research, explained at the AGU Fall Meeting that the thermospheric model might be useful to predict variations in storm time and atmospheric drag on some earth satellites. If the thermosphere's dynamics are better understood, he reasoned, more accurate predictions of a satellite's orbital decay can be made. The model may also help to predict the effects of communications equipment and magnetic forces on power grids.