Thermosphere-ionosphere General Circulation Model Research Articles

Atmospheric Waves and Their Possible Effect on the Thermal Structure of Saturn's Thermosphere

AbstractAtmospheric waves have been discovered for the first time in Saturn's neutral upper atmosphere (thermosphere). Waves may be generated from instabilities, convective storms or other atmospheric phenomena. The inferred wave amplitudes change little with height within the sampled region, raising the possibility of the waves being damped, which in turn may enhance the eddy friction within the thermosphere. Using our Saturn Thermosphere Ionosphere General Circulation Model, we explore the parameter space of how an enhanced Rayleigh drag in different latitude regimes would affect the global circulation pattern within the thermosphere and, in turn, its global thermal structure. We find that Rayleigh drag of sufficient magnitude at midlatitudes may reduce the otherwise dominant Coriolis forces and enhance equatorward winds to transport energy from poles toward the equator, raising the temperatures there to observed values. Without this Rayleigh drag, energy supplied into the polar upper atmosphere by magnetosphere‐atmosphere coupling processes remains trapped at high latitudes and causes low‐latitude thermosphere temperatures to remain well below the observed levels. Our simulations thus suggest that giant planet upper atmosphere global circulation models need to include additional Rayleigh drag in order to capture the effects of physical processes otherwise not resolved by the codes.

Magnetosphere–atmosphere coupling at Saturn: 1 – Response of thermosphere and ionosphere to steady state polar forcing

We present comprehensive calculations of the steady state response of Saturn’s coupled thermosphere–ionosphere to forcing by solar radiation, magnetospheric energetic electron precipitation and high latitude electric fields caused by sub-corotation of magnetospheric plasma. Significant additions to the physical processes calculated in our Saturn Thermosphere Ionosphere General Circulation Model (STIM–GCM) include the comprehensive and self-consistent treatment of neutral–ion dynamical coupling and the use of self-consistently calculated rates of plasma production from incident energetic electrons. Our simulations successfully reproduce the observed high latitude temperatures as well as the latitudinal variations of ionospheric peak electron densities that have been observed by the Cassini Radio Science Subsystem experiment (RSS). We find magnetospheric energy deposition to strongly control the flow of mass and energy in the high and mid-latitude thermosphere and thermospheric dynamics to play a crucial role in driving this flow, highlighting the importance of including dynamics in any high latitude energy balance studies on Saturn and other Gas Giants. By relating observed H3+ column emissions and temperatures to the same quantities inferred from simulated atmosphere profiles we identify a potential method of better constraining the still unknown abundance of vibrationally excited H2 which strongly affects the H3+ densities. Our calculations also suggest that local time variability in H3+ column emission flux may be largely driven by local time changes of H3+ densities rather than temperatures. By exploring the parameter space of possible high latitude electric field strengths and incident energetic electron fluxes, we determine the response of thermospheric polar temperatures to a range of these magnetospheric forcing parameters, illustrating that 10keV electron fluxes of 0.1–1.2mWm−2 in combination with electric field strengths of 80–100mVm−1 produce H3+ emissions consistent with observations. Our calculations highlight the importance of considering thermospheric temperatures as one of the constraints when examining the state of Saturn’s magnetosphere and its coupling to the upper atmosphere.

Role of thermosphere‐ionosphere coupling in a global ionospheric specification

In this paper we present our recent research development in the area of ionospheric specification by means of data assimilation of ground‐based observations. NOAA's Total Electron Content specification methodology (namely, a Gauss‐Markov Kalmanfilter with an empirical model of the ionosphere as a background model) over the continental United States has lately been expanded to the multiregional domains and to the entire globe. Analyses of the global TEC maps reveal clear signatures of thermosphere‐ionosphere coupling, in both dynamical and compositional nature, even though the underlying specification methodology does not take thermospheric effects into account. This suggests that ground‐based observations of electron density contain some information about the state of the thermosphere. By using a thermosphere ionosphere general circulation model in a prototype Ensemble Kalman filter (EnKF), we examine the role of thermosphere‐ionosphere coupling in a global ionospheric specification. Observing system simulation experiments, designed for a global network of ionosondes, suggest that ionospheric data assimilation considerably benefits from self‐consistent treatment of thermosphere‐ionosphere coupling in a forecast model as well as in assimilation schemes, both of which can be achieved inherently by using the EnKF.

Validation of the Coupled Thermosphere Ionosphere Plasmasphere Electrodynamics model: CTIPE‐Mass Spectrometer Incoherent Scatter temperature comparison

New requirements for specification and forecast of the space environment and the availability of unprecedented amounts of real‐time data are now driving the development of data assimilation schemes for the thermosphere and ionosphere. Such schemes require accurate knowledge of any biases affecting the models. Finding the biases is not trivial and requires significant effort. Here we present a first step in the validation of a coupled thermosphere ionosphere general circulation model in preparation for its inclusion in a data assimilation scheme. We present a comparison between the Mass Spectrometer Incoherent Scatter (MSIS) radar empirical model neutral temperatures and the Coupled Thermosphere Ionosphere Plasmasphere Electrodynamics (CTIPE) neutral temperature predictions for three solar cycle conditions (F10.7 = 70, 125, and 200), three geomagnetic activity conditions (Kp = 1, 3, and 7), and three seasons (equinox, summer, and winter). The CTIPE model was run for each case with constant inputs until a diurnally reproducible (“steady state”) global temperature pattern was obtained. MSIS predictions were generated for “perpetually constant” equivalent conditions. The temperature comparisons are performed on a 300 km altitude shell. We present global temperature averages, area‐weighted root mean square differences, and zonally averaged temperature comparisons. CTIPE temperatures at 300 km altitude are lower than MSIS if Joule heating calculations do not include small‐scale E field variability. This is the first global assessment of a general circulation model for the thermosphere over such a wide range of geomagnetic and solar conditions.

Theoretical study of the low‐ and midlatitude ionospheric electron density enhancement during the October 2003 superstorm: Relative importance of the neutral wind and the electric field

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 (∼ −70°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 × 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 × 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.

A high-resolution, three-dimensional, time dependent, nested grid model of the coupled thermosphere–ionosphere

First results are presented from a 3-D, time dependent, high resolution, nested grid model that has been developed to study mesoscale processes in the global, coupled thermosphere–ionosphere system. This new Thermosphere–Ionosphere Nested Grid (TING) model, which is an extension of the National Center for Atmospheric Researchs thermosphere–ionosphere general circulation model (NCAR–TIGCM), runs on a UNIX workstation. The TING model simultaneously calculates global (coarse resolution) and local (high resolution) distributions of neutral and plasma winds, temperature and composition. It is comprised of two coupled codes—a global TIGCM and an adjustable nested grid code which uses the same solvers as the TIGCM, but has higher spatial and temporal resolution. The size, location and level of nesting of the high resolution grid(s) are adjustable to suit the specific application. The coupling between the coarse (TIGCM) grid and the nested interior grids is via a one-way interaction scheme. In this scheme, the TIGCM output influences the nested grid model by providing initial conditions and temporally evolving boundary conditions, but the outputs from the nested grid are not permitted to influence the TIGCM. Diurnally-reproducible results of the TING model are presented for solar-maximum, winter solstice, geomagnetically-quiet conditions. The TING model successfully simulates well-known thermosphere–ionosphere features that are smeared or not modeled at the spatial resolutions used in standard TIGCMs. These include the sub-auroral electron density trough, the polar cap hole and the polar cap tongue of ionization.

The thermospheric-ionospheric storm of Dec 8, 1982: Model predictions and observations

The Thermosphere-Ionosphere General Circulation Model (Roble et al., 1988) and the Coupled Thermosphere-Ionosphere Model (Fuller-Rowell et al., 1987) are used to simulate thermospheric-ionospheric behaviour during the magnetic storm of December 8, 1982. A comparison of these predictions with observations indicates that both models overestimate the daytime increase of the O/N 2 ratio and the associated positive ionospheric storm effects. Also there is no indication of a large enhancement in the O/N 2 ratio in the evening sector of the winter hemisphere.

Geomagnetic activity effects on thermospheric tides: a compendium of theoretical predictions

The theoretical effects of auroral activity on thermospheric tides are investigated using simulations from the National Center for Atmospheric Research thermosphere-ionosphere general circulation model. Simulations for March, June, and December during solar cycle minimum and maximum are presented for three levels of geomagnetic activity representing quiet, moderate, and disturbed conditions. The mean, diurnal, and semidiurnal components of the model neutral temperatures and horizontal winds at geographic longitude 70 ° W at 17.5 ° N, 42.5 ° N, and 67.5 ° N are examined. The model predicts that the effects of varying geomagnetic activity are larger at solar cycle minimum than solar cycle maximum and increase with increasing latitude and altitude. In general, the predicted mean temperatures increase, the mean zonal winds become more westward and the mean meridional winds more equatorward as geomagnetic activity increases. In the upper thermosphere, the effects of geomagnetic activity on the mean and tidal neutral winds and temperatures are such that diurnal temperature amplitudes decrease, diurnal zonal winds generally decrease at low and middle latitudes and increase at high latitudes, and diurnal meridional winds increase; semidiurnal amplitudes generally increase. At low altitudes in high latitudes, increasing levels of geomagnetic activity cause tidal amplitudes to increase strongly, peaking near 140 km. At middle latitudes, the lower thermosphere diurnal winds increase in March and June and decrease in December. At low latitudes, the effects at low altitudes are generally small and complex. Comparison with observational and other theoretical results is inconclusive because of a lack of data.

Lower thermospheric neutral winds at Søndre Strømfjord: A seasonal analysis

Data obtained by Søndre Strømfjord incoherent scatter radar during recent Lower Thermospheric Coupling Study (LTCS) campaigns have been analyzed to examine seasonal changes in high‐latitude lower thermosphere neutral winds. Data included in this study were obtained with a 2 × 160 μs multipulse as well as with higher‐resolution multipulse and alternating code techniques and were analyzed with different auto correlation function (ACF) fitting routines. We report neutral wind results obtained during LTCS experiments with the new acquisition and analysis system and compare these results with those acquired using earlier versions of the software. We investigate the effects of different assumptions in the ACF fitting program on the data return and quality. Wind results are intercompared to examine average seasonal structure and variability. Strong semidiurnal oscillations characterize high‐latitude lower thermospheric winds at 105 km irrespective of season; however, summer amplitudes are nearly a factor of 2 greater than those observed during winter. Average winter winds appear to have more variability than those deduced for summer. Equinox winds at fall and spring show similar oscillations but are nearly reversed in phase. At 115 km, diurnal oscillations grow and their amplitudes often exceeds typical semidiurnal amplitudes. Tidal amplitudes observed in radar data are compared with the predictions of several models, including National Center for Atmospheric Research thermosphere‐ionosphere‐general‐circulation model (TIGCM) [Roble et al., 1988], global scale wave model (GSWM) [Hagan et al., 1994], and Forbes‐Vial model (FV91) [Forbes and Vial, 1991]. These comparisons suggest that theoretical predictions significantly underestimate tidal amplitudes calculated from Sondrestrom incoherent scatter radar measurements.

Assimilative mapping of ionospheric electrodynamics in the thermosphere‐ionosphere general circulation model comparisons with global ionospheric and thermospheric observations during the GEM/SUNDIAL period of March 28–29, 1992

Satellite and ground‐based observations from March 28 to 29, 1992, were combined in the assimilative mapping of ionospheric electrodynamics (AMIE) procedure to derive realistic global distributions of the auroral precipitation and ionospheric convection which were used as inputs to the National Center for Atmospheric Research (NCAR) thermosphere‐ionosphere general circulation model (TIGCM). Comparisons of neutral model winds were made with Fabry‐Perot measurements and meridional winds derived from ionosondes. The peak equatorward winds occurred 1–2 hours later in the model. Gravity waves launched from high‐latitude Joule heating sources reached the equator in about 2 hours and agreed with observed variations in the height of the maximum electron density (hmF2) and in the meridional winds. Joule heating events produced minima in the O/N2 ratio that moved equatorward and usually westward in longitudinal strips which lasted about a day. Changes in the O/N2 ratio and in the peak electron density (NmF2) were strongly correlated so the observed daytime NmF2 values for stations near 50° magnetic latitude were generally reproduced by AMIE‐TIGCM on the second day of the simulation. The AMIE‐TIGCM underestimated the electron density after midnight by up to a factor of 2 in midlatitudes, while the modeled F2 layer was about 35 km lower than the observations at midnight. Shifting the model winds 2 hours earlier at night could double the NmF2 at 0400 LT and increase hmF2 by 20 km. NmF2 could also be increased at night by realistically increasing the TIGCM nighttime downward fluxes of O+ at the upper boundary.

Cellular structures in the high‐latitude thermosphere

An organized density (and pressure) structure was recently discovered in the neutral thermosphere at high‐latitudes. The structure consists of two to four high‐ and low‐density regions having diameters of 1000 to 2000 km. The density in each region is enhanced or depleted from the hemispheric average by up to 30%. The structure is thus a significant feature of the near‐Earth space environment at high‐latitudes. We refer to each distinct region of enhanced or depleted density as a density “cell.” The cells extend upward from about 120 km into the upper thermosphere, and once formed they remain approximately fixed with respect to the geomagnetic pole. A parametric study of the density cell morphology for different magnetic activity levels is described for equinox solar minimum using the National Center for Atmospheric Research thermosphere ionosphere general circulation model (NCAR‐TIGCM). Density data were sought to verify the existence of the structures first predicted by the NCAR model. The TIGCM simulations were used to predict the large density perturbations observed by the S85‐1 satellite in a circular sun‐synchronous orbit near 200 km altitudes. The most obvious manifestation of the cells was the presence of density peaks located near 70°Λ on the dayside and nightside, and a density minimum near the magnetic pole. Since high‐latitude densities are generally expected to increase during magnetic activity, the low densities over the pole are perhaps the most interesting feature of the cell structure discussed here. The satellite data confirm the existence of the cellular structure over a range of magnetic activity levels. The discovery of the cells is important because the structure provides a unifying framework for the analysis and interpretation of high‐latitude data from both past and future experiments. The cells result from various forms of coupling between the ionosphere and thermosphere. The cell formation is quantitatively consistent with concepts from dynamic meteorology.

Upper thermosphere winds and temperatures in the geomagnetic polar cap: Solar cycle, geomagnetic activity, and interplanetary magnetic field dependencies

Ground‐based Fabry‐Perot interferometers located at Thule, Greenland (76.5°N, 69.0°W, Λ=86°) and at Søndre Strømfjord, Greenland (67.0°N, 50.9°W, Λ=74°) have monitored the upper thermospheric (∼240‐km altitude) neutral wind and temperature over the northern hemisphere geomagnetic polar cap since 1983 and 1985, respectively. The thermospheric observations are obtained by determining the Doppler characteristics of the (O I) 15,867‐K (630.0‐nm) emission of atomic oxygen. The instruments operate on a routine, automatic, (mostly) untended basis during the winter observing seasons, with data coverage limited only by cloud cover and (occasional) instrument failures. This unique database of geomagnetic polar cap measurements now extends over the complete range of solar activity. We present an analysis of the measurements made between 1985 (near solar minimum) and 1991 (near solar maximum), as part of a long‐term study of geomagnetic polar cap thermospheric climatology. The measurements from a total of 902 nights of observations are compared with the predictions of two semiempirical models: the vector spherical harmonic (VSH) model of Killeen et al. (1987) and the horizontal wind model (HWM) of Hedin et al. (1991). The results are also analyzed using calculations of thermospheric momentum forcing terms from the thermosphere‐ionosphere general circulation model (TIGCM) of the National Center for Atmospheric Research (NCAR). The experimental results show that upper thermospheric winds in the geomagnetic polar cap have a fundamental diurnal character, with typical wind speeds of about 200 m s−1 at solar minimum, rising to up to about 800 m s−1 at solar maximum, depending on geomagnetic activity level. These winds generally blow in the antisunward direction, but are interrupted by episodes of modified wind velocity and altered direction often associated with changes in the orientation of the interplanetary magnetic field (IMF). The central polar cap (>∼80 magnetic latitude) antisunward wind speed is found to be a strong function of both solar and geomagnetic activity. The polar cap temperatures show variations in both solar and geomagnetic activity, with temperatures near 800 K for low Kp and F10.7 and greater than about 2000 K for high Kp and F10.7. The observed temperatures are significantly greater than those predicted by the mass spectrometer/incoherent scatter model for high activity conditions. Theoretical analysis based on the NCAR TIGCM indicates that the antisunward upper thermospheric winds, driven by upstream ion drag, basically “coast” across the polar cap. The relatively small changes in wind velocity and direction within the polar cap are induced by a combination of forcing terms of commensurate magnitude, including the nonlinear advection term, the Coriolis term, and the pressure gradient force term. The polar cap thermospheric thermal balance is dominated by horizontal advection, and adiabatic and thermal conduction terms.

Equatorial penetration of magnetic disturbance effects in the thermosphere and ionosphere

The neutral dynamic and electrodynamic coupling between high and low latitudes, and the mutual interactions between these two processes, are investigated. For 22 March 1979, when a sudden increase in magnetic activity occurred, we have analyzed the following experimental data: (a) neutral densities and cross-track neutral winds as a function of latitude (0°–80°) near 200 km from a satellite-borne accelerometer; (b) hourly mean H-component magnetic data from the Huancayo Observatory (0.72°S, 4.78°E; dipole geomagnetic coordinates) magnetometer; and (c) hourly mean foF2 measurements from the ionosonde at Huancayo. Comparisons are also made with a self-consistent thermosphere-ionosphere general circulation model and with observationally-based empirical models of winds and density. In concert with the increase in magnetic activity to K p levels of 5–7, a nighttime (2230 LT) westward intensification of the neutral wind approaching 400 ± 100 ms −1 occurred near the magnetic equator on 22 March 1979, accompanied by a 35% increase in neutral mass density. About 2 h after each of two substorm commencements associated with periods of southward IMF, ∼100γ and ∼200γ reductions in the daytime Huancayo H-component (corrected for ring current effects) are interpreted in terms of ∼0.5 and ∼1.0 mVm −1 westward perturbation electric fields, respectively. An intervening 2-hour period of northward IMF preceded a positive equatorial magnetic perturbation of about 200γ. Time scales for field variations are a few hours, suggesting that processes other than Alfven shielding are involved. Variations in f 0 F2 (∼ ± 1.0 MHz) over Huancayo are consistent with the inferred electric fields and magnetic variations. Similar equatorial perturbations are found through examination of other magnetic disturbances during 1979.

Magnetic activity dependence of high‐latitude thermospheric winds and densities below 200 km

Satellite‐based measurements are utilized to elucidate the latitudinal, local time, and magnetic activity dependence of winds and densities in the scantily observed atmospheric region between 170 and 220 km above 45° magnetic latitude. One data set consists of atmospheric densities from high‐accuracy (time resolution ≈ 5‐6 hours) orbital analyses of three Doppler Beacon satellites in orbit during 1973. The perigees of these satellites are generally restricted to 160‐180 km, 1200‐1400 LST, and geographic latitudes greater than about 45°. Statistical relationships are derived between the density changes and the planetary magnetic index, Kp, and the 5‐hour mean of the auroral electro jet index, . The former relationships are compared with those derived from the MSISE90 empirical model (Hedin, 1991), which is found to overestimate the rate of increase of density with respect to Kp. Another data set consists of densities and cross‐track winds from the Satellite Electrostatic Triaxial Accelerometer (SETA) experiment for the March 21 to April 9, 1979, period, which includes several intervals of elevated magnetic activity. Besides comparing various time series, the data are also binned according to 10° latitude increments and unit increments of Kp to derive trends. Some typical results include the following, corresponding to average changes in the 45° to 65° magnetic latitude band as Kp is increased from 1 to 6: (1) for the nightside (≈ 2230 LT), a change in cross‐track (nearly zonal) wind from 25 ± 25 m s−1 (eastward) to −125 ± 25 m s−1 (westward), and an increase of about 20% in density; and (2) for dayside (≈ 1030 LT), a change in cross‐track wind from 25 ± 25 m s−1 (eastward) to 125 ± 25 m s−1 (eastward), and a density increase of 25%. For some individual sudden enhancements in magnetic activity, changes in winds and densities can be more than double the above average values. Comparisons are also made with the NCAR TIGCM (National Center for Atmospheric Research thermosphere‐ionosphere general circulation model) simulation for the complete 20‐day interval, and with recent empirical models of densities (Hedin, 1991) and winds (Hedin et al., 1991), with data points in each case derived for the satellite paths, instrument orientations, and sampling rates identical to that of the SETA experiment.

Thermospheric tides simulated by the National Center for Atmospheric Research Thermosphere‐Ionosphere General Circulation Model at equinox

Simulations of the equinox thermospheric structure by the National Center for Atmospheric Research thermosphere‐ionosphere general circulation model (TIGCM) are presented. Calculations were made for both solar cycle minimum and solar cycle maximum under geomagnetically quiet conditions. Emphasis here is on the tidal structures at geographic longitude 70°W. Comparisons are made with observations and empirical model predictions of the thermospheric tides from the extended mass spectrometer and incoherent scatter radar model (MSIS) (Hedin, 1991) and the harmonic wind model (HWM) (Hedin et al., 1991). In general, the TIGCM tidal simulations are broadly consistent with available data. Agreement is best for the neutral temperatures and worst for the zonal winds. Investigation of the latitudinal structure of the tidal meridional velocities and temperatures near 300 km reveals important differences between the TIGCM and the empirical MSIS and HWM models. For example, the TIGCM predicts nearly a threefold increase in the diurnal temperature amplitude from solar cycle minimum to maximum with little latitudinal variation in the amplitudes from the equator to about 50°. The empirical MSIS model predicts a somewhat smaller variation of the the diurnal amplitudes over the solar cycle with the amplitudes decreasing with latitude. Resolution of the discrepancies requires acquisition of simultaneous observations from widely separated locations. Ideally, observations would be made in both the lower and upper thermosphere in order to help determine the contribution of the tides excited in the lower atmosphere to the thermospheric structure.

Mars mesosphere and thermosphere coupling: Semidiurnal tides

The National Center for Atmospheric Research thermosphere ionosphere general circulation model for Earth has been modified to examine the three‐dimensional structure and circulation of the upper atmosphere of Mars (MTGCM) (Bougher et al., 1988b, 1990). Recent examination of Mariner 9 UVS airglow measurements taken during a global dust storm provide evidence of large temperature variations and atomic oxygen distributions uncorrelated with solar activity and the corresponding MTGCM in situ driven winds. We suspect significant forcing of the thermosphere from below as a result of upward propagating gravity waves or tides generated by solar heating of airborne dust (Stewart et al., 1992). The effects of upward propagating tides are introduced into the MTGCM by appropriately specifying its lower boundary condition according to classical tidal theory. We initially adapt the terrestrial scheme used by Fesen et al. (1986) to a Mars model appropriate to Mariner 9 near‐solar‐minimum conditions. Estimates of the amplitude and phase of the likely dominant semidiurnal (2,2) mode at the mesopause (∼100 km) are specified for a range of possible lower atmosphere dust conditions. MTGCM simulations contrasting tidally driven fields with solar‐only forced ones show a dramatic change in the horizontal and vertical wind patterns, whereby the global temperature and oxygen distributions are also modified significantly. This semidiurnal component predominates below 135 km, while the in situ solar‐driven diurnal component is largely dominant above. Constructive interference serves to enhance midafternoon exospheric temperatures toward Mariner 9 observed values. The thermospheric response and the altitude of penetration of these semidiurnal tides is found to be much greater during solar minimum periods when dissipation due to viscosity and thermal conductivity is diminished from that at solar maximum. Finally, the Martian response during dusty periods is predicted to be much larger than that typically observed for Earth. Martian dust‐driven tides, especially during solar minimum time periods, cannot be ignored when addressing the Mars thermospheric structure and dynamics.

Acceleration, heating, and compositional mixing of the thermosphere due to upward propagating tides

The National Center for Atmospheric Research thermosphere‐ionosphere general circulation model (TIGCM) is utilized to evaluate the role of upward propagating diurnal and semidiurnal tides in determining the zonal mean states of the thermosphere and ionosphere above 100 km. Differences between various zonal mean fields from TIGCM simulations are examined with and without tidal forcing at the lower boundary. The following effects are observed: (1) A westward jet of the order of 10–30 m s−1 in the equatorial lower thermosphere and weaker eastward flanking mid‐latitude jets are induced by eddy and molecular dissipation of the tidal motions. (2) Salient characteristics of the equatorial jet are independent of tidal phase. (3) Increases in the zonal mean N2 and O2 number densities of the order of 10–30% are produced above 110 km and between ± 30° latitude, mostly by virtue of the net heating due to tidal dissipation; these are accompanied by electron density depletions of the order of 10–15% above about 160 km due to the enhanced chemical loss. (4) A low‐latitude (< 30°) depletion (∼ 30–50%) of atomic oxygen occurs due to the increase in effective recombination rate within a tidally displaced air parcel. The latter may explain the low‐latitude depression in the OI (5577 Å) airglow intensity from satellite‐based observations. In addition, variations in the zonal mean densities of NO, N(4S), and N(2D) of the order of 30–60% are induced by reactions with the tidally modified distributions of O, O2, and temperature. The occurrence frequency of these effects is influenced by the amplitude of diurnal propagating tide assumed at the lower boundary of the TIGCM, which according to radar observations is exceeded about 20–40% of the time, depending on season.

Interactive ionosphere modeling: A comparison between TIGCM and ionosonde data

Results from a time dependent geomagnetic storm simulation of the coupled thermosphere and ionosphere using the new interactive thermosphere‐ionosphere general circulation model (TIGCM) (Roble et al., 1988) of the National Center for Atmospheric Research are compared with F2‐ layer data obtained from a latitudinal chain of East Asian ionosonde stations situated close to the −165° magnetic meridian and separated by about 5° in magnetic latitude. This is among the first extended comparisons (10 days) between the TIGCM modeled ionosphere and data, where the effects of neutral dynamics on the ionosphere are studied using a global, fully interactive thermosphere‐ionosphere model. The ionosonde stations provide latitudinal coverage that extends from 15° to 50° magnetic north. Hourly values from both the simulation results and ionosonde data for hmF2, ƒoF2, and meridional neutral winds, for the period March 19–28, 1979, are fitted in latitude using Legendre polynomials, and variations from quiet‐time values are displayed in latitude‐UT coordinates. Color graphics for both the simulation and data are used to illustrate the equatorward penetration of ionospheric disturbances and their dependence on Kp, storm time, and local time. Observed effects are interpreted in terms of plausible electric field, neutral wind, and neutral composition changes during the storm period and where possible are related to TIGCM predictions. It is shown that the TIGCM ionosphere responds very well to varying geomagnetic conditions. The agreement between data and model for critical frequency ƒoF2, is very good at 45° magnetic north, but the agreement decreases equatorward. The agreement for critical height hmF2, however, is good through the whole region of comparison. A test of the general procedure used to derive winds from ionosonde data is also presented. Winds derived from the model's ionosphere hmF2 compare well with winds computed by the TIGCM. This agreement shows the strong coupling between meridional neutral winds and critical height at mid‐latitudes and gives some validity to the methods of computing disturbance winds from variations in F2 layer height.

Geomagnetic activity effects on the equatorial neutral thermosphere

The effects of geomagnetic activity on the equatorial neutral thermosphere are investigated with mass spectrometer measurements from the Atmosphere Explorer E (AE‐E) satellite and simulations generated by the National Center for Atmospheric Research thermosphere/ionosphere general circulation model (TIGCM). A study of the local time dependence of the equatorial geomagnetic storm response concentrates on a disturbed period from March 20 (day 79) to March 31 (day 90), 1979. This interval was the subject of an intense data‐gathering and analysis campaign for the Coordinated Data Analysis Workshop 6, and global TIGCM predictions are available for the specific conditions of the storm as a function of universal time. The AE‐E measurements demonstrate that significant geomagnetic storm‐induced perturbations of upper thermospheric N2 and O densities extend into the equatorial zone but are mainly restricted to the midnight/early morning sector. The qualitative features of the observations are reproduced by the TIGCM, although in general, the model simulations overestimate the storm temperature and density enhancements, primarily in the nighttime thermosphere. This suggests that either the nighttime cooling rates in the TIGCM are too small or that the specified auroral forcings of the model are too persistent.

The polar lower thermosphere

The Earth's mesosphere and lower thermosphere are the least explored regions of the Earth's atmosphere. The observations that have been made in this region, however, indicate that it is a dynamically active region, especially the polar lower thermosphere where most of the auroral energy that impacts the Earth's atmosphere is deposited. This energy is redistributed globally by the thermospheric wind system and there are important dynamic, chemical, radiational, and electrodynamic couplings that occur between the lower thermosphere and the middle atmosphere. There are also couplings with the upper thermosphere, ionosphere, and magnetosphere. A brief review of the available observations important for understanding global dynamic processes in the lower thermosphere is given. Results of simulations made with the NCAR thermosphere/ionosphere general circulation model (TIGCM) are presented to illustrate interactions of polar lower thermospheric chemistry and dynamics. The calculations show that major neutral gas constituents' (O 2, N 2, and O) number densities in the lower thermosphere are influenced primarily by dynamics, whereas the minor neutral constituents of the odd nitrogen system are influenced by both dynamics and chemistry. Furthermore, both major and minor constituent number density changes, as well as temperature changes, have an important influence on the electron number density distributions in the thermosphere. Results of time-dependent simulations with the TIGCM also suggest that a significant amount of NO is generated in the lower thermosphere during disturbed geomagnetic conditions which could be transported to the middle atmosphere. It is important to consider both chemistry and dynamics in determining the transport of constituents between the mesosphere and thermosphere.