E. C. Ridley

A thermosphere/ionosphere general circulation model with coupled electrodynamics

A new simulation model of upper atmospheric dynamics is presented that includes self-consistent electrodynamic interactions between the thermosphere and ionosphere. This model, which we call the National Center for Atmospheric Research thermosphere-ionosphere-electrodynamic general circulation model (NCAR/TIE-GCM), calculates the dynamo effects of thermospheric winds, and uses the resultant electric fields and currents in calculating the neutral and plasma dynamics. A realistic geomagnetic field geometry is used. Sample simulations for solar maximum equinox conditions illustrate two previously predicted effects of the feedback. Near the magnetic equator, the afternoon uplift of the ionosphere by an eastward electric field reduces ion drag on the neutral wind, so that relatively strong eastward winds can occur in the evening. In addition, a vertical electric field is generated by the low-latitude wind, which produces east-west plasma drifts in the same direction as the wind, further reducing the ion drag and resulting in stronger zonal winds.

Thermospheric General Circulation with Coupled Dynamics and Composition

Abstract A general circulation model has been developed for the atmosphere above 97 km. It uses a 5° latitude × 5° longitude grid and 24 vertical levels in increments of 0.5 scale height. The prognostic variables are horizontal winds, temperature, and the mass mixing ratios of atomic and molecular oxygen, which are obtained using hydrodynamic equations and which include vertical transport by realistic models of molecular diffusion. All the prognostic variables are in near diffusive equilibrium in the vertical as the top of the model is approached. Realistic ion drag is included in the model equations for horizontal winds, including the rapid polar drifts of magnetic field fines due to magnetospheric convection. Excellent agreement is achieved between the calculated and observed global averaged composition, provided a reasonable amount of vertical eddy mixing is included in the compositional equations over the lowest model scale height. Calculations are carried out for solar minimum equinox conditions. The...

Meridional Circulation in the Thermosphere I. Equinox Conditions

Abstract The mean meridional circulation and latitudinal variation of temperature in the thermosphere are considered for equinox conditions. With regard to these parameters there have been serious discrepancies between observational indications and theoretical expectations. A numerical model of the zonally symmetric thermospheric circulation is formulated and solved using a finite-difference initial value approach to steady-state solutions. Solutions are obtained for three different prescriptions of forcing terms: solar heating alone, solar heating plus an effective momentum source due to diurnal variations, and inclusion of a high-latitude heat source representing Joule dissipation of electric current systems. It is concluded that the Joule heating is essential for bringing theoretical predictions into agreement with observations but that the global mean of the required heating during geomagnetically quiet periods is necessarily small compared to global mean solar heating at the same levels.

Venus mesosphere and thermosphere temperature structure. II. Day-night variations

Abstract A two-dimensional nonlinear hydrodynamic model has been developed for studying the global scale winds, temperature, and compositional structure of the mesosphere and thermosphere of Venus. The model is driven by absorption of solar radiation. Ultraviolet radiation produces both heating and photodissociation. Infrared solar heating and thermal cooling are also included with an accurate NLTE treatment. The most crucial uncertainty in determining the solar drive is the efficiency by which λ A solar radiation is converted to heat. This question was analyzed in Part I, where it was concluded that essentially all hot atom and O( 1 D) energy may be transferred to vibrational-rotational energy of CO 2 molecules. If this is so, the minimum possible euv heating occurs and is determined by the quenching of the resulting excess rotational energy. The hydrodynamic model is integrated with this minimum heating and neglecting any small-scale vertical eddy mixing. The results are compared with predictions of another model with the same physics except that it assumes that 30% of λ A radiation goes into heat and that the heating from longer-wavelength radiation includes the O( 1 D) energy. For the low-efficiency model, exospheric temperatures are ≅300°K on the dayside and drop to

The high latitude circulation and temperature structure of the thermosphere near solstice

Abstract The neutral gas temperature and circulation of the thermosphere are calculated for December solstice conditions near solar cycle maximum using NCARs thermospheric general circulation model (TGCM). High-latitude heat and momentum sources significantly alter the basic solar-driven circulation during solstice. At F -region heights, the increased ion density in the summer hemisphere results in a larger ion drag momentum source for the neutral gas than in the winter hemisphere. As a result there are larger wind velocities and a greater tendency for the neutral gas to follow the magnetospheric convection pattern in the summer hemisphere than in the winter hemisphere. There is about three times more Joule heating in the summer than the winter hemisphere for moderate levels of geomagnetic activity due to the greater electrical conductivity in the summer E -region ionosphere. The results of several TGCM runs are used to show that at F -region heights it is possible to linearly combine the solar-driven and high-latitude driven solutions to obtain the total temperature structure and circulation to within 10–20%. In the lower thermosphere, however, non-linear terms cause significant departures and a linear superposition of fields is not valid. The F -region winds at high latitudes calculated by the TGCM are also compared to the meridional wind derived from measurements by the Fabry-Perot Interferometer (FPI) and the zonal wind derived from measurements by the Wind and Temperature Spectrometer (WATS) instruments onboard the Dynamics Explorer ( DE −2) satellite for a summer and a winter day. For both examples, the observed and modeled wind patterns are in qualitative agreement, indicating a dominant control of high latitude winds by ion drag. The magnitude of the calculated winds (400–500 m s −1 ) for the assumed 60 kV cross-tail potential, however, is smaller than that of the measured winds (500–800 m s −1 ). This suggests the need for an increased ion drag momentum source in the model calculations due to enhanced electron densities, higher ion drift velocities, or some combination that needs to be further denned from the DE −2 satellite measurements.

Venus mesosphere and thermosphere: III. Three-dimensional general circulation with coupled dynamics and composition

Abstract The National Center for Atmospheric Research (NCAR) thermospheric general circulation model (TGCM) for the Earths thermosphere has been modified to examine the three-dimensional (3D) structure and circulation of the upper mesosphere and thermosphere of Venus (VTGCM). This model used the parameterizations from our earlier two-dimensional (2D) Venus model, including eddy diffusion and wave-drag parameterizations, and the 15-μm cooling scheme from Bougher et al . (S W. Bougher, R. E. Dickinson, E. C. Ridley, R. G. Roble, A. F. Nagy, and T.E. Cravens 1986, Icarus 68, 284–312). Processes unique to the Earths thermosphere (ion drag, magnetospheric convection, etc.) are removed, but the TGCM computational framework is retained. A symmetric version of the VTGCM is used first to simulate the mean subsolar-to-antisolar variation of observed composition and temperatures, as determined from Pioneer Venus data and subsequent emperical models. The VTGCM equatorial fields are shown to be largely consistent with previous symmetric 2D model fields of Bougher et. al . (S. W. Bougher, R. E. Dickinson, E. C. Ridley, R. G. Roble, A. F. Nagy, and T. E. Cravens 1986, Icarus 68, 284–312). The VTGCM is then used to examine the 3D character of the Venus asymmetric circulation and structure. A prescribed Venus retrograde zonal wind profile has been superimposed on the mean subsolar-to-antisolar circulation, and results show that the major (heavy) species and temperatures are not greatly affected by this superrotation. A shift in the exospheric temperature minimum to LT = 2 AM occurs, which is consistent with the convergence of the horizontal winds after midnight. Many of the observed features of the Venus thermosphere can be reproduced by the 3D VTGCM. Calculated terminator winds reach 230 m sec − , with exospheric temperatures ranging from 309 (day) to 136°K (night). Model atomic oxygen concentrations show little diurnal variation along constant pressure surfaces. Prescribed wave drag is primarily responsible for this weak global circulation, which is consistent with the observed day-night contrast in calculated densities and temperatures. Furthermore, the incorporation of a retrograde zonal momentum source helps to simulate observed asymmetries in Venus thermospheric fields. The present VTGCM thus serves as a useful benchmark upon which to incorporate additional minor constituents and test new self-consistent parameterizations for wave drag and superrotation.

Thermospheric tides at equinox: Simulations with coupled composition and auroral forcings: 2. Semidiurnal component

Thermospheric tidal components calculated by the National Center for Atmospheric Research thermospheric general circulation model have been revised by including the effects of self-consistent composition couplings and auroral processes. Calculations are presented for equinox conditions at solar cycle minimum and maximum. The diurnal neutral winds and perturbation temperatures predicted by the model are compared with radar observations and the MSIS-86 model at low, mid, and high latitudes. Sensitivity tests indicate that the feedback between composition and dynamics affects the diurnal tidal temperature amplitudes up to 50% and the diurnal winds by about 15%. The calculated phases are within an hour of uncoupled composition calculations. Auroral processes dominate the diurnal waves at high latitudes, where the wind amplitudes may be double or triple those calculated for solar forcing only. The high-latitude energy and momentum sources must be included in the model formulations in order to reproduce observations of the September 1984 Equinox Transition Study exospheric temperatures.

Venus Mesosphere and Thermosphere II. Global Circulation, Temperature, and Density Variations

Recent Pioneer Venus observations have prompted a return to comprehensive hydrodynamical modeling of the thermosphere of Venus. Our approach has been to reexamine the circulation and structure of the thermosphere using the framework of the R. E. Dickinson and E. C. Ridley (1977, Icarus 30, 163-178), symmetric two-dimensional model. Sensitivity tests were conducted to see how large-scale winds, eddy diffusion and conduction, and strong 15-/xm cooling affect day-night contrasts of densities and temperatures. The calculated densities and temperatures are compared to symmetric empirical model fields constructed from the Pioneer Venus data base. We find that the observed day-to-night variation of composition and temperatures can be derived largely by a wavedrag parameterization that gives a circulation system weaker than predicted prior to Pioneer Venus. The calculated mesospheric winds are consistent with Earth-based observations near 115 km. Our studies also suggest that eddy diffusion is only a minor contributor to the maintenance of observed day and nightside densities, and that eddy coefficients are smaller than values used by previous one-dimensional composition models. The mixing that occurs in the Venus thermosphere results from small-scale and large-scale motions. Strong COz 15-/xm cooling buffers solar perturbations such that the response by the general circulation to solar cycle variation is relatively weak.

Meridional Circulation in the Thermosphere. II. Solstice Conditions

Abstract The mean meridional circulation and latitudinal variation of temperature in the thermosphere are calculated for solstice conditions. The heat and momentum sources that drive the thermospheric circulation are solar EUV and UV heating, high-latitude heating due to auroral processes, and a momentum source due to the correlation of diurnal variations of wind and ion drag. The results show a solar-driven, summer-to-winter circulation that is modified by the high-latitude heat source. The high-latitude heat source reinforces the summer-to-winter circulation in the summer hemisphere, but reverses the circulation in the mid-latitude winter hemisphere at F-layer heights with transition from one cell to another in the midlatitude winter hemisphere. Below about 150 km, however, the summer-to-winter circulation is maintained at all latitudes. The zonal winds at midlatitudes are generally eastward in the winter hemisphere and westward in the summer hemisphere. At F-layer heights, there is a significant temper...

A Numerical Model for the Dynamics and Composition of the Venusian Thermosphere

Abstract The structure, composition and winds of the mesosphere and thermosphere of Venus are investigated using a nonlinear time-dependent hydrodynamic model. The assumption that all variables depend only on altitude and distance from subsolar point allows a two-dimensional formulation of the problem. Within this framework the model provides an entirely self-consistent treatment of the multi-component fluid. The system solved consists of four time-dependent equations for motion, temperature, and the distributions of O and CO, and two diagnostic equations representing continuity and hydrostatic balance for the total fluid. The model is forced by absorption of solar radiation which provides heating of molecules and dissociation of CO2 into CO and O. A large-scale circulation is calculated, the gross features of which-resemble those derived in an earlier simplified model, consisting of a single cell with rising motion on the dayside, sinking motion on the nightside, and a day-to-night horizontal flow. This ...