The polar lower thermosphere

Place. Space Sci., Vol. 40, No. l/3, pp. 271497, 1992 Printed in Great Britain.

THE POLAR

0

0032-0633/92 S5.00+0.00 1992 Pergamon Real plc

LOWER THERMOSPHERE R. G. ROBLR

High Altitude Observatory, National Center for Atmospheric Research,* Box 3000, Boulder, CO 80307, U.S.A. (Received 17 September 1991) Abstract-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 dynamkahy 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 thermosphereand 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 proceasesin the lower thermosphere is given. Results of simulations made with the NCAR thermosphere/ionosphere general circulation model (TIGCM) are presented to ilhtstrate interactions of polar lower thermoaphericchemistry and dynamics. The calculations show that major neutral gas constituents’ (02, N,, and 0) number densities in the lower thermosphere are influenced primarily by dynamics, whereas the minor neutral constituents of the odd nitrogen system are inthrenced by both dynamics and chemistry. Furthermore, both major and minor constituent number density changes, as well as temperature changes, have an important inthrencc 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.

1. INTRODUCIION The aeronomy of the Earth’s polar regions has received considerable attention during the past decade. With increases in trace gases released from human activity, there is concern that increased greenhouse warming could cause the polar troposphere to warm rapidly, with the possibility of melting the polar ice caps. Arctic pollution and haze in the polar troposphere are also environmental problems of concern. In the stratosphere, the discovery of the Antarctic ozone hole has caused a considerable increase in aeronomic polar research in an attempt to identify the cause of the hole and to evaluate its potential impact on the biosphere. In the mesosphere, observers have detected a secular trend in mesospheric temperatures (Clancy and Rusch, 1989; Aiken et al., 1991; Hauchecome et al., 1991) and in the occurrence frequency of noticulent clouds (Gadsden, 1990). Suggestions have been made that perhaps these perturbations may be due to enhanced methane (Thomas et al., 1989) and carbon dioxide concentrations (Roble and Dickinson,

l The National Center for Atmospheric Research is sponsored by the National Science Foundation.

1989). In the upper thermosphere, there has been considerable research on the dynamics of the coupled thermosphere and ionosphere system spurred on by data from the NASA Dynamics Explorer satellite and from a variety of coordinated ground-based campaigns such as CEDAR and WITS. Our understanding of the dynamics of the upper thermosphere has increased considerably as a result of these investigations. The lower thermosphere, between 90 and 200 km, has not been investigated to the extent that other polar atmospheric regions have, yet it is an important region of our atmosphere. Most of the auroral energy impacting the Earth’s atmosphere is deposited in the polar lower thermosphere. Joule heating caused by the dissipation of intense auroral current systems provides an important heat source for the neutral atmosphere. Chemical species such as nitric oxide and atomic oxygen are generated by aurora1 processes and these species may be transported to the middle atmosphere where they can perturb the chemistry of that region. The ionospheric wind dynamo is driven primarily by thermospheric winds in the lower thermosphere and the electric fields that are generated map effectively upward where they can influence tbe ionospheric F-region structure and magnetosphere inter271

R. G.

212

actions, and downward where they can couple into the Earth’s global electric circuit (Roble, 1991). In this paper, a brief review of the available measurements and modeling studies of the polar lower thermosphere is made in an attempt to define the large-scale dynamics, temperature, and compositional structure of the region and its response to aurora1 inputs. Simulations of lower thermssphere dynamics are also made using the NCAR thermosphere/ ionosphere general circulation model (TIGCM) primarily to illustrate the dependence of the polar lower thermosphere structure and composition on aurora1 processes. The results show that variations in major neutral constituent densities of 0, OS, and Nr are caused primarily by dynamic processes. Variations in minor neutral constituents [N(2D), N(‘S), and NO], as well as lower thermosphere electron and ion densities, are controlledmainly by chemical processes driven by solar and aurora1 ionization and dissociation. It is shown that aurora1 production of NO during geomagnetic disturbances is highly variable and of sufficient magnitude to enhance the globally averaged reservoir of thermospheric odd nitrogen. The aurorally produced NO is long-lived in the thermosphere and is subject to a downward transport into the middle atmosphere where it can couple into middle atmosphere chemistry. A flux of minor neutral constituents from the lower thermosphere has a potential for perturbing the chemistry of the middle atmosphere that has been described by Nicolet (1989). Brief reviews of upper and lower thermosphere dynamics are presented in the next two sections. In Section 4, the NCAR TIGCM used for the simulation of lower thermosphere dynamics is described and the results of steady-state diurnally reproducible simulations of lower thermosphere structure are presented in Section 5. Time-dependent TIGCM simulations are discussed in Section 6 and an overall summary of the derived results are presented in Section 7. 2. UPPER

POLAR THERMOSPHERE

There has been considerable research during the past decade in an attempt to define the basic structure and dynamics of the polar thermosphere above about 200 km and to determine its response to solar and aurora1 variability. This research was motivated in part by data obtained from the NASA Dynamics Explorer Satellite Program and by data from various coordinated ground-based campaigns such as CEDAR and WITS. The results obtained from these research efforts have been reviewed by a number of authors : Killeen (1987a) ; Killeen and Roble (1988) ; Fuller-Rowe11 et al. (1988) ; Kazimirovsky (1988) ; M.

ROBLE

H. Rees (1989) ; Smith et al. (1988) ; Hemandez and Killeen (1988); Crowley (1991); and D. Rees (1989). Only a brief review relevant to the interpretation of upper and lower thermosphere coupling interactions will be given here. It has become clear from the results of measurements and simulations with thermospheric general circulation models that the upper thermosphere structure and dynamics are controlled mainly by the absorption of solar e.u.v. radiation and by the energy and momentum inputs associated with aurora1 processes. During quiet geomagnetic conditions solar e.u.v. heating drives a global circulation that at Fregion heights flows basically from the dayside of the Earth to the nightside. At low and mid-latitudes the dynamics are controlled by an approximate balance between pressure and ion drag forces as discussed by Killeen and Roble (1984). The dynamics at F-region heights is also influenced, to a minor extent, by the dissipation of upward propagating tides from the middle atmosphere and by plasma mass and energy exchange between the ionosphere and plasmasphere. At high latitudes the basic thermospheric structure and dynamics have been shown to be strongly influenced by the auroral heat and momentum sources (Hays et al., 1984 ; Crowley et al., 1989a, b ; Fuller-Rowe11 et al., 1987, 1988 ; Hagan, 1988 ; Hemandez et al., 1990; Killeen et al., 1988 ; Killeen and Roble, 1984, 1986, 1988; Larsen and Mikkelsen, 1983 ; Mayr et al., 1988 ; McCormac et al., 1987,1988 ; Meriwether et al., 1984, 1988 ; Ponthieu et al., 1988; Rees and Fuller-Rowell, 1987, 1988a; Roble et al., 1987a, c, 1988; Sica et al., 1989). The two-cell pattern of ionospheric convection, driven by the interaction of the solar wind with the Earth’s magnetic field, imparts a momentum source to the neutral gas that in the upper thermosphere is sufficiently strong to drive the neutral wind in a similar two-cell circulation pattern that follows but lags the pattern of ion convection. In the upper thermosphere, the ion drag momentum source dominates the Joule heat source and therefore the irrotational winds essentially follow the ion drift pattern (Mayr and Harris, 1978 ; Fuller-Rowe11 and Rees, 1980 ; Roble et al., 1982 ; Mikkelsen and Larsen, 1983 ; Killeen and Roble, 1984; Larsen and Mikkelsen, 1983, 1987; Clark et al., 1988 ; Fuller-Rowell, 1990). Results determined from measurements made by the Dynamics Explorer satellite have shown that the neutral wind speed in the polar cap increases with geomagnetic activity and the magnitude cross-polar cap potential drop of the ionospheric convection pattern. Typical geomagnetic quiet wind speeds are 200-300 m s-’ increasing to 800-1000 m s-’ during intense geomagnetic activity (McCormac et al., 1987). Anti-

The polar lower thermosphere

213

sunward wind speeds in the dusk and dawn aurora1 solar and geomagnetic activity. At a given constant regions vary from approximately 300 to 800 m s-’ pressure surface, the mass mixing ratios of O2 and Nz as the aurora1 Kp index increases from 0 to 7. As increase and 0 decmases as geomagnetic activity in~ma~etic activity incrcascs, sunward~ntisunw~d creases. The wm~tion~ properties of the upper therreversal boundaries move to lower geomagnetic latimospherc in the polar region am well described in the patudes and the geomagnetic width of the antisunward pers of Hedin and Carignan (1985) and Hedin (1987). Ground-based neutral wind and temperature polar cap neutral wind flow increases from about 25” to 45’. McConnac et al. (1987) present analytic for- measurements in the high latitude polar upper thermosphere have been reviewed by Smith et al. (1988) mulae that relate the neutral gas wind speed in various and by Hemandex and Killeen (1988). high latitude regions and the position of sunward~tis~ward boundaries with the auroral AE and K,, indices. In addition, McConnac et al. (1985, 1987), Thayer er al. (1987), Meriwether and Singh (1987), 3. LOWER POLAR THERMOSPHERE Rees et al. (1988), Sica et 01. (1989), Stewart et al. The lower thermosphere is a transitional region (1988), and Hemandez et al. (1991) have shown that lying between the geostrophic dynamics of the lower the polar neutral circulation responds strongly to atmosphere and in the ion drag and viscous controlled changes in ionospheric convection associated with dynamics of the upper thermosphere. It is also a variations in the B,, component of the interplanetary region where a transition between eddy and molecular magnetic field (IMF). In the Northern Hemisphere, when the BYcomponent is positive the evening cell is transport processes occur. Perhaps the most distinguishing large-scale feature of this transition region enhanced relative to the morning cell and when the is the continuous presence of atmospheric tides and BYcomponent is negative the opposite occurs. Varitheir day-to-day va~ability. Upward propa~ting ations in the neutral wind pattern mirror similar varidiurnal and semidiurnal tides clearly dominate the ations in the ion drift pattern. These results indicate dynamics of the low and mid-latitude lower thermothat the ion drag momentum source in the upper sphere. There have been many single station studies thermosphere is the dominant force acting on the of lower thermospheric tides but only recently have neutral winds. Ponthieu et al. (1988) have shown that there been attempts to derive global tidal structure the neutral winds do not follow the instantaneous ion from coordinated measu~ent campaigns, such as drift pattern since the time constant for momentum during the CEDAR Lower Thermosphere Coupling transfer from the ions to the neutrals ranges from tens Study (LTCS). The results from the initial study have to hundreds of minutes depending on the local ion been reported in a special section of the February density. Only during periods of steady ion drag forc1991 Journal Geophysical Research. Reviews of the ings do neutral winds and ion convection have similar circulation patterns. Even during periods when the B,, dynamics of the mid- and low latitude lower thercomponent of the XMF is northward, Killeen er al. mosphere have also been given by Forbes and Groves (1987), Forbes (1982a, b, 1987), and Crowley (1991). (1985) have shown that the neutral winds follow the The dynamics, temperature, and compositional complex ion drift pattern with sunward flow in the structure of the polar lower thermosphere, however, middle of the polar cap. Empirical models of F-region neutral winds have been constructed by Killcen er al. is not well defined. Rocket vapor trail measurements have provided much of the wind data for the polar (1987b), Batten et al. (1987) and Hedin et al. (1988). lower thermosphere (Heppner and Miller, 1982). In addition to the neutral winds, the basic temThese m~suremen~, however, give only instanperature and wm~sitio~l structure of the polar taneous vertical profiles of winds in a local region upper thermosphere have been studied in great detail although multiple rocket vapor trails have been used and the many measurements that have been made to construct regional patterns (Mikkelsen er al., 1987 ; during the past two decades have been incorporated Larsen et al., 1989). The wind measurements show in a progression of empirical models such as the Mass complex vertical wind variations that indicate a tranSpectrometer/Incoherent Scatter (MSIS-83, MSIS-86 and MSIS-~) of Hedin (1983,1987,1991) as well as sition between the tidal-to-ion drag dominance of the wind fields. Furthermore, the measurements suggest others. The polar upper thermosphere shows cona very wmplex response of these winds to auroral siderable temperature and composition variation with forcings. solar and geomagnetic activity that is well described in Another source of information on the dynamics of the MSIS-90 model. In general, the neutral gas temthe polar lower thermosphere are measurements from perature, 0, 4, and Nz number densities increase at high latitude incoherent scatter radars. Johnson et al. any given altitude in the thermosphere with increasing

214

R. G.

(1987), Johnson (1991), Johnson andvirdi (1991) and Mikkelsen et al. (1987) have presented data from the Chatanika, Alaska and Sondrestrom, Greenland, incoherent scatter radars that clearly illustrate that the lower thermospheric winds respond to aurora1 forcings down to about 90-95 km. Kuntake and Schlegel (1991) have summarized 5 years of EISCAT radar data that clearly identify both tidaveatures and wind responses to geomagnetic activity-in the polar lower thermosphere between 100 and 120 km. Mazaudier et al. (1987) have also presented radar observations that show a wind response to aurora1 activity in the midlatitude thermosphere. Avery et al. (1989) have also discussed tidal data in the high latitude mesosphere and lower thermosphere. The above data all indicate aurora1 perturbations to the tidal wind system in thegolar lower thermosphere, but it is difficult from ffie available observations to construct a large-scale pattern. The numerical studies by Fesen et al. (1991a, b, c) show that the tidal structure in the NCAR TIGCM can be perturbed by aurora1 forcings. Mikkelsen and Larsen (199 1) and Walterscheid et al. (1986) also present calculations showing an influence of aurora1 processes on atmospheric tides. Killeen (pers. comm., 1991) has used 6 months of atomic oxygen green line wind measurements made by the Fabry-Perot interferometer on the Dynamics Explorer spacecraft to derive a wind pattern that is highly suggestive of an ion drag controlled circulation in the polar lower thermosphere. There have been numerous temperature and wind determinations in the lower thermosphere from spectroscopic measurements and green line doppler profile measurements made primarily in the aurora1 zone of the lower thermosphere (e.g. Rodger and Stewart, 1990; Wiens et al., 1988), but it has been difficult to use these measurements to derive a large-scale pattern because of uncertainties in the airglow emission heights. Nevertheless, the measurements suggest a general increase in neutral gas temperature and winds with aurora1 activity. Even though there is considerable uncertainty about the atomic oxygen number density in the lower thermosphere, as discussed in the September 1988 special issue of Planetary and Space Science, there are recent measurements by Hecht et 92. (1991) that show that the atomic oxygen number density decreases considerably during geomagnetic storms. This depletion is accounted for in empirical models such as MSIS (Hedin, 1988), but the observations suggest a much greater depletion during large storms than the model predicts ; but again, there are insufficient observations to derive the large-scale structure of the oxygen depletion in the polar lower thermosphere during geo-

ROBLE

magnetic storms. Mayr et al. (1990) have performed calculations suggesting that atomic oxygen recombination is an important heat source for the upper mesosphere and it depends upon aurora1 activity. Nitric oxide in the lower thermosphere has been known to increase dramatically with aurora1 activity for a long time (Cravens and Stewart, 1978), but it is only recently that the detailed structure of the response has been measured over an extended period of time. Barth et al. (1988), Barth (1990), Siskind et al. (1989a, b), and Gerard et al. (1990) all present data that illustrate the enhancement of NO with both solar and aurora1 activity. During geomagnetic storms NO is enhanced at high latitudes and the equatorward extent of the NO density perturbation is related to geomagnetic activity. At times, during large storms, perturbations have been observed to extend in latitude to the equator. Kasting and Roble (1981), Gerard and Roble (1988), Gerard et al. (1990), Rees and Fuller Rowe11 (1988b), and Maeda et al. (1989) have all examined the zonally averaged structure of the lower thermosphere using two-dimensional models and their results indicate a substantial dynamic response of the lower thermosphere to aurora1 activity. Thermospheric general circulation model studies by FullerRowe11 and Rees (1981, 1984), Roble et al. (1982, 1987a), Killeen and Roble (1984), Rees and FullerRowe11 (1990), and Gundlach et al. (1988) all calculate a significant dynamic response of the lower thermosphere to aurora1 activity; but all of these model studies generally lack sufficient experimental data to substantiate the magnitude of the calculated lower thermosphere response. 4. THERMOSPHERE/IONOSPHERE CIRCULATION

GENERAL

MODEL

The NCAR thermosphere/ionosphere general circulation model (TIGCM) solves the primitive equations of dynamic meteorology but adapted to the physics appropriate to thermospheric heights. The dynamic equations of the thermospheric neutral gas have been described previously by Dickinson et al. (1981, 1984), Roble et al. (1982, 1988), and Killeen and Roble (1984). The inclusion of upward propagating tides from the middle atmosphere into the model has been described by Fesen er al. (1986,199 1a, b, c), and Forbes et al. (1987b, 1991). The empirical model of high latitude aurora1 inputs has been described by Roble er al. (1982) and Roble and Ridley (1987). Time-dependent inputs to the model have been described by Roble et al. (1987a) and Crowley et al. (1989a). The basic aeronomical processes and the

215

The polar lower thermosphere

of the coupled ionospheric model in the TIGCM have been described by Roble et al. (1987b, 1988). Briefly,, the TIGCM includes a ~lf~o~istent aeronomic scheme of the coupled thermosphere and ionosphere. The chemical reactions included in the model at present are listed in Tables 1 and 2. These reactions are only a subset of possible thermospheric and ionospheric chemical reactions ; however, they are the main ones that control the~osphe~c and ionospheric global structure. In future versions of the TIGCM the aeronomic processes will be expanded to include additional chemistry such as metastable species and various excited states. The model calculates global distributions of the neutral gas temperature, zonal, meridional and vertical winds, the major neutral gas ~o~tituen~ number densities of 4, 0 and Nz, and the minor neutral gas constituent number densities of N(2D), N(‘S), and NO. The interactive Eulerian model of the ionosphere solves for the global distributions of O+, 0:) N: , NO+, N+, and N,, It also solves for the global dist~butions of the electron and ion tern~~tu~s, T, and Z, respectively. Mutual couplings between the thermosphere and ionosphere oamr at each model time step and at each point of the geographical grid. The TIGCM has an effective 5” latitude by longitude horizontal grid with 25 constant pressure surfaces in the vertical extending approximately between 95 and 500 km in altitude with a vertical resolution of two grid points per scale height. The model time step is 5 min and it takes about 20 min of the NCAR CRAY details

Y;MP CPU time to simulate 1 day. At least five simulated days are required to achieve a diurnally reproducible solution of the coupled thermosphere and ionosphere system. The TIGCM uses the empirical model of Richmond et al. (1980) to specify the low and mid-latitude electric &lds and the Heelis et al. (1982) empirical model to specify the high latitude electric fields associated with ionospheric convection. Aurora1 particle inputs are specified using the Hemispheric Power Index derived from satellite measurements, and the empirical model of aurora1 particle precipitation as described by Fuller-Rowe11 and Evans (1987). The solar e.u.v. inputs are determined from the empirical solar flux model of Hinteregger (1981) that uses the daily Fr,,., and 81&ay average F,,,, as inputs. The e.u.v. and U.V. flux &de1 specifies the solar tlux in 59 wavelength bands and emission lines from 5 to 175 nm. These wave length intervals have been specified by Torr et al. (1979) for the solar e.u.v. and by Torr and Torr (1985) for the solar U.V. The amplitudes and phases of the upward propagating semidiumal tides from the 2-2 to the 2-6 tidal modes are obtained from the model of Forbes and Vial (1989, 1991) for the month under consideration and the amplitude and phase of the upward propagating diurnal tide is specified as described by Forbes et al. (1991). 5. TEMPERATURE, COMPOSFTIONAL, AND DYNAMIC S~U~URE

OF THE LOWER

~ERM~~~E

The NCAR TIGCM has been run for a number of different cases to illustrate the importance of high

TABLEI. Nxwra~~-NEWIKUCQ-REACTtONSANDRAlBStJSEDIN~MODEL Reaction rates and mfereuces Reactions N(‘S)+O181-N0+0+1.4

Rate co&icient eV

N(1D)+O~br-NO+O(1D)+1.84eV N(‘S)+NO~N~~O+Z.~

eV

References*

B, = 4.4 x lO”‘*‘exp (-3220/T,)

a

fi2 = 5.0 x lo-=

b

pa = 3.4 x lo-”

a

N~D)+O~N(‘S)+O+2.38

eV

/?* = I .o x lo- ‘3

b

NcD)+eB)-N(‘S)+e+2.38

eV

/I, = 3.6x 10-‘“,.(T,/300)‘”

b

N(ZD)+NO~N2+0+5.63

eV

j16= 7.0 x 10-I’

C

N(3D)bt.N(‘S)+hv

j?, = 1.06 x 10-J

d

NO+hv8’-N{‘S)+O

j?s = 4.5 x IO--$exp (- 10-a [N(OJlo=I

NO+hv},,_,~NO++c

*exp (-5.0 x lo-” [N(O,)])

e

o+o+M-Lo*+o

y , = 9.59 x 10-k exp (480/T,)

f

O+Oz+MAO,+M

y2 = 2.15 x lo-”

g g

exp (345/T,,) for M = 0 and 4 = 8.82 x 10a3’exp (575/T,,) for M = Nt

* Refcrcuces a-g for the rate cocfkients are given in Roblc ef al. (1987b).

+0+1.555

(10%) N(‘S) + N(‘!J) + 5.82 eV (90%) N(‘S) + N( 2D) + 3.44 eV

N: +e*

l

lo-”

X lo-”

300 K < r, < 1700 K

(T,+T,)/2

(T,/300)2, 1700 < T2 < 6000 K

(T,/300)‘,

300 d T, < 6000 K

(T,/300)*

a2 = 1.6 x IO-’ (300/T~)""s for Tc > 1200 K a2 = 2.7 x IO-’ (300/TJo7 for T. < 1200 K a, = 1.8 x lo-’ (T,/300)-“.m, where T,=0.667T+0.333Tn, T2= 0.636377,+0.363T,,., and TR =

lo-”

TR 2 1500 K

lo-”

1.483 x lo-”

(T,/300)+8.6x (T,/300)+ r, < 1500 K

4.2xlo-’ (300/Te)o's

(Z’R/3OO)o.2

(300/2.,)“.u

a, =

4.0 X lo-” 2.0X lo-”

4.4

1.2 X lo-”

5.2 x lo-”

1.4 X lo-”

lo-”

lo-”

KS =

K, =

Kg =

K5 =

K4 =

Kj =

K, =

KI = 2.73 x lo-“-1.155~

x lo-l6 (T,/300)‘,

x lo-”

Reaction rate 1O-‘2 (T,/300)+1.073

(T,/300)‘+9.65

-7.74x

x2 = 1.533 x lo-“-5.92~

-5.17x

K, = 2.82x lo-”

References a-h for the rate coefficients are given in Roble er al. (1987b).

NO+hvl+,~NO++e

(15%) 0(3P)+O(3P)+6.95 eV (85%)OCP) +O(‘D) +4.98eV

0: +eA

eV

(20%) N(‘S)+0+2.75 eV (80%) N(2D)+0+0.38 eV

+N+0.98

eV

NO+ +eL

N++O*O+

eV

0: + N(‘S) + 2.486 eV

N++02-%N0++0+6.6W

N+ + 4 fy

eV

0: +NOAN0++02+2.813

eV

eV

+N(2D)+0.70

eV

0: +N(‘S)ANO++0+4.21

Nt +OaNO+

O+(‘S)+N,*NO++N(‘S)+1.0888

0+(‘S)+02AO:

Reactions

Reaction rates and references

h

g

e f

d d

d

C

b

a

a

References*

E

zf

P

P

The polar lower thermosphere latitude aurora1 processes on the dynamics, thermal, and compositional structure of the polar thermosphere. Results are presented as maps of total fields for the case under consideration, as well as difference fields for a particular TIGCM run minus a control case. The results illustrate the polar lower thermosphere response to aurora1 inputs. The solar Fro., and Ii10.7 are 150x 10ez2 W mm2 Hz-’ for all simulations. 5.1. March equinox conditions 5.1.1. Solar radiative forcing only. This first TIGCM run, considering solar radiative forcing only, is a hypothetical case, because in reality there is an aurora always present in the polar regions. However, this TIGCM simulation does illustrate the underlying solardriven structure at high latitudes upon which the aurora1 perturbations are superimposed. The basic temperature structure, circulation and number densities of 4,0, and NO at 120 km for equinox conditions are shown in Fig. 1. The temperature and circulation pattern over the polar region shows a characteristic semidiurnal pattern indicating that the structure of the lower thermosphere is strongly influenced by the upward propagating semidiurnal tidal modes. The lowest temperatures occur near 0l:OO L.T. and the number densities of O2 and NO decrease toward the pole whereas the number density of 0 increases toward the pole. This simulation without aurora1 forcings indicates the basic solar radiativedriven circulation and structure of the lower thermosphere polar cap. X1.2. Solar and aurora1forcings. The aurora1 processes incorporated into the TIGCM have been described by Roble and Ridley (1987). For these simulations the empirical ion convection model of Heelis et al. (1982) is used considering a cross-polar cap potential drop of 70 kV. This pattern at OOSO U.T. over the Northerm Hemisphere polar cap is shown in Fig. 2a. The aurora particle input to the aurora1 oval is obtained from the Fuller-Rowe11 and Evans (1987) empirical model considering 16 GW of hemispherical power input. Using this level of particle input the calculated TIGCM electron density at 120 km is shown in Fig. 2b. The aurora1 oval electron densities are clearly enhanced over the solar background at OOzOO U.T. for this equinox simulation. When aurora1 particle precipitation of 16 GW and ionospheric convection with a cross-polar cap potential drop of 70 kV are included as forcings in the TIGCM simulation, the temperature, circulation, and 0, O2 and NO number densities at 120 km respond as shown in Fig. 3. These aurora1 inputs are for moderate levels of geomagnetic activity with K,, between 2 and

211

4. The resultant total fields are for steady, diurnally reproducible fields responding to steady aurora1 inputs at the indicated levels. At this level of geomagnetic activity, the lower thermosphere clearly responds to the aurora1 inputs. The circulation increases in magnitude and tends to follow, but generally lag, the two-cell pattern of ionospheric convection. The temperature pattern develops a dusk-todawn temperature drop across the magnetic polar cap. This pattern and its relationship to the ionospheric convection pattern in the NCAR TIGCM have been discussed previously by Roble et al. (1982) and Killeen and Roble (1984). It was shown that the lower thermosphere is a transition region between the tidal dynamics of the lower atmosphere and the ion drag controlled dynamics of the upper thermosphere. During quiet geomagnetic conditions, when ion drag in the lower thermosphere is small, the circulation, temperature, and compositional structure are primarily influenced by tidal dynamics similar to that shown in Fig. 1. However, during enhanced geomagnetic aurora1 particle precipitation greatly activity, increases the electron density and the combined effects of increased ion drag and enhanced electric fields produce a circulation pattern similar to the two-cell pattern that occurs in the upper thermosphere.. The twocell pattern of neutral circulation penetrates deeper into the lower thermosphere during enhanced geomagnetic activity and then retreats upward as geomagnetic activity subsides. Thus, the lower thermosphere experiences a variable momentum forcing, as well as variable particle and Joule heat sources, during geomagnetic activity. The neutral gas number densities also respond to geomagnetic activity as shown in Fig. 3. Enhanced Joule heating causes the entire atmosphere to expand upward, but along a constant pressure surface, the heavier molecular constituents, such as O2 and N2 increase their mixing ratio while the mixing ratio of 0 decreases. It has been shown by Hays et al. (1973) and Mayr and Volland (1973) that upward vertical motions from enhanced Joule and particle heating in the polar regions increase the molecular species and decrease the atomic species in the lower thermosphere. Bums et al. (1989a, b, 1991) have examined the various terms in the major species composition equation in the TIGCM and have shown that vertical motions are mainly responsible for the compositional changes in the lower thermosphere. In the upper thermosphere, advection and molecular diffusion contribute to the overall compositional structure, however in the lower thermosphere these terms are generally small. Nitric oxide calculated by the TIGCM during enhanced aurora1 activity is shown in Fig. 3d. In com-

R. G. ROBLE

278 mcM

NEUTRALTEMPERA-

(Dw; K)

TICCYOZNWBER

DlBUSTY x 1X10

{C&f-3)

b

TlCCM 0 NUMBERDEtWlY

t l.Elt

(CM-3)

12

b

Fio. 1. Cc&noms (a) Neutral gas temperature

18

OF TIGCM CALCUU?ED.

6) and neutral wind vectors (m s-l) ; (b) Oz number density (cnP) ; (c) 0 number density (cm-p; and (d) NO number density (cm-‘)3 at 0090 U.T. and 120 km for equinox conditions for the case of solar radiative forcings only. The outer circle perimeter is 42.YN latitude.

The polar lower thermosphere

LOCAL TIHE

'I'KXY ELECTRONDENSITY(CM-3) 12

0 LOCAL TIIIE

279

R. G.

280

ROBLE

~CXl4 NEUTRAL TEMPERATURE (DEG I()

(b)

‘W.XM

0 NUMBER DENSlTY x

l.Eii

(Cl&3)

TIGCMNO NUN3ER DENSITYx 1.E6 (CAM)

FIG. 3. CoNMuas OF TIGCM CALCULATED. (a) Neutral gas temperature 4) and neutral wind vectors (m s-l); (b) Oz number density @II-~); (c) 0 number density (~rn-~); and (d) NO number density (cmv3), at 120 km for equinox conditions at OOSOU.T. for the case of solar radiative and aurora1forcings.A he~~he~~ amoral particle input of 16 GW and a cross-polar cap potential drop of 70 kV is used in the simulation. The outer circle primeter is 42S”N latitude.

The polar lower thermosphere

paring with Fig. Id, it is seen that NO is greatly enhanced in the vicinity of the aurora1 oval by particle precipitation and to a lesser extent by the higher neutral temperatures that increase the temperature dependent reaction rate of N(‘S) + O2 -+ NO + 0. This reaction mainly controls the NO density in the thermosphere above about 150 km. The overall NO density over the entire polar cap is greatly enhanced by aurora1 processes, compared with the case of solar radiative forcing only, especially in the aurora1 oval. The TIGCM calculations are generally consistent with the measurements of lower thermospheric NO densities made by the Solar Mesosphere Explorer (SUE) spacecraft (Barth, 1990; Siskind et al., 1989b) that clearly show high latitude enhancements that are correlated with aurora1 activity. 5.2. December solstice conditions A similar set of TIGCM runs, as described above, were run for December solstice conditions and the results for solar radiative forcings only and for solar with aurora1 forcings are shown in Figs 4 and 5, respectively. For the case of solar radiative forcing only, the winter polar lower thermosphere displays a cold low pressure vortex with a circumpolar counterclockwise circulation centered on the pole. This circumpolar vortex is modulated by the semidiurnal tidal structure with minimum wind speeds occurring at 09:OO and 21:00 L.T. The maximum wind speed is around 80 m s-’ eastward. The number density patterns at 120 km are also centered on the pole. Minimum densities for O2 and NO and maximum densities for 0 occur at the North geographic pole as shown in Fig. 4. When aurora1 activity increases, particle precipitation is sufficiently strong to enhance the ion drag forcings in the lower thermosphere and give rise to the development of a two-cell circulation pattern in the neutral gas that follows, but lags by about 4 h, the two-cell pattern of ion convection, as shown in Fig. 5. The maximum wind speed of about 120 m s-’ occurs in the center of the convection pattern. A cold low pressure circulation develops near the center of the morning cell, that at this particular Universal Time is over the North geographic pole. The overall temperature of the polar cap increases by about 100 K from the case of solar radiative forcing only with maximum temperatures occurring in the throat of the neutral gas convection-driven circulation pattern. The O2 and NO number densities at 120 km increase and the 0 density decreases in response to aurora1 activity. The O2 density maximum and the 0 density minimum are centered on the North polar region at this Universal Time. The NO density, however, shows

281

a clear enhancement in the aurora1 oval and it is caused by intense particle precipitation dissociating Nb with the N subsequently reacting with O2 to prdduce NO. Thus, even during winter solstice conditions, the temperature+ circulation, and compositional structure of the polar lower thermosphere aear to be influenced by the high latitude aurora1 f&cings to such an extent that the basic cold lower thermospheric vortex, that forms during extremely quiet geomagnetic activity, is considerably altered.

5.3. Zonal mean response to aurora1 forcings To illustrate the global response of the entire thermqsphere to aurora1 forcings, difference fields of the z&al mean structure between two case studies are ct&tructed. The difference fields are between a TIGCM run with an aurora1 hemispheric power input of 16 GW and a cross-polar cap potential drop of 70 kV minus a TIGCM run with 5 GW and 30 kV for these same inputs, respectively. This represents the zonally averaged structure of the steady-state, diurnally reproducible, response between a prolonged period of moderate geomagnetic activity minus a prolonged period of quiet geomagnetic activity. The zonal mean neutral gas temperature difference field and difference fields for 02, N2, 0, NO, N(‘D), NCS) and N, at 00 : 00 U.T. are shown in Figs 6 and 7 for the equinox simulation. The neutral gas temperature difference, shown in Fig. 5a, is largest at high latitudes and becomes progressively smaller at the lower latitudes. Maximum temperature differences are 200 K in the upper thermosphere in both polar regions. The zonal mean is calculated at 00 : 00 U.T. and there is some hemispherical asymmetry at this Universal Time caused by the displaced geomagnetic and geographic poles. A true zonal average should also include a Universal Time average over a day, however the average. over Universal Time is neglected for simplicity. A Universal Time average will not change the results or conclusions of the paper. Significant temperature differences on the order of 25-50 K occur down to the lower thermosphere. At the lower boundary of the model, 95 km, the temperature differences are artificially set to zero and thus the model cannot determine the depth into the Earth’s atmosphere that the effects of aurora1 variability can penetrate. The percent zonal mean difference fields for 02, N2, and 0 for equinox solar minimum conditions are shdwn in Fig. 6b-d, respectively. The differences in major constituent composition produced by aurora1 processes are almost entirely driven by neutral dynamics instead of atmospheric chemistry. Even though aurora1 particle precipitation can dissociate O2 and

282

R. G. ROBLE TIGCM 02 NUMBERDENSllYz

nGcNNnrrrutTExPERATuRE(DEcK)

l.ElO(CM-3)

12

TIGCY ONUMBERDENSllYx l.Ell (CM-3)

TXGCNNO NUMBERDENSITYx

(d

FIG.4.SAME AS Ro. 1 EXCEPTPOR DECEMEBRSOLSTKXc!o~~rno~~.

l.E7(W-3)

The polar lower thermosphere

nccYNEmuTGYPWnTuRE(DECK)

TIGCNO3NUMBCRDBwnrx

283 lSlO(CM-3)

19

TJCa 0 MMBQiDE%IT'rr l.Etl(CM-~) 12

TICCMNO NUMBERDEN3lTYx i.GI(cY-3)

R. G.

284

nmt UT -

1W

-w

xv2 0.00

mm

ZbNAL NBAN

-40 -30

ROBE

ox-3) IX DUferencd

lmD~@EC)

30

60

00

k)

FIG.6. TONALLY AVERAGED TIGCM DIFFERENCE PELDS FOR TKE CASE OF MODERATE GEOMAGMETIC ACTIV~Y (16 GW, 70 kV) MINUS QUIET GMIMAGNETIC ACTIVITY (5 GW, 30 kVj POR EQUINOX SOLAR MEDIUM CONDITIONS o() ; (b), (C) AND (d) ARE PERCENT NUMBER DENSITE (FM 7 = 150); (a) NEUTRAL GAS TEMPERATURE DIFFERENCE PIELDS FOR o,, N2 AND 0, ilk?PEC!TIWLY.

N2, the amount dissociated is small relative to the background reservoir of these species. The amount of additional 0 produced by aurorai particle precipitation is also small compared with its background. Thus, the effects shown in Fig. 6b-d are caused by dynamics affecting the mass mixing ratios along constant pressure surfaces and not by the~osphe~c chemistry. In the lower thermosphere, upward motion produced by aurora1 heating, enhances the molecular constituents O2 and N,, and causes the 0 density to decrease. In the polar lower I thermosphere, 0 decreases on the order of #-SO% and N2 and 0, increase on the order of SO-loO%, respectively, near 150 km. In the upper thermosphere, the densities of

all species increase in response to the general expansion of the atmosphere in response to the aurora1 heat source. At low latitudes the densities of all three species increase at fixed altitude levels in response to enhanced temperatures caused by sinking motion that is driven by the high latitude aurora1 heat source. Atomic oxygen densities increase by about 10% at 150 km at low latitudes and O1 densities decrease. The above-described compositional changes occur in the TIGCM between two steady-state runs and represent equilib~um conditions. During periods of rapidly varying aurora1 forcings, the compositional response will also vary rapidly at any given location and time.

The polar lower thermosphere

FlG.7. ZONALLYAVERAGEDnGCMDIPWREN~~mELDSRTHE~OFIrIODERA'IEOI(OMAONETICACIlVITY (I6GW,70kV)~1~usot~~r GEOb(AONBnCACTMTY (5GW, 30 kV) POR-ox SOLAR MEDIUM CONDITIONS (Fto., = 150); (a), (h), (c) AND(d) ABEraacmrr mnraaa oausrrv ~nmimm FOR NO, N(%), None,

The calculated xonal mean difference fields for NO, N(‘S), N?D), and N, for the equinox conditions are shown in Fig. 7a4, respectively. Aurora1 particle precipitation dissociates N2 at high latitudes and the resulting N reacts with 0, to produce NO. As a result, the NO densities increase greatly at high latitudes and by changing compositional structure and temperature, even at mid-latitudes. Several hundred percent increases occur in the lower thermosphere from the pole down to 60” latitude. NO increases at low

-LY.

latitudes are relatively small. The increase in NO in the polar lower thermosphere depktes N(%) below about 250 km as shown in Fig. 7b. Since NO has a larger density than N(%) in the lower thermosphere, it destroys N(‘S) through the %annabilistic** chemical reaction NO+N(‘S) -+ N*+O. Above about 250 km, the N(‘S) number density increases at fixed altitudes due to the upward expansion of the atmosphere caused by auroral heating.

286

R.

G.

Figure 7c shows the increase in N(2D) caused by aurora1 activity and also by compositional and temperature changes associated with thermospheric dynamics. In the lower thermosphere, the large enhancements are caused primarily by particle precipitation but some enhancement occurs because of 0 depletion and the consequent reduction of N(2D) quenching by 0. The zonal mean difference field of electron density, shown in Fig. 7d, indicates enhancements from particle precipitation in the lower thermosphere, depletions in the upper thermosphere from enhanced recombination caused by increases in the N2 and 0, number densities, and also plasma horizontal transport due to enhanced ion drifts. Enhancements in electron densities at low latitudes are caused by an increase in the solar production from enhanced 02, 0, and N2 number densities at fixed altitudes. Thus, the changes in the global distribution of electron density from dynamic and chemical effects are quite complex. In these calculations the effects of altered magnetosphere/ionosphere plasma heat and mass exchange are not included. There should, thus, be additional electron density variation superimposed upon these dynamic and chemical perturbations caused by magnetosphere/ionosphere plasma exchange processes primarily in the upper ionosphere. A similar set of difference fields for December solstice conditions are presented in Figs 8 and 9. The zonal mean neutral gas temperature increase, shown in Fig. 8a, is about the same in both hemispheres except that the area of enhancement is much more extensive in the summer hemisphere. In the winter hemisphere, the enhanced temperatures are confined to the vicinity of the polar cap. This is consistent with the mean meridional circulation having a basic summer-to-winter hemisphere flow with aurora1 effects superimposed upon this flow as discussed by Roble et al. (1977). Minimum temperatures occur at about 35” latitude in the winter hemisphere. The temperature enhancements in the lower thermosphere, near 150 km, are greater in the winter hemisphere. Since significant solar heating is present in the summer polar cap region, aurora1 effects only add to the intense solar heating. In the winter hemisphere, in the absence of sunlight, aurora1 heating has a much more pronounced influence on the total heating and causes a larger dynamic response. The zonal mean percent difference fields for 02, N2, and 0 are shown in Fig. 8b, c and d, respectively. The major compositional effects are also confined to the vicinity of the winter polar cap in the Northern Hemisphere, whereas in the summer Southern Hemisphere the effects are distributed over a greater latitudinal

ROBLE

extent. In the polar lower thermosphere, 0 is depleted and O2 and N, are enhanced. Minimum compositional variations occur in the winter hemisphere near 40” latitude. This is the region where the mean mass flow stream function of the two hemispherical cells converge. The calculated zonal mean difference fields for NO, NeS), N(‘D), and N, are shown in Fig. 9a, b, c and d, respectively for the solstice case. As with the major species, the compositional effects for these minor species are greatest in the winter polar region. NO is enhanced in the lower thermosphere and NCS) is depleted as a result of chemical reactions in the odd nitrogen system. N(2D) is enhanced by particle precipitation and the calculated electron density displays a distribution that is similar to that calculated during equinox conditions but with asymmetries caused by the seasonal differences. These simulations illustrate steady-state patterns, and considerable variability in thermospheric composition will also occur with variable aurora1 forcings. 5.4. Time-dependent response The NCAR thermosphere general circulation model (TGCM) has been used by Roble et al. (1987a), Crowley et al. (1989a, b) and Fesen et al. (1989), and the TIGCM has been used by Burrage et al. (1991) and Codrescu et al. (1991) to study the time-dependent thermospheric response to realistic time-dependent aurora1 forcings. Roble et al. (1987a) and Forbes et al. (1987a) studied thermospheric dynamics for the 22 March 1979 magnetic storm calculated by the TGCM. This was an isolated storm with the period preceding the storm being geomagnetically quiet for nearly 2 days. The TGCM was run until a diurnally reproducible pattern was obtained for the geomagnetic quiet prestorm conditions. The time variations of particle precipitation and cross-polar cap potential drop, based on satellite measurements, were then introduced as time-dependent forcings into the model and the model was run for 22 March using a 5 min time step. The results of that simulation showed that the thermosphere responded dramatically to the energy and momentum inputs during the storm. Large- and medium-scale disturbances were launched during the event that propagated to equatorial latitudes. The results also showed that the storm response was different in the upper and lower thermosphere. In the upper thermosphere, the winds generally followed the two-cell pattern of ionospheric convection with a lag of only 1/2-l h. In the lower thermosphere there was a pronounced asymmetry between the circulation cells on the dawnside and duskside of the polar cap. The lower thermosphere circulation features, once estab-

The polar lower thermosphere nccMNa~wca(cm-s) VT- 0.eeae?ULrwItxDW.raW8l

nccy UT-

0 twmn DDISTY (eu-3) 0.00toluyuwI%oirf-1

FIG. 8. SAMEZ CAPTIONAS FORFIG. 5 EXCEPT FORDECZMBER SOLSTICE CONDITIONS.

lished, tended to persist several hours after the storm time forcings subsided. The results also showed changes in the mass mixing ratios of the major neutral constituents along constant pressure surfaces in both the upper and lower thermosphere. The storm simulation for 22 March 1979 has heen repeated, but using the TIGCM. Furthermore, the simulation was continued, using time-dependent auroral forcings, for 19 additional days ; with an overall coverage between 20 March and 10 April 1979. TIGCM histories were recorded every hour during the simulation period. The overall time period was geomagnetically disturbed and a considerable amount of variability was displayed by calculated thermospheric and ionospheric fields. The time variation

of the Kp index for the first 10 days of the period is shown in Fig. 10. The time-dependent responses of the neutral gas temperature and meridional winds at 120 km are shqwn in Fig. 11 for the 10 day period 20 March-30 March 1979. The figures show contours of temperature and winds as a polar orbiting satellite would view the lower thermosphere, remotely, at a constant solar local time of 15:OOL.T., similar to the orbits of the SME or, at various times, the Dynamics Explorer spacecraft. The results show that the lower thermosphere at 120 km has considerable variability introduced by variable aurora1 activity. The neutral gas temperature and equatorward neutral winds increase at high latitudes in response to enhanced aurora1 par-

288

R. G. ROBLE TICCN UT -

Y’ICCY NO NUMBERDENSITY (CM-S) 0.00 ZONALNUN (X Diffcrcace) UT 600

boa

500

500

N4S NUHBEB 0.00 ZONAL

DENm MEAN

(N-3) (X wfmaccl

g4@3 f p300

FIG. 9. SAMECAPTION

AS FOR

FIG. 6 EXCEPT

ticie precipitation and Joule heating and then decrease as aurora1 activity subsides. At times, the aurora1 activity is impulsive, launching disturbances that propagate equatorward. At other times, persistent heating results in a mean overturning of the atmosphere from high to mid-latitudes. A dominant feature in the pattern is a diurnal variation caused by the offset geomagnetic and geographic poles. The calculated 0, 02, Nz, and NO number densities at 120 km along the same 15:OOL.T. orbital path are shown in Figs 12 and 13. The results show that at high latitudes O2 and NO increase and 0 decreases as geomagnetic activity increases. At mid-latitudes 0 increases and O2 decreases with geomagnetic activity. There is an overall build-up of temperature, winds O2 and NO densities throughout the period as a result of the persistent heating during the storm.

POR DE~EMEER

SOLSTICE CONDITIONS.

The time variation of the globally averaged NO density in the 100-160 km height range is shown in Fig. 14 for the entire 21 day period. Prior to 22 March geomagnetic quiet conditions existed with most of the NO produced by the Sun. However, as geomagnetic activity increases on day 79 the globally averaged NO increases dramatically. Most of the NO production occurs at high latitudes and it is sufficiently large to cause a substantial increase in the globally averaged value. As auroral activity subsides the NO density decreases primarily by solar photodissociation (Nicolet and Peetermans, 1980). During equinox conditions both polar regions are illuminated by sunlight resulting in photochemical lifetime of about a day. In addition, substantial gradients of NO occur near the the lower boundary of the model near 95 km indicating that a portion of the NO is being transported

The polar lower thermosphere TICCN INPUTS FOR HARCH/‘iPRlL. ?,.I’..r.#.r.r.l.,.

1979

289

that constant pressure surface. Thus, atomic oxygen concentrations are depleted in the lower thermosphere 6.5 in response to aurora1 heating. These calculations are b.0 consistent with the observations of Hecht et al. (1991) 5.5 that show significant depletions of atomic oxygen in 5.8 the polar lower thermosphere during the great geo4.5 magnetic storm of February 1986. The changing O/O2 4.0 ratio along constant pressure surfaces also has an "y 3.5 influence on the odd nitrogen system calculated in the 3.0 model. NO is produced by both solar radiation and 2.5 particle p~ipi~tion in the polar lower thermosphere 2.0 and it is subjected to transport by the wind system. In 1.5 addition, the partitioning of number density in the 1.0 odd nitrogen system is dependent upon temperature .5 and changes in the mixing ratios of the major neutral 0 constituents. In the TIGCM calculations it is shown 79 80 81 82 83 84 85 86 87 88 89 that NO changes occur, not only in the polar lower DAY OF 1979 thermosphere, but globally primarily as a result of FIG. 10.TIME-DEPFMENT VARIATION OFTHEKpGEDMAGN~~TIC changes in dynamics and composition. These changes INDEXBF,TWEEN 20 MARIAAND10 APRIL 1979. in composition also inSuence the electron density globally which in turn feeds-back on the.dy~~ of the entire thermosphere. It thus appears that the entire to the mesosphere through the lower boundary. The NO flux to the mesosphere is quite variable and is lower thermosphere is inthtenced by aurora1 processes in the polar lower thermosphere. related to geomagnetic activity. Thus, thermospheric The TIGCM has also been run for a 21 day period generated NO has a potential for downward transport 20 March-10 April 1979, using realistic time-depeninto the mesosphere. dent forcings. The results show the time history of density variations as a polar orbiting satellite at a ii. CONCLUSIONS constant solar local time (S.L.T.) of IS:00 h would observe along its orbital path. They indicate that the The NCAR TIGCM has been used to investigate lower thermosphere at 120 km has considerable variathe global dynamic response of the lower therbility induced by the variable aurora1 activity during mosphere to auroral inputs inditing mainly on the compositional changes in the polar lower therthis moderately disturbed geomagnetic period. Most of the perturbations are confined to latitudes greater mosphere. The results have shown that the basic comthan about 40” but at times disturbances can propapositional structure of the polar lower thermosphere that is established by solar radiational pro~ses is gate to the vicinity of the equator. There is also a pronounced diurnal variation at a given locagreatly altered by aurora1 particle precipitation and tion induced by the diurnal motion of the geoionospheric convection. Because these aurora1 inputs are highly variable the dynamic and compositional magnetically controlled aurora1 zone about the geostate of the lower thermosphere also illustrate congraphic pole. The results show that Nt and Or increase and 0 decreases in a complex manner in response to siderable variability. In genera& the results show that g~ma~etic activity. The odd nitrogen densities also in the polar lower the~osphe~ N2 and O2 increase respond but the major effects are confined to the and 0 decreases at altitudes below 200 km in response to aurora1 activity. These compositional changes are higher latitudes. As NO increases, N(%) decreases indicating a tight coupling in this thermospheric odd entirely caused by dynamic distributions, primarily by nitrogen system. The calculated NO density enbanceupward vertical motion driven by aurora1 particle prements seen at equatorial latitudes are considerably cipitation and Joule heating. In the lower thermosphere, upward vertical motions enhance conless than NO densities that have been observed by the stituents heavier than the mean mass and deplete SME satellite (Siskind et al., 1989a; Barth, 1990; constituents lighter than the mean mass along conGerard et al., 1990). It appears that transport, temstant pressure surfaces. At a given altitude in the lower perature and compositional readjustment in the low ~e~osphe~ densities change in response to both latitude lower thermosphere is insuffi~ent for increaschanges in the height of a constant pressure surface ing the NO densities to the SME observed levels. The and an adjustment of the mass mixing ratios along low latitude enhancements observed by SME during 7.8

R. G. ROBLE

290

TlGCM NEUTRAL TEMPERATURE (DEC K) MARCH-APRtLl979 SLT= 15.00HT=lZO.O B0

b0 40

g 20 =I ICI 0 z -I

-20 -40 -60 -00

79

90

81

82

03 DAY

CORlDUR

84

OF

85

86

87

88

89

1979

FROM IS0 TO 8%

R-f RR

TIGCH NEUTRAL MERIDIONAL WIND (M/S) PARCH-APR[Ll979 SLT= 15.00HT=l20.0 00

60

-40 -60 -80

79

80

81

82

83

84

85

86

87

88

89

DAY OF 1979 coNTouRFR0Y400?0SooDY40 FIG. 11. CONIWJMOF (a)

NEUTRALOAST~MPERATURE (K)AND(~) MERIDIONAL NEUTRAL WIND 120 km PORA 10 DAY PERIOD 20-30 MARCH1979.

(m s- ‘) AT

The figures show the time history of these fields as a polar orbiting satelfite at a constant solar local time (S.L.T.) of 15:OOh would observe aIong its orbital path.

The polar lower thermosphere

291

TIGCM 02 NUMBER DENSITY (CM-3) MARCH-APRIL 1979'SLT= 15.00HTw120.0

60 40

IW l-

0

I: -20 -40 -60 -80 79

80

81

82

83

84

DAY CONTOUR

OF

85

86

87

08

09

1979

FROM 2 4 TO 4.5

BY 3

TIGCM 0 NUMBER DENSITY (CM-3) MARCH-APRIL 1979 SLT- 15.00HT=120.0 80 60 40 E 3 tE

20 0

u. -I -20 -40 -60 -80 79

80

61

82

83

84

DAY

OF

95 86 1979

87

68

99

COMTOURPRONS?OlN..I FIG. 12. SAME CMIION AS FIG. I1 EXCEPTFOR (a) O1 (x 10”’ cm-)) D2N2lTlES

AND(b) 0 (x 10” cmm3) NUMBER

292

R. G. ROBLE

TICCM NO NUMBER DENSITY (W-3) MARCH-APRIL 1979 SLT= 15.00HT=120.0

60 40 0” 20 3 I0 c” 5 -20 -40 -60 -80

79

e0

81

82

83

84

95

86

07

88

99

1979 coNTOUR FROM .aTO6 Fy .E DAY

OF

TIGCM N2 NUMBER DENSITY (CM-3) MARCH-APRIL 1979 SLT= 15.00HT=120.0 80

60 40 g

20

=, c CI c 5

0 -20 -40 -60 -80

79

80

81

82

83

84

85

86

DAY OF 1979 coNTouumoY l.OTo2.2BY.2

87

88

89

293

The polar lower thermosphere LOCI0 TICCY NO NUMBER DENSITY (W-3) Clobel Yttn March 19 - 28). 1979

DAY OF 1970 lYL

WDIUUY

TYD. -mvrN~.D

-

J&C10 TJCCY NO NUMBER DtNSJTY (CM-3) Global Yttn March 29 - AprJJ 7. 1979

loo Be

g0

19

91

92

93

94

95

16

97

98

DAY OF 1DtB

FIG. 14. TIME VARIMYON

OF THE GLOW

MEAN PROFILE OF lo&a

PERIOD

DURING THE20 DAY (NO) NUMBER DENSITY

20 MARCH-IO APRIL 1979.

294

R. G.

geomagnetic storms are more hkely caused by enhancements in X-ray and e.u.v. solar radiation during the storm-causing solar disturbance rather than equatorial transport of aurorally produced NO (Cravens and Killeen, 1988) or global compositional and temperature adjustments. Another possible source may be from low latitude aurora1 particle precipitation during the storm. Even with the low latitude dititepancy between the calculated and observed behavior of NO during storm, the results of the TIGCM calculation suggest that the aurora1 enhancements alone are significant causing a considerable increase in the calculated globally averaged density profile of NO. The TIGCM calculates surges in the globally averaged NO density profile following major geomagnetic disturbances with a decay time on the order of aday. The de-cay is attributed to solar photodissociat$ which is on the order of a day in the lower thermosphere and also to downward transport to the lower thermosphere. At high latitudes the downward transport may be significant for the middle atmosphere (Brasseur and Nicolet, 1973 ; Solomon et al., 1982; Garcia et al., 1984, 1987). The results presented in this paper are from a model that is based on our current theoretical understanding of the aeronomic processes in the lower thermosphere. There is an important need for more coordinated measurement campaigns and a satellite with a capability for measuring the dynamics, temperature, and compositional structure of the lower thermosphere in order to improve our overall understanding of this important region of our atmosphere.

RRFRRENCTS

Aiken, A. C., Chanin, M. L., Nash, J. and Kendig, D. J. (1991) Temperature trends in the lower mesosphere. Geophys. Res. L.&t. 18, 416. Avery, g. K., Vincent, R. A., Phillips, A., Manson, A. H. and Fraser, G. J. (1989) High latitude tidal behaviour in the mesosohere and lower thermosnhere. J. atmos. terr. 1 Phys. 51,595. Barth, C. A. (1990) Reference models for thermospheric NO. Ada. Space Res. 10, 103. Barth, C. A., Tobiska, W. K., Siskind, D. E. and Chary, D. D. (1988) Solar terrestrial counlina: low latitude thermospheric nitric oxide. Geophys.kes.-Lett. 15,92. Batten, S. M., Rees, D. and Fuller-Rowell, T. J. (1987) A numerical data base for VAX and personnel computers for the storage, reconstruction, and display of global thermospheric and ionospheric models. Planet. Space Sci. 35, 1167. Braaseur, G. and N&let, M. (1973) Chemospheric processes of nitric oxide in the mesosphere and stratosphen. Planet. Space Sci. 21,939. Burns, A. G. Killeen, T. L. and Roble, R. G. (1989a) Pro-

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cesses responsible for the compositional structure of the thermosphere. J. geophys. Res..94,367O. Bums. A. G.. Killeen. T. L.. Crowlev. G., Emerv. B. A. and Roble, R. G. (1989b) On. the mech&ms res&nsible for high-latitude thermospheric composition variations during the recovery phase of a geomagnetic storm. J. geophys. Res. 94, 16,961. Bums, A. G., Killeen, T. L. and Roble, R. G. (1991) A theoretical study of thermospheric composition perturbations during an impulsive geomagnetic storm. J. oeophys. Res. 96, 14,153. B&age: M. D., Abreu, V. J., Fesen, C. G., Roble, R. G. and Orsini. N. (1991) Geomaanetic activitv effects on the equatorial. neutral thermosphere. J. geo&ys. Res. (in press). Clancy, R. T. and Rusch, D. W. (1989) Climatology and trends on mesospheric (58-90 km) temperatures based upon 1982-1986 SME limb scattering protiles. J. geophys. Res. 94,3377. Clark, M. A., Larsen, M. F. and Mikkelsen, I. S. (1988) An analysis of the response of the thermospheric normal modes to temporally varying convection at high latitudes. J. geophys. Res. 93,12,893. Codrescu, M. V., Roble, R. G. and Forbes, J. M. (1991) Interactive ionosphere modeling : a comparison between TIGCM and ionosonde data. J. geophys. Res. (in press). Cravens, T. E. and Killeen, T. L. (1988) Longitudinally asymmetric transport of nitric oxide in the E-region. Planet. Space Sci. 36, Il. Cravens, T. E. and Stewart, A. I. (1978) Global morphology of nitric oxide in the lower E-region. J. geophys. Res. 83, 2446. Crowley, G. (1991) Dynamics of the Earth’s thermosphere : a review. Rev. Geophys. Suppl., pp. 1143-I 186. Crowley, G., Emery, B. A., Roble,-R. G., Carlson, H. C., Jr. and KniDD. __. D. J. (1989a) Thermosnheric dvnamics during September 18-19,. 1984,‘l. Model Emulations. J. geophyi Res. 94, 16,925. Crowley, G., Emery, B. A., Roble, R. G., Carlson, H. C., Jr., Salah, J. E., Wickwar, V. B., Miller, K. L., Oliver, W. L., Bumside, R. G. and Marcos, F. A. (1989b) Thermospheric dynamics during September 18-19, 1984, 2. Validation of the NCAR thermospheric general circulation model. J. geophys. Res. 94, 16,945. Dickinson, R. E., Ridley, E. C. and Roble, R. G. (1981) A threedimensional general circulation model of the thermosphere. J. geophys. Res. 86,1499. Dickinson, R. E., Ridley, E. C. and Roble, R. G. (1984) Thermospheric general circulation with coupled dynamics and composition. J. atmos. Sci. 41, 205. Fesen, C. G., Crowley, G. and Roble, R. G. (1989) Ionospheric effects at low-latitudes during the March 22,1979, geomagnetic storm. J. geophys. Res. 94,5405. Fesen, C. G., Dickinson, R. E. and Roble, R. G. (1986) Simulation of thermospheric tides at equinox with the NCAR thermospheric general circulation model. J. geophys. Res. 91,447 1. Fesen, C. G., Richmond, A. D. and Roble, R. G. (1991a) Auroral effects on midlatitude semidiumal tides. Geophys. Res. Lett. 18,412. Fesen, C. G., Roble, R. G. and Ridley, E. C. (1991b) Thermosphere tides at equinox : simulations with coupled composition and aurora1 forcings, 1. Diurnal component. J. geophys. Res. %, 3627. Fesen, C. G., Roble, R. G. and Ridley, E. C. (1991~) Thermosphere tides at equinox : simulations with coupled com-

The polar lower thermosphere position and amoral forcings, 2. Semidiumal component. J. geophys.Res. 96, 3647. Forbes, J. M. (1982a) Atmospheric tides, 1. Model descrip tion and results for the solar diurnal component. J. geophys.Res. 87.5228. Forbes, J. M. (1982b) Atmospheric tides, 2. The solar and lunar semidiumal components. J. geophys. Res. 87, 5241. Forbes, J. M. (1987) Modelling the propagation of atmospheric tides from the lower to the middle and upper atmosphere. Phys. ScriptaT18,240. Forbes, J. M. and Groves, G. V. (1987) Atmospheric structure between 80 and 120 km. Ado. Spuce Res. 7,135. Forbes, J. M., Roble, R. G. and Fesen, C. G. (1991) Acceleration, heating, and compositional mixing of the thermosphere due to upward-propagating tides. J. geophys. Res. (in press). Forbes, J. M., Roblc, R. G. and Matcos, F. A. (1987a) Thermospheric dynamics during the March 22,1979, magnetic storm, 2. Comparisons of model predictions with observations. J. geophys. Res. 92,6069. Forbes, J. M., Vincent, R., Fraser, G., Avery, S., Bowhill, S., Clark, R., Gmisinger, K.. Mattson, A., Roper, R. and Tsuda, T. (1987b) Gn the speci6cation of TGCM tidal lower boundary conditions from radar wind measurements during the June 1984 GTMS period. AL. Space Res. 7,295. Forbes, J. M. and Vial, F. (1989) Monthly simulations of the solar semidiumal tide in the mesosphere and lower thermosphere. J. atmos. terr. Phys. 51, 663. Forbes, J. M. and Vial, F. (1991) Semidiurnal tidal climatology of the E-region. J. geophys. Res. 96, 1147. Fulkr-Rowell, T. J. (1990) Model simulations of the electrodynamics of the high latitude thermosphere and ionosphere with the magnetospheric inuut defined by statistical or empirical models. Adv; Space ies. 10,153. _ Fuller-Rowell. T. J. and Evans. D. S. (1987l Heiaht-integrated Pederscn and Hall &&ctivi~y pa&ns%tferred from the TIRO-NOAA satellite data. J. geophys. Res. 92, 7606. Fuller-Rowell, T. J. and Rees. D. (1980) A three-dimensional, timedcpendent global model of the thermosphere. J. atmos.Sci. 37.2545. Fuller-Rowe& T. J. and Rees, D. (1981) A three-dimensional, time-dependent simulation of the global dynamical msponse of the thermosphere to a geomagnetic substorm. J. utmos. terr. Phys. 43,701. Fuller-Rowell, T. J. and Rees. D. (1984) Interpretation of an anticipated long-lived vortex in the lower thermosphere following simulation of an isolatul substorm. Planet. Space Sci. 32,69. Fuller-Rowell, T. J.. Rees, D., Quegan, S., Moffett, R. J. and Bailey, G. J. (1987) Interactions between neutral thermospheric composition and the polar ionosphere using a coupled thermosphere-ionosphere model. J. geophys. Res. 92,7744. Fuller-Rowe& T. J., Rees, D., Quegan, S., Moffett, R. J. and Bailey, G. J. (1988) Simulations of the seasonal and universal time variations of the high-latitude thermosphere and ionosohere usinn a cou~lai. threedimensiomd model. Pure ajpl. Geophis. 127, i89.. Gadsden, M. A. (1990) A secular change in noticulent cloud ocmrma. J. atmos. terr. Phys. 52,247. Garcia, R. R., Solomon, S., Avery, S. K. and Reid, 0. C. (1987) Transport of nitric oxide and the D-region winter anomaly. J. geophys. Res. 92D, 977.

295

Garcia, R. R., Solomon, S.. Roble, R. G. and Rusch, D. W. (1984) A numerical study of the response of the middle atmosphere to the 1l-year solar cycle. Pkmet. Space Sci. 32,411. Gerard, J.-C., Fesen, C. G. and Rusch, D. W. (1990) Solar cycle variation of thermospheric nitric oxide at solstice. J. geophys. Res. %,12,235. Gerard, J.-C. and Robk, R. G. (1988) The role ofnitric oxide on the aonally averaged structure of the thermosphere: solstia conditions for solar cycle maximum. P&net. Space Sci. 36,271. Gundlach. J. P., Larsen, M. F. and Mikkelsen, I. S. (1988) A simple model describing the nonlinear dynamics of the dusk/dawn asymmetry in the high latitude tbennospheric flow. Geophys.Res. Len. 15,307. Hagan, M. E. (1988) Effects of geomagnetic activity in the winter thermosphere, 2. Magnet&By disturbed conditions. J. ueophvs. Res. 93.9937. Hauchecomk A., ~Chenin, G. L. and Keckhut, P. (1991) Climatology and trends of the middle atmospheric temperature (33-87 km) as seen by Rayleigh Lidar over the south of France. J. geophys. Res. (in press). Hays, P. B., Jones, R. A. and Rees. M. H. (1973) Auroral heating and the composition of the neutral atmosphere. Pkmet. Space Sci. 21, 559. Hays, P. B., Killeen, T. L., Spencer, N. W., Wharton, L. E., Roble, R. G., Emexy, B. A., Fuller-Rowell, T. J., Rees. D., Frank, L. A. and Craven, J. D. (1984) Observations of the dynamics of the polar thermosphere. J. geophys. Res. 89.5597. Hecht, J. H., Strickland, D. J., Christensen, A. B., Kayser, D. C. and Walters&id. R. L. (1991) Lower thermospheric composition changes derived from optical and radar data taken at Sondre Stromf~ord during the great magnetic storm of February, 1986. J. geophys. Res. 96,5757. Hedin, A. E. (1983) A revised thermospheric model based on mass spectrometer and incoherent scatter data : MSIS 83. J. geophys. Res. 88, 10,170. Hedin, A. E. (1987) MSIS-86 thermospheric model. J. geophys. Res. 92.4649. Hedin, A. E. (1988) Atomic oxygen modellin8 in the upper thermosphere. P&met.Space Sci. 36,907. Hedin, A. E. (1991) Extension of the MSIS thermosphere model into the middle and lower atmosphere. J. geophys. Res. %, 1159. Hedin, A. E. and Carignan, G. R. (1985) Morphology of thermospheric composition variations in the quiet polar thermosphere from Dynumics Exp/orer mcasurcmcats. J. geophys. Res. 90.5269. Hedin, A. E., Spencer, N. W. and Killeen, T. L. (1988) Empirical global model of upper thermosphere winds based on Atmostdere and Dynamics Explorer satellite data. J. geophys.kes. 93,9959: Heelis, R. A., Lowell J. K. and Spiro, R. W. (1982) A model of the high-latitude ionospheric convection pattern. J. geophys. Res. 87,6339. Hemmer. J. P. and Miller. M. L. (19821 Thermosohericwinds &-high latitudes fro& chemkal r&se observations. J. geophys. Res. 87, 1633. Hemandez,, G. and Killeen, T. L. (1988) Gptical measurements of winds and kinetic temperatures in the upper atmosphere. Adu. @ace Res. 8,149. Hemandez, G., McCormac, F. G. and Smith, R. W. (1991) Austrial thermospheric wind circulation and interplanetary magnetic geld orientation. J. geophys. Res. 96, 5777.

296

R. G. RQBLE

Hernandez, G., Smith, R. W., Roble, R G., Gress, J. and Clark, K. C. (1990) Thermospheric dynamics at the South Pole. Geophys. Res. Len. 17, 1255. Hinteregger, H. E. (1981) Representations of solar EUV fluxes for aeronomi~l appf~tions. Adv. Space Res. 1,39. Johnson, R. M. (1991) Sondrestrom incoherent scatter radar observations during the lower thermosphere coupling study: Sentember 21-26.1987. J. oeotihvs. Res. %. 1081. Johnson, R: M. and Virdi,‘T. S. (199l)‘H&latitude lower thermospheric neutral winds at EISCAV and Sondrestrom during LTCS I. J. geophys. Res. 96, 1099. Johnson, R. M., Wickwar, V. B., Roble, R. G. and ~u~ann, J. G. (1987) Lower ~e~osphe~c wind at high latitude: Chatanika radar observations. Ann. Geophys. 5A, 383. Kasting, J. F. and Roble, R. G. (1981) A zonally averaged chemicaldynamical model of the lower thermosphere. J. geophys. Res. 86.964 1. Kazimirovsky, E. S. (1988) Thermosphere dynamics. J. ammos.fem. Phys. 50,889. Killeen, T. L. (1987a) Energetics and dynamics of the Earth’s thermosphere. Rev. Geophys. 25,433. Killeen, T. L., Craven, J. IX, Frank, L. A., Ponthieu, J.-J., Spencer, N. W., Heelis, R. A., Brace, L. H., Roble, R. G., Hays, P. B. and Carianan. G. R. (19881 On the relationship between dynamics of the polar thermosphere and the morphology of the aurora: global-scale observations from Dy~ics Explorers 1 and 2. J. geophys. Res. 93, 2675. Killeen, T. L., Heelis, R. A., Hays, P. B., Spencer, N. W. and Hanson, W. B. (1985) Neutral motions in the polar thermosphere for northward interplanetary magnetic field. Geophys. Res. Lert. 12, 159. Killeen, T. L. and Roble, R. G. (1984) An analysis of the hip-latitude the~osphe~c wind pattern calculated by a ~e~o~he~c general circulation model, I. Momentum forcing. J. geophys. Res. 89,7509. Killeen, T. L. and Roble, R. G. (1986) An analysis of the high-latitude thermospheric wind pattern calculated by a thermospheric general circulation model, 2. Neutral parcel transport. J. geophys. Res. 91, 11,291. Killeen, T. L. and Roble, R. G. (1988) Thermosphere dynamics: ~nt~butions from the first 5 years of the Dynamics Explorer program. Reu. Geophys. 26,329. Killeen, T. L., Roble, R. G. and Spencer, N. W. (1987b) A computer model of global therrnospheric winds and temperatures. Ado. Space Res. 7, 207. Kuntake, M. and Schlegel, K. (1991) Neutral winds in the lower thermosphere at high latitudes,from five years of EISCAT data. Ann. Geophys. 9, 143. Larsen, M. F. and Mikkelsen, 1. S. (1983) The dynamic response of the high-latitude thermosphere and geostrophii adjustment. J. geophys. Rex 88,3158. Larsen, M. F. and Mikkelsen, I. S. (1987) The normal modes of the thermosphere. J. geophys. Res. 92,6023. Larsen, M. F., Mikkelsen, I. S., Meriwether, J. W., Jr., Niciejewski, R. and Vickery, K. (1989) Simultaneous observations of neutral winds and electric tields at spaced locations in the dawn aurora1 oval. J. geophys. Res. 94, 17,235. Maeda, S., Fuller-Rowe& T. J. and Evans, D. S. (1989) Zonally averaged dynamical and compositional response of the thermosphere to amoral activity during September 18-24, 1984. J. geophys. Res. 94, 16,869. Mayr, H. G., Dube, M., Harris, I., Hedin, A. E. and Herrero, F. A. (1988) On the structure and circulation of the polar

thermosphere under magnetically quiet conditions. J. atmos. terr. Phys. SO, 983. Mayr, H. G. and Harris, I. (1978) Some characteristics of electric field momentum coupling with the neutral atmosphere. J. geophys. Res. 83,3327. Mayr, H. G., Harris, I. and Dube, M. (1990) Polar thermosphere Joule heating and redistribution of recombination energy in the upper mesosphere. J. atmos. ferr. Phys. 52, 103. Mayr, H. G. and Volland, H. (1973) Magnetic storm effects in the neutral composition. Planet. Space Sci. 20,379. Mazaudier, C., Richmond, A. D. and Brinkman, D. (1987) On the thermospheric winds produced by aurora1 heating during magnetic storms and associated dynamo electric fields. Ann. Geophys. SA, 443. McCormac, F., Kilieen, T. L., Bums, A. G., Meriwether, J. W.. Jr.. Roble. R. G.. Wharton. L. E. and Snencer. N. WI (1988) Polar: cap diurnal temperature variations I observations and modeling. J. geophys. Res. 93.7466. McCormaq F. G., KiilePn, T. L., Gombosi, E., Hays, P. B. and Spencer, N. W. (1985) ~nfi~ration of the highlatitude thermosphere neutral circulation for IMF By negative and positive. Geophys. Res. Lett. 12, 155. McCormac. F. 0.. Killeen. T. L.. Thaver. J. P.. Tschan. C. R., Hemandez, G., .Ponthieu, J: J: and’ Spencer; N. W. (1987) Circulation of the polar thermosphere during geomagnetically quiet and active times as observed from DE-2. J. geophys. Res. 92, 10,133. Meriwether, J. W., Jr., Killeen, T. L., McComac, F. G., Burns, A. G. and Roble, R. G. (1988) Thermospheric winds in the geomagnetic polar cap for solar minimum conditions. J.geoph&. Res.-93, 7478. Meriwether. J. W.. Jr.. Shih. P.. Killeen. T. L.. Wickwar. V. B. and Roble, ‘R. d. (1984) Nighttime thennospheric winds over Sondre Stromtjord, Greenland. Geophys. Res. Left. 11,931. Meriwether, J. W., Jr. and Singh, P. (1987) On the nighttime signatures of thermospheric winds observed at SondreStrom, Greenland, as correlated with IMF parameters. Ann. Geophys. JA, 329. Mikkelsen, I. S. and Larsen, M. F. (1983) An analytic solution for the response of the neutral atmosphere to the highlatitude convection pattern. J. geophys. Res. 88,8073. Mikkelsen, I. S. and Larsen, M. F. (1991) A numerical modeling study of the interaction between the tides and circulation forced by high latitude plasma convection. J. geophys. Res. 96, i203: Mikkelsen. I. S.. Larsen. M. F.. Kelly. M. C.. Vickerv. J.. Friis-Christen&r, E. J:, Meriwethe;,‘W., Jr.‘and Shih, P: (1987) Simultaneous measurements of the thermospheric wind profile at three separate positions in the dusk aurora1 oval J. geophys. Res. 92,4639. Nicolet, M. (1989) Aeronomic chemistry of ozone. Planer. Space Sci. 37, 1621. Nicolet, M. and Peetermans, W. (1980) Atmosphericabsorption in the O2 Schumann-Runge band spectral range and photodissociation rates in the stratosphere and mesosphere. Planet. Space Sci. 2%.85. Ponthieu, J.-J., Kiileen, T. L., Lee, K.-M. and Carignan, G. R. (1988) Ionosphere thermosphere momentum eoupling at solar maximum and solar minimum from DE-2 and AE-C data. Phys. Scripta 37,447. Rees, D. (1989) Antarctic upper atmosphere investigations by optical methods. Planet. Space Sci. 37,955. Rees, D. and Fuller-Roweil, T. J. (1987) Comparison of theoretical models and observations of the ~e~~he~

The polar lower thermosphere and ionosphere during extremely disturbed geomagnetic conditions during the last solar cycle. Adu. Space Res. 7, 21. Rees, D. and Fuller-Rowe& T. J. (1988a) The CIRA theoretical thermosphere model. Ada. Space Res. 8.27. Rees, D. and Fuller-Rowe& T. J. (1988b) Understanding the transport of atomic oxygen within the thermosphere, using a numerical global thermospheric model. Planer. Space Sci. 36,935.

Recs, D. and Fuller-Rowell, T. J. (1990) Modelling of Eregion auroral winds. Ada. Space Res. 10,197. Rees, D., Fuller-Rowell, T. J., Quegan, S., Moffett, R. J. and Bailey, G. J. (1988) Simulations of the seasonal variations of the thermosphere and ionosphere using a coupled 3-D global model. including variations of the IMF. J. ofmos. rerr. Phys. SO, 903. Rees. M. H. (1989) Physics und Chemistry of the Upper Atmosphere. Cambridge University Press, Cambridge. Richmond, A. D., Blanc, M., Emery, B. A., Wand, R. H., Fejer, B. G., Wocdman, R. F., Gangbly, S., Amayenc, P., Behnke, R. A., Calderon, C. and Evans, J. V. (1980) An empirical model of quiet-day ionospheric electric fields at middle and low latitudes. J. geophys. Res. 85,4658. Roble, R. G. (1991) On modeling component processes in the Earth’s global electric circuit. J. utmos. few. Phys. 53, 831. Roble, R. G. and Dickinson, R. E. (1989) How will changes in carbon dioxide and methane modify the mean structure of the mesosphere and thermosphere? Geophys. Res. Len. 16, 1441. Roble, R. G., Dickinson, R. E. and Ridley, E. C. (1977) Seasonal and solar-cycle variations of the zonal mean circulation in the thermosphere. J. geophys. Res. 82,5493. Roble, R. G., Dickinson, R. E. and Ridley, E. C. (1982) The global circulation and temperature structure of the thermosphere with high-latitude plasma convection. J. geophys. Res. 81,1599.

Robld, R. G., Forbes, J. M. and Marcos, F. A. (1987a) Thermosnheric dvnamics during the March 22.1979 maanetic sto&n (1) Model simulat:ons. J. geophis. Res. 95, 6045. Roble, R. G., Ridley, E. C. and Dickinson, R. E. (1987b) On the global mean structure of the thermosphere. J. geophys. Res. 92,874s.

Roble, R. G., Emery, B. A., Killeen, T. L., Reid, G. C., Solomon, S., Garcia, R. R., Evans, D. S., Hays, P. B., Carignan, G. R., Heelis, R. A., Hanson, W. B., Winningham, D. J., Jr., Spencer, N. W. and Brace, L. H. (1987c) Joule heating in the mesosphere and thermosphere during the July 13, 1982, solar proton event. J. geophys. Res. 92,6683.

Roble, R. G., Killeen, T. L., Spencer, N. W., Heelis, R. A., ReiK, P. H. and Winningham. D. J., Jr. (1988) Thermospheric dynamics during November 21-22, 1981: Dynamics Explorer measurements and thermospheric gen-

291

era1 circulation model predictions. J. geophys. Res. 93, 209. Roble, R. G. and Ridley, E. C. (1987) An auroral model for the NCAR thermospheric general circulation model (TGCM). Ann. Geophys. SA, 369. Roble, R. G., Ridley, E. C., Richmond, A. D. and Dickinson, R. E. (1988) A coupled thermosphere/ionosphere general circulation model. Geophys. Res. Lett. 15,1325. Rodger, A. S. and Stewart, R. D. (1990) Lower ther-mospheric wind measurements near 60” L from 5577 A Doppler interferometry. AL. Space Res. 10, 187. Sica, R. J., Hemandex, G., Emery, B. A., Roble, R. G., Smith, R. W. and Rees, M. H. (1989) The control of aurora1 zone dynamics and thermodynamics by the IMF dawn-dusk ( Y) component. J. geophys. Res. 94, II,92 1. Siskind, D. E., Barth, C. A. and Roble, R. G. (1989a) The response of thermospheric nitric oxide to an auroral storm, 1. Low and middle latitudes. J. geophys. Res. 94,16,885. Sikind, D. E., Barth, C. A., Evans, D. S. and Roble, R. G. (1989b) The response of thermospheric nitric oxide to an aurora1 storm, 2. Aurora1 latitudes. J. geophys. Res. 94, 16,899. Smith, R. W., Rees, D. and Stewart, R. D. (1988) Southern Hemisphere thermospheric thermodynamics : a review. Rev. Geophys. 24,59 1. Solomon, S., Reid, G. C., Roble, R. G. and Cmtxen, P. J. (1982) Photochemical coupling between the thermosphere and lower atmosphere, 2. D-Region ion chemistry and the winter anomaly. J. geophys. Res. 87,122l. Stewart, R. D., Rodger, A. S. and Dudeney, J. R. (1988) Thermospheric wind response to driving forces in the vicinity of the Harang discontinuity. Planet. Spuce Sci. 36, 225. Thayer, J. P., K&en, T. L., McCormac, F. G., Tschan, C. R., Pontbieu, J.-J. and Spencer, N. W. (1987) IMF Bydependent neutral wind signatures for Northern and Southern Hemispheres from Dynamics Explorer-2 data. Ann. Geophys. SA, 363. Thomas, G. E., Olivero, J. J., Jensen, E. J., Schroeder, W. and Toon, 0. B. (1989) Relation between increasing methane and the presence of ice clouds at the mesopause. Nature 338,490. Torr, D. G. and Torr, M. R. (1985) Ionization frequencies for solar cycle 21: revisuL J. geophys. Res. 90.6675. Torr, M. R., Tot-r, D. G., Ong, R. A. and Hinteregger, H. E. (1979) Ionization frequencies for major thermospheric constituents as a function of solar cycle 21. Geophys. Res. Lett. 6,771. Walterscheid, R. L., Sivjee, V. V., Schubert, G. and Hamwey, R. M. (1986) Large amplitude semidiurnal temperature variations in the polar mesopause : evidence of a pseutide. Nature 324,341.

Wiens, R. H., Shepherd, G. G., Gauh, W. A. and Kosteniuk, P. R. (1988) Optical measurements of winds in the lower thermosphere. J. geophys. Res. 93,5973.