Dynamics of the low-latitude thermosphere: Quiet and disturbed conditions

Journal qf Atmospheric andSolar-Temstrd

PII: SOO21-9169(96)00154-7

Physrcr. Vol. 59. No 13. pp. 1533-1540, 1997 %I 1997 Elsevier Scmm Ltd All rights reserved. Printed in Great Brmin 1364-6826197 $17.00+0.00

Dynamics of the low-latitude thermosphere: quiet and disturbed conditions T. J. Fuller-Rowell,’

M. V. Codrescu,’

B. G. Fejer,’

W. Borer,3

F. Marco?

and D. N. Anderson3 ‘CIRES, University of Colorado / NOAA Space Environment Laboratory, 325 Broadway, Boulder, CO 80803, U.S.A.; 2CASS, Utah State University, Logan. UT 84322, U.S.A.: ‘Phillips Lab., Hanscom AFB, MA 01731, U.S.A. (Received 3 October 1995; accepted 9 September 1996) Abstract-Low-latitude dynamics. electrodynamics, and plasma density structure are closely linked. Dynamically driven electric fields initiate the equatorial ionization anomaly. Between the latitudes of the anomaly crests, steep gradients in ion density span more than three orders of magnitude. Zonal winds accelerate in response to the severe deficit of plasma, and reduced ion drag, at the dip equator. Zonal winds give rise to a vertical polarization field, causing plasma to drift with the neutrals and further diminish ion drag. Signatures of neutral temperature are associated with the winds; cooling appears in the zonal jet itself and there is slight warming on either side. Chemical heating is suggested as the mechanism responsible for the temperature feature, but this has yet to be confirmed. During geomagnetic disturbances, large-scale waves propagate efficiently from the remote high-latitude source region. The strength of the waves and the circulation changes depend on local time; the strongest and most penetrating waves arise on the nightside,

where they are hindered least by drag from the low ion densities. The rapid arrival of waves to low latitudes may be the cause of the electrodynamic drift that has been observed to follow a rise of geomagnetic activity within four hours. Winds at low latitudes respond to sources from both polar regions. The changes are manifest by the arrival and interaction of a series of waves from high latitudes that propagate well into the opposite hemisphere. Lower altitudes, below the F-region. respond more slowly because propagation speeds are limited in the cooler, dense lower thermosphere. Finally, during solstice, bulges enriched in molecular nitrogen migrate. over a period of a day or so, from their high latitude source to low latitudes. Characteristic negative phases can result, depleting the ionosphere and further feeding electrodynamic change. The timing of low-latitude electrodynamic signatures in response to geomagnetic disturbances is, at least in part, closely connected to global dynamical time scales. Numerical models are used to illustrate the response of the upper atmosphere during quiet and magnetically disturbed conditions, and are used to elucidate the important physical processes. 0 1997 Elsevier Science Ltd

INTRODUCTION

The equatorial upper atmosphere is an excellent illustration of coupling between neutral dynamics, plasma density structure, electrodynamics, and energetics. The sequence is initiated by solar heating, generating pressure gradients, which drive the thermosphere into motion. The same radiation ionizes the neutral gas. The zonal neutral flow across the horizontal magnetic field. near the magnetic equator, drives a dynamo (Rishbeth, 1971; Heelis et al., 1974; Stening, 1981; Crain et a/., 1993). After sunset, the E-region conductivity drops and the F-region dynamo currents no longer close through the E-region. The combination of conductivity gradients and the F-region dynamo give rise to a postsunset enhancement of the zonal polarization electric field, or equivalently a vertical

plasma drift (Woodman, 1970; Fejer et ul., 1991). It is this vertical E x B drift that redistributes F-region plasma and is the primary driver of the evening equatorial ionization anomaly (EIA) (Anderson, 1973; Anderson and Mendillo, 1983). The neutral flow, the driver of the dynamo fields, is itself affected by ionospheric change in two ways. At the dip equator the vertical plasma drift depletes the ionosphere, reduces ion drag, and permits the neutral gas to accelerate (Anderson and Roble, 1974; Richmond et al., 1992). The dynamo action also creates a vertical electric field which allows the ions to drift with the zonal neutral wind (Rishbeth, 1971), further reducing ion drag. Since the winds are the driver of the dynamo fields in the first place the system demonstrates an interesting feedback potential. In the simulations presented here for quiet geomagnetic con1533

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T. J. Fuller-Rowe11 et al. 5x106

DE-2 (Orbit 7153, Day 82324) 3 I I I , I I I s , I , I I , , , I 1 , I I I , ?? ,

Example of the equatorial temperature and wind anomaly observed by Dynamics Explorer 2. The figure drsplays the electron density (upper panel), zonal winds (middle panel). and temperature (lower panel) at low latitudes on 20 November 1981 at 1900 h LST. The zonal winds and temperatures, from the empirical HWM87 and MSIS models, respectively, are also shown for comparison. From Raghavarao ef al. (1991).

ditions feedback of the neutral dynamics to the generation of the dynamo fields is not treated selfconsistently. Observations of plasma density and neutral winds at low latitudes from the Dynamic Explorer (DE) satellite are shown in Fig. 1, from Raghavarao et ul. (1991). The figure shows a latitude cut through the upper thermosphere in the postsunset sector at 18.9 LT. The altitude of the satellite changes by over 100 km through the orbit but much of the structure is latitudinal. The data show the depletion of plasma density at the dip equator and the peaks either side

associated with the EIA. The zonal neutral wind is inversely correlated with the plasma density as expected from ion drag. The third panel in Fig. 1 shows the temperature structure associated with the feature referred to by et al. (1991) as the equatorial temRaghavarao perature and wind anomaly (ETWA). The temperature shows an apparent reduction at the dip equator coinciding with the depletion in plasma density and maximum in zonal wind. On each side, near the peaks in ion density, the temperature appears elevated. During quiet geomagnetic times the complex interaction between the components arises naturally from the initial diurnal solar heat source. The response of the low latitudes to time dependent geomagnetic events is less clear. Analysis of the Jicamarca ion drift observations suggests a coherent time sequence in the response. The initial penetration electric field and magnetospheric shielding (Fejer and Scherliess, 1995) is followed by a chain of events initiated by global dynamical time-scales; such as the passage of largescale gravity waves, development of a new pattern of global circulation, and transport of species affecting ionospheric conductivity. This paper will review some of the dynamical signatures of the low latitude thermosphere during quiet and disturbed conditions. The aim is to understand the dominant physical mechanism responsible for the structure, and illustrate some of the characteristics with reference to a numerical model. The mode1 used for the disturbed conditions (Fuller-Rowe11 et al., 1994) has been used to explain many of the seasonal, local time, and Universal Time dependencies in the neutral and ionosphere response to geomagnetic storms. For the quiet conditions a hybrid model is used based on the same thermosphere model combined with a parameterized ionosphere that captures the extreme gradients in the equatorial latitudes.

MODELING

QUIET GEOMAGNETIC

CONDITIONS

To model quiet geomagnetic conditions, the equatorial upper atmosphere has been simulated by combining a neutral thermosphere model (Fuller-Rowe11 and Rees, 1980; Fuller-Rowe11 et al., 1994) with a parameterized ionosphere mode1 (PIM; Daniel1 and Anderson, 1995; Daniel1 et al., 1995). PIM is based on a fit to the output from a series of numerical simulations from two physically-based ionospheric models, one for high and mid latitudes from Schunk and Sojka (1995) the other for low latitudes from Anderson (1973). For these simulations of quiet geomagnetic

Low-latitude

disturbance

conditions there is no feedback from the neutral atmosphere to the ionosphere. The climatological model of vertical and horizontal electric fields from Fejer (199 1) was used as the external driver of ionospheric drift in the Anderson ionosphere/plasmasphere O+ model, and both used the neutral atmosphere from the MSIS and HWM87 models. The equatorial ion drifts are based on measurements from the longitude sector of the Jicamarca Radar Observatory, Peru; the drifts at longitude 324” (Brazilian sector) have comparable pre-reversal peak values but reversal times are slightly different, as indicated by Ionosonde data (Batista rt al., 1986) and satellite measurements (Fejer et al., 1995). The purpose here is to understand the mechanism causing the equatorial temperature and wind anomaly (ETWA; Raghavarao et al., 1991). The feature is characterized by fast, zonal, neutral wind, flowing between the crests of the equatorial ionization anomaly (EIA), and a temperature minimum collocated with the maximum wind (see Fig. 1). Figure 2 illustrates the ETWA, as simulated by the combined model, at the December solstice and high solar activity, F10.7 = 200. Part (a) shows a latitude slice through the PIM ionosphere near 400 km altitude, at 324” longitude and at 1.2 h Universal Time (UT); the local time is 22.8 h, and the figure covers f 60” geographic latitude. At the dip equator (- 5” latitude), the ion or electron density drops steeply, by three orders of magnitude in 15” latitude; this is in excellent agreement with the Dynamics Explorer observations (Fig. 1). The zonal neutral wind in the model (Fig. 2b) responds to the reduction in ion drag and accelerates to nearly 200m/s, again in excellent agreement with the observations. The temperature structure is more difficult to simulate and to understand. Our first attempt at reproducing the temperature feature is shown in Fig. 2c. At first glance the coincidence of the temperature minimum with the wind jet appears to be reproduced, albeit with reduced magnitude in comparison to the observations. On closer inspection, it became clear that this temperature feature is driven by the tidal forcing from the lower atmosphere. Signatures of the (2,2) and (2.4) Hough modes imposed near the lower boundary propagate well into the upper thermosphere (Fuller-Rowell, 199.5). In other longitude sectors, and at other Universal Times, the tidally-driven temperature feature no longer aligns neatly with the modeled wind structure. Tides are linked to processes defined by the geographic frame but the wind feature with which we are dealing here is associated with the ionospheric troughs and peaks which follow magnetic dip latitude.

effects

1535 Electron density M-3

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’

-30

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I

IJL

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t/.

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Fig. 2. Simulation of the ETWA with a hybrid thermosphere/ionosphere model. The figure shows ion density from PIM (a). zonal neutral winds (b) and neutral temperature (c) from -60’ to +60” latitude, at 1.2 UT, local time 22.8 h, at an altitude of about 400 km in the upper thermosphere.

The cause of the temperature structure has been postulated by a number of authors. A similar feature was observed in the Nz density structure by Hedin and Mayr (1973). They showed data between 400 and 500 km obtained from OGO 6 neutral mass spectrometer at 1700 LT indicating elevated N, densities 20” either side of the magnetic equator. The increased densities were believed to reflect an underlying tem-

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T. J. Fuller-Rowe11

perature structure. Hedin and Mayr (1973) suggested that the anomaly could result from latitude variations in ion drag associated with the EIA. At latitudes of enhanced electron density they postulated that the energy flow from the dayside to the nightside of Earth is damped, and thus the amplitude of the temperature and density variation would be enhanced. Anderson and Roble (1974) investigated the consequences of the changing ion drag on neutral winds, as the ionospheric F-layer rises and falls in response to the post-sunset upward and downward drifts. The neutral gas accelerates as the layer rises and ion drag is reduced, and then decelerates when the layer falls and ion drag increases. They invoked a continuity argument to suggest an upwelling of air to feed the zonal acceleration and a subsequent downwelling in response to the deceleration. They suggested that adiabatic cooling and heating would produce a temperature signature, possibly contributing to the perturbation observed near midnight at low latitudes. Raghavarao et (I/. (1993) favored the mechanism of Hedin and Mayr (1973) and suggested that the vertical wind structure that is observed would be consistent et al. (1993) with this interpretation. Raghavarao showed that downwelling occurs in the region of high eastward wind velocity. bounded by upwelling at the anomaly peaks. An increase in Joule heating at the anomaly crests has also been suggested, but does not appear to have sufficient magnitude. The electric fields are small and direct Joule heating from the polarization fields creates only small heating rates. The other Joule heating source comes from the relative balance between internal and kinetic energy. In regions where ion drag is largest, at the anomaly peaks, the wind is reduced and the temperature rises from the conversion of kinetic energy to internal energy. Between the anomaly peaks, the trough of ion density no longer extracts this kinetic energy as heat. The temperature difference that can be ascribed to this Joule mechanism is related to the change in the magnitude of the wind veocity, which from the observed values amounts, at most, to a 15 K temperature difference between the peaks and trough of the anomaly; this is considerably less than the 50 to 1OOK observed by the Dynamics Explorer satellite. Results from the numerical simulations support none of these mechanisms. The first mechanism, horizontal transport, appears to work in the wrong sense. The increased zonal wind at the dip equator carries the hot dayside gas into the evening sector, generating local warming. The second mechanism, adiabatic cooling, requires that upwelling coincides with the temperature reduction. The observations, however, show

et al.

Chemical

180 Longitude

heating

225

270

31.5

360

Fig. 3. Ion chemical heating rate from the recombination of O+ at 12 UT, between +60’ geographic latitude: the peak heating rate exceeds 80 J/kg/s.

a clear downwelling (Raghavarao et al., 1993). which would drive adiabatic heating and therefore put this explanation also in doubt. The observed vertical wind signature suggests a heat source at the crests of the anomaly, where the winds are damped. We suggest that the most likely explanation is chemical heating, where a large fraction of the 13.61 eV ionization energy (Rees, 1989) from recombination is converted to heating of the neutral gas. At the peaks of the EIA, ion density values near 4 x l0i2m-3 are common at 300 km altitude, well into the late evening (out of sunlight). The two-stage recombination process of Of ions is exothermic and is controlled by the rate of the reaction with the neutral atmosphere molecular species O2 and N,. The combined heating rate at 300 km for an atmosphere at an exospheric temperature of 1000 K is over 200 K/h. Figure 3 shows the chemical heating rate at 12 UT between +60” geographic latitude; the peak heating rate exceeds 80 J kg-’ s-‘. A chemical heat source at the peak electron density drives vertical winds in the sense observed by DE (Raghavarao et al., 1993) with upwelling at the crests of the EIA and downwelling between them.

MODELING

DISTURBED

GEOMAGNETIC

CONDITIONS

Although remote from their source at high latitudes, clear signatures of geomagnetic activity appear at low latitudes. Numerical simulations have been performed to reproduce the observed variations and to aid in their interpretation. A geomagnetic storm is defined by the development of the ring current as indexed by D,,. During these periods the atmosphere is driven by a large increase in the magnetospheric convection electric field, which is mapped to the highlatitude ionosphere. At low latitudes, the first and

1537

Low-latitude disturbance effects almost instantaneous signature is of the penetration electric field (see Fejer and Scherliess, 1995 and review by Fejer, 1997, in this issue. for further discussion of these processes). Magnetospheric shielding is then expected to reduce this field within an hour of the onset of geomagnetic activity; this has been shown both experimentally (Fejer and Scherliess, 1995) and theoretically (Spiro et al., 1988). For simulation of disturbed conditions the Coupled Thermosphere Ionosphere Model (CTIM) (FullerRowe11 et ul., 1994) is used. In this model the ionosphere and thermosphere are computed self-consistently except at low latitudes, within 25” of the equator, where an empirical ionosphere is used. The magnetospheric forcing for the simulated storm is defined by an increase in the cross polar cap potential (CPCP) from a background level of 45 kV to a peak of 130 kV, for a period of 12 h. The aurora1 oval intensifies and expands in concert with the convection pattern. Details of this forcing is described more fully by Fuller-Rowe11 rt al. (1996). The initial response at high latitudes is that Joule heating raises the temperature of the thermosphere while ion drag drives high-velocity neutral winds. The heat source drives global wind surges, from both polar regions, which propagate to low latitudes and into the opposite hemisphere. Figure 4a illustrates the change in meridional wind in the upper thermosphere, near 300 km altitude. in response to a storm at the December solstice. as predicted for the 0” longitude sector. This figure shows the k70~ geographic latitude region. The surge has the character of a large-scale gravity wave with a phase speed of about 600 m/s; the arrival of this surge at low latitudes some three to four hours later is the storm’s first dynamical signature there. The electrodynamic response at Jicamarca (Fejer and Scherliess, 1995) indicates a change on this time scale, which may be a consequence of the passage of such a wind surge. Burns and Killeen (1992) showed DE observations of wind surges crossing the equator and Fesen et al. (1989) showed clear signatures in the ion density structure associated with the passage of such waves. After the waves from each hemisphere interact, a second surge develops (see Fig. 4a), reaching the equator after about 10 h. At lower altitudes (150 km) the phase speed is slower than in the upper thermosphere. so the first wave arrives at low latitudes after about 6 h and the second after 12 to 14 h. The response of the low-latitude electrodynamics to the neutral dynamical changes will, therefore, be a complex mix as the various waves at each altitude arrive at different times. After the magnetospheric disturbance ends. the dynamics in the upper thermosphere are dampened

within about 6 h by the action of viscosity and ion drag. At lower altitudes, damping is reduced; this allows winds to persist into the recovery phase, well after the source has ceased (Fuller-Rowell, 1995). The global dynamic response, although complex, is divergent at high latitudes in both hemispheres. Vertical winds are driven by the divergent wind field and barometric thermal expansion. Vertical winds driven by the divergent wind field propel material through pressure surfaces, and carry molecular-rich gas to higher levels. Then, the composition ‘bulge’ of increased mean molecular mass is transported by both the storm-induced and background wind fields simu(Fuller-Rowe11 et al., 1994). The numerical lations suggest that the prevailing summer-to-winter circulation at solstice transports the nitrogen-rich gas to mid and low latitudes in the summer hemisphere over the day or two following the storm (Fuller-Rowell et al., 1996). Figure 4b illustrates the equatorward transport of the mean mass increase during the first day of a storm at the June solstice. The response of the upper thermosphere to a 12 h disturbance is shown near 300 km altitude and at 270” longitude. In the recovery from the storm. from 0 to 12 UT, the background summerto-winter circulation transports the composition to low latitudes. The altered neutral--chemical environment subsequently depletes the F-region ionosphere to produce a ‘negative phase’. The ionospheric changes will then feed into the electrodynamics through changes in conductivity. Composition changes persist for at least two days following major geomagnetic disturbances; the resulting ionospheric and electrodynamic changes are expected, and are observed, to have a similar lifetime (Fejer, 1997, this issue).

SUMMARY

The low-latitude ionosphere, neutral wind and temperature, and electrodynamics are a closely coupled, interacting system. Solar extreme ultraviolet (EUV) heating initially drives the dynamics, but the geometry of the geomagnetic field at low latitudes gives rise to narrow features both in the ionosphere (the EIA) and in the neutral atmosphere (the ETWA). The PIM ionosphere captures the extreme gradients of electron density associated with the equatorial ionization anomaly in the evening sector. The modelled dynamic response is in excellent agreement with the observed equatorial wind and temperature anomaly (ETWA). Tides produce a temperature characteristic at low latitudes similar to the DE observation, but it fails to align with the winds and electron density at all

T. J. Fuller-Rowe11 et al.

538

CTIM-DIFF meridional wind Ims) 100

Minimum

0

gsD - ut5 CPCP(kV)

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UT (hours) Pressure level 12.0 Latitude 270.0 Minimum -1.479, Maximum 2.528, Contour interval

150

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3

Fig. 4. Response of the change in meridional wind (a) and mean molecular mass (b) in response to storms at December and June solstice respectively. The figure shows the difference in the parameters between the storm simulations (runcodes gs0 for December, and gs5 for June) and a quiet reference day (runcodes ~15 for December. and a06 for June). The driven phase of the simulated storm commences at 12UT and lasts for 12h.

1539

Low-latitude disturbance effects longitudes, nor ‘generates the observed vertical winds. Chemical heating from the exothermic recombination of O+ appears to be the most likely cause of the temperature feature, and would reproduce the sense of the observed vertical winds. Interaction between tides and chemical heating is possible, either reinforcing or canceling the temperature feature. The density structure observed by Hedin and Mayr (1973) is consistent with the chemical heating hypothesis. During magnetospheric disturbances, low-latitude winds are affected by wave surges from the high-latitude source region; these wave surges arrive on a variety of time-scales. The first waves to arrive, from north and south polar regions, do so within 4 h of the onset of geomagnetic activity, propagating as internal gravity waves in the hot upper thermosphere. These waves would interact with the low latitude ionosphere directly. An electrodynamic response of the equatorial

vertical drift is apparent at Jicamarca on a similar time-scale (Fejer, 1997). Lower altitudes respond more slowly because gravity wave propagation speeds in the cooler denser lower thermosphere are slower. The global dynamical changes drive a disturbance dynamo often following, but sometimes competing with, the magnetospheric penetration

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Acknowledgements-Parts of this work were performed under NSF grant: ATM-9416557, NASA grant: NRA-92SSI-SR and T-050. and USAF support.

REFERENCES

Anderson D. N.

1973

Anderson D. N. and Mendillo

Anderson

Batista

M.

1983

D. N. and Roble R. G.

1974

I. S., Abdu M. A. and Bittencourt

J. A.

1986

Burns A. G. and Killeen T. L.

1992

Crain D. J., Heelis R. A. and Bailey G. J.

1993

Daniel1 R. E. and Anderson

1995

D. N.

Daniel1 R. E., Brown L. D., Anderson Fox M. W., Doherty P. H.. Decker Sojka J. J. and Schunk R. W. Fejer B. G.

D. N., D. T.,

Fejer B. G.

1991 1997

Fejer B. G. and Scherliess

Fejer B. G., dePaula Hanson W. B. Fesen C. G., Crowley

Fuller-Rowe11

1995

L.

1995

E. R., Heelis R. A. and

1995

G. and Roble R. G.

1989

T. J. and Rees D.

1980

A theoretical study of the ionospheric F region equatorial anomaly ~ I. Theory. Planet. Space Sci. 21, 4099419. Ionospheric conditions effecting the evolution of equatorial plasma depletions. Geophys. Res. Left. 10,541l 544. The effect of vertical E x B ionospheric ion drifts on the F-region neutral winds in the low latitude thermosphere. J. Geophys. Res. 79, 5231-5236. Equatorial F region vertical plasma drifts: seasonal and longitudinal asymmetries in the American sector. J. Geophys. Rex. 91, 12055512064. The equatorial neutral wind response to geomagnetic forcing. Geophys. Res. Letr. 19, 977-980. Effects of electrical coupling on equatorial ionospheric plasma motions: When is the F region a dominant driver in the low-latitude dynamo?. J. Geophys. Res. 98,6033-6037. Realtime operational ionospheric specification and forecast models. AIAA. 95.0549,33rd Aerospace Sciences Meeting. PIM: A global ionospheric parameterization based on first principles models. Radio Science 30, 1499-l 510. Low latitude electrodynamic plasma drifts: A review. J. Amos. Terr. Phys. 53, 677-693. The electrodynamics of the low-latitude ionosphere: recent results and future challenges. J. Amos. Ten-. Phys. 59, 146551482. Time dependent response of equatorial ionospheric electric fields to magnetospheric disturbances. Geophvs. Res. Len. 22, 851-854. Global equatorial ionospheric vertical plasma drifts measured by the AE-E satellite. J. Geophys. Res. 100, 576995776. Ionospheric effects at low latitudes during the March 22. 1979. geomagnetic storm. J. Geoph)>s. Res. 94, 540555417. A three-dimensional, time-dependent, global model of the thermosphere. J. Amos. Sci. 31, 2545-2567.

1540

T. J. Fuller-Rowe11

Fuller-Rowe11 Quegan S. Fuller-Rowe11

T. J., Codrescu

M. V., Moffett

R. J. and

1995

T. J.

Fuller-Rowe11 T. J., Codrescu M. V., Rishbeth Moffett R. J. and Quegan S.

H..

P. C., Moffett

1996

1973

Hedin A. E. and Mayr H. G. Heelis R. A., Kendall Windle D. L.

1994

R. J. and

1974

Raghavarao R., Wharton L. E., Spencer N. W., Mayr H. G. and Brace L. H. Raghavarao R., Hoegy W. R., Spencer N. W. and Wharton L. E.

1991

Rees M.H.

1989

Richmond

Rishbeth

A. D., Ridley E. C. and Roble R. G.

H.

1993

1992

1971

Schunk R. W. and Sojka J. J. Spiro R. W.. Wolf R. A. and Fejer B. G.

1995

Stening

1981

R. J.

Woodman

R. F.

1970

et al. Response of the thermosphere and ionosphere to geomagnetic storms. J. Geiphys. Res. 99, 3893-3914. The dynamics of the lower thermosphere. AGU Geophys. Monograph 87,23-36. On the seasonal response of the thermosphere and ionosphere to geomagnetic storms. J. Geophjw. Res. 101, 2343-2353. Magnetic control of the near equatorial neutral thermosphere. J. Geophys. Res. 78, 1688-1691. Electrical coupling of the E and F regions and its effect on F region drifts and winds. Planet. Space Sri. 22, 743-756. An equatorial temperature and wind anomaly (ETWA). Geophys. Res. Lett. 18, 1193-l 196. Neutral temperature anomaly in the equatorial thermosphere-a source of vertical winds. Geoph.vs. Res. Lett. 20, 1023-1026. Physics and Chemistry of the Upper Atmosphere. Cambridge Atmospheric and Space Science Series, Cambridge University Press. A thermosphere/ionosphere general circulation model with coupled electrodynamics. Geophys. Res. Lrtt. 19,601-604. Polarization fields produced by winds in the equatorial F-region. Planet. Space Sci. 19, 357-369. USU model of the global ionosphere. STEP Handbook. Penetration of high-latitude electric field effects to low latitudes during SUNDIAL 1984. Ann. Geophys. 6, 39-50. J. A two-layer ionospheric dynamo calculation. Geophys. Res. 86,3543%3550. Vertical drift velocities and east-west electric fields at the magnetic equator. J. Geophys. Res. 75, 6249% 6259.