A modelling study of the coupled ionospheric and thermospheric response to an enhanced high-latitude electric field event

0032-0633,‘93 $6.00 + 0.00 Pergamon Press Ltd

Planet. Space Sci.. Vol. 41. NO. 1, pp. 45-56, 1993 Printed in Great Britain.

A modelling study of the coupled ionospheric and thermospheric response to an enhanced high-latitude electric field event G. H. Millward,’ S. Quegan,’ R. J. Moffett,’ T. J. Fuller-RowelI’

and D. Rees’

’ Department of Applied and Computational Mathematics. University of Sheffield. P.O. Box 597, Sheffield SlO 2UN. U.K. ’ CIRES. University of Colorado/NOAA, Space Environment Laboratory, 325 Broadway, Boulder, Colorado, CO 80303, U.S.A. ’ Atmospheric Physics Laboratory, University College London. 67-73 Riding House Street, London W 1P 7PP, U.K. Received in final form 25 August 1992

Abstract. A modefling study of the effect of a shortlived, localized enhancement in the high-latitude dawnside convection electric field has been made using the Sheffield/UCL/SEL (Boulder) coupled ionosphere/ thermosphere model. The conditions imposed on the model are intended to represent a single fluctuation of the type often observed in this region and thought to be responsible for a number of dynamic effects in both the ionosphere and thermosphere. Results from the simulation show frictional heating of both ions and neutrals producing an expansion of the atmosphere. The ionospheric expansion, when combined with greatly enhanced chemical reaction rates, leads quickly to the establishment of an ionospheric trough accompanied by significant increases in the proportions of the molecular ions NO+ and 0:. The initial expansion of the atmosphere is followed by a subsequent relaxation, leading to the formation of a large-scale gravity wave which propagates equatorwards and polewards from the event region. The dynamics of the complete coupled system during the event are studied, with particular attention to coupling and feedback processes.

1. Introduction Satellite and incoherent scatter radar measurements of the high-latitude ionosphere usually reveal a considerable amount of structure. particularly in the absence of sunlight. A typical crossing of the polar cap by satellite reveals a number of peaks and troughs in ion density and. although observations and theory indicate a number of different causes, all are intimately related to properties of ionospheric convection (Rodger er nl., 1992). Grebowsky et al. (1983) studied a class of high-latitude troughs Corrrsporrderrce ro : G. H. Millward

(HLTs) which they defined as being characterized by marked troughs in the 0+ density at the altitudes of the OGO 6 satellite, accompanied by similar H’ behaviour or enhanced NO+ densities. Their most telling conclusions are that HLTs on the duskside and on the dawnside exhibit different relations to precipitation and electric field boundaries. On the duskside, the mean location of this class of HLTs is equatorward of the average trapped particle boundary and the average location of the region of largest convection electric fields. This can be understood in terms of the definition of HLTs used by Grebowsky et al. (1983) together with the normal theory of the duskside midlatitude trough (Quegan et al., 1982) and the properties of sub-aurora1 ion drifts (Spiro et al.. 1979). From early morning through to the afternoon, however, the mean location of the HLTs and the region of largest convection electric fields are coincident. whilst still remaining equatorward of the trapping boundary. (It must be stressed that this is a statistical, not a case-by-case, coincidence.) On this basis. Grebowsky et al. (1983) attributed the HLTs observed in their study to fast convection. Caseby-case studies (Killeen ef al., 1984; Baron and Wand, 1983 : Winser et al., 1986) also show that channels of fast convection are associated with electron density depletions. Travelling ionospheric disturbances (TIDs) observed at mid-latitudes have been linked to the propagation from high latitudes of atmospheric gravity waves (AGWs). Such waves are thought to originate as dynamic variations in the local atmosphere under the effect of intense Joule heating and Lorentz forcing, and therefore as a direct consequence of fluctuations in the convection electric field. Several authors have dealt with the high-latitude sources of atmospheric gravity waves (Chimonas and Hines, 1970 : Francis. 1975 ; Richmond, 1978, 1979). More recently. several worldwide atmospheric gravity wave studies (WAGS) have directly linked observations of TlDs seen at mid-latitudes to observations of electric field fluctuations at high latitudes (Williams et al., 1991, 1992). In this paper we use a coupled ionosphere/thermosphere

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G. H. Millward et al. : Ionospheric-thermospheric

model to study the effects on ionospheric and thermospheric behaviour of a localized short-lived enhancement of the convection electric field in the dawn sector. The enhancement is meant to be representative of the largest electric field fluctuations seen in this sector at high latitudes (e.g. the electric field spike described and analysed by Winser et al., 1990). As would be expected, the enhancement gives rise to increased Joule heating, atmospheric expansion and ionospheric decay, the latter leading to the formation of an ionospheric trough. These effects of enhanced convection have been discussed by several authors (Fuller-Rowell, 1984; Schunk et al., 1975, 1976). However, such discussions have been unable to deal properly with the nonlinear feedback mechanisms which link ionospheric and thermospheric behaviour under these conditions. The results in this paper are novel in that the coupled model allows us to examine these mechanisms in detail. We show that modifications of the local thermosphere play an important and complex role in the changes occurring within the ionosphere. The ionospheric changes themselves affect both the amount of energy deposited in the thermosphere by Joule heating and the amount of momentum transferred by ion drag. These two processes significantly affect the thermospheric dynamics. One of the most interesting dynamical consequences of the model conditions is the launch and propagation of an atmospheric gravity wave. Although the study was motivated by the desire to understand and quantify the formation of a high-latitude trough, the results are of wider relevance. Intense and fluctuating electric fields are frequently observed at high latitudes (Winser et al.. 1986, 1988a,b; Jones et al., 1988; Gombosi and Killeen. 1987; de la Beaujardiere and Heelis, 1984 ; Williams et al., 1990 ; HlggstrGm and Collis, 1990), and the processes induced by these fields will be similar to those investigated in this study. This is likely to be true even when the event occurs in sunlight, so that the model results have a bearing on the effects of fluctuating fields in the cusp region (Lockwood and Smith, 1989; Todd et al., 1986), although this is not discussed in this paper. The aim of this paper is to illustrate the ionospheric and thermospheric consequences of a short-lived intense electric field event, to identify the dominant processes causing these effects and to examine their time-dependent behaviour. Following a brief description of the coupled model and the input parameters used for the study in Sections 2 and 3, we consider the response of the ionosphere/thermosphere system in Section 4. The time-dependent behaviour of the processes giving rise to number density. momentum and energy changes are dealt with here, particular attention being given to the various coupling and feedback processes at work. Section 5 contains our conclusions.

2. Model description The global coupled model of the ionosphere/ thermosphere developed in collaboration between the University of Sheffield and University College, London,

response to high-latitude electric field event

has been described previously (Fuller-Rowe11 et al., 1987). For a given (possibly time-dependent) convection electric field, the model calculates the time-dependent global three-dimensional structure of the temperature, density, composition and vector velocity of the neutral atmosphere, and of the density, temperature and vector velocity of the ions O+ and H+, by solving the nonlinear equations ofcontinuity. momentum and energy. The concentrations of the molecular ion species O;, NO+ and N: are calculated using a simpler solution scheme which assumes chemical equilibrium and the electron concentration at each point is then taken to be the sum of the concentrations of the individual ionic species O+, H+, NO+, 0: and N$ (assuming charge neutrality). The solution is performed numerically on a global grid with resolutions of 2” in latitude and 18” in longitude. Each grid point rotates with the Earth to define a noninertial frame of reference in a spherical polar coordinate system, each longitude sweeping through all local times during a 24 h simulation. In the vertical dimension, the thermospheric code uses 15 grid points corresponding to pressure levels. The pressure levels are separated by a distance equivalent to one scale height, the bottom level forming a boundary defined to have a pressure of 1 Pa at an altitude of 80 km. In contrast, the ionospheric part of the code is divided vertically into 90 grid points along the field lines from a base altitude of 100 km to a maximum distance of 10,000 km, with coupling between heights in the field-aligned direction.

3. Model inputs In this study the model was run globally for December solstice using a time step of 1 min, the simulation starting at 18:OO U.T. and running until 18:30 U.T. (event times (r30 min). Initial conditions were appropriate to steady state (in a diurnal sense), this data set having been calculated under conditions of F,o,7 = 185, Tiros precipitation activity level 7 (about K,, = 3) (Fuller-Rowe11 and Evans, 1987) and Millstone Hill electric field level 7 (Foster et al., 1986). The F,, , and geomagnetic activity levels subsequently remained constant. Output from the simulation was studied in detail over the geographic latitude range from 60” to 84”. and within the morning sector from 03:36 to 12:OO L.T. To produce the enhanced electric field event, the model was subjected to a dynamic fluctuation of the convection electric field. the field having been taken from results produced by the Rice University magnetosphere model (Wolf et al., 1991). This electric field was chosen because, in the dawn sector, the field shows a strongly peaked structure in latitude which produces a narrow channel of fast ion convection dominated by its eastward component. The enhanced event was produced by boosting (and subsequently relaxing) the field globally over a time scale of 10 min to create a temporal electric field spike. Although the convection electric field was boosted globally, the intention of the modelling study was to look at the effects of localized field enhancement in the dawn sector only. The contradiction here does not prove to be a problem,

G. H. MjiIw~~d et ai. : ionospheric-thermaspt3eric

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as long as effects produced by electric fiefd enhancement in other sectors do not propagate into the region of interest within the period of the study. Over the relatively short time scales of the simulation, no such interference is passible and thus, for our purposes. the enhancement of the electric field can be considered to have been restricted to the dawn sector alone. Xn Fig. la and b, polar plots of the Northern Hemisphere show the ion velocity vectors produced as a direct result ofE x B convection, where E is taken from the Rice electric field modei. Figure Ia shows the ion velocities produced by the normal Rice field, while Fig. fb shows the vectors when the fiefd has been boosted gjobafly {in this case by a factor of 2.3). In the simulation presented here, the Rice field was increased iinearfy from its normal value (Fig. la) to the enhanced state, boosted by a factor of 3 (similar to Fig. I b. but with vectors of slightly greater magnitude) over the first 5 min of the event. The field was then decreased linearly in exactly the same way, such that at an event time of 10 min the convection was again of normal rn~~~~~~~e. Figure 2 shows a plot of the two components of the

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Fig.2. Southward ff and eastward (-----f components of the convection electric field plotted against latitude for an event time of 5 min (greatest field enhancement)

convection electric field (positive southwards and eastwards) plotted against latitude at an event time of 5 min (i.e. maximum fidd ~hanceme~t~. In this figure, and in afI subsequent figures showjng parameters plotted against latitude, the Iongitude is 162%; thus at the start of the event (I 8:OOU.T.> the local time is 04:48 L.T, The southward electric field component clearly dominates here and shows a strongly peaked structure in latitude with a maximum value (at 74”N) of about 200 mV m- ‘. The zonal and meridional components of the ion convection velocity at 300 km are plotted against latitude for three diFferent event: times in Fig, 3a and b. Peak eastward velacities at the centre of the channel of around 4 km s-l are produced at an even? time of 5 min.

4. Model results In describing the resrrlts obtained from this study. we response and then the ionospheric response to the event. However, it must be stressed that the model used here consists of a fully coupled and dynamic ionosphere/thermosphere system, This model is therefore capable nf producing realistic simuiations of the coupling and feedback between ionospheric and thermospher~c processes which have previousiy been considered separately. We shall return to this point EnSection 4.3. first describe the thermospheric

Fig* X, Potat plots showing ion vefocity vectors resultirtg directly from Ex % convection, with E having been taken from the Rice rnagnet~s~~e~c mod&. s Normai convection : b enhanced convectian

The thermospheric response to the imposed efectric field enhancement is characterized by an expansion of the atmosphere under the effect of Joule heating, followed by a subsequent relaxation. The heights of the neutral air pressure levels are plotted against latitude, at event times of 5, IO, 20 and 30 min, in Fig. 4a, b, e and d, respectively. Also shown are the ckmges in the rner~dj~~a~~vert~ca~ wind vectors compared with the winds at event time zero, i.e. the arrows show the components of u(t) - u(O), where u(r) is the neutral gas velocity at time t. The expansion of

G. H. Millward ef al. : Ionospheric-therrnospheric

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Fig. 3. a Zonal component of the ion velocity (positive eastwards) plotted against latitude for event times of 1 (-). 5 (----) and 10 min (---.-); b meridional component of the ion velocity ), 5 (----) and 10 min (positive equatorwards) plotted against latitude for event times of 1 ((-.-.-)

the atmosphere at earlier event times (Fig. 4a, b) is due to temperature increases at higher altitudes caused by Joule heating. The Joule heating per unit mass peaks in the Fregion, and it is here that the greatest expansion occurs. The upwelling created by this atmospheric expansion produces enhanced densities of all thermospheric constituents (0, 02, N2) at a fixed height. Figure 5a and b shows the molecular densities of 0, and N2 at 300 km plotted against latitude for four event times. Both show increases of

around a factor of 3, with maximum values being attained at an event time of 10 min. These increases are particularly relevant to the chemistry of the ionosphere and are discussed further in Section 4.2. The winds, driven vertically by the expansion. are probably slightly overestimated in this simulation, since the assumption of hydrostatic equilibrium. implicit in the constant pressure levels of the thermosphere code. cannot be strictly applied to a dynamic situation such as this one. By an event time of

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Fig. 4. Heights of neutral air pressure levels 7-15 plotted against latitude for four different event times: a 5 min : b 10 min : c 20 min; and d 30 min. Vectors show the changes in the neutral velocities in the vertical/meridional plane (relative to their initial values. see text). For clarity, vectors representing velocities of less than 50 m s- ’ are not shown

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Latitude ldeg) Fig, 5. a Density of N2 at 300 km plotted against latitude for event times of 5 min (-), 10 min (----). 15 min (-.--.-) and 30 min (.. ...) ; b density of 0, at 300 km plotted against latitude for event times of 5 min (-), 10 min (----), 15 min (-.-.---) and 30min (.....)

10 min, the maximum expansion of the neutral air has already occurred. At the centre of the event, the vertical motion is just beginning to show a collapse of the thermosphere (see Fig. 4b). Figures 4c and d show the further relaxation of the atmosphere in the event region and the propagation of a large-scale atmospheric gravity wave. To understand fully the physical processes and their relationship during the event, it is helpful to consider the significant terms in the equations of momentum and energy which arise in the pressure level coordinate system of the neutral code. We first discuss the energy balance of the thermosphere. Figure 6 shows a plot against event time of all the significant terms in the energy equation at 162”E, 74”N and pressure level 12 (approximately 300 km). The terms included here are: vertical advection of heat. adiabatic

heating/cooling and Joule heating. Contributions from viscous heating, particle heating, horizontal advection, vertical heat conduction, solar heating and infrared cooling are insignificant and are not shown. The figure shows clearly the dramatic increase in Joule heating at earlier event times, a subsequent decrease and then a levelling out after an event time of about 10 min, such that the steady-state Joule heating rate after the event is only about half of its initial value. The decrease in the Joule heating rate after the event compared with its initial value occurs as a direct result of ionospheric decay during the event (discussed in Section 4.2). This effect, which is clearly dependent upon feedback between the ionosphere and thermosphere, causes the ion-drag term in the momentum equation to show similar behaviour (see Fig. 7). The other terms which have a significant effect in the

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terms in the energy equation at pressure level 13, longitude 162”E and latitude 74”N plotted against event time. The terms shown are Joule heating (- .-.--), vertical advection of energy (----), adiabatic cooling (. ....) and the total heating rate (--)

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Event Time 1mins) Fig. 7. Zonal components of the significant momentum equation

terms at pressure level 12, longitude 162”E and latitude 74”N plotted against event time. The terms shown are ion drag (-. -). vertical advection of momentum (. . .), pressure term (----) all the terms

and the total forcing

(-)

which is the sum of

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time of 5 min and then decreases as the electric field is relaxed, attaining a roughly constant value after an event time of 10 min. Decay of the ionosphere during the event means that the steady-state value of the ion-drag after the event is only about half of its initial value (as discussed above for Joule heating). In addition to ion-drag, vertical advection of momentum acts to decelerate the gas, a peak value being attained at an event time of around 12 min. The physical process at work here is that, as gas upwells through the pressure levels, it drives a positive gradient in the height profile of zonal wind velocity. At a given pressure level, gas with less initial momentum is introduced from below. This manifests itself as a negative acceleration, although it must be remembered that this arises from a vertical transport of gas across the pressure levels. The upwelling of gas through the pressure levels is associated with a divergence in the meridional wind flow, and is identical to that which produces cooling in the energy balance equation. In contrast, the meridional wind is driven by the gradient in the neutral air pressure which occurs with respect to latitude across the event region. In the pressure level coordinate system, an increase in the local pressure at a fixed height manifests itself as an increase in the height of the associated pressure level. The two are completely equivalent. On the poleward and equatorward sides of the event region (as shown in Fig. 8a and c) the forcing of the meridional wind is dominated by the raising of the pressure level which peaks at 74”N, producing a meridional pressure gradient. Equatorwards (Fig. 8a), the pressure term shows a positive peak at an event time of around 8 min (the time of maximum atmospheric expansion) indicating equatorwards forcing. Likewise, Fig. 8c shows a negative peak at the same time and for the same reason, this time the forcing being polewards. An important difference between the meridional forcing on each side of the event region is that in Fig. 8a the meridional component of ion-drag acts to enhance the peak forcing due to the pressure gradient, whereas in Fig. 8c it acts in opposition. This factor is responsible for producing a greater overall amount of forcing on the equatorward edge than on the poleward. Vertical and horizontal ad-

energy balance equation are vertical advection of heat and adiabatic cooling. These two terms have a cooling influence which peaks at an event time of about 12 min. The cooling produced by these terms is dependent upon upwelling through the pressure levels. Cooling due to vertical advection of heat arises because of the positive gradient in the temperature profile of the thermosphere. Upwelling of gas relative to the pressure levels means that, on a given pressure level, cooler gas is being introduced from below. Such gas also cools adiabatically, since upwards motion across the levels of constant pressure means that the pressure of the gas is decreasing. The upwelling itself results from continuity considerations and the fact that during the event there is a divergence in the meridional wind. The cause of this divergence is a pressure gradient in the meridional direction (expressed as a raising of the pressure levels), itself caused by the expansion of the atmosphere in the region of the event. Thus, we see here a full three-dimensional coupling of the momentum and energy equations with vertical transport (advection) of gas playing a particularly important role. The sources of momentum driving the eastward and southward components of the neutral wind are markedly different. Figures 7 and 8 show the relevant terms in the zonal and meridional directions, respectively, plotted against event time. These are : ion-drag, pressure gradient and advection (transport) of momentum. In Fig. 7, only the vertical component of advection is shown, since the horizontal contribution is insignificant. In Fig. 8, both the horizontal and vertical components of advection are shown. Momentum sources due to viscous coupling and Coriolis are also included in the calculations of the neutral dynamics, but are insignificant during this simulation and thus are not shown. In Fig. 7. the data are plotted at the centre of the event (74-N). Figure 8 shows three plots corresponding to three adjacent latitudes in the model [(a) 72 N. (b) 74~N. (c) 76’ N] with plot (b) again at the centre of the event. Positive values here indicate forcing in the eastward and southward directions. In the zonal direction. the forcing at earlier event times is dominated by ion-drag which shows a peak at an event

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Fig. 9. a The height of the neutral air pressure levels 6-15 at longitude 162”E and latitude 74”N, plotted against event time; b temperature of the neutral gas on the pressure levels whose heights are given in a plotted against event time. In the model the temperature on pressure level 15 is identical to that on level 14

vection of momentum increase in opposition to both of the initial pressure gradient peaks, with a maximum response at around an event time of 13 min. At the centre of the event (Fig. 8b), the situation is less clear due to a complex response of the pressure term, which is of relatively small magnitude. Figure 9a shows the height of the neutral air pressure levels at 162”E and 74”N plotted against event time with Fig. 9b showing the neutral temperature on the same levels produced as a result of the dynamics described above. The zonal, meridional and vertical components of the neutral wind velocity on pressure level 12 are shown plotted against event time in Fig. lOa, b and c with the three figures showing each of the components at the latitudes 72”N, 74N and 76”N. respectively. It is noted that. during the event, the greatest increases in meridional wind vel&ity (for instance those on the equatorward edge, Fig. 10a) are much greater than the greatest increases in zonal wind. As shown above. the zonal wind is dependent initially on ion-drag, and the meridional wind upon the pressure gradient across the

event region (and thus dependent on the Joule heating and its extent in latitude). Thus, we see that for this event the dominant motion of the neutral gas is perpendicular to the direction of forcing by the ions. This is very different from the steady-state situation in which winds driven by ion convection have a tendency to move in the same direction as the ions. The vertical component of the wind shows a sharp spike at the beginning of the event (Fig. lob). This velocity is the real velocity relative to fixed heights and is equal to the sum of the motion of the pressure levels and upwelling through the levels. After an event time of 10 min, the vertical velocity remains positive (or zero), although the pressure levels are clearly moving downwards at this time (Fig. 9a). Therefore, relative to these pressure levels, there must be a positive upwards velocity. It is this positive velocity which is responsible for the adiabatic and advection cooling effects and the deceleration of the zonal gas. It is noted that for this event, characterized by enhanced eastwards ion convection, the gravity wave propagates,

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for F-region altitudes, at right angles to the Lorentz forcing of the ions. It is therefore apparent in this case that the source of the gravity wave is Joule heating of the neutral gas by ion friction and not Lorentz momentum forcing. Further studies of the AGW as it continues to propagate to mid- and low-latitudes have been undertaken and will be presented fully in future work.

response to high-latitude electric field event

account the “anisotropic” behaviour of the ion temperature. Critical to an understanding of the ionospheric response to enhanced ion temperatures are the chemical processes at F-region heights resulting in loss of O+ ions. These are : 0++N2-+NO++N, with reaction

o++o,

4.2. Ionospheric response The ion temperature

(r)

as :

in the model is calculated

T, = Tn+$$V,-VA’.

+o+o;,

(3)

with reaction rate k?. The decay rate of O+ due to these reactions is given by :

W+l ~ dt = -(k, [NJ +k2P~W+l,

(1)

where T, is the temperature of the neutral gas, &I, is the mean molecular mass, k is Boltzmann’s constant and Vi and J’” are the three-dimensional vector velocities of the ion and neutral gases respectively. The second term on the right-hand side of this equation is the contribution to the ion temperature from frictional heating with the neutral gas. At the peak of the event (in both latitude and time), with relative ion-neutral velocities of about 4 km s-‘, this term becomes dominant, yielding ion temperatures in excess of 10,OOOK (Fig. 1 I). The squared dependence of the ion temperature on the velocity produces a much “sharper” structure with respect to latitude than the velocity itself. It should be pointed out that, for convection velocities of the magnitude produced in this event, there is. in reality, no single parameter T,. The ion temperature along the field line, which is solely responsible for fieldaligned diffusion, is different from the temperature perpendicular to the field, the latter of which makes the major contribution to the effective temperature T,*, important in the chemical recombination processes discussed below. In future, when the ratio of the two components is known with certainty, refinements to the model will take into

(2)

rate k,, and :

(4)

where [N?], [O,] and [O+] denote the densities of the respective constituents, and the rate coefficients k, and kz (units cm3 s-l), taken from Torr and Torr (1979), are given by : k, = 1533x

10-“-5.920x

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(6) The effective temperature

T,, is given by :

(7)

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Fig. 11. Ion temperature at 300 km plotted against latitude for event times of 1 min (--). 5 min (----) and 10 min (-._.--)

where M, z M(N,) in equation (5) and M, = M(0,) in equation (6). As Tew increases, both k, and k2 initially decrease. attaining minimum values for T,, of 1050 and 1650K. respectively. Above these values, k, and k? increase very sharply. Thus, as the relative ion-neutral velocity increases, there is no increase in the rate of O+ decay until the convection velocity has exceeded about 1 km s ’ . For ion convection velocities of around 4 km s- ’ , k, and kz are greatly enhanced. Figures 13a and b show the values of k, and k2 at 300 km plotted against latitude for event times of 1, 5 and IO min. Both coefficients show increases of over 2 orders of magnitude coincident with the position of greatest field enhancement, peak values being attained at an event time

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(deg)

Fig. 12. a Rate coefficient k, (0+-N?) at 300 km plotted against latitude for event times of 1 min (--). 5 min (----) and 10 min (---.--_): b rate coefficient kz (0+-O,) at 300 km plotted against latitude for event times of 1 min (---), 5 min (----) and 10 min (- .-. -)

of 5 min. In addition, and as previously mentioned, the densities of NZ and 0: at 300 km show increases during the event of about a factor of 3. with peak values being reached at an event time of about 10 min (Fig. 5a and b). The increases in [N?] and [0,] help, at a fixed height, to keep the 0’ decay rate high at later event times. However, care must be taken in considering ionospheric effects at a fixed height. The vertical motion of the ionosphere, which is similar at F-peak heights to that of the associated neutral pressure levels. means that a more natural framework for analysing the chemical decay reactions is based on the pressure levels themselves. On pressure level 12, the densities of N2 and 0: actually decrease during the initial stages of the event-and therefore act in opposition to the increased 0’ decay rate! A subsequent increase in the densities of both constituents occurs at later event times, producing a factor of 1.5 increase (relative to initial values) at an event time of about 22 min. Analysed in this frame. the very marked increase in the 0’ decay rate is seen to be wholly due to the enhancements in k, and lit. Accompanying the increase in O- decay is a redtstribution of the plasma. Thermal expansion. a further consequence of the greatly increased ion temperature, produces a large upward flux of O+ ions out of the F-region. The response of the ionospheric ion densities to the greatly enhanced convection is shown in Fig. 13a, b. c and d for event times 0. S. 10 and 15 min. respectively. Each plot shows vertical density profiles for ions 0’. NO’ and 0: and the electrons (calculated simply as the sum of the densities of the three positive ions). at the point of greatest field enhancement (74 N). The combined effect of the increased O+ decay rate and the thermal expansion is seen. after 5 min to have produced an ionosphere depleted to about 40% of its initial density at F2-peak heights. In addition there is an increase in h,,,F2 such that the ion depletion at a fixed height of (say) 300 km is greater than the changes in N,,,F2. Accompanying the ionospheric decay are large increases in the proportion of both NOandOT, with NO’ being the more significant. This causes the transition height from NO’ to O+ dominance to

increase from 200 to nearly 300 km in the first 5 min of the event. It should be remembered that, in this simulation. the profiles of NO+ and 0: are due to ionospheric chemistry alone [expressed mainly in equations (5) and (6). but involving other reactions as well]. It is noted that during the event we have a situation in which the dominant ion at around 300 km is no longer 0+ but NO+, and thus a full simulation should allow for diffusion of this ion. The subsequent relaxation of the electric field (after an event time of 5 min) results in a downward flux of O+ ions back into the F-region so that at later event times (Fig. 13~. d) the ionosphere has been partially re-established.

4.3. Ionosphere/thermosphere

coupling and feedback

Figure 14 shows the processes affecting the neutral and ionized gases in the presence of strong convection electric fields (after Rodger et al., 1992). The coupling and feedback between processes is to be noted here, as is the fact that for interpretation in any physical context, consideration must be made of the different characteristic time scales and the transport of the effects out of the source region. An example of the coupling and feedback produced in the model is the fact, already mentioned in Section 4.1. that the ionosphere, which is responsible for the dynamic input of energy and momentum to the neutral atmosphere, is itself severely eroded in the process. Thus, it is seen in Figs 6 and 7 that the terms relating to Joule heating and ion-drag are much smaller after the event than immediately before it. There are two important points here. Firstly, it seems likely (though as yet not fully tested) that there is a limit to the amount of energy and momentum which the ionosphere can deliver into the thermosphere during an event such as this. The reasoning here is that if a larger electric field spike (either in magnitude or duration) is applied then there will also be a corresponding increase in the

G. H. MiIlward et al. : Ionosphe~c-the~osphe~c

54

response to high-latitude

electric field event

(bf

Ion Dens&y

(rn-?

), NO+ (----). 0: (--.-.--) and e- (.‘*‘e) Fig. 13. Height profiles of density for ions Oi (for four different event times : a 0 min (initial profiles) ; b 5 min : c 10 min ; and d 15 min STRONG ELECTRICFIELD

, JOULE

1

THERMAL

UPWELLING SPECIES

EXPANSION

I OF HOLECULAR IO,. NO. N, I

-

CHANGE TO PRESSURE GRADIENT

TI ENHANCED

i MODIFY NEUTRAL WIND

+_, ENHANCED PLASMA DIFFUSION

I

t

1 INCREASED

FIELD ALIGNED ION MOTION

/

I

i

1

ION HEATING

HEATING

1

I

I FIELO

RECOMBINATION

ENHANCED RECOMEINATION COEFFICIENT k! & k,

ALIGNED

n

ACCELERATION

*

f\

I

--

4

I ION NEUTRAL COUPLING TIME INCREASED

REDUCTION OF ELECTRON CONCENTRATION

t

I

IONlSATlONTROUGH FORMED

Fig. 14. Geophysical processes occurring in regions of strong electric field and their interrelations decay of the ionosphere. either due to increased reaction rates or prolonged decay time, and thus there will be a tail-off in the effectiveness of increasing the field. A second and related point is that, with the ionosphere in its depleted state after the event, a subsequent event of similar magnitude occurring (say) 30 min after the first would not be able to input as much energy to the thermosphere, and thus all of the thermosphere effects we have noted (i.e. AGW, etc.) would be of diminished stature. Thus, a series of enhanced electric field events occurring in the absence of ionospheric production would result in smaller thermospheric responses each time. This

argument assumes that the enhancement of the electric field extends over a considerable distance in longitude, so that the region in which enhancement of the field occurs has not been completely resupplied with plasma from regions where the ionosphere has not been previously depleted.

5. Conclusions In studying the ionospheric and the~ospheric response to a dynamic ~uctuation in the convection electric field, it

G. H. Millward et al. : Ionospheric-~he~osphe~c

response to high-latitude electric field event

is not sufficient to describe any process in isolation. In explaining, for example, the response of the neutral wind in the eastward direction, it is impossible to ignore the three-dimensional dynamics and energetics of the coupled ionosphere/thermosphere system as a whole. The sharp increase in the convection electric field produces large eastward ion velocities. Frictional heating of both ions and neutrals produces greatly enhanced ion and neutral temperatures and an expansion of the atmosphere in the event region. Thermal expansion of the ionosphere, coupled with greatly enhanced reaction rates for the decay of ionospheric 0’. leads to the formation of a high-latitude ionospheric trough and an increase in the proportions of NO’ and Oz, such that NO+ becomes the dominant ion to over 300 km. The expansion of the atmosphere produces a latitudinal pressure gradient across the event region leading to a divergent meridional wind. In the zonal direction the pressure gradient is insignificant (since the enhancement in the field extends completely around the dawn sector) and the zonal wind is initially dominated by ion drag, forcing the gas eastwards. The Joule heating and Lorentz forcing of the neutral gas decrease as the electric field is relaxed. UpweIling of

gas relative to the pressure levels, the result of a divergent meridional wind, gives rise to cooling (via vertical advection of energy and adiabatic cooling) and a deceleration in the zonal direction (vertical advection of momentum). The cooling produces a relaxation of the atmosphere in the event region and the propagation of a large-scale AGW. The largest component of the wind produced during the event is in the meridional direction. Acknolrledgentents. Discussions with P. J. S. Williams have been very helpful. This work has been partly supported by SERC under grants GREi90571 and GRGj33973 and by CIRES! University of Colorado~~OAA Space En~~~ronrn~n~ Laboratory.

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55

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response to high-latitude electric field event

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theor-