The generation of vertical thermospheric winds and gravity waves at auroral latitudes—I. Observations of vertical winds

Plmer.SpaceSci.,Vol.32,No.6,pp.661~84, Printed in Great Britain.

0032-0633/84$3.00+0.00 Pergamon Press Ltd.

1984

THE GENERATION OF VERTICAL THERMOSPHERIC WINDS AND GRAVITY WAVES AT AURORAL LATITUDES-I. OBSERVATIONS OF VERTICAL WINDS D. REES,* R. W. SMITH,? P. J. CHARLETON,* F. G. McCORMAC,t *Department

of Physics

N. LLOYD* and AKE STEEN1

and Astronomy,

University College London, Gower Street, London WClE 6BT, U.K. t Ulster College, The Northern Ireland Polytechnic, Newtownabbey, Co. Antrim BT37 OQB, Northern Ireland $ Kiruna Geophysical Institute, Kiruna, Sweden (Received 26 July 1983) Abstract-Observations of vertical and horizontal thermospheric winds, using the 01 (3P-ID) 630 nm emission line, by ground-based Fabry-Perot interferometers in Northern Scandinavia and in Svalbard (Spitzbergen) have identified sources of strong vertical winds in the high latitude thermosphere. Observations from Svalbard (78.2N 15.6E) indicate a systematic diurnal pattern of strong downward winds in the period 06.00U.T. to about 18.00 U.T., with strong upward winds between 20.00 U.T. and 05.00 U.T. Typical velocities of 30 m s- 1 downward and 50 m s- I upward occur, and there is day to day variability in the magnitude (3&80 m s-l) and phase (+/- 3 h) in the basically diurnal variation. Strong and persistent downward winds may also occur for periods of several hours in the afternoon and evening parts of the aurora1 oval, associated with the eastward aurora1 electrojet (northward electric fields and westward ion drifts and winds), during periods of strong geomagnetic disturbances. Average downward values of 3&50m s- ’ have been observed for periods of 4-6 hat times oflarge and long-lasting positive bay disturbances in this region. It would appear that the strong vertical winds of the polar cap and disturbed dusk aurora1 oval are not in themain associated with propagating wave-like features of the wind field. A further identified source is strongly time-dependent and generates very rapid upward vertical motions for periods of 15-30 min as a result of intense local heating in the magnetic midnight region of the aurora1 oval during the expansion phase of geomagnetic disturbances, and accompanying intense magnetic and aurora1 disturbances. In the last events, the height-integrated vertical wind (associated with a mean altitude of about 240 km) may exceed lo&150 m s- ‘. These disturbances also invariably cause major time-dependent changes of the horizontal wind field with, for example, horizontal wind changes exceeding 500 m s-l within 30 min. The changes of vertical winds and the horizontal wind field are highly correlated, and respond directly to the local geomagnetic energy input. In contrast to the behaviour observed in the polar cap or in the disturbed afternoon aurora1 oval, the ‘expansion phase’ source, which

corresponds to the classical ‘aurora1 substorm’, generates strong time-dependent wind features which may propagate globally. This source thus directly generates one class of thermospheric gravity waves. In this first paper we will consider the experimental evidence for vertical winds. In a second paper we will use a threedimensional time-dependent model to identify the respective roles of geomagnetic energy and momentum in the creation of both classes of vertical wind sources, and consider their propagation and effects on global thermospheric dynamics.

mid-latitude station (Fritz Peak), which he ascribes to the passage of gravity waves from an assumed aurora1 (high-latitude) source. Spencer et al. (1976, 1982) have observed vertical winds in the thermosphere by means of a specially adapted satellite-borne mass spectrometer-the Wind and Temperature Spectrometer (WATS). Their observations, at altitudes generally above 300 km, indicate that the largest vertical winds (of the order of 100-200 m s-l) occur in the vicinity of the aurora1 oval and during disturbed geomagnetic conditions. The satellite vertical wind observations do not allow an unambiguous distinction between those structures in vertical winds which are associated with quasi-stationary features of the thermospheric wind field, driven by slowly-varying geomagnetic sources of energy and momentum, and

1. INTRODUCTION

In low- and mid-latitude regions of the thermosphere, the vertical wind velocity, averaged over spatial regions of the order of 1000 km and periods of the order of 1 or 2 h, and associated either with the diurnal ‘breathing’ of the atmosphere, responding to solar e.u.v./u.v. heating, or to the more dramatic hemispheric or global circulation cells generated by high-latitude heating during geomagnetically disturbed periods, will not exceed values of the order of 2-5 m s-l at altitudes of about 300 km (Geisler and Dickinson, 1968; Hernandez and Roble, 1979). Hernandez (1982) has recently described observations of local and short period oscillations, up to 50 m s- 1amplitude and about 40 min period, from a Fabry-Perot interferometer at a 667

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those which are generated in response to rapidly timedependent energy and momentum sources. The data of Hernandez (1982) relate predominantly to the passage of propagating waves from a distant (assumed aurora1 oval) source. In the middle and upper thermosphere (above 200 km), vertical wind velocities of several tens to more than 100 m s-r can be generated in quasi-stationary or time-dependent situations respectively by intense local heating (l&100 mW me*) and acceleration of the thermosphere by geomagnetic phenomena. These large vertical winds have extremely important effects on the energy balance of the thermosphere and combine with strong horizontal winds to provide the dominant mechanism for the large-scale redistribution of major and minor atmospheric species within the thermosphere during and following periods of elevated geomagnetic activity (Mayr and Volland, 1974 ; Priilss, 198 1). The measurement of vertical winds in the thermosphere by optical interferometric techniques is difficult due to the relatively small amplitude of vertical winds and the experimental problems of retaining adequate short and long term stability (or at least calibration) of the absolute “zero velocity reference” of a typical instrument. Obtaining high time resolution with a limited photon flux, and the need to describe the horizontal wind field during the period of the vertical wind observations have been major factors in impeding a detailed empirical study of some very interesting thermospheric phenomena. The observations described here have been made by two distinct instruments. They reflect a combination of improving technology as applied to Fabry-Perot interferometry, and the introduction of the experimental methodology necessary to make unambiguous determinations of vertical winds. Observations of the vertical wind must be placed in the context of the complete thermospheric wind field, and also related to the causative mechanisms. In this paper we will present the wind observations with a description of the geophysical phenomena which appear to be associated with the generation of vertical winds. In our second paper, using the UCL three-dimensional, timedependent global thermospheric model (Fuller-Rowe11 and Rees, 1980), we examine the generation of vertical winds under a variety of geophysical situations. Heat sources, buoyancy effects and the divergence of horizontal advection are all involved in determinining the vertical wind speed, and its characteristic periods, scale sizes and history. 2.

INSTRUMENTATION

During 1976/77, a piezo-electric scanning FPI (Smith and Sweeney, 1980) was operated from Skibotn,

et al.

Northern Norway (20E, 69N). Vertical winds were measured by this instrument on two nights in 1977. On the night of 7/8 November the instrument scanned alternately between the zenith and the North direction (zenith distance 60”) with a time resolution of the order of 15 min per pair of observations. On 22/23 October the instrument looked only at the zenith with a time resolution of about 7 min. These time resolutions were consistent with a 1 sigma error of about 10 m s- I, given the existing aurora1 enhancement of the 01 630 nm emission. With some minor modifications, this same instrument was used at Longyearbyen (Svalbard) during the 1980/81 and 81/82 winters. Data from a total of seven periods of 24-h observations represent an extremely interesting survey of the diurnal behaviour of the vertical wind in the thermosphere. Since the instrument was measuring, at the same time as the vertical winds, either the horizontal winds or the ion drifts, the time resolution is of the order of 30-60 min and short period fluctuations are thus not observed. During the winters of 1980/81, 81/82, 82/83, a new instrument based on a stable and rugged etalon construction, and obtaining a large multiplex advantage from the use of an Imaging Photon Detector (Rees et al., 1980,198l; McWhirter et al., 1982) was used at Kiruna, northern Sweden. In 1980/81 and 81/82, the instrument was used primarily to measure horizontal winds at a zenith distance of 60 degrees. In 1982/83, the instrument, which has a throughput comparable to the TESS described by Hernandez (1982), was used to scan the zenith, the four cardinal directions and also the Northwest and Northeast directions. The standard time resolution for a sequence of six measurements of horizontal components, the vertical component and a neon calibration lamp spectrum, with a S.E. of 10 m s - ’ was 15 min, with this being reduced to 8 min or to 4 min during lines of bright 630 nm emission. 3. THE DATA

(i) 7 November 1977 (Skibotn) Data from 2 nights’ observations in 1977 (Skibotn) are shown in Fig. 1. The ground-based magnetic data are also shown, as is the intensity of the 630 nm emission, and the zenith distance of aurora1 arcs as deduced from a meridian-scanning photometer. On 7/8 November, the instrument alternated between North and zenith directions, while on 22123 October, the instrument exclusively viewed the zenith, so that no simultaneous measurements of the horizontal wind field are available on this latter occasion. On November 7/8, prior to 21.25 U.T., the vertical wind was downward, 2G50 m s- r, but changed to upward about

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FIG. 1. DATA FROM SKIBOTN, 1977. Panel (a) shows a compilation of measurements made between 21.00 and 01.00 U.T. on the night of 22123 October. Panel (b) shows a similar compilation for 2O.Otk24.00 U.T. on 7 November. Subsections (i)(v) of each panel are on the same time axis, and show, respectively, the zenith intensity at 630 nm, the vertical component of thermospheric wind, the thermospheric temperature at the height ofthe wind measurement, the meridional elevation of aurora1 arcs at 630 nm and the X-component of the local magnetogram.

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20 m s- ’ during the overhead passage of aurora1 arcs at the time of a modest (150 nT) negative magnetic bay disturbance. As the arcs moved from the zenith and the bay subsided, the vertical wind became downward again (20-50 m s-i) before turning upward again after 23.00 U.T. This latter event was not associated with either (local) magnetic or aurora1 arc activity. The meridional wind to the North of Skibotn was weakly (100-150 m s-i) equatorward throughout this period.

(variously 412 h), while during the daytime period the trend is reversed, and for about 6 h the winds are systematically downward. The maximum amplitudes rarelyexceed 1OOms - ’ ; there are significant day to day fluctuations in the amplitude and phase of this diurnal “breathing” of the thermosphere. A statistical average of the data in January 1980, while confirming the trend, shows that the maximum upward wind (50 m s-l) is at 24.00 U.T., and that the maximum downward wind (30 m s-i) occurs about 10.00 U.T.

(ii) 22/23 October 1977 (~~j~o~n) On the night of 22/23 October (1977), the trend of the vertical wind was to change from a downward wind of about 30 m s-l in the period up to 21.50 U.T. to about 50 m s-i upward about 22.00 U.T. There were then some short period oscillations between modest downward values and larger upward values before a longer period of downward winds between 23.50 and 01.00 U.T. The geophysical record shows that the downward winds in the early evening coincided with a small positive bay, and there was a strong relationship between the periods of upward winds, negative excursions (in AX) of the magnetometer, associated with the expansion phase of substorms, about 22.00 U.T. and near 23.50 U.T., and the transit ofauroral arcs near the zenith. There was no particular relationship between the magnitude of the vertical wind, the actual magnetic perturbation or the intensity of the 630 nm emission, although the combination of time resolution and errors (10 m s ‘) may mask a significant result. The data suggest that the maximum duration of the periods ofstrong upward winds are similar to the typical sample periods of observations (i.e., 15-30 min). The large apparent magnitude of the upward winds, 5-10 times the standard deviation, and the association with aurora1 and magnetic events, suggests that the phenomena is real despite the natural caution of accepting and interpreting data with wide fluctuations between successive data points.

(iv) November/December 1980 (Kt’runu) In Fig. 3, three particular events relating to the behaviour ofthe horizontal thermospheric windduring theexpansion phase ofgeomagneticdisturbanceevents (aurora1 substorm), which were observed during the Energy Budget Campaign (Offermann, 1981; Rees et al., 1982, 1984) are shown. However, on these occasions, wind data were only measured in the four cardinal directions. Each of these three disturbances had their brightest aurora1 emissions and their eiectrojets centered to the South (equatorward) of Kiruna. Comparing the horizontal wind changes with the ground-based magnetic records shows that the major change in the horizontal winds was a sharp reduction in a pre-existing equatorward wind observed to the North of Kiruna, which coincided with the period of peak intensity of the aurora1 input during the substorm. Sometimes, close to the peak of the disturbance, the wind to the North of Kiruna blew poleward for a short period (30 November, 02.00 U.T. and 19 November at 23.00 U.T.). Depending on the local time relative to magneticmidnight, thezonal wind response varied. A major increase in the eastward wind velocity usually occurred, particularly to the East of Kiruna and in events occurring after magnetic midnight, indicating that there was an enhanced eastward ion drag acceleration, associated with the aurora1 and magnetic disturbance, and which was a major factor in the high-latitude thermospheric response. Equatorward of Kiruna, there was a surge of the pre-existing southward wind immediately following the magnetic disturbance. After the end of the major energy input, the zonal winds and the meridional winds equatorward of Kiruna slowly moderated, but often stayed at values higher than those preceding the disturbance for periodsofa few hours (subject to further activity). The meridional wind to the North of Kiruna returned equatorward after the end of the substorm, anditssubsequent equatorward value usually exceeded that preceding the disturbance for a period of 3b-60 min. The phase of the reduction of the meridional wind poleward of Kiruna coincided with the most rapid upward winds shown in the earlier Skibotn data, and

(iii) ~u~~ury 1980 (~u~~bur~) In Fig. 2, the results of some 7 nights’ observations from Longyearbyen (Svalbard) in January 1980 are shown, Since there are rarely more than two observations of the vertical wind component per hour, it would not be reasonable to expect that the short-term fluctuations, which appear to dominate the Skibotn data in response to aurora1 arcs and individual aurora1 and magnetic disturbances, would stand out clearly in these data. Indeed, the data show only slow long-period trends, the most important of which is that during the the thermospheric wind is “nighttime” period, systematically upward for a period of several hours

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FIG.2. DATAFROMLONGYEARBYEN,S~ITZBERGEN,INJANUARY~~~~SHOWINGTHEVERTICALCOMPONENTOFTHE THERMOSPHERICWINDASAFUNCTIONOFU.T.PMTTEDONA COMMONTIMEAXIS.

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(v) 23 November 1982 In Fig. 4, a comprehensive set of wind observations from 23 November 1982, is shown. In this case, the lineof-sight wind components from six horizontal locations are available, in addition to the zenith observation (and an independent reference spectral calibration). This

data set described precisely the sequence of events pieced together from the earlier Skibotn and Kiruna data. There were two step-like increases of the IMF total field IBI about 10.00 U.T. (5-12 nT) and 17.00 U.T. (12-> 20 nT) timed at the ISEE- spacecraft, upstream from the Earth in the solar wind. During this period, the IMF ‘Z’ component was oscillating between southward and northward with a quasi-period of about 2 h, and with an increasingly southward tendency until

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23.00 U.T. The ‘Y’ component had been mainly negative before 17.00 U.T., became mainly positive between 17.00 and 22.00 U.T. oscillated between 22.00 U.T. and 01.00 U.T. (24 November) then settled negative, with a northward IMF and strong IBl until 16.00 U.T. (24 November) when it switched southward to trigger additional major geomagnetic disturbances. On 23/24 November, ISEE- was a long way from the direct Sun-Earth line and it is therefore not realistic to expect a perfect correlation between short period fluctuations of the IMF at the spacecraft and the detailed geomagnetic response. Nevertheless, the increasing southward IMF tendency until about 23.00 U.T. has to be the direct cause ofincreased geomagnetic activity and the thermospheric wind response.

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Between 18.OOand 18.40 U.T., westward winds to the West of Kiruna and southward winds to the North of Kiruna intensified from 100 to 300 m s-i and 25&450 ms-i respectively, although there was relatively little local magnetic activity, and the aurora1 arcs, while moving equatorward and brightening, were “quiescent” in form. The impulsive aurora1 and magnetic disturbance apparently started with an aurora1 brightening (630 nm) to the South (18.50) and West (19.00) which, later, spread to the northern part of the sky (by w 19.20). The magnetometer showed a negative pulse in AX, and a positive pulse in AZ (400 nT) starting at 19.00 U.T. and with a maximum at 19.20 U.T. This current system (electrojet), as with the aurora1 (630 nm) intensities,

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FIG. 3. THERMOSPHERIC WINDSOBSERVED FROMKIRUNA(ESRANGE) USING THE 630 nm (01) EMISSION LINE (- 240 km ALTITUDE). (a) Meridional windmeasured -400 km North of Kiruna. Zonal wind measured - 400 km West of Kiruna.

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Meridional wind measured -400 km South of Kiruna. Zonal wind measured -400 km East of Kiruna. Positive winds are eastward or equatorward. Dates are: 1l/12, 19/20 and 29/30 November 1980.

maximized equatorward of Kiruna. The region of bright 630 nm emission was associated with a very bright and active visual aurora1 event during the period up to 19.20 U.T. This current system apparently decayed before a stronger electrojet, centered more over Kiruna, built up about 19.40 U.T. The impulsive response of the thermospheric winds appears with the dramatic decrease of the equatorward wind components to the North, Northeast and Northwest, coincident with the start of the aurora1 brightening(630nm)in theSouth,about 18.50U.T.The

maximum response in these horizontal wind components, and in the upward vertical wind component overhead Kiruna is about 19.10 U.T., when the magnetic and aurora1 disturbances are still increasing! The closest set of wind observations to 19.10 U.T. show that there was a 160 m s-l upward wind over Kiruna, while the winds Northwest, North and Northeast of Kiruna all reversed in sense from towards to away from Kiruna, by 350, 5.50 and 300 m s-r respectively. The correspondence between the wind changes in the

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various directions observed from Kiruna is emphasized in the presentation of six “snapshots” of the circulation about Kiruna shown in Fig. 5. Here the line-of-sight wind components in the six horizontal viewing directions are plotted for measurement sequences centred at 18.00, 18.30, 18.50, 19.10, 19.50 and 20.00 U.T. (closest observations from each direction). It should be noted that the zonal winds measured to the East and West of Kiruna did not appear to participate in the outward explosion, although the southward wind to the South of Kiruna increased to 300 m s-i from its initial value of 100 m s-l. While there are large vertical winds during this event, and probably over an extended region, these winds cannot play a significant role in confusing the deduced horizontal winds, observed at a zenith distance of 60”. The maximum effect of the larger (upward) vertical winds, resolved along the observing directions, would

be about 75 m s- ’ (away from Kiruna) but usually the resolved values would be much less, and contrasting with observed meridional wind change of 500 m s- i. (vi) 4 February 1983 (Kiruna) All of these observations were recorded under ideal cloudless polar winter observing conditions from an altitude of 350 m above sea level, when there was a very low degree of scattering in the troposphere. In Fig. 6 the data for a similar event observed on 4 February 1983 are shown. Early in the afternoon and evening a “Great Red” aurora occurred over Scandinavia, following a strong Storm Sudden Commencement at 16.00 U.T. This event, regrettably, tripped the twilight safety overload devices used to protect the Imaging Photon Detector of the FabryPerot interferometer from operation in bright twilight or daylight (Sun above -3” depression angle).

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FIG. 4. THERMOSPHERIC WINDSOBSERVED IN THEN, NE, E, S, W, NW AND VERTICAL DIRECTION FROMKIRUNA GEOPHYSICAL INSTITUTE ON THENIGHTOF 23/24 NOVEMBER1982. The major focus of interest is the correspondence between changes observed in each of the viewing directions in

the period 18.0&20.00 U.T. before, during and after the strong disturbance near 19.00 U.T. Observed are indicated by the crosses (+), and the 01630 nm intensity by the continuous line.

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of 90 min, as the event decayed (the peak intensities observed correspond to about 10 kR of 630 nm). A Westward Travelling Surge passed South of Kirunda about 20.40 U.T., propagating from the magnetic midnight region (as can be seen from the delay between the intensity increases in the East, zenith/South and West directions). As might be anticipated from the high overall level of solar wind/magnetospheric disturbance (IMF lB1 > 20 nT), this Westward Travelling Surge was associated with a very strong “aurora1 substorm”, and the thermospheric wind response was similarly strong-upward winds of 50-150 m s-i occur for a 40-min period, and the break

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southward winds to the North of Kiruna (400 m SC’ preceding the Westward Travelling Surge) decreased rapidly to very small values for a period of45 min before returning to 400 m s- ’ equatorward. (vii) 7/8 December 1982 (Kiruna) On the night of 7/8 December 1982, two similar events occurred. The first (Fig. 7) occurred at the very early time of 17.00 U.T., 4 h before magnetic midnight, and marked the extreme westward excursion of a Westward Travelling Surge which was located equatorward of Kiruna and indicated by the 01630 nm intensities and by ground-based magnetometer data. This event caused very little change in horizontal winds,

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The generation of vertical thermospheric winds and gravity waves at aurora1 latitudes but nevertheless generated a brief period of 100 m s- ’ upward vertical wind. A second and much stronger event (in aurora1 and magnetic terms) occurred about 19.50 U.T. In this event, however, the Westward Travelling Surge passed overhead and to the North of Kiruna. As a result, the response in the vertical wind to this second event was much smaller than that induced by the first. The pre-existing downward wind decreased temporarily by 50 m s- ’ at the time of passage of the Westward Travelling Surge, but the wind did not move upward. There was a rapid reversal of the initially northward meridional wind flow during this event while at the same time a fairly homogeneous westward zonal wind was stopped rather abruptly and over the entire region (800 km diameter) sampled by the groundbased Fabry-Perot interferometer. These observations of a Westward Travelling Surge passing over or poleward of Kiruna provided a rather spectacular

illustration of the effects of the abrupt convection plasma flow changes across the Westward Travelling Surge western boundary. The signatures in the North and Northwest directions are possibly the strongest we have yet observed of the horizontal wind flow changes associated with the passage of the Westward Travelling Surge (WTS). (viii) 17 December 1982 (Kiruna) Following a Storm Sudden Commencement at 08.05 U.T. on 17 December 1982, there was a period of strong and systematic downward vertical winds above Kiruna (Fig. 8) accompanied by extreme horizontal winds. The general behaviour of the vertical wind was quite distinct from that observed in the night-time events described previously, and more comparable to that seen in the Svalbard data. At the peak of a very strong positive bay event (+750 nT max) following the Storm Sudden

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FIG. 6. THER~OSP~~IC WINGSOBSERVED IN THEN, NE, E, S, W, NW AND VERTICAL DIRECTION FROMKIRUNA GEOPHYSICAL INSTITUTE ON THBNIGHTOF 3/4 FEBRUARY1983.

The major focus ofinterest is the correspondence between changes observed in each of the viewing directions the period between 18.30 and 19.30 and between 19.40 and 21.30 U.T.

Commencement, very strong westward thermospheric winds (maximum of 900 m s- ‘) were observed for a period of several hours. The vertical wind component was persistently downward during this event, although thevaluefluctuated between80ms-‘at 14.00U.T.and 16.00 U.T. and nearly zero about 15.00 U.T. Due to the relatively high intensity of 01 630 nm emission throughout this event, a time resolution of 8 min was maintained for the complete sequence of wind observations (six horizontal, vertical and calibration). Throughout this extended event, it appears that the main eastward electrojet (and by inference aurora1 oval and regions of stronger northward electric fields) was always located equatorward of Kiruna.

in

4. DLSCUSSION

The ground-based observations have described two apparently distinct phenomena of the vertical thermospheric wind. The data from Svalbard and from an afternoon (large) positive bay event at Kiruna describe a persistent vertical wind of several tens of metres per second and with, in the case of the Svalbard data what appears to be a basic diurnal variation (daytime downward, night-time upward). The Kiruna data of 17 December 1982 is, we believe, due to the same phenomenon that causes the daytime downward winds above Svalbard. Under both these conditions, there is strong ion drag and Joule heating in the thermosphere

679

The generation of vertical thermospheric winds and gravity waves at aurora1 latitudes

hundred metres per second. In addition, strong divergences of this basically-horizontal neutral gas flow can exist. The strong and systematic divergence of the strong sunward flow can overcome the competing effect of Joule and friction heating in some parts of the aurora1 oval and the polar cap, causing, at least at the altitudes probed by the 01 630 nm observations, an apparent collapse of the upper thermosphere. The magnitude of the mean vertical winds (- 30 m s-r or 100 km h-‘) and their duration (3-6 h) is a graphic illustration of the intense perturbation of the thermosphere by geomagnetic processes, noting that while the 17 December 1982 observations relate to a rare and powerful event, the Spitzbergen data reflect geomagnetic conditions. These mainly “average” persistent vertical winds exist in regions where the mean horizontal velocity is high (several hundred

over a widespread region (in local time) of the eastward electrojet. This region will be associated with a positive bay magnetic disturbance if the intensity and mean energy of aurora1 electrons is sufficient to enhance the Hall conductivity (height-integrated). At lower values of mean aurora1 electron energy (and maybe total flux) the Pedersen conductivity is virtually always enhanced above 16&200 km (Sugiura et al., 1983) so that at such altitudes ion drag can excite significant sunward winds even in the absence of a ground-based magnetic perturbance or acceleration of sunward winds at lower (12&160 km) altitudes (Rees, 1971; Heppner and Miller, 1982 and Rees et al., 1983). The cause of the strong and systematic downward winds is that, as we will discuss in detail in our second paper, the ion drag wind acceleration throughout the F region creates (nearly) sunward neutral wind velocities of several

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D. REEK et al. (bl

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FIG. 7. THERMOSPHERIC WINDSOBSERVEDIN THEN, NE, E, S, W, NW ANDVERTICALDIRECTION FROMKIRUNA GEOPHYSICALINS~TUTE ON THENIGHTOF 7 DECEMBER1982. The major focus ofinterest is the correspondence between changes observed in each of the viewing directions in the period 19.4G21.00 U.T. metres

per second).

pass

through

downward

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the

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or upward

gas parcels of

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to

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short

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the magnetosphere neutral

significantly less than 1 h), so that the

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total

or upward

latitude.

movement

is relatively

limited. (midnight

disturbances

aurora1 oval during an expansion

magnitude

and duration

do cause a strong thermosphere. translation, zontal

in longitude

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heating (particle

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of the upward vertical

vertical

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is a major

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ion-drag

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in thermo-

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with

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periods (probably downward

can cause significant

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altitudes).

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of those at the greater wind prior equatorward,

to the which

The generation

of vertical thermospheric

winds and gravity

carries the air bearing the excited oxygen atoms significantly equatorward (300 m s-r for 100 s = 30 km). The explosion results in an outward-expanding wave, whose consequences are most easily seen and described in relationship to the meridional wind, since the zonal wind response is complicated by the zonal existance ofthe disturbed region around the night-time part of the aurora1 oval and the complete enhancement of the ion drag acceleration. The initial equatorward propagating wave travels easily towards the midlatitude night-time region, slowly decreasing in amplitude with distance from the source. A poleward wave is also launched which has to battle against the ubiquitous equatorward wind prior to the disturbance. In many cases the local energy input is adequate to overcome temporarily the equatorward wind, so that locally and temporarily the meridional wind is driven poleward North of the disturbed region (as on 23 November 1982). Once the energy input decays at the end of the substorm, however, the persistent antisunward day to night flow over the polar cap, which is itself significantly enhanced by the effects of the largerscale energy and momentum input during the entire geomagnetic disturbances of which the expansion phase (aurora1 substorm) is only a part, overcomes the

681

waves at aurora1 latitudes

poleward surge, and a second equatorward wave occurs, which generally exceeds the speed of the preceding equatorward wind. The effect of the poleward wind can be likened to the creation of a dam acting against the ubiquitous anti-sunward thermospheric flow over the polar cap. At the end of the disturbance the dam collapses against the antisunward cross polar cap flow, introducing a second successive equatorward-propagating wave. There is an indication that the zonal wind acceleration by ion drag is enhanced significantly prior to the aurora1 expansion phase, indicating that the large-scale convective electric field (and possibly F-region conductivities) are considerably enhanced during the directly-driven phase of the geomagnetic disturbance, and quite independently of the timing and magnitude of the expansion phase/aurora1 substorm. The direction of the zonal wind acceleration, except during the aurora1 expansion phase and passage of the WTS, appears to be closely related to the average temporal evolution of the convective electric field as described by semi-empirical models. During the aurora1 expansion phase and passage of a WTS, the sense of ground-based magnetic perturbations, and the thermospheric wind response (zonal component) reflect the highly distorted

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FIG. 8.WIND OBSERVATIONS FROM KIRUNA GEOPHYSICALINSTITUTEON 17 DECEMBER1982. The observations (W, NW and Zenith) demonstrate the large downward vertical winds observed at the time of the strong westward winds during the large positive bay disturbance. The average downward wind over the period 13.5(r17.C4l U.T. is over 40 m s-’ (a total downward motion of 400 km!).

convection

and

current

systems

which

exist

during

these phenomena. From a careful survey of about 200 nights of FPI observations from Kiruna since 1980, it is clear that during an aurora1 expansion phase or WTS occurring between 18.00 U.T. and 02.00 U.T. (21.05 magnetic local time (M.L.T.) at Kiruna), and when the peak aurora1 intensities and ionospheric currents are equatorward of Kiruna, the wind changes will follow those observed on 23 November 1982. When the major activity is centred poleward of Kiruna (7 December at 19.30 U.T.), equatorward wind surges are observed, but the strong upward winds (i.e. >lOO m s-i) are not observed (from Kiruna). This selection is simply explained on geometrical considerations and by the spatially limited region of intense heating. The downward winds which occur outside the region of strong heating, and after the end of the intense aurora1

activity and heating, are normally of more moderate (3650 m s-i) magnitude. Allowing for the sampling statistics of the ground-based FPI as affected by weather, local time and the latitudinal centre of aurora1 activity and heating activity, this class of strong vertical wind disturbance is very common and, in any 24-h period with magnetic activity of K, 2 3, several will occur. The horizontal wind disturbances of the middle and upper thermosphere associated with such events readily propagate in the night-time thermosphere, so that during periods of even moderate geomagnetic activity, the entire night-time thermosphere is continuously perturbed by strong waves emanating from both polar regions. Observations such as Hernandez (1982) Lloyd et al. (1972) and Rees et al. (1977) are among many instances of local observations of (some of) the consequences of such propagating disturbances of the neutral thermosphere.

The generation

of vertical thermospheric

5. SUMMARY

In regions of the polar thermosphere under the direct influence of magnetospheric energy and momentum sources, vertical winds of tens of metres per second or more are a ubiquitous feature of the wind field of the upper thermosphere. In periods of moderate geomagnetic activity, there are regions of systematic vertical winds, which are observed from Svalbard, with a diurnal variation (maximum upward of 80m s-r at 24.00 U.T., maximum downward of 50 m s-i at 10.00 U.T.). Such winds, and large systematic downward winds (average 40 m s-i over 6 h), at times oflarge disturbances, on the poleward boundary of the afternoon aurora1 oval, appear to be associated with divergences (downwards) or convergences (upwards) of the ion drag-induced horizontal winds acting in addition to the variety of high latitudinal heat sources. Much stronger (5G200 m SC’) upward winds are observed associated with the expansion phase of geomagnetic disturbances (aurora1 substorm and Westward Travelling Surge). These intense upward winds are of short duration (typically 10-30 min) and are frequently surrounded in space (and time) by regions of weaker (3(r50 m s-i) downward winds. These strongly time- and space-dependent vertical winds have to be placed in the context of very large associated disturbances of the horizontal wind field which can be summarized as follows. (i) The directly-driven phase Before any strong local aurora1 or magnetic disturbances occur, there is a period, typically one to several hours, when the aurora1 oval expands, and there is considerable enhancement of thermospheric wind flow. Prior to magnetic midnight (-21.00 U.T.), the enhancement usually produces strong westward winds, while later in the night strong eastward and southward winds will be produced during this initial phase of the geomagnetic disturbance. (ii) The expansion phase (aurora! substorm) Associated with the onset of higher active aurorae, with the centre of aurora1 and magnetic activity to the South (equatorward) of Kiruna, the southward wind in the North and the line-of-sight components in the Northeast and Northwest directions, both initially towards Kiruna, abruptly decrease by several hundred metres per second. At the same time the southward wind in the South increases significantly. The short period of strong upward winds (50-200 m s-r) coincides with the maximum poleward surge in the wind components measured North of Kiruna.

winds and gravity

waves at aurora1 latitudes

683

Within about 30 min of the peak meridional wind disturbances (poleward of Kiruna) the upward winds generally reverse order to moderate downward values (-50 m SC’) or oscillate with a similar magnitude, while the meridional wind poleward of Kiruna returns equatorward often with a value exceeding that during the directly-driven phase. The meridional winds North and South of Kiruna frequently remain at high values (30&500 m s ‘) for several hours. The behaviour of the zonal wind during the expansion phase is more complex, and the complexity appears to be due to the time-dependent changes of polar ion convection which occur in the region of the Westward Travelling Surge. Prior to magnetic midnight, the strong westward wind acceleration of the directly-driven phase is halted and may be revised by the passage of the Westward Travelling Surge (i.e. 7/8 December 1982), despite the outward explosion indicated in the meridional winds. The longitudinal extension ofthe disturbed region around the night-time aurora1 oval appears to be partly responsible for the discrepancy between zonal and meridional wind behaviour. Near to magnetic midnight and post-magnetic midnight, there is an eastward wind acceleration during the directly-driven phase which is further intensified by the expansion phase/aurora1 substorm. In this instance the sources of zonal ion drag acceleration and pressure gradients induced by strong heating are complementary while at pre-magnetic midnight, they tend to oppose. The above description is valid for aurora1 substorms at magnetic midnight and post-midnight, or the passage of a Westward Travelling Surge pre-magnetic where the major electrojet currents, midnight, precipitation and heatingareequatorward ofKiruna. If the centroid of activity passes poleward of Kiruna (or even overhead), the sequence of thermospheric responses are identical but, for geometrical reasons, the strong upward winds are not observed (i.e. overhead Kiruna). The reversal of the equatorward meridional wind poleward of Kiruna is also no longer observed, but only because of the poleward translation of this region and its masking by a region of bright 01630 nm emission exploding equatorward from the region being strongly excited and heated. In our second paper, theoretical and numerical models simulating these observations of strong vertical winds in the high lateral thermosphere will be described in detail. Acknowledgements-We would like to express our appreciation of the great assistance of the staffs of Kiruna Geophysical Institute and the University of Tromso in providing facilities

684

D. REEK et

to aid these observations being made. The University College London and Ulster Polytechnic observing programmes were both supported by grants from the UK Science and Engineering Research Council. Drs. G. Rostaker and S. 1. Akasofu contributed considerably to relating the local wind, aurora1 and magnetic observations to the general development of magnetospheric disturbances.

REFERENCES Fuller-Rowell, T. J. and Rees, D. (1980) A three-dimensional, time-dependent, global model of the thermosphere. J. Atmos. Sci. 31, 2545. Geisler, J. E. and Dickinson, R. E. (1968) Vertical motions and nitric oxide in the upper mesosphere. J. atmos. terr. Phys. 30, 1505. Heppner, J. P. and Miller, M. L. (1982) Thermospheric winds at high latitudes from chemical release observations. J. geophys. Res. 87, 1633. Hernandez, G. (1982) Vertical motions of the neutral thermosphere at midlatitude. Geophys. Res. Lett. 9, 555. Hernandez, G. and Roble, R. G. (1979) On divergences of thermospheric meridional winds at midlatitudes. Geophys. Res. Lett. 6, 294. Lloyd, K. H., Low, C. H., McAvaney, B. J., Rees, D. and Roper, R. G. (1972) Thermospheric observations combining chemical seeding and ground-based techniques. 1. Winds, turbulence and the parameters of the neutral atmosphere. Planet. Space Sci. 20, 761. Mayr, H. G. and Volland, H. (1974) Magnetic storm dynamics of the thermosphere. J. atmos. terr. Phys. 36,2025. McWhirter, I., Rees, D. and Greenaway, A. H. (1982) Miniature imaging photon detectors. III. An assessment of the performance of the resistive anode IPD. /. phys. E: Sci. Instrum. 15, 145. Offermann, D. (1981) Scientific objectives and experimental set-up of the Energy Budget Campaign, in BMFT-FB-W 81-052: Sounding Rocket Program Aeronomy Project: Energy Budget Campaign 1980 Experiment Summary (Edited by Offermann, D. and Thrane, E. V.), pp. 1 l-45. PrBlss, G. W. (198 I) Latitudinal structure and extension of the polar atmospheric disturbance. J. geophys. Res. 86,2385.

al

Rees, D. (1971) Ionospheric winds in the aurora1 zone. J. Brit. Interplan. Sot. 24233-246. Rees, D., Charlton, P., Carlson, M. and Rounce, P. (1984) High latitude thermospheric circulation during the Energy Budget Campaign. J. atmos. terr. Phys. (submitted). Rees, D., McWhirter, I., Rounce, P. A., Barlow, F. E. and Kellock, S. J. (1980) Miniature imaging photon detectors. J. phys. E: Sci. Instrum. 13, 763. Rees, D., McWhirter, I., Rounce, P.A. and Barlow, F. E.(1981) Miniature imaging photon detectors. II. Devices with transparent photocathodes. J. phys. E: Sci. Instrum. 14,229. Rees, D., Rounce, P. A., Charleton, P., Fuller-Rowell, T. J., McWhirter, I. and Smith, K. (1982) Thermospheric winds during the Energy Budget Campaign: ground-based Fabry-Perot observations supported by dynamical simulations with a three-dimensional, time-dependent thermospheric model. J. Geophys. 50,202. Rees, D., Rounce, P. A. and Killeen, T. L. (1977) Winter Constituents Anomaly-Trace Sounding Rocket Campaign. Daytime neutral wind measurements in the mesosphere and lower thermosphere. J. Geophys. 44,47. Rees, D., Fuller-Rowell, T. J., Gordon, R., Killeen, T. L., Hays, P. B., Wharton, L. E. and Spencer, N. W. (1983) A comparison of wind observations of the upper thermosphere from the Dynamics Explorer satellite with the predictions of a global time-dependent model. Planet. Space Sci. 31, 1299. Smith, R. W. and Sweeney, P. J. (1980) Winds in the thermosphere of the northern polar cap. Nature, Lond. 284, 437. Spencer, N. W., Theis, R. F., Wharton, L. E. and Carignan, G. (1976) Local vertical motions and kinetic temperature from AE-C as evidence for aurora-induced gravity waves. Geophys. Res. Lett. 3, 313. Spencer, N. W., Wharton, L. E.,Carignan,G. R. and Maurer, J. C. (I 982) Thermosphere zonal winds, vertical motions and temperature as measured for Dynamic Explorer. Geophys. Res. Lett. 9, 953. Sugiura, M., Iyemori, T., Hoffman, R. A., Maynard, N. C., Burch. J. L. and Winningham. J. D. (1983) Relatiotlships between field-aligned currents, electric fielhs and partidle precipitation as observed by Dynamic Explorer-2. NASA Tech. Mem. 85025, May 1983.