Middle atmosphere and lower thermosphere processes at high latitudes studied with the EISCAT radars

Pergamon

Middle atmosphere and lower thermosphere processes at high latitudes studied with the EISCAT radars J. R~TTGER EISCAT Scientific Association, P.O. Box 812. S-981 28 Kiruna, Sweden (Receiwd

in,final form 28 Fehruar~~ 1993; uccepirrl25Muy 1993)

Abstract-The

middle and upper atmosphere and the ionosphere at high latitudes are studied with the EISCAT incoherent scatter radars in northern Scandinavia. We describe here the investigations of the lower thermosphere and the E-region, and the mesosphere and the D-region. In the amoral zone both these altitude regions are influenced by magnetospheric processes, such as charged particle precipitation and electric fields, which are measured with the incoherent scatter technique. Electron density, neutral density, temperature and composition are determined from the EISCAT data. By measuring the ion drifts. electric fields, mean winds, tides and gravity waves are deduced. Sporadic E-layers and their relation to gravity waves, electric fields and sudden sodium layers are also investigated with EISCAT. In the mesosphere coherent scatter occurs from unique ionization irregularities. This scatter causes the polar mesosphere summer echoes (PMSE), which are examined in detail with the EISCAT radars. We describe the dynamics of the PMSE, as well as the combination with aeronomical processes. which could give rise to the irregularities. We finally outline the future direction which is to construct the EISCAT Svalbard Radar for studying the ionosphere and the upper, middle and lower atmosphere in the polar cap region.

1. 1NTRODU~ON The radar technique has evolved as one of the essential ground-based tools for studying the ionosphere and the upper atmosphere as well as the middle atmosphere. In particular the conventional incoherent scatter technique is most appropriate for these studies, which are performed with the radars of the EISCAT Scientific Association. Coherent scatter from mesospheric irregularities has also been detected with the EISCAT radars and has attracted wide attention in order to understand the scattering process as well as the aeronomic and dynamic phenomena leading to these irregularities. EISCAT is operating high power VHF and UHF radar systems for studying the aurora1 ionosphere and the upper and middle atmosphere in northern Scandinavia (e.g. RC~TTGER, 1991 a). This allows investigations of the most relevant region of the middle atmosphere, the polar mesosphere, where the merging of effects from the magnetosphere and ionosphere with the phenomena originating below in the troposphere and stratosphere takes place. Furthermore, the polar mesopause in summer is the coldest region of the Earth’s atmosphere, which has obvious effects on the dynamics and aeronomy of this region. Certain phenomena in the polar atmosphere and the amoral ionosphere, namely, charged particle precipitation, Joule heating and Lorentz forcing result from magnetosphere-ionosphere coupling. Magneto-

spheric particles cause a variety of effects, for instance the aurora and an enhancement of the electron density. Magnetospheric electric fields propagate downwards into the lower thermosphere and mesosphere, where they cause dissipative currents to flow, resulting in Joule heating. Changing electric fields cause varying plasma drifts whereby Lorentz forcing deposits momentum in the lower thermosphere and upper mesosphere and atmospheric waves can be generated. The main effects of particle precipitation and Joule heating on the lower thermosphere and mesosphere are sketched in Tables I and 2, respectively. These effects from above merge in the polar mesosphere and lower thermosphere with effects from below. These latter are tides and gravity waves, which deposit momentum and dissipate energy into turbulence and heat in the mesopause region (see FORBES, 1983. for discussions). Further interesting phenomena, observed with EISCAT in the lower thermosphere, are aurora1 sporadic E-layers. These are thought to result from the interaction of neutral dynamics and electrodynamics affecting the ionization structure and composition. The relative importance of the effects originating below and above the polar mesopause, where they merge, are not well known and need further detailed studies. Moreover, the coupling of the dynamics and aeronomy in the mesopause region leads to interesting blends of scattering irregularities created by rather poorly understood mechanisms. A particular example 1173

J. R&TGER

II74 Table I. Synopsis of the rlYects of energetic cipitklting

from the magnetosphere

particles preinto the lower aurora1

ionosphere, which could cause the high-latitude mesospheric wind and wave systems to be changed .-- _-___l-l _I_---___

rationale for Further studies of the coupling between the Arctic upper, middle and lower atmosphere with EXSCAT.

PRECIPlTRTit4~ PARTtCtES loniz&on

CoGions Dissociation i -Recomblnatior? ---‘~

Ion &mistry-

I lon,Etectron Heating

I Ionization Transport i Variation of Composition

I

NeLlk3i

&J:orzt

1

Excitation

i I Changes T,,, UV.Absorption

Ii_

i

Heating

I

Changes T,

1

I

Neutral Temperature, Winds and Waves

in the Lower Thermosphere and Masosphere

of these latter processes and mechanisms is the polar mesosphere summer echoes (PMSE). which are regularly observed with the EISCAT VHF radar and occasionaliy also with the EISCAT UHF radar. WC describe the dynamics of these echoes as well as the interplay of dynamics, chemistry and aeronomy, which WCbelieve govern these phenomena. ElSCAT is contributing considerably to the corresponding investigations. A few exampies of refcvant results obtained with EISCAT arc prosenfed here to dcmonstrare the coupling between the upper and the middle atmosphere. Where pertinent we also refer to other related work. We finally emphasize the scientific

of

Table 2. Synopsis of etlkcts Joule heating resulting from magnetospheric electric fields and ionization en~an~~~eilts in the aurora5 lower thermosphere and upper mcsosphere. This heating could lead to a change of the high-latitude upper mesospheric wind and wabe systems under disturbed conditions -.” JOULE HEATING i

Temperature Changes in the Thermosphere and Upper Mesosphere

1

I

Neutral -and Plasma Instabilities

-Atmospheric Waves

i Shears, convection

I

i

Tuib:lence i

Mesospheric Winds and Turbulence

Eddy Transport Combosltion

I Thermospheric Winds

-

I - lonizatiorl ..-.l___

THERtlOSPWERE

AND

MESOSPHERE

The inclusion of incoherent scatter radars (ISR) is an appropriate complement to other techniques which are applied for studying the middle atmosphere. The EISCAT UHF and VHF radars, locared near fromsn in northern Norway at 69.5 N and 19.2 E, comprise a proper combinatian to observe the ionosphereethermosphere. mcsospherc, lower stratosphere and troposphere in the polar region with the incoherent and coherent scatter modes. The EISCAT VHF and UHF radars operate on center frcqucncies of224 and 93 1 MHz funtit 1990 : 933 MHz) with peak powers of about 2 x I .5 MW and 1.5 MW, and eff’ective antenna apertures of 3250 and 520 m’. respectively. Tristatic measurements arc performed with the remote UHF receiving sites in Kiruna (67.9 N. 70.4 El. Sweden, and SodankyfL (47.4’ N. 26.6 Et, Finland. Whereas tnesosphe~~-stfatosphere-tniposphcrc (MST) radar investigations gcncrally allow studies of atmospheric turbulence. waves and neutral winds (see. for instance, R~~TTGER, 1984), the incoherent scatter radar invesGgations allow studies of electron density, ion velocity. temperature, neutral density. composition and the number density of positive and negatitre ions in the mesosphere (see for instance HALL. 1989 ; COLLIS and RBTTGER, 1990). The experimental possibilities of the ElSCAT radars to study the mesosphere and D-region were outlined by J-A Hoz i*f (ri. (1989a). T~,I~GNE& f I’%), WA~~R~R~ rt ui. (f989) and tA I-bz er 4. ~tYt%n) have described the data acquisition procedures for mcsospheric observations by EISCAT. R~TTGER (199la) has summarized EISCAT applications for studying the aurora1 ionosphere and thcrrnosphcre with the incoherent scatter technique. There is a typical difference in the characteristics OT incoherent scatter and riot-~ncol?crent scatter, displaying also different ionospheric and atmospheric features. The non-incoherent scatter process causes radar echoes. which are commonly referred to as coherent or turbulence scatter echoes and are detected by MST radars. The coherent scatter is due to refractivity ~rre~~~~a~t~esoriginating from a variety of neutraI atmosphere and aeronomic processes. The incoherent scatter results from scattering by free electrons in the ionized atmosphere. Incoherent scatter is also rcferrcd to as thermal or Thomson scatter. We later set that afso enhanced Thomson scatter can occur

EISCAT and middle atmosphere and lower thermosphere I”“,“1

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1805

1800 UNIVERSAL

TIME

I. Electron density contour plot (lowest contour = IO” m ‘, and contour interval = 5. 10”’ mm ‘) observed on 16 November 1983 with the EISCAT UHF radar. At the beginning and end of this observation interval precipitating particles enhanced the electron density. In the middle of the interval a thin sporadic E-layer occurred at around 107 km. The lower diagram shows the direction of the ionospheric electric lield, measured simultaneously with EISCAT (from Fig.

TUKUNEY et trl., 1985). under

certain

spectra

of

aeronomic circumstances. The power incoherent scatter echoes are determined by the thermal motions of electrons and ions. The power spectra of coherent scatter echoes arc controlled by motions of the irregularities, which arc mostly affected by neutral motions. Usually the incoherent scatter spectra are broader than the coherent scatter spectra, since thermal velocity fluctuations are greater than the neutral atmosphere velocity fluctuations. The upper panel of Fig. I (from TUKUNEN et cd.. 1985) presents a contour plot of electron density deduced with the incoherent scatter method. This figure shows a composition of electron density layers caused by particle precipitation and sporadic E-layer. The enhanced electron density due to particle precipitation (c.g. around 1807 UT) is fairly short-lived and related to the aurora. It is extended vertically over several tens of kilometers. On the contrary, the sporadic F layer is extended vertically over at most I km. This layer is thin and horizontally stratified. As we will discuss later, the aurora1 sporadic E-layers result from neutral dynamic, ionospheric and electro-dynamic effects. The latter are related to the ionospheric electric field (lower panel of Fig. I), which is simultaneously mcasurcd with the tristatic EISCAT UHF radar. Except for sporadic Elayers, the features of common electron density profiles are rarely thin and stratified. They often show vertically extended structures. as common

processes

1175

seen at 1807 UT in Fig. I (upper panel), indicating pronounced horizontal gradients of electron density which are created by structured beams of magnctospheric-ionospheric particles reaching the middle atmosphere. An example of two types of phenomena of magnetospheric origin and of middle atmosphere origin is shown in Fig. 2 (from RGTTGERef rd.. 1990b). This figure exhibits height-time intensity plots of scattered power. as well as upward and downward velocity, measured with the EISCAT UHF radar. The upper panel shows the merging of two processes : (I) the enhancement of power (= electron density) due to particle precipitation between about 00 and 01 UT covering altitudes 72Z96 km made manifest by incoherent scatter echoes; and (2) thin bite-outs of scattered power (= depletions of electron density), which later result in echo enhancements around 86 km altitude (made manifest by coherent scatter echoes) that are assumed to originate from aeronomic and dynamic effects. The two lower panels show large vertical velocity oscillations below 88 km at periods between IO and 25 min. which are caused by gravity waves. One may traditionally assume that the echo enhancement occurring after 0010 UT around 86 km in Fig. 2 is caused by mesospheric turbulence. It is very unlikely, however. that coherent scatter due to turbulence can occur at the short Bragg scale of 16 cm corresponding to the EISCAT UHF radar. Since larger scale density fluctuations in the middle atmosphere decay according to a power law into smaller scale fluctuations. which finally end up as thermal fluctuations. coherent scatter is more likely at longer radar wavelengths (such as EISCAT VHF or lower frequencies). and incoherent scatter dominates the shorter wavelength radar observations (such as EISCAT UHF). It was shown by Rii-r’rc;eR (‘I (I/. (1990b) that these enhanced 933 MHz echoes in Fig. 2 are due to polar mesosphere summer echoes (PMSE), which were simultaneously observed with the 46.9 MHz CUPRI VHF radar which was operated at the EISCAT radar location. Processes, such as neutral turbulence, temperature inversions or clustering of ions, which give rise to the scattering of radar waves, are often controlled by neutral atmosphere dynamics in the middle atmosphere. Occasionally, such coherent echoes due to scatter from irregularities induced by neutral turbulence were detected in the mesosphere below 70 km with the EISCATVHF radar (e.g. Co~~ketd.. 1992). The more frequent and inspiring radar echoes, such as the PMSE observed by EISCAT between about 80 and 90 km. arc caused by aeronomic processes, as WC

shall describe later. The PMSE structures also display wave and turbulence features as well as certain layering, strata or laminae, manifesting horizontal stratifications and steep vertical gradients in the atmosphere. We shall discuss the incoherent scatter investigations in Section 3. In Section 4 dynamic processes are addressed, which were observed with incoherent scatter and coherent scatter. In Section 5 the interplay of dynamic and aeronomic phenomena studied by means of incoherent and coherent scatter is considered.

HUUSKONEN,1989). We notice the low mesopause at lower temperatures in the summer month of July. HUUSKONEN(1989) observed that day to day variations of temperature below I10 km, where ion and electron temperature should be equal to the neutral temperature, can be 50 K (see altitude region around IO5 km at 17 July 1984). Huuskonen explains this variability by Joule and particle heating. It is unknown, however, how large the temperature variation is at the lowest observational altitudes around 9.5 km. KOFMAN ct at. (1986) had reported results of the deviation of neutral mass. collision frequency and temperature in the lower thermosphere ; they assumed that during geomagnetically disturbed conditions the 3. AERONOMY OF THE LOWER THERMOSPHERE AND assumption of thermal equilibrium between electrons MESOSPHERE AND COUPLING FROM THE and ions in the lower thermosphere is not correct. ,MAGNETOSPHERE MAVKtt d. (1990) have emphasized that the energy dissipation by Joule heating causes a redistr~butioll In the lower the~osphere the incoherent scatter and depletion of atomic oxygen due to wind induced becomes influenced by collisions between ions and diffusion and the temperature decreases at 90 km. neutrals and it is dominated by collisions in the mesosphere. From the width of the incoherent scatter spec- Latitudinal and time variations of Joule and particle heating rates measured with EISCAT in the lower trum the temperature, neutral density, the negative and positive ion density and the mass of the ions in thermosphere had been reported by DUFXXN(1986). the mesosphere can be estimated (e.g. HALL. 1989). The temporal variations were more pronounced than the latitudinal variations. HANSENet at. (1991) have pointed out that care has The ISR observations of D-region electron density to be taken when applying these standard techniques. The neutral density is deduced from the collision fre- are directly related to ionospheric radio wave absorption (e.g. RANTApr al., 1985). COLLISand K~RKWOO~I quency, which can be determined from the spectrum as well as the vertical ion drift and the electric field ( 1987) have noted significant changes in electron density and absorption with small horizontat scale sizes (NYGRBN rt al., 1987). The ion drift is deduced from of some 20 km. Temporal changes happen within less the Doppler shift of the scatter spectrum. In the mesothan a minute to several tens of minutes (COLLISand sphere the ion drift corresponds to the neutral wind velocity and in the thermosphere-ionosphere (F- KIKKWOOLI.1987), when heating rates should change by up to two order of magnitudes. Also COLLISand region) the ion drift corresponds to the plasma drift RIETVELD(1990) have studied the latitudinal as well from which the electric field is deduced. The electron as the diurnal variation of electron density. The mean density is determined from the scattered power. latitudinal variations extend over a few degrees and 3.1. Electron density, neutral density and tenqwratuw usually peak in the aurora1 zone around 60-70”N, which is consistent with the model input from the There exist quite a few investigations of electron DE-2 satellite (RORLEet al., 1987). These heating rates density structures of the D-region during quiet concommonly maximize around I IO-125 km due to Joule ditions (c.g. TLJRUNENet al., l988), during aurora1 heating of ions and neutrals. Around 100 km altitude substorms (e.g. COLLISet nl., 1986) and during solar proton events (e.g. COLLIS and RIETVELD, 1990). the Joule and particle heating rates are about equal at IO -’ W mm ’ and, during times of low or moderate When the electron density is sufficiently high, the incodisturbance, an order of magnitude lower in the mcsoherent scatter echo from the n-region can be properly analyzed in terms of other parameters, which are pause region around 90 km. BANKS(I 979) has pointed out that there is a second maximum caused by the deduced from the TSR spectrum, such as neutral density and temperature. Results obtained with the heating of electrons in the mesosphere around 70 km. Although there are some estimates of heating rates EISCAT radars are in general agreement for the neutral density and temperature with model values (FLA of the middle atmosphere in high latitudes, it is difficult to assess the large-scale or global effect on the er al.. 1985 ; HUUSKONENet al., 1986). Figure 3 shows middle atmosphere (RORLE et d., 1987). The geoexamples of collision frequency, which is proportional graphical extent of these effects in the mesosphere to the neutral density, and temperature for a summer is hardly known, even for the widespread polar cap and a winter day in the lower thermosphere (from

EISCAT and middle atmosphere and lower thermosphere processes

EISCAT UHF RADAR

1177

1-2 JULY 1988

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Fig. 2. Incoherent and cohereut scatter observations with the EISCAT 933 MHz UHF radar. Upper panel : height-time intensity plot of scatteredpower (O- 15 dB power difference between white and black). Center panel : intensity plot of upward velocity (0 to + IO m s- ‘). Lower panel: intensity plot of downward velocity (0 to - 10 m s- ‘). (From R~TTGERef cd.. 1990b.)

J. R~TTGER

1178

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Fig. 6. Self-normalized height-time-intensity diagrams of polar mesosphere summer echoes (PMSE) observed with 300 m height resolution with the EISCAT 224 MHz VHF radar (after LA HOZ CI ul.. 1989b).

u

EISCAT

and middle atmosphere

and lower thermosphere

processes

1179

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Fig. 7. Upper panels : self-normalized dynamic spectra of vertical velocity of polar mesosphere summer echoes observed in single range gates with the EISCAT 224 MHz VHF radar (after LA Hoz et al., 1989b). The Doppler frequency of 16 Hz corresponds to a vertical velocity w = 10.7 m s- ‘. Lower panel : simplistic model of vertical displacement z’ of PMSE lamina, which would result in the observed vertical velocity w’ displayed in the upper panels (from R~TTGER cf al., 1990a).

1180

J. RBTTGER

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and middle atmosphere and lower thermosphere processes

TEMPERATURE

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Fig. 3. Ion temperature T,. electron temperature T, and ion-neutral collision frequency Y,,deduced from EISCAT UHF radar data. The solid lines show the best fit model. (From HUUSKONEN, 1989.)

absorption, which result from solar proton events. There is also a large spatial and temporal variation in the magnitude of the electric fields, which drive the Joule heating. ROBLE et 01. (1987) expect that significant, but variable, Joule heating rates occur in the mesosphere during long lasting intense solar proton events. The influence of these events on the middle atmosphere had already been pointed out by BANKS (1979), who estimated Joule heating rates and a temperature increase of the mesopause region of the order of a few K per day. These investigations, supported by ISR observations, have to be continued in order to quantify their effects on the middle atmosphere. 3.2. Particle,fluxes

and cmnposition

in the mesosphere

During magnetospheric substorm events energetic particles ionize the E- and D-region, change the conductivity and, together with electric fields, cause the electrojet current in the aurora1 oval, where heating of the neutral atmosphere results (see BREKKE, 1988). Solar proton fluxes enter the terrestrial atmosphere over the whole polar cap and cause the so-called solar proton events (SPE) or polar cap absorption (PCA). The high energy protons (with energy up to several 100 MeV) cause a substantial enhancement of the atmospheric ionization down to less than 50 km in the polar cap, which can last over several days. HARGREAVESrl al. (1987) have used proton fluxes to deduce ion-electron production rates as high as 4 x 10’ crn~. 3s- ’ around 60 km during a PCA. Using EISCAT ISR electron density profiles, they were able

to find the effective electron-ion recombination coefficient profile (z = 10 ’ cm3 s- ’ at 90 km and s( = 10 ‘cm’s_’ at 70 km), which is consistent with results from other methods. Gradual changes of the recombination coefficient in the mesosphere were explained by progressive changes of mesospheric photochemistry (e.g. COLLIS and RIETVELD, 1990). KOFMANcr al. (1984) fitted EISCAT UHF radar data to models and found good agreement with simultaneous rocket data obtained during the CAMP campaign (KOPP c’t rd., 1985). These results presented for the first time the ISR-deduced temperatures in the polar summer mesopause of around 120 K. KOFMAN et al. (I 984) noted the presence of negative ions below 75-80 km. This was later proved by HALL er al. (1988), who further found indications of the high masses of positive ions of the order of IO&230 amu in the summer mesosphere. It was postulated that proton hydrates exist in the mesosphere, for instance, H+(H>O),,. with n = 5-8 in winter and n = 5-13 in summer. These numbers are not unreasonable. since BJ~~RNand ARNOLD (1981) have observed values as high as n = 21 with rockets. TURUNEN et al. (1988) have measured neutral temperature and negative ion number density, which were consistent with models. They observed a layer of increased positive ion mass and suggested that it could be related to a layer of proton hydrates in the vicinity of noctilucent clouds. TURUNEN (1992) has done very detailed studies of EISCAT radar observations during a PCA event. Figure 4 shows profiles of electron density, ratio of number

1182

J. R~TTGEK

_____ L7 10’0

2 4 6 a [Negative ions] i [Electrons]

10”

Electron density [me31

90

1

c)

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s

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80

.9

,’

,

2 -

=-___

8.5 70 75

3oJJL

i

J 80

(50 200 250 Temperature [K]

Mean ion?kasF[arG]

Fig. 4. (a) Electron density profiles (solid lines) on 14 August 1989 measured with the EISCAT UHF radar during a polar cap absorption event. The raw densities are given by dashed lines. (b) Ratio of negative ions

to electrons

(dashed

line

corresponds

ISR data and model (dashed line). (d)

to different

Neutral

chemical

model). (c) Mean ion mass deduced from profiles, corresponding to (c). (From TURUNFY.

temperature 1992.)

densities of negative ions and electrons, mean ion mass and temperature in the mesosphere, which Turunen deduced using an advanced ion chemistry model (BURNS et al., 1991). This model includes all reactions due to recombination of positive ions with electrons, photodissociation of positive ions, electron photodetachment of negative ions, photodissociation of negative ions, electron attachment to neutrals and recombination. Minor constituents are ion-ion included, and night- and day-time conditions are separated. We note in Fig. 4 that the mesopause is around 85 km at temperatures around 130-150 K. which is fairly typical for the summer months in high latitudes. The mean ion mass of 50 amu in the mesopause is somewhat smaller than reported by HALL et al. (1988). The number density of negative ions increased below 80 km around midnight (00:05), and it was reduced substantially at 02:55, when the mesosphere became sunlit and UV radiation was detaching electrons from the negative ions. When the number density of negative ions is large the electron density is correspondingly small,andviceversa. COLLIS~~~RIETVELD(~~~O)~~~ RIETVELD and COLLIS (1992) have also done detailed studies of these day-night differences and the corresponding ion chemistry. They have shown that. at

sunrise at heights above 70 km, electrons are released by UV photodetachment of high-affinity negative ions, which may be NO,. Below 66 km altitude the increase of electron density is delayed by about 30 min, indicating that neutral oxygen is a controlling species in producing free electrons in these altitudes. High resolution electron density profile ISR measurements in the mesosphere (D-region) are also used to investigate the characteristics of the energetic particle precipitation during geomagnetic disturbances. This allows us to obtain information on the temporal changes on the hardness of the energy spectrum of precipitating electrons (COLLIS et cd., 1986; DEVLIN er ul.. 1986; HALL et d.. 1992). which originate in the magnetosphere. Discrete layers of Dregion ionization, which were found by COLLIS and KIRKWOOD (1990), were, on the other hand, unlikely to be produced by particle precipitation rather than by the influcncc of metallic ions.

4.

DYNAMICS OF THE LOWER THERMOSPHERE AND MESOSPHERE

The plasma or ion drift velocity is measured with EISCAT from the middle atmosphere to the exosphere. At thermospheric heights the drift velocity

EISCAT and middle atmosphere and lower thermosphere processes

is directly related to the electric field, mapping from the magnetosphere to the ionosphere. In the lower thermosphere the plasma drift is controlled by the neutral wind and the electric field. Knowing the electric field from the F-region measurements, the neutral wind velocity is deduced in the lower thermosphere. Due to the high collision frequency the plasma drift in the mesosphere is equal to the neutral air velocity.

Investigations of mean and tidal winds as well as gravity waves in the lower thermosphere and mesosphere are done with EISCAT. MEYER et ul. (1987) used EISCAT data to extend the upper limit of prevailing wind profiles between 70 and 90 km obtained with rockets and MST radar between December 1983 and February 1984 during the MAP/WINE campaign. KIRKW~O~ (1986) evaluated semi-diurnal variations of wind velocity, ion temperature and collision frequency between 92 and 120 km. The semidiurnal amplitudes of these parameters also show seasonal variations, which KIRKW~OI) (1986) used to develop an adjustment of the models. VIRDI and WILLIAMS (1989) had observed the semi-diurnal tide in the winds in the lower thermosphere up to 120 km. SCHLIXXL and R0TTGb.R (1987) and RBTTGER and MEY~~R(1987) measured tidal winds in the altitude region from 70 to I IO km. It was proved that the periodic wind variations with height in the lower thermosphere and upper mesosphere are caused by the semi-diurnal tide (VIRDI and WILLIAMS, 1986; WILLIAMSand VIRDI, 1989). HUUSKONENet al. (1991) noticed that the day-to-day variability of the semidiurnal tide observed in the altitude range lO(~l60 km was partially related to geomagnetic activity as well as to modulations by a two-day planetary wave. This is consistent with observations made in Canada by MANSON and MEEK (1991). who found small changes in the mean winds and tidal amplitudes due to variations of magnetic activities. NYGR~N ef cd. (1992) assumed that the day-to-day variability of the semi-diurnal tide observed at EISCAT may also be due to interactions with gravity waves. During prolonged D-region ionization in solar proton events (SPE) long-period waves were observed in the mesosphcric meridional velocity (RIETVELD et cd.. 1992). In Fig. 5 large velocity amplitudes and vertical wave structures with descending phase can be seen. The relationship between these strong velocity amplitudes and the SPE needs to be studied further (e.g. ROTTGEK, 1993a). COLIJS et ul. (1992) found very strong shear regions caused by these waves below 70 km. A thin turbulence layer was generated by these

1183

shears, which could be detected by the radar as a coherent scatter echo on top of the background incoherent scatter echo. HALL et al. (1987) observed vertical velocity variations of 2-3 m s ’ by atmospheric gravity waves in the altitudes 74-90 km, which they attributed to generation by temperature changes due to the solar terminator. Velocity variations due to gravity waves are frequently observed in the mcsosphere and it can be assumed (WILLIAMS et cd., 1989) that they modulate the appearance of the so-tailed polar mesospherc summer echoes.

Strong mesospherc backscatter echoes were observed by 50-MHz MST radars at polar latitudes in summer (c.g. ECKLUND and BALSLEY, 1981 ; CZECHOWSKYct nl.. 1989). These echoes arc confined to about SO-95 km with a clear maximum around 85 km. These mesosphere echoes were also observed by the EISCAT 224-MHz radar (HOPPE et cd., 1988) and by the EISCAT 933-MHz radar (see Fig. 2). Examples of observations on 224 MHz are shown in Fig. 6. Thin laminae as well as turbulent broadening are characteristics of these PMSE structures. RBTTGER rt cd. (1988) had introduced the name polar mcsosphcre summer echoes (PMSE), because these echoes are typical for the polar mesosphere in summer. A furthct justification for using this special name is that these echoes are connected to the very cold polar mesosphere and the occurrence of hydrated cluster ions. which reduce the ambipolar diffusivity due to their mass and size. The cold polar mesosphcre is associated with global dynamics, causing upwelling and adiabatic cooling of air in polar regions in summer. Also a relation of the PMSE to gravity waves (HALL. 1990) and long-period waves in the mesosphere exists, as we shall explain later. The observations of R~~TTGERct cd. (1990b) with the EISCAT radars support the dcpendcnce of PMSE on the clcctron density profiles. but question a clear relation to neutral air turbulence due to the frequently very narrow spectral widths (corresponding to less than I m s ’ r.m.s. velocity fluctuations), the very strong power and the frequency dependence of the PMSE observed on 50. 224 and 933 MHz (e.g. R~~TTGERpt al.. 1988. l990b : RBTTGER and LA Hoz, 1990). It is clear that these PMSE are quite intermittent in power over short time scales, of the order of minutes, but the average diurnal variation is quite persistent. CZECHOWSKYrt 01. (1989) report quasi-periodicitics in 50 MHz. PMSE echo power variations related to fractions of a day, which suggest neutral dynamical influences. On the other hand, RISHRETHet 01.(I 988)

1184

J. RBTTGEK

23-Ott-89

65

60

Fig. 5. Northward velocity profiles measured on 23 October 1989 during a polar cap absorption event with the EISCAT UHF radar. Each profile is an average over 2 min and is offset by 50 m SS’ to the right of the previous ones. The solid lines show the estimate of the descending velocity maxima. (From RIETVELD (>Icrl., 1992.)

noticed quasi-periodic variations on time scales of several tens of minutes in the EISCAT PMSE which apparently correlated with geomagnetic activity. WILLIAMSet al. (1989) described a case of PMSE, observed with the EISCAT VHF radar, where the maximum signal intensity corresponded to the maximum upward velocity of a gravity wave. Possibly, variations in particle precipitation or adiabatic cooling related to the vertical velocity due to the wave as well as wave steepening and breaking at the mesopause may have played a role in this effect. However, LA Hoz et al. (1989b) noticed an event where altitude and velocity oscillations were exactly coherent but 90’ out of

phase. LA Hoz et a/. (1989b) related this to the vertical advection of thin layers. Similar events were reported also by FRITTS et al. (1990). HOPPE and HANSEN (1988) claimed to find evidence in EISCAT bistatic UHF radar data that the neutral temperature and density in the mesosphere were modified as air parcels were vertically displaced. The conventional breaking of waves into turbulence, which could explain periodic echo power variations, are not very regularly seen in the Doppler spectra of the EISCAT VHF radar PMSE. There are certainly wavelike undulations (e.g. LA HOZ et al., 1989b) or certain stepwise displacements (e.g.

EISCAT

and middle atmosphere

R~TTGER et al., 1990a) detected in the thin structures of the PMSE, which can be explained by gravity wave influences. The EISCAT observations show that there can be vertical velocities in the mesopause up to 10-15 m s- ‘. However, the ‘oscillations’ are frequently non-sinusoidal, triangular shaped or sawtooth-like, as shown in Fig. 7. These features are taken as an indication of gravity wave steepening and tilting (R~~TTGERet al., 1990a). In the bottom part of Fig. 7 a vertical displacement of a thin lamina is modelled. which can give rise to the observed Doppler spectra. We assume that this steepening, causing a ridge in the laminated structure, is due to solitary waves, as was recognized in the polar mesosphere by WIDDEL (1992). The EISCAT observations show that gravity wave breaking into turbulence is not remarkably frequent in the polar mesopause, although it distorts the PMSE laminae when they occur (see lower panels of Fig. 6). We propose that other properties such as these nonlinear effects and/or wave-wave interactions undoubtedly have to be considered in the polar mesopause. Steepening of long period waves in the middle atmosphere was theoretically treated by MOBBS (1985). We have sketched in Fig. 8 the velocity and temperature structure of an internal wave, which is steepening. Rocket and lidar measurements show that temperature excursions of some 10 K can occur in the mesosphere (Schmidlin. personal commun., 1992), and it is usually assumed that these are due to waves (e.g. CHANIN and HAUCHECORNE. 1987 ; HAUCHECORNY et ul., 1987). MURAOKA et al. (1987) have observed characteristic velocity variations due to long-period internal waves in the mesosphere with the MU radar in Japan. As we shall explain in Section 5.2, the low temperature sectors of these long-period

and lower thermosphere

processes

1185

waves will cause electron density depletions and clustering of ions. These occur in thin laminated and sometimes quasi-periodical structures, which often move downward. VAN EYKEN et al. (1991) reported that the PMSE strata could be inclined to the horizontal to some degree. LA Hoz et ul. (1989a), R~~TTGERet al. (1988, 1990a) and FRANKE et al. (1992) using the EISCAT VHF radar, point out that the structures seen by the EISCAT radars can often be thinner than a few 100 m. Typical examples of such laminae are displayed in the upper two panels of Fig. 6. In the lower panels of Fig. 6 more diffuse layers are shown, which exhibit certain wavelike or overturning structures. which we attribute to the Kelvin Helmholtz-instability. The overturning is an indication of turbulence generation. We argue that this neutral turbulence distorts pre-existing laminae and increases the turbulent velocity fluctuations but does not always enhance the radar backscatter crosssection at the EISCAT radar frequencies. There is obviously a similarity between these described dynamical features of PMSE and the well known noctilucent clouds (NLC, e.g. WITT, 1962). Similar characteristics are their geographical appearance in polar latitudes, their seasonal variations, their altitude of occurrence as well as their thin layered and in particular their wavelike and steepened structures. Also the appearance of polar mesospheric clouds (PMC), observed with the Solar Mesosphere Explorer satellite by OLIVERO and THOMAS (1987), correlates with the PMSE over Alaska (JENSENet cd., 1988). The physical mechanisms leading to PMC and NLC on the one hand and PMSE on the other are surely somewhat different, but they must be undoubtedly related. This issue remains somewhat unclear due to the fact that

I

To(,) Fig. 8. Schematic drawing of the horizontal (x)-vertical (z) variations (center). Left : the vertical velocity profiles t:(x,J. Right: the temperature profiles I-(x,,) at location x,, of a gravity wave during steepening (thick lines). The arrows in the center indicate the air flow and the inclined lines show the isentropes.

1186

J. R~TTGER

TAYLOR et al. (1989) and WITT et ul. (personal commun., 1992) did not find an obvious relationship between the PMSE observed with EISCAT and the NLC observed in northern Scandinavia. TAYLORet al. (1989) claimed that the appearances of these two phenomena are not closely related, at least during the period of their observations. REID (1990) has given an explanation for this disparity, namely that the particles affecting the PMSE would be too small to produce visible clouds. It is obvious, in any case, that the variety of wave structures in PMSE and NLC observations are very similar and they display the dynamics of the mesopause region. 5. THE

INTERPLAY OF DYNAMICS AND AERONOMY

The most evident phenomena in the middle atmosphere and lower thermosphere, which are related to dynamic and aeronomic effects, are sporadic E-layers (Fig. I) and the polar mesosphere summer echoes (Figs 2 and 6). It is found that electrodynamic effects also control the aurora1 sporadic E-layers and it is supposed that there may be also an influence of electrodynamics on the PMSE. In the following sections we discuss EISCAT radar observations of these two phenomena, sporadic E-layers (E,) and polar mesosphere summer echoes (PMSE); both are characterized by a horizontally layered structure. We also address briefly the similarities of sporadic E-layers and sudden sodium layers (SSL) observed by lidar. These Es layers should not be confused with thin aurora] ionization layers occurring in connection with pulsating aurora (WAHLUNDet al.. 1989) or with the wide ancmalous ion layers at about 100 km altitude, which are related to nitric oxide or metallic ions (Km~woon. 1991). These will not be discussed here. We also do not address plasma irregularities occurring in the amoral E-layer, which cause coherent scatter, which is observed with EISCAT (MOOKCROFTand S(XL.FGEL. 1988) and COSCAT (MCCREA et al., 1991).

It is well known that thin layers (z 1 km) of enhanced electron density are observed in the lower thermosphere (see Fig. I) and sometimes even down to altitudes around 90 km in the upper mesopause. These are sporadic E-(&)-layers (e.g. TURUNENet al., 1985). which are usually assumed to be generated by wind shears due to tidal or gravity wave motions acting on heavy ions of meteoric origin. Using the EISCAT UHF radar, TURUNENet al. (1985) found that ionospheric electric fields play a role. To deduce

the characteristics of sporadic E-layers, such as electron density, the abundance of metallic ions and the vertical ion velocity, a good height resolution of a few hundred meters is necessary (TURUNENrt af., 1988). NYGR& (1989) and WUUSKONEN and TUR~NEN (1990) have reviewed the studies of sporadic E-layers using the EISCAT radars. The original idea to explain the creation of sporadic E-layers demands a convergent vertical ion velocity created by neutral wind shears. Although it is often stated that the wind shear mechanism cannot work at high latitudes, where the Earth’s magnetic is almost vertical, sporadic E-layers are frequently detected with the EISCAT UHF radar (e.g. TURUN’EK et al., 1985, 1988; LANCHESTER et cd., 1991). It was also reported that E, is associated with aurora1 arcs drifting through the radar beam and the corresponding electric fields arc affecting the formation of the layers. It turns out that the electric field can shift layers. which were generated by the wind shear mechanism, vertically. If the electric field is strong enough and the right direction. a convergent vertical ion velocity results. This effect can create sporadic E-layers as was predicted by NVGRBNer al. (1984) and proved experinlentally by T~RUXEN et ul. (1985). This was later studied in some detail by KIRKWOODand COLLIS (1989) and KIRKWOODand VONZAHN (1991). who pointed out that thin metallic ion layers can be formed above 105 km if the electric field direction is between northward and westward (see Fig. I). Slightly thicker metallic ion layers can be formed between 90 and 105 km, if the electric field has a southward component (KIRKWOOD and VONZAHN, 1991). KIRKWOODrt ol. (1991) have modelled the sporadic E-layer formed by the westward electric field and found that an additional eatward wind is necessary to increase the electron density in the sporadic E-layer. Figure 9 shows the simultaneous occurrence of a sporadic E-layer in the electron density (,V,.) and a sudden sodium layer in the sodium (Na) profile. Sudden neutral sodium layers (SSL) are observed between about 85 and 105 km with lidars and rockets (VON ZAHNof al., 1989). A review of sudden sodium layers and their relation to sporadic E-layers has been published by HANSENand VONZAHN(1990). Some neutral sodium layers are assumed to be linked to sporadic E-layers. This was shown by comparison with ionosonde measurements (VON ZAHN and HANSEN. 1988) and with EISCAT incoherent scatter radar measurements (KIRKWOODand Cor.trs. 1989). The latter authors used EISCAT UHF radar data and found that the action of a gravity wave, generated by aurora1 Joule heating, could be a mechanism responsible for the simultaneous

occurrence

of sporadic

E-layers

and

EISCAT

and middle atmosphere

sudden neutral sodium layers. It was recently noted, again by KIRKW~OD et al. (1991), that electron density enhincements, measured with IS radar, did indeed correlate well with the sodium layers (Fig. 9). It is proposed that sodium forming the sudden layers is released from upper atmospheric dust by energetic aurora1 particles (VON ZAHN et al., 1988; HANSEN and VON ZAHN, 1990). The studies of sodium layers, together with incoherent scatter observations, is of particular interest in high latitudes where aurora1 particle precipitation occurs and where the lowest mesopause temperatures are found. TURUNEN et al. (1992) have classified the sporadic E-layers observed with EISCAT at aurora1 latitudes. They suggest that one type is related to particle precipitation and electric fields, the second type is due to waves in the neutral atmosphere. The third type, TURUNEN et al. (1992) suggest, is caused by the fact that metal ions can be swept from below 100 km by the electric field to the upper E-region. The deduction of composition in addition to temperature and electron density in an &-layer needs a particular analysis algorithm, which was applied by HUUSKONEN et al. (1988). TURUE;W et al. (1988) noticed that the heavy ions are most probably Fe+ and/or Mg+. Interestingly, the good altitude resolution of the EISCAT radar allowed them to find a double layer near 100 km altitude, composed of Fe+ in the upper and of the lighter ion Mg+ in the lower portion of the double sporadic E-layer (HUUSKONEN cut al., 1988). So far deduction of the sodium ion density has not been possible from the incoherent scatter data. The seasonal as well as the geographical variations of these layers are believed to be related to the mesopause temperature, affecting the reaction rates of chemical loss processes for sodium. The horizontal

and lower thermosphere

processes

1187

extent of sporadic E-layers has been investigated by COLLIS and TURUNEN (1987), and COLLIS (1990) has studied the patchiness of the layers with the EISCAT UHF radar. It is found that the electron density gradients at the edges of sporadic E-layers can be equally large, both in the horizontal and vertical direction. EISCAT observations have also shown that, although layer intensities can be horizontally very patchy, their mean horizontal structure matches the vertical descent of the layer. The vertical propagation (see Fig. 10, from TURUNEN et cd., 1992), which is mostly backward, results from atmospheric tides and gravity waves, which is modified by the effect of ionospheric electric fields (COLLIS and TURUNEN, 1987). It is noticed from Fig. IO that the sporadic E-layers can descend down to the upper mesopause region around 90 km. 5.2. Polur mewsphere sunmer echoes In Section 4.2 we introduced PMSE in view of their relation to mesospheric dynamics. Here we summarize the discussions about the mechanism creating the irregularities, which scatter radar waves. and discuss their relation to mesospheric aeronomy. Reviews of PMSE observations and theory have recently been prepared by CHO and KELLEY (1993), HOPPE et al. (1993) and R~TTGER (1993b). Here we summarize the observations done with the EISCAT radars and the related theories (see also R~TTGER, 1993c, for further details). Due to collisions between neutral particles and ions, the neutral turbulence is generally causing irregularities in the electron density structures, which then scatter radar waves. This process is referred to as coherent scatter and is also called turbulence scatter. The neutral atmosphere turbulence fluctuations arc

Fig. 9. Sporadic E-layer (N,) and sudden sodium layer (Na) measured, respectively, by the EISCAT UHF radar between 1854 and 1857 UT and by lidar between 1855 and 1858 UT on 25 February 1990 over Andoya (from KIRKWOOD ct cd., 1991).

J. R~~TTGEK

1188

properly described by the well-known Kolmogoroff spectrum of neutral turbulence. The question of where the viscous subrange of the Kolmogoroff spectrum begins in the mesosphere was discussed by COLLIS et al. (1992). How well the neutral atmosphere fluctuations transform into the ionized atmosphere fluctuations and whether the Kolmogoroff spectrum can be unambiguously applied also for the electron gas is not known. It was, for instance, suggested by KELLEY rt rd. (1987) that fluctuations at short scales of meters or less can exist in the electron gas of the mesosphere. This can happen in the presence of heavy cluster ions. These fluctuations are characterized by an inner scale, which is smaller than the Kolmogoroff inner scale of neutral turbulence. This scatter is still coherent although the scattering does not reflect the turbulence flucluations. Compared with the incoherent scatter spectra from the mesosphere. the spectra of PMSE are frequently extremely narrow (Fig. 11). corresponding to very small velocity fluctuations and thus very weak turbulence. The power of PMSE is often very strong, which is not characteristic for scattering from weak turbulence. Considering this evidence, R&TGER et al. (1988) and R~~TTGERand LA Hoz (1990) concluded that the conventional turbulence scatter theory indeed fails to explain the echoes on 224 MHz, but that enhanced electron density gradients should be considered. These gradients could be mixed by viscousconvective subrange turbulence through an enhanced Schmidt number. R~~TTGERet ul. (1990b) reached a conclusion from their observations of PMSE on 933 MHz that a further scattering from irregularities created by another mechanism must be considered, although some enhancement of turbulence during certain PMSE events could not be excluded. They cautioned that enhanced radar echo power cannot be

EISCAT

2 JULY 1987

Z = 05.9 km

VUF

considered to be an immediate indicator of turbulence intensity, rather than an indicator of decreasing gradient scale lengths of the electron density and of clustering of the ions. KELLEY et al. (1990) found that their PMSE data, compared with rocket data, can be explained by a decrease in gradient scale length, and that the turbulence intensity was enhanced. The diffusive subrange of the electron gas was at much smaller spatial scales than the viscous subrange of the neutral gas. However, it is evident that a sufficient background electron density is necessary for these processes to work. The background electron density in the high latitude ionosphere is frequently determined by precipitating particles. This means that there arc coupling processes between the magnetosphere and the mesosphere involved in the processes leading to PMSE. It is evident that the PMSE scatter process does not immediately result from turbulence, although certain turbulence features can be seen in the PMSE phenomena. This assumption results from the evident inconsistencies in the ambiguous viscous subrange limit and the frequently very persistent structures of PMSE. It was already reasonably excluded by HOPPE rr a/. (1988) and R~~~TGERef al. (1988) that conventional turbulence (coherent) scatter or incoherent scatter could cause these PMSE. The latter was excluded because the echo strength would correspond to an unrealistically high electron density, and the former was excluded due to very narrow spectra and high scattered power. R~~TTGERet al. (1990a) have estimated that the typical backscatter cross-section per unit volume of these mesosphere echoes is around IO I2 m ’ on 46.9 MHz and around IO- Ix n- ’ on 933 MHz. This means that the backscatter radar cross-section of the 3 m-scale irregularities is six orders of magnitude larger than that of the 16 cm-scale, and

Radar

9 JULY 1987

EISCAT

Z = 83.8 km

2

UHF

RADAR

JULY 1988

I

k-

Af=267Hz

-cl

Af=267Hz

+

Z=826

0Ll

I

J

868 km

Fig. 1 I, Normalized power spectra of coherent scatter polar mesospheric summer echoes. measured with the EISCAT VHF radar (left and center panel) and with the EISCAT UHF radar (right panel at 84.7 km and the next higher range gate). The right-hand side spectra cover the frequency range +225 Hz. A weak and broad spectral component due to incoherent scatter can be noticed in the UHF spectra at z = 82.6 and 86.8 km (from R&TCER and LA Hoz, 1990 and R~TTGERet al., 1990b).

EISCAT and middle atmosphere and lower thermosphere processes the wavenumber dependence follows approximately k m4‘, where k is the Bragg wave number of radar backscatter. Neither the frequency dependence of pure turbulence scatter nor that of pure incoherent scatter follows this dependency. HOPPE et al. (1990) have described multi-frequency PMSE observations and also ruled out chemically induced fluctuations as the scatter process. 5.2.1. The efict of’hecq~ ions. KELLEY et al. (1987) have pointed out that there is no evidence that abnormally high levels of turbulence exist in the polar mesosphere. Strong turbulence scattering would be needed to explain the strong power of PMSE on frequencies of 50 MHz and higher (i.c. on the EISCAT frequencies 224 and 931 MHz). They have mentioned that an extension of electron density fluctuations to small scales could be responsible for the strong summer echoes. This effect could be caused by a reduction of the ambipolar diffusion coefficient in the presence of heavy positively charged cluster ions, which occur in the cold polar mesopause (KOPP ef ul., 1985; TURUNEN, 1992). Since KELLEY et al. (1987) noted that unreasonably heavy cluster ions would be needed, CHO et al. (1992a) pointed out that the physical size of the particles strongly affects the diffusivity. The reduction of the diffusion coefficient could lead to electron density fluctuations at spatial scales which are much smaller than those of the neutral air turbulence. This would lead to enhanced backscatter crosssections even at radar wavelengths which are much shorter than the Kolmogoroff microscale of neutral turbulence. In the considered wave number range of some m ’ the neutral fluctuations in the mesosphere are in the viscous subrange and the electron fluctuations are in the diffusive subrange of the spatial wave number spectrum, assuming reduced electron diffusivity. The deviation of the neutral fluctuation spectrum from the electron fluctuation spectrum can be expressed by the Schmidt number (KELLEY et al., l987), which is given by the ratio of the molecular viscosity coefficient of the neutral gas to the diffusion coefficient of the electrons. When this coefficient is one the electrons, as passive scalars, and the neutrals have about the same one-dimensional fluctuation spectrum. This means that the fluctuations in the electron gas are statistically a replica of those of the neutral gas. For Schmidt numbers larger than one, which can occur due to large ion masses (KELLEY et al., 1987) and in particular due to the increased size of the charged particles (CHO (‘t al., 1992a), the electron gas has a fluctuation spectrum which differs from the neutral spectrum. In other words, the electron density fluctuations become decoupled from the neutral fluc-

1189

tuations. Since the radar backscatter cross-section depends on the electron density fluctuations, the radar echo power can no more be used to deduce neutral turbulence fluctuations. According for instance to KELLEY et al. (1987), KELLEY and ULWICK (1988), R~~TTGERet al. (1990b) and CHO et (11. (1992a), this should happen in the polar mesopause in summer where heavy and large hydrated cluster ions are present and thus cause strong PMSE, as observed by EISCAT. Besides the verification of heavy ions in the polar mesopause in summer by rockets (e.g. KOPP et al., 1985). measurements of the incoherent scatter ion line spectrum should also indicate the presence of heavy ions. Observations with the EISCAT 993-MHz IS radar in summer (COLLIS et al., 1988 ; TURUNEN et ul.. 1988 ; ‘TURUNEI*;,1992) actually have shown spectral narrowing, which could result from heavy ions. COLLIS rt ul. (1988) have estimated that their observations can be explained by the presence of cluster ions with mass of the order of 500 a.m.u. and pointed out the intermittent occurrence of heavy ions in very narrow layers. ROTTGERef al. (I 990b) have noticed a similar spectrum-narrowing effect during simultaneous PMSE observations with the CUPRI 46.9-MHz radar and just before the onset of the PMSE on 933 MHz. However, they related to the occurrence of narrowband PMSE in addition to the common wide-band incoherent scatter echoes. HALL and BREKKE (1988) have used the EISCAT VHF radar to deduce Schmidt number profiles for a typical summer and a winter month. It is noticeable that high summer Schmidt numbers of 3-5 are occurring in narrow layers of at most a few kilometers thickness around 88 km. whereas the winter numbers are much closer to one and only gradually increase with altitude. Some caution. however, has to be applied in the interpretation of the incoherent spectra, which could partially be spoilt by simultaneously occurring PMSE, which per SC have a narrower spectrum. This could result in a spurious reduction of the spectrum width and consequently to too large a Schmidt number. HALL (1990) has used an ion-chemical model to investigate the degree of hydration in the polar summer mesosphere. He considered in particular the effect ofgravity wave perturbations of temperature and neutral density and the corresponding response of cluster ion population. It was shown that periodic changes in clustering occur and the ion mass can become as large as some 100 a.m.u. These variations result in periodic changes of the scale of the viscous-convective subrange and could explain the observations of quasiperiodic PMSE observed with the EISCAT VHF radar by WILLIAMSet nl. (1989).

1190

J. R~TTGER

In addition to the layers of heavy ions. deep depletions of electron density or bite-outs are detected in the summer mesopause region by rockets (KELLEY and ULWICK, 1988) and incoherent scatter radar (R~TTGER et al., 1990b; see also Fig. 2). Besides the formation of negative ions, which is unlikely during daytime, the depletions in electron density can be caused by an increase in the electronion rccombination coefficient z z mf’T_“, where the constants a and h are positive and in the order of 1; m, is the ion mass and T is the temperature. In the regions of low temperature, which occur in certain phases of gravity waves (see Section 4.2), the electron density would be depleted due to enhanced recombination. This effect would be strongly amplified when the ion mass increases due to the formation of heavy cluster ions which form in the low temperature wave phases. Gradients at the top and bottom of these depletions, mixed by weak, remnant turbulence, will enhance the scatter cross-section and cause PMSE. This turbulence could be in the viscous-convective subrange due to an enhanced Schmidt number. REID (1990) has noted that scavenging of electrons by small ice particles in the mesopause could cause the bite-outs ofelectron density. He also mentioned that the sharply defined bite-outs with steep gradients may be taken as evidence for steep temperature gradients caused by atmospheric waves. These suggestions can be likely explanations of laminated structures seen in PMSE, as we suggested in Section 4.2, but further quantitative studies are needed. 5.22. The c$iJc.t cfrlust~~ plasma. Another explanation for PMSE has been proposed by HAVNES et al. (1990). who considered that dust particles (e.g. of meteoric origin) could be charged to substantial positive surface potentials by a photoelectric effect in the cold mesopause. A charged dust particle would be dressed by a cloud of electrons being controlled by the charge and motion of the dust particle, which was assumed to result in a substantial enhancement of the radar backscatter cross-section. It could also explain the narrow Doppler spectra of the EISCAT PMSE signals, reflecting the low thermal motion of heavy dust particles. Whether this proposed new mechanism is applicable remains to be discussed, since the resulting elIect could be that of conventional incoherent scatter from an ensemble of independent electrons. Furthermore, this effect of scattering from dressed dust is independent of the radar wavelength (in the referred ranges down to several 10 cm), which cannot bc supported by the observations. The role of charged aerosols was also investigated by CHO rt ~1.(1992a) and HALL et al. (1992). CHO t’t rd. (1992a) disagree with HAVNES ef al. (1990) that

charged aerosol incoherent scatter can explain PMSE at VHF. They presume that the enhanced Schmidt number is responsible for the VHF radar scatter and they could explain the UHF radar scatter by positively charged aerosols with charge number of the order of 100. HAGFORS(1992) has proved that the scatter crosssection due to dust is only strongly enhanced for quite large charges on the dust and the enhancement over the incoherent scatter (Thomson) scatter is relatively modest. Hagfors found only some 4 dB difference in cross-section between 224 and 933 MHz. He mentions that the cross-section could be enhanced if the dust particles would be involved in turbulent motions, but he follows the consensus of observers that neutral turbulence at the scales corresponding to the EISCAT radar wavelengths are unlikely in the mesosphere. A basically similar conclusion was reached by LA Hoz (1992). HAVNES rt al. (1992) suggest that falling dust interacting with a neutral gas vortex can cause steep gradients in electron density. which could reflect radar waves. It is speculated that very few of these reflecting vortices in the radar beam could explain the braided structures in PMSE spectra as observed with EISCAT by R~TTGER and LA Hoz (1990). Since these braided structures occur only occasionally it needs to be studied if this effect can explain the PMSE in general. Studies are also needed to verify such small vortex dimensions in the neutral gas. Radar interferometer investigations are necessary, allowing high-resolution sensing, to prove the existence of these small-scale structures in the PMSE. These experiments are being done with the EISCAT VHF radar. Recently PMSE were observed even on 1.3 GHz with the Sondrestrom radar (CHO pt d.. 1992b). They suggest that these echoes are enhanced incoherent (Thomson) scatter from a layer of charged aerosols. These observations again prove that turbulence scatter has to be discarded. CHO ct al. (1992b) speculate that their I.3 GHz observations can be explained by enhanced Thomson scatter, provided that the charge number per particle is in the order of IO. 5.2.3. Other. plwnomcvur causing PMSE. Regardless of the detailed applicability of these theoretical invcstigations. which all seem to suffer from the problem of high charge numbers on particles, WC assume that certainly charging of heavy and large ions. aerosol, dust or ice particles has to play a role in the creation of certain structures in the PMSE. Micro-scale structures (in the range of the radar wavelength) would scatter the radar waves or certain small-scale structures (larger than the radar wavelength) show up as laminae or other observed structuring in the PMSE. The possibilities of charging were further discussed by R~TTGER and LA Hoz (1990), who suggested that charge sep-

EISCAT

and middle atmosphere and lower thermosphere processes

aration by ice particles in mesospheric convection could happen. GOLDBERG (1989) has measured electric fields in the mesosphere with rocket probes, and has pointed out the possible influence of mesospheric electric fields on noctilucent clouds, which we assume would also affect the PMSE. Stratification in the mesospheric electric field and the disappearance of electrons in noctilucent clouds were measured with rockets by CROSKEYet al. (I 990). This could be related to stratifications in PMSE and the electron density bite-outs. TRAKHTENGERTS(1992) has discussed the electric space charge waves that can be excited by the interaction of gravity waves in a shear flow which could cause scattering. These processes, however. need fairly large diameters and high charge densities of the particles. We should also investigate the possibility of double diffusion as occurs in the ocean, which was excluded by RBTTGER (1980) to explain stratospheric radar laminae, but could well apply in the mesospherc. This diffusion, resulting from different diffusion coefficients of constituents such as different aerosols and ice particles in the ion species, mesopause. could cause a laminated structure in the mesosphere. HOPPE (1993) has formulated a model for molecular ditrusion causing a layering. Also lateral convection of air masses in the mesosphere can cause such laminae in the mesosphere (e.g. R~~TTGER,1980). We recognize that all the present explanations of PMSE observed by EISCAT, which relate to multiply charged particles, need charge numbers in the order of 10 or more or particular deviations from random particle distributions. It needs to be verified experimentally and theoretically if this is observable and conceivable. All these processes, which could create the PMSE. also require the ionization of the mesosphere, which is affected by magnetospheric particles precipitation in the aurora1 zone. This comprises a particular coupling mechanism from the magnetosphere to the mesospherc. We regard the further combination of radars operating at different frequencies, observing coherent, usual and enhanced incoherent scatter, as an essential tool to study these dynamical and aeronomical phenomena. The combination of such observations with other instrumentation such as lidar and rockets will be demanded, too.

6. CONCLUSIONS AND OUTLOOK Particular phenomena in the polar atmosphere and the aurora] ionosphere, namely particle precipitation, Joule heating and electric fields as well as vertical transport of constituents and momentum, which result from magnetosphere--ionosphere coupling, have an impact on the middle atmosphere as we have

II91

outlined. Furthermore dynamic processes, such as tides and gravity waves, which originate in the lower atmosphere and the middle atmosphere, propagate upwards and dissipate in the thermosphere. These mutual coupling processes, which affect the structure, dynamics and aeronomy of the middle atmosphere and lower thermosphere. take place uniquely in the high latitude mesosphere. The polar mesopause is the predominant region of the Earth’s atmosphere where the effects from above, the magnetosphere and ionosphere, merge with the effects from below. the mesosphere, stratosphere and troposphere. As we have shown, the EISCAT UHF and VHF radars have certainly demonstrated the ability to study coupling processes from above to below in the lower thermosphere and mesosphere of the aurora1 zone. Several mesosphere and D-region investigations have been done with EISCAT to prove the impact of precipitating electrons and protons on the D-region electron density, composition and dynamics. How the coupling processes between the magnetosphere and the ionosphere. the thermosphere and the middle atmosphere take place and how &-layers and polar mesosphere summer echoes develop in the polar cap region is so far unknown, since appropriate radar instruments are not operated at polar latitudes, higher than those of EISCAT. In view of global change CHO and KIILLEY (I 992) remind us of the possibility of anthropogenic effects, such as the increase of methane gas, which diffuses upwards into the mcsosphere, where it can increase the change for notilucent clouds. Since we are aware of a relation between notilucent clouds, ice particles, hydrated ions and the polar mesosphere summer echoes, the understanding of the latter and their continuous observations is regarded as a possible indicator for global change. This holds in general for ionospheric observations. which can be very sensitive indicators for long-term trends. RISHRETH( 1990) has for instance argued that a signature of the greenhouse effect could occur in the ionosphere. This supports the ongoing efforts for continuing ground-based ionosphere and middle atmosphere observations, especially in high latitudes. There are also particular phenomena in the Arctic stratosphere and troposphere which need to be studied by radar (e.g. RBTTGER, 199lb), in addition to the multitude of other measurements which are already applied, for instance, for ozone studies in the Arctic and Antarctic. With radar observations it is additionally possible to explore dynamic processes over a wide scale range from planetary and synoptic scale disturbances to small-scale atmospheric gravity waves and clear air turbulence. The wind field variations

J.

1192

RGTTGER

occurring in the polar vortex can be monitored by radar. In Arctic regions it is of special interest to investigate with radar the exchange processes between the troposphere and the lower stratosphere, in particular the variation of the tropopause height and the dynamics of tropopause foldings and fronts. The possibility to study vertical transport between the troposphere and the stratosphere, and the transport within the stratosphere by means of the mean vertical motion and by turbulent diffusion, is a major research task. The transport and deposition of energy and momentum by atmospheric gravity waves in the lower stratosphere, which can also be measured with radars, have an impact on the mean stratospheric circulation and the Arctic stratospheric temperature. It is known that the polar stratospheric clouds control the ozone depletion. The formation and the dynamics in and around polar stratospheric clouds should also be investigated by radar. EISCAT constructs a further research radar station on Svalbard to study the Earth’s atmosphere in the polar region of the Arctic (BJBRNAet aI., 1991). The

plans of EISCAT include, besides the studies of the ionosphere and its coupling to the magnetosphere and

the solar wind, also the investigations of the middle and the lower atmosphere. In view of the possible changes of the atmosphere due to anthropogenic influences it is essential to incorporate these investigations of the Earth’s environment into these new scientific concepts of EISCAT. In continuing the present operations of the EISCAT radars in northern Scandinavia and preparing for the operation of the EISCAT Svalbard Radar (ESR) in the Arctic, we have an appropriate ground-based system of radars covering, in addition to the detailed studies of the polar cap ionosphere and magnetosphere, studies of the middle atmosphere and the coupling with the upper and lower atmosphere at high latitudes. Acknou,l~~~rmenrs-The EISCAT Scientific Association is funded by CNRS (France), MPG (Germany), NFR (Norway), NFR (Sweden), SA (Finland) and SERC (U.K.). The

author is indebted to many colleagues for valuable input and discussions. John Cho’s constructive comments on this manuscript are very welcome. Parts of Sections 4 and 5 of this paper are an augmented version of a paper published in the Proceedings of the NATO Advanced Research Workshop on ‘Coupling Processes in the Lower and Middle Atmosphere’ in Loen, Norway, 1992.

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1195