,4dv. Space Res. Vol. 9, No. 5, pp. (5)l6~(5)l72,1989 0273—117789$0.00+ .5’)Pnnted in Great Britain. All rights reserved. Copyright © 1989 COSPAR

RECENTD-REGION RESEARCHUSINGINCOHERENTSCATTERRADAR

C. Hall

TheAurora! Observatory,P.O. Box953, N—9001Tromsø,Norway

ABSTRACT

An attempt has been made to review some of the more recent developments in incoherent scatterresearch relating to the D—region of the ionosphere, and most accent has been placed on themost recent research. As a preamble, the basic relevant incoherent scatter theory is intro-duced, but without so much mathematics as to confuse the reader. Research has tended toconcentrate on three distinct areas: (i) electron density and energy distribution, (ii) ionchemistry and (iii) neutral dynamics, corresponding to the three parameters directly measuredby the radars: power, spectral width and bulk Doppler shift respectively. This review isthus divided into corresponding sections.

INTRODUCTION

The last decade has seen considerable advancement in the operations of incoherent scatterradar. The systems have become more reliable enabling regular measurement programs andallowing time to be spent on development or more advanced experiments rather than maintenanceof hardware. These more sophisticated experiments have included radar modulations optimisedto measure in the D—region, arguably the part of the ionosphere least understood. Due tothe rapid variations of electron density with both time and height, combined with complexchemistry, the D—region makes high demands on an incoherent scatter radar trying to probe it.An experiment running on a UHF system must be able to obtain good signal with minimal elec--tron densities, be able to resolve spectra down to widths of a few Hertz, yet be able tomeasure spectra as wide as 1500 Hz only 20 km higher up. Furthermore the rapid variationwith height demands high height resolution, and rapidly varying precipitation demands hightime resolution. In the last few years particularly, much interest has focused on the D—region from both sides of the Atlantic, and from low to high latitudes.

This paper attempts to review the more recent developments regarding D—region research.The technical aspect of radar modulation will not be touched upon, since it is of a ratherspecialised nature and not easily accessible to the general readership. Instead, the exist-ing incoherent scatter theory is summarised and the latest developments introduced. In thesubsequent sections the experimental aspect is reviewed. Three main parameters are actual-ly measured by incoherent scatter radars in the D—region, these being power, bulk Dopplershift and spectral width. All other parameters are derived. The discussion of research istherefore divided into three main sections: power is usually used to deduce electrondensity and hence information on negative ion concentrations and also for derivation of pre—cipitating particle energy spectra and predicted cosmic noise absorption; the bulk Dopplershift provides us with insight into neutral atmosphere dynamics, in particular, gravitywaves; spectral width gives information on ion mobility, mass and negative ions, and henceion chemistry.

OVERVIEWOF ThEORY

Our starting point will be the work of Fukuyama and Kofman /1/ and their explicit expres-sions for spectral shape and backscattered power in terms of various mttnosp,heric parameters.It must not be forgotten however that the foundadons for this work were laid by many re-searchers. Incoherent scatter theory is generally formulated in terms of the Maxwell—Vlasov equations combined with the dressed test—particle principle or from a hydrodynanticapproach. This is quite justified in the E— and F—regions because collisions are infrequent.However, to apply these methods to the D—region, where collisions are important, may requiremore justification than has been the custom hitherto. Both /2/ and /3/ have proposed waysof accounting for the collisions by use of relaxation terms and recently /4/ have comparedthese two approaches, demonstrating that the former is a better model for their observations.

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(5)164 C. Hall

Indeed, the work of /4/ gives us a concise resumé of the development of the expressionsgiven by /1/ from the Vlasov equation starting point via /2/. /1/ have furthermore includedthe hydrodynamic approach of /5/ in order to account for three types of charged particle:electrons, positive ions and negative ions, the last being an important feature of the lowerionosphere.

Let us define what we mean by “collisions are important”, as it is in such a regime that theformulations of /1/ are valid. We define the “collision dominated regime” as that in whichcollisional damping dominates over Landau damping, and in particular that the ratio of radarwavelength to ion mean free path is large. This ratio is essentially an ion—neutral colli-sion frequency normalised to the radar frequency. For this regime, then, /1/ give the shapeof the ion component of the incoherent backscatter as

S(k,w)dw = a (1+2X)&’tPd~P (1)(2(1+A)a

2+1)2 + 4(~2+1)2~2O2

where S(k,o~) is the power at probing wave number k, and Doppler frequency to; a is a constantproportional to the electron density; X is the ratio of number densities of negative ions toelectrons; ~ = (kDY’, D being the Debye length; ~P= V

1~/v’2kV, and is the positive ion—neutral collision frequency ~‘jn normalised to the radar frequency, V being the thermal speedof the positive ions; 8 = w/I2kV. Here /1/ have assumed that tj is the same for positive ionsas for negative ions. We shall return to the validity of equation (1) later. We now noticethat the spectrum is expected to assume a so—called “Lorentzian” shape. If we integrateover frequency we obtain the backscattered power. /6/ have conveniently expressed thispower and also the spectral width in terms of easily understandable atmospheric parameters:

Power = bNe(1+2X)/(2(1+X)+~2)(1+~2) (2)

Width = c(2(1+X)+a2)/(1+~2)T/mivin (3)

where b and c are constants, T is the temperature, assumed to be the same for electrons,ions and neutrals and mi is the positive ion mass. /6/ tell us that neglecting the Debyelength effect, so that we can ignore cC2, will lead to an overestimate of the spectral widthof about 10%. It is, however, not too difficult to iteratively compute cC2 and Ne using thesuggestion given by /7/.

In addition to these parameters, we are able to measure the Doppler shift of the wholespectrum relative to the transmitted frequency. This Doppler shift may then be used todeduce the bulk plasma velocity directed along the scattering vector, i.e. along the bi-sector of the transmitted and received beam directions. A later section is devoted toresults of investigations into dynamics by this method.

It would seem, then, that if we are prepared to make some assumptions, parameters such aselectron density and ion mass may be available from incoherent scatter data. Indeed, if weare prepared to assume that no negative ions are present, e.g., above say 90 km altitude atnight, or 70 km during sunlit conditions, the electron density obtained from the back—scattered power will be fairly representative of the ionosphere, provided we are assuredthat backscatter comes from the positive ion component of the spectrum alone. Many re-searchers have made this very assumption and their work is summarised elsewhere in thisreview.

Unfortunately, however, contributions to power may come from other sources. /8/ have pro-posed an additional scattering mechanism involving chemically induced structure fluctuationsin the electron gas. This scattering would manifest itself as a narrow spike superimposedon the ion component. The mechanism is felt to be questionable, however and no firm obser-vational evidence exists to support it. More important are the consequences of measuring atwavelengths approaching scales of structure size. Such structures might be scatteredregions of high electron density gradient with height, which would give rise to Fresnelscattering /9/. Another possibility is scattering from turbulence due to decreased electronmobilities in the presence of heavy positive ions (e.g. water clusters) /10/. Evidence forthese last two mechanisms has been seen, however, and is briefly discussed in a later sec-tion.

Although the more exotic scattering mechanisms do not fall within the category of incoherentscatter, they do introduce important consequences for the extraction of atmospheric para-meters from spectral widths. Since we are restricting our discussion to the collision domi-nated regime, we have also committed ourselves to a fixed spectral shape. it is impossible,therefore to determine say X without a priori knowledge of m~, Vin and Ne. This is incontrast to the E— and F—region incoherent scatter spectral analysis where different para—meters affect the shape in different ways. Both /6/ and /11/ discuss combining spectral

D-Region Researchusing IncoherentScatterRadar (5)165

width information with neutral atmosphere models.

An alternative approach is to recognise the dangers in trying to estimate ion mass forexample, and to satisfy ourselves with estimating the molecular diffusion coefficient, givenby 0 = 2KBT/miVi

0. We can see that 0 is directly proportional to the spectral width. If weare prepared to assume that the electron and ion diffusion is quasi—ambipolar, we maythen proceed to estimate the Schmidt number, which is the ratio of neutral kinematic vis-cosity to 0. This somewhat obscure parameter is akin to the more familiar Prandtl number,in which D is replaced by thermal diffusivity. These concepts are discussed by /12/ andapplied to the ionosphere by /10/.

ELECTRONDENSITY RELATED STUDIES

Perhaps the easiest parameter to estimate from incoherent backscatter, is the electrondensity, particularly if we neglect negative ions. Indeed, early experiments failed toobtain good signal—to—noise ratio in the negative ion regime anyway, unless some degree ofprecipitation was present. The highly sensitive Arecibo system has nontheless been able toyield reliable electron densities from at least as low as 64 km altitude with only photo—ionization as the production mechanism /13/. In the auroral zone /14/ have also measuredelectron densities, this time from the EISCAT 933 MHz system and compared the results withboth rocket—borne Faraday rotation experiments and partial reflection methods (Figure 1).Meanwhile, /15/ have compared EISCAT derived electron densities with X—ray fluxes measuredon balloons; as might be expected, the injection of high energy primary electrons correlateswell with enhanced X—ray flux. Various researchers have utilised the electron density pro-files to deduce the particle energy spectra. A method of inverting the profile was proposedby /16/, using data from the Chatanika system /7/ and alternative algorithms have since beendescribed in the literature, e.g. /17/. The hardening of precipitation during substormgrowth has subsequently been investigated, e.g. /18/ and /19/. Examination of the develop-ment of the electron density at 10 second time resolution by /19/ revealed considerableinternal structure within what had previously been observed as a single “spike” in the rio—meter response at substorm onset. The pre—growth phase of the substornt exhibits steadilyincreasing particle fluxes but with no high energy component; during the onset, however,within 1 minute, increasingly harder particles appear, the hardest particles appearing some40 seconds after the actual onset. Figure 2 shows some typical energy spectra correspondingto varying conditions.

150 a) 860131 1520 UI

~

~~i5o b)8601311600U1 ~ ~2

10’ 10’ 1010 10” 100 t _M’4—~ J~t0 aes— ma~~nt5MEAN Ne PER CUBIC METRE - 5”

‘o_o- N~PRE~ N~EISCAT

lonosonde lf0E)

Fig. 1. Electron densities from The Fig. 2. Derived electron energy distributionsEISCAT 933 MHz system, extended down— at three times during the recovery phase of anwards from 90 km using differential auroral substorm, 23rd March 1985, observedabsorption data from 2.75 MHz. with the EISCAT 933 MHz system (after Devlin et

al.).

(5)166 C. Hall

A similar region of interest has been the combination of incoherent scatter data with thatfrom satellites. One example of this is how satellite—measured proton fluxes may be com-bined with EISCAT electron densities to investigate recombination coefficients /20/. In thisparticular case, the time—development of the recombination coefficient profile through apolar cap absorption was noted, revealing the inadequacy of using a static model for recom—bination coefficient. Hydrated ions have an order of magnitude higher affinity for electronsthan molecular ions, so these hydrated ions will tend to recombine rapidly at the onset ofprecipitation, leaving the molecular ions to recombine more slowly. A more chemistry—oriented study has been made recently, /21/, but employing the same methodology as /20/,here the electron densities being measured at Arecibo.

Comparison of cosmic noise absorption measured by riometers with that predicted by the in-coherent scatter derived electron density has enjoyed some popularity amongst EISCAT workersin particular /6/, /19/, /20/, /22/, /23/, and also at Sondrestram /24/. Such comparisonsprovide a means of independently checking the validity of the derived profile, although thetechnique is prone to error. Some form of electron—neutral collision frequency model mustbe employed to compute the predicted cosmic noise absorption, and unfortunate choices ofmodel may give rise to a systematic offset. Furthermore, the antenna beam widths of rio—meter and radar tend to be very different. An example of such comparison is shown in Figure3, and clearly the agreement is generally good. The spatial separation of the instrumentsin this case was around 20 km, whereas both surprisingly good agreementand total disagree-ments have been observed by the same authors /23/ over spatial separations of 120 km, but inunpublished work. This has indicated both blanket and highly localised types of precipita-

tion in the evening sector. 1TTII2100 0110 2120 2130 21’0 2150 2200 iT

— (NA AS PREDICTED ABSORPTION Co,-,P0000FROM 0E14 11 DATA FROM RIOMETER CHARTS

ILAAAII0000LED PIOMETERI

Fig. 3. Comparison of cosmic noise absorption at 30 MHz as measured by a riometerwith that predicted by EISCAT operating at 933 MHz.

DYNAMICS RELATED STUDIES

Less easy to estimate, but somewhat more reliable is the bulk Doppler shift of the spectrum.From this we are able to deduce the bulk neutral air motion along the scattering vector bymaking two prime assumptions: the first is that the ion—neutral collision frequency is highenough that the ion drift is representative of the neutral motion, the second is that theion drift is constant throughout the scattering volume. Normally, the horizontal componentof the wind totally dominates the vertical, so it is usual to measure the components sepa-rately in the mesosphere. The vertical component can be measured by either directing theantenna beam vertically, or by employing a bistatic method as shown in Figure 4. The multi—

TROMSO FAUNA

Fig. 4. Schematic of the bistatic scattering geometry used to estimate verticalneutral wind. The transmitter—receiver baseline forms the base of an isosceles tri-angle. This system enjoys the advantage of low system noise at the receiving sta—tion, but has the disadvantage of increased range to the scattering volume (afterNoppe and Hansen).

D-Region Research using Incoherent Scatter Radar (5)167

static EISCAT system is particularly well suited to the latter method at 933 MHz, and to theformer at 224 MHz. In the monostatic case, height resolution is determined by the trans—mitted pulse length, whereas in the bistatic case it is determined by the geometry and thebeam—width. The bistatic method has failed to gain popularity due to its inability todetermine wind profiles free from space—time ambiguity.

The bistatic method has been successfully employed by /6/ and later /27/ on the EISCAT 933MHz radar. However, the majority of workers in this field have resorted to the monostaticmethod. At Arecibo vertical velocities have been obtained as low as 59 km, and profilesobtained up to 90 km as shown by /25/. Long periods were observed around 90 km (4 hours),and it was proposed that gravity waves below the mesopause were fed by the diurnal tide.Tidal effects have been more extensively investigated during early 1984 by EISCAT, /26/.Zonal and meridional drifts have been measured in the height range 80—105 km and subsequent-ly compared with winds determined using falling spheres and foil clouds released by rockets.It was noted that the height range 85—105 km exhibited strong semi—diurnal tide motion, butthe first 5 km of the observed height interval (80—85) exhibited only small windspeeds. Theauthors present their results as a representation of the mean state of the undisturbed springhigh latitude mesosphere/lowerthermosphere,and an example is given in Figure 5. Concen-trating on the vertical component only, and using the EISCAT 224 MHz system, /28/ have madeextensive use of vertical wind profile time series to search for mesospheric gravity waves.It was concluded that these waves are far from uncommon. In particular a wave of 40 minuteperiod, and horizontal phase velocity of 130—180 rn/s has been identified as being launchedby the dawn terminator. The terminator had long been suggested as a mechanism for generationof gravity waves, but only at the high—latitude observatories is the terminator subsonic,and in this instance the wave’s horizontal velocity coincided almost exactly with that ofthe terminator /29/. Due to the monochromacity of this wave and its retention of constantamplitude with height, it was possible for the authors to estimate the energy dissipation,which in turn agreed favourably with similar values obtained by in—situ methods elsewhere.Continuing this work at EISCAT /30/ were able to detect similar waves, but not attributableto the terminator this time. Figure 6 shows such an example; here only velocities towardsand away from the radar are distinguished, and a wave with downward progressing phase isclearly seen, the period being around 20 minutes. The cutoff of the wave around 90 km istypical and is attributable to damping at the strong temperature inversion of the mesopause,possibly combined with non—linear electrodynamic driving.

16 Feb 1982. 16 Feb 1981.

•D06E ‘ ,

14111 14111TO~ ‘ OS;

—ISO -120 —AR 3 50 100 150 —ISO .100 —50 0 50 100 150

v/I,.”1

Fig. 5. Examples of meridional and zonal wind profiles measured with EISCAT at 933MHz from a study of tides (after R~ttger and Meyer).

LISCAT D-REGIORR ACE (GEN-tI-YHEA)VEWrICAI. DRIFT (H/SIFROM 2)104/27 22~0OO0 To 22/04/27 0200 00

NO DATA

~:~ -2200 2240 2320 0000 0040 0t20

TIME UT

Fig. 6. Vertical air motion versus time and height observed with EISCAT at 224 MHz.The shaded areas denote downward motion. Oscillations with downward progression inphase are clearly seen from 2320 onwards (after Turunen et al.).

(5)168 C. Hall

INTERPRETATION OF SPECTRALWIDTh

The computation of spectral width requires only the fitting of a Lorentzian to the measuredspectrum. Alternatively, we might note that the Fourier transform of a Lorentzian is anexponential, thus it is possible to fit a straight line to the logarithm of the correlationfunction, once we have removed the bulk Doppler shift. However, the interpretation of theresulting spectral widths in terms of atmospheric parameters is somewhat subjective. Theapproach to interpretation is furthermore season and latitude dependent: mid and low lati-tudes do not need to account for auroral precipitation, and the low temperatures encounteredat the summer mesopause in polar regions; in summer, the ion species, particularly at highlatitudes tend to be varied, whereas in winter only two or three types tend to dominate;negative ions tend to be absent at mesopause heights in summer at high latitudes, because ofthe continual sunshine. In order to deduce any one parameter from the spectral width (thatis, apart from molecular diffusion coefficient), the remaining parameters must be estimatedfrom models or measured independently.

Estimates of the negative ion to electron number density ratio (X) were made at Arecibo by/31/, by assuming an ion mass of 31 amu. First, the authors assumed no negative ions andcomputed the collision frequency as a function of height. This was then compared with theCIRA 1972 atmosphere and a very strong deviation from the model below 75 km altitude attri-buted to negative ions. The collision frequency profile was then relaxed to allow estimationof the negative ion density profile. An example of an unrelaxed profile is shown in Figure7, and resulting estimates of negative ion density in Figure 8. Examination of these colli-sion frequency profiles in more detail revealed to /32/ the presence of a step in the pro-file at around 86 km. Although this is not so clear in the example data shown here, otherprofiles exhibit a very distinct ledge. The authors have interpreted this as a possibletransition from simple molecular ion species to hydrates, and estimate the mass to be 61 amuThe strong departure from the model attributable to negative ions occurs well below theheight suggested as the transition to hydrates, so these two aspects of the ion chemistryare not confused in this case. Using the same technique, /33/ has subsequently seen evi-dence for presence of negative ions in the 85—90 km region. The presence of negative ionsbelow the mesopause is relatively well understood, but the presence of layers higher up isnot. /33/ proposes the presence of heavy negative ions possibly related to noctilucentclouds, although it is perhaps questionable that temperatures at the low latitude summermesopause would be low enough for accretion. Indeed at higher latitudes /34/ have observeddiscrete reductions in electron density at such heights by in—situ methods, and it ispossible that this is due to attachment of electrons to form negative ions or negativelycharged particles. These layers of negative ions were observed during the sunset period,and as a result /33/ has gone on to investigate the sunrise and sunset transitions in themesosphere above Arecibo. A decrease in spectral width was noted at both transitions. Thespectral narrowing at sunset was attributed to a possible combination of reduction in thetemperature to neutral density ratio and positive ion mass, and the narrowing at sunrise todestruction of the negative ions which had formed during the night hours.

1OO~- ICIRA 1972 13 JUNE 78 11—12 HRS

~:: z >“ .31 - ~ i;2MER 1978

70:

Fig. 7. Deduced ion—neutral collision fre— Fig. 8. Examples of profiles of negativequency o~from Arecibo data (dashed line), ion to electron number density ratio de—Corresponding model profiles derived for rived from Arecibo data. The evening valuesdifferent ion masses are also shown. The are to be treated with caution but the noon—departure from the models below 75 km is time values are seen to be very reasonableattributed to presence of negative ions not (after Ganguly et al.).accounted for in the analysis (after Gangulyet al.).

D-Region Researchusing IncoherentScatterRadar (5)169

At high latitudes, similar investigations have been made, but using a broader variety ofstrategies. The first investigation of negative ion presence by the EISCAT system wascarried out by /6/ at 933 MHz. These authors chose to plot theoretical spectral widthversus height for various negative ion to electron number density ratios. They then super-imposed the measured values of spectral width hence giving an indication of which negativeion concentrations best fitted the data. The theoretical widths were computed for twodifferent temperature profiles using the neutral atmosphere model of /35/. This model issemi—empirical, and allows the user to select temperatures at certain heights in order tofix the model to actual measurements. This feature was employed by /23/, who obtained thetemperature at 85 km from hydroxyl measurements, and the stratopause temperature from acombination of satellite and rocket soundings. Then assuming a constant ion mass in muchthe same way as /32/, they were able to estimate the negative ion to electron number densityratios as shown in Figure 9. There is a similarity between these profiles and that of /32/,for collision frequency, and indeed, when the step in the profiles of /23/ was relaxed, theauthors were able to make qualitative estimates of positive ion mass. The same techniquewas later employed by /28/ to make more extensive investigations into negative ion concen-trations. The collision frequency profiles determined by /32/ show a great similarity tothose estimated by /36/, who suggest similar interpretations in terms of negative ions andpositive ion mass.

GEN-11A DSPEC290, 10111/65 0154 UT 23/09/IS 2151 UT

~To~iii ~ S

90~ 10111115 0156 UT ~. 90] 23/08/95 OlSS UT

4 5 S S T

90 15/11/65 2044 UT 03/09/85 3157 07

I 5 1 1 OT2~iii66AT

90 15/11/IS 2105 UT 23/09/85 3351 UT

Ti A ~ S I

Fig. 9. Examplesof profiles of negative ion to electron number density ratio derivedfrom EISCAT 933 MHz data. The persistent knee is thought to be due to the assumptionof constant ion mass, and relaxation of this assumption allows us to estimate the ionmass below the knee relative to that above it. The summer data suggests larger massesbelow a lower transition height.

A similar investigation has been made by /36/ who, however, recognise that their measure-ments are from a rather different latitude and so decline to estimate ion mass, suggestingthat departure of temperature from model values could also be responsible for the effectthey see. In a further study, the same authors /37/ have attempted to estimate positive ionmasses. Here they detect very narrow spectral widths at or near the summer mesopause. Thefrequency used was 933 MHz, thus virtually excluding contamination by the scattering mecha-nism suggested by /10/. Very high orders of ion hydration were indicated by the data (e.g.20, assuming proton hydrates), and there is a strong possibility that these heavy ions couldbe related to presence of noctilucent clouds. In addition to the work of /28/, /30/ havealso made investigations into negative ion concentrations using the same method, again at224 MHz and at high latitude, but in spring.

A rather different approach to the question of obtaining negative ion information from in-coherent backscatter has been employed by /38/. The results of a large number in—situelectron density and positive ion density profiles have been combined by /39/ to provide anempirical formula for estimating A from electron and neutral number densities. This formulawas obtained from rockets launched from Andøya Rocket Range in Norway (69°N, 16°E) and istherefore valid for use with EISCAT derived electron density profiles, but possibly notelsewhere. Figure 10 shows a typical A profile determined by this method, and is clearly ingood agreement with previous measurements obtained in situ. The authors then inserted theseprofiles into the expression for spectral width, along with independent temperature profilesdeduced using the method of /23/. The model of /35/ provides not only temperature but alsoneutral number density, and hence /38/ were able to estimate mean positive ion mass profiles,as in Figure 11. The figure includes the corresponding electron density profiles, and thus

(5)170 C. Hall

Z 1km)

__ >70

0 5 10 15 0 20 40 60 80 ~ iO~ 1010 10~A rn )a mu) eleCtrons flT

3

Fig. 10. Negative ion to electron number Fig. 11. Mean positive ion mass from EISCATdensity ratios derived from EISCAT 224 MHz 224 MHz data and correspondingelectron den—electron densities (see text) for 12th sities. The harder morning Sector precipi—February 1987. tation has destroyed the cluster ions down

to 75 km.

Z 1km)

I90-

) 870717

8S~ ,‘/.~—c\ 1005 UT8707171040 UT

1 2 3 4 5Sc

Fig. 12. Profiles of the somewhat exotic Schmidt number (see text) from the EISCAT224 MHz system. These were estimated from data recorded around the time of obser-vation of coherent echoes from the same height, but the data here is from the in-coherent scatter signal. A clear reduction in electron mobility is indicated centredat 88 km.

shows how, during particle precipitation, cluster ions are destroyed to the depth of pene-tration of the particles. Using the neutral density model, /38/ were also able to estimatethe kinematic viscosity, and hence, combining this with the molecular diffusion coefficientsobtained from the radar and assuming the diffusion to be quasi—ambipolar, have estimated theSchmidt number. This parameter may be used as an indication as to what degree the electrongas acts as a passive scalar for the neutral gas, or qualitatively, how likely it is thatthe mechanism of /10/ will manifest itself. An example of typical summer Schmidt profilesis given in Figure 12 and layers exhibiting high values can be seen. These layers coincidewith the coherent echoes reported by /40/, are thought to coincide with noctilucent cloudsand the mesopause, and are further thought to be attributable to heavy positive ions of thekind reported by /37/. It is also possible that these layers may coincide with the phenome-non observed by /33/ and described above. It might be stated that the heavier the ions, themore suspect the resulting mass estimates /41/. To investigate this further we need toreturn to the more basic theory described, for example, by /42/ and /43/.

D-Region Researchusing IncoherentScatterRadar (5)171

SUMMARY

The early simplifications to the Vlasov equation, have been compensatedby the addition of arelaxation term to account for the high collision frequencies found in the D—region.Expressionshave been developed using a hydrodynamic approach to describe the incoherentscatter mechanism. Since then, researchers have tended to use perhaps over—simplifiedversions of these expressions to deduce information of physical parameters from the in-coherent scatter spectrum. Nevertheless, much important work has been done to investigateelectron density distribution with height and development of particle energy distributionsduring precipitation events of various kinds. Early work to deduce collision frequencyprofiles has lead to estimates of negative ion to electron density ratio profiles by variousmethods, and this in turn has turned attention to the possibility of estimating ion mass andhence investigating the ion chemistry near the mesopause. These investigations, combinedwith recent attempts to obtain incoherent scatter in the VHF band have suggested that thesimplified theory may be inadequate, and alternative descriptors of the atmosphere have beenintroduced to minimise the assumptions made. Incoherent scatter radar is also now provingto be an indispensible tool for the study of neutral atmosphere dynamics, particularlygravity waves in the mesosphere. This leads to improved understanding of the mechanismsresponsible for producing turbulence and hence the possibility to estimate turbulent para-meters themselves.

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