Solar Mesosphere Explorer measurements of polar mesospheric clouds (noctilucent clouds)

Solar Mesosphere Explorer measurements of polar mesospheric clouds (noctilucent clouds) GARY

E.

THOMAS

Planetary and Atmospheric Sciences. and The Laboratory Atmospheric and Space Physics. University of Colorado, Boulder, CO 80309, C.S.A

Department

of Astrophystcal,

(Received lnjnoljorm

6 Fehruar~

for

1984)

Abstract-The Ultraviolet Spectrometer experiment on board the Solar hlesosphere Explorer satellite has measured scattering of sunlight from the polar mesospheric cloud layer in the 024.3 gm spectral range. The layer is manifested in the limb-scanning measurements as large increases in radiance at heights near 80 km. at latitudes 60-90‘N and 60-90’S and during the summer season. They are stmilar to noctilucent clouds with respect to their height, geometrical thickness (less than 3.5 km) and morphology. However, as shown b! DONAWE et al. (1972), they are much brighter and occupy the entire polar ‘cap’ area, a region largely inaccesible to ground-based observation. Themeasurements have revealed a forward-scattering asymmetry. whtch increases with layer brightness. This behaviourshows that the brighter clouds are composed ofparticles withradiiaccordingtoMietheoryuptoO.O7~m,at least forthelimitedset ofdatastudtedsofar.Thevariation of layer brightness with asymmetry factor is consistent with the layer betn_g limited by the available water content of the atmosphere. The calculated water content of the particles. assuming them to consist of pure water ice, is about 100 pg mY2, provided the particle distribution is monodisperse. This corresponds to the total amount ofatmospheric water vapor residing in a vertical column above 80 km for a water vapor mixtng ratio of 1.2 ppmv. This is consistent wtth the amount of water vapor believed to exist at mesopause heights (a few ppmv). The large amounts or ice reported by DONAHUE et nl. (1972) are too large b! a factor of ten. Their corrected values are consistent with the present analysis. A brief description IS giLen concerning additional research topicc which are being pursued using the extensive SME data base. uhich now consists of tile complete simmer seasons from-Ii)81 to the present

INTRODUCTION

The Ultravtolet Spectrometer Experiment (UVS) the Solar Mesosphere Explorer (SME) spacecraft been recording scattered light from thin layers in high-latitude summertime mesosphere since launch 6 October 1981. These layers were discovered DOXAHUE

er

measurements

al.

(1972)

at visible

from

satellite

wavelengths.

relationships of the particle radii, optical depths and equivalent water vapor column mass are then described. This is followed by a brief descriptton of the potential of the large SME data base for future studies

on has the on by

EXPERIMENT

photometer

In the ultraviolet

(PMC) are more optically dense than in the visible: however, this advantage is more than offset by the lower solar flux in the U.V.The principal advantage of u.v. observattons is that the wavelengthsare sufficiently close to the particle size that non-Rayleigh effects in the scattering may be observed. The properties of the UVS instrument and the method of limb-scanning are brielly discussed. This is followed by a description of a typical height prolile. The occurrence frequency of the PMC throughout the 1982 northern summer season is described. Attention is then focussed on a selected data set during northern summer 1983. These data were taken at two scattering angles and reveal the presence of forward scattering behaviour. This permits the estimation of the mean particle sizes through Mie scattering theory. The (u.v.) the polar

mesospheric

clouds

819

The UVS experiment is a two-channel spectrornct. The spectral resolution ic 0.0015 pm and the channels are separated by 0.03 ‘jtm. The field of view is O.O:J i. 0.74 which projects to 3.5 x 35 km at the Earth’s Iamb. The objectives of the ShlE mission were described h! THOM.G tii ul. (1980) were given

and

the first results

from

thr UVS

in RL-SCH cr trl. t 1983).

The SME spacecraft spins at 5 rpm in a cartwheel mode. so that the four atmospheric experiments normally scan the limb forward of the spacecraft motion and approximately in the plane of the orbit. A complete height profile of the radiance from 100 km down to 20 km is obtained approximately every 1 degree of latitude near the equator. The nearly-polar. sun-synchronous orbit allows essentially pole-to-pole

CAKY E. THOWG

820

coverage at a local time of 3 pm (determined at the equator crossing as the spacecraft moves northward). The data coverage is 6-7 orbits per day, tape-recorded mainly over the day-time western hemisphere. The highest latitude viewed by the SME instruments is 82.5..

HEIGHT DEPENDENCE

PROFILES

AND SEASOSAL

OF THE SCAlTER14G

LAYER

A bright PMC in the Southern hemisphere is shown in Fig. 1. Also shown are the normal radiance profiles due to Rayleigh scattering from gas molecules in the middle atmosphere. Thedifference in the two profiles at lower heights is due to strong ozone absorption in channel 1 (0.265 pm) and to lesser ozone absorption in channel 2 (0.296 pm). The geometry of the southern observations with scattering angles near SO”causes the southern clouds to be enhanced by up to a factor of 10 over their Northern hemisphere counterparts where the scattering angle is near 130’. Modelling of the layer by an optically-thin homogeneous layer shows that the geometrical thickness must beless than 3.5 km.The true altitude of the layer is not necessarily the observed tangent height of the maximum. This is due to several factors. the most important being the high degree of hor~zolltal inhomogeneity (see Fig. 3). A spatiallylocaiized layer would generally appear lower than its true height, due to the Earth’s curvature. It is possible in principle to perform a two-dimensional inversion in order to derive both the true layer height and its spatial variation along the orbit track. The determination of accurate layer heights is a difficult one. involving the removal of Earth curvature e!Tects and the acquisition

Fig. I. C’v’S rau data versus tangent height at 8I! Sand at 54 S. Solid curve -channel I (0.265 pm). Dashed o_tr\e-channel7 (W96 pm). The abscissa is logarithm of the count rates per

integration period. The curves connect data points collected ever! 3.5 km rn tangent height.

of accurate pointing information from a spinning spacecraft. The radiance at peak brightness of the cloud shown in Fig. 1 is 1.6 MRayleighs A-‘. The maximum slant optical depth is about 0.5. an unusually high value. The vertical optical depth is about 90 times fess than in slant viewing. This ‘amplification factor’ (DOXAHUE er al.. 1972) is independent of the geometrical thickness. provided it is much smaller than the field of view. The derivation of the physical properties, such as the mean number density of scatterers, requires among other things a knowledge ofthe particle size distribution and a suitable scattering theory for non-spherical ice crystals. This subject will be discussed in the next section. The signal due to Rayleiph scattering from the background gas is generally small (less than 50 counts per integration period) and can usually be ignored. When the cloud radiances are also weak it becomes difficult to separate out the two phenomena, since only an inflection occurs in the height profile. rather than a true maximunl as shown in Fig. 1. The separation was achieved in the computer processing by first fitting a parabola through all the data points aboLe 70 km. The height profile of the diflerence between the data and this parabolic fit more cleari! re\eals the presence of a cloud. The total radiance at that point is interpreted to be the cloud r,tdiance. However. a~ 101~ count rates statistic,+1 fluctuations of the signal ma!’ yield spurious result>. YU;ian!tif these cases have been removed b> rejecting the data unless the maximum occurs at the same hetght rn both channels. Using the above definition of a PhlC occurrence we have analyzed a complete season of data to stud? temporal and spatial behavior of layer height. peak radiance and occurrence frequency. Space permits only a discussion of the latter. the ratio of profiles containing PMC 10 the total number of profiles within a latitude band. This quantity is plotted in Fig. 2 for three different latitudes for the entire northern 1981 summer season. Each data po’int represents a dail: average ofdata from six orbits, The smooth curves are eleven-da! running averages at each latitude. A striking feature is the abruptness of the onsct. particularly at the highest iatltude. Also notable is the steep latitudinal variation. In the lowest latitude zone shown (50-60 N) the layer is quite sporadic. This is consistent with the behaviour of noctilucent clouds (NIX). which. as DQSAHUU er ul. (1972) have suggested. may be the ‘ragged edge’ of the more dense polar layer. In the 7040 zone the phenomenon is ubiquitous. but as will be shown, quite variable in bri~htness.Ti~eonset ofthe PMC,definedas the time when thecloudinessfactor isequal to halfofits peak value. was about 5 June in 1982 at high latitude.

Measurements

of polar mesospheric

clouds (noctilucent

clouds)

811

1974; WITT er ul.. 1976). These authors asslgned upper limits for a ofO.I-O.13 ~~.GADsDE~’ (1976) has sho\\n that the results ofTozer and Beeson can be interpreted either in terms of small particles with a < 0.05 pm, or large particles with radii 0.24 or 0.29 jmi. In the most elaborate theoretical study to date. TUR(.O el (I/. (I 9821 obtained mean values of SO.1 llrn in eleven different NLC models. We have derived an independent estimate for the mean particle size from SME measurements This IS accomplished by use ofdata collected during a specialI> designed operational sequence uhich allowed radlancc measurements to be taken at two different scattering angles. The ratio of the two radiances IS equal to the ratio of the appropriate scattering phase functions. Fag. 2. Fraction of PMC occurrence in 10 bins centered at 55 which in the Mie theory is quite sensitive to the size 65” and 75’ versus day number for the 19dZ northern summer parameter x, provided .Y> I. season.Thedatapointsaredailyaverages Thesohdcurvesare eleven-day running averages of the dail! values. The details concerning the data analysis and the theoryaregiveninTr+oMAsand McK~~(l9831. Briefly. the spacecraft was commanded to collect data in the This agrees quite well with the results of DONAHUE el al. ‘trailing-limb’ mode, following its normal data-tahlng (1972). The corresponding date at which the operation in the ‘leading-limb’ mode. This allowed the phenomenon is dying away was 13 .August. The same colume ofspace to be viewed (at slightI> different behaviour of NLC occurrence at lower latitudes is times) at two different scattering angles. about 50 and similar. but different in detail (Foc;Lt and H.~LYXWITZ. I30 Fortunately, the spin-auls of the spacecraft \\ a\ 19661. very nearly normal to the orbit plane durlnp the 198; northern summer season and so the lines ofsight In the tuo modes were very closely comcident tn space at thl\ PHYSICAL PROPERTIES Ot THE time. L.Al’ER PARTICLES In Flg.3areshown thedataforsixorblta.eau\ composition and concentration. UnfortunateI>, we still spin number. Each data point is separated b! a greatdo not have direct evidence that the particles are watercirclearc ofabout 0.8-‘. Since the orblt is not quite polar. ice crystals. although this appears to be generalI> the projection of the orbit approuimatel! follows the accepted. The current situation regarding this question 80- latitude circle at its closest approach to the pole has been reviewed by AVASTEer trl. ( 1WJI and GADSIX\ Much of the data shown \vas taken near this latitude (1983) In the absence ofany other credible theories, the Thelongitudecoverageforall theseorhitsisfrom about assumption is made that the scatterers are small ice I50 E to about 30 W. The trailing-limb data hake been particles of index of refraction I.30 - O.*i and that Mie linedupwiththeleading-limbdatasothattheq.appl!~to theory for spherical scatterers applies. The latter the same spatial region. Actual&, only about a third of assumption is justified on the basis that for size the data shown were taken \vhen the fields of CIS\\ parameters .Y= ?na,i < 4, the assumption of equal overlapped (that is were ivithln 0 74 or 35 km of one volume spheres allows one to use \lle theory e\en for another in the horizontal planci. This IS an Important irregular particles (MUGXAI and Wlscohlnr. 1980. point in view of the large changes in the brlghtnes\ BOHKENS, 1983). (In the above. (I 1s the radius of the which can occur over the space of e\en a single spinsphere of equal volume and I is the wavelength.) This scan. means that the Mie theory is adequate for the UVS Figure 3 illustrates the extreme spatial variability of measurements up to particle radii as large as 0.15 /cm. the PMC and shows large systematic dilTerence\ bet\+een the leading-limb data (backward scatterlnp) Previous attempts to deduce the mean particle size ha\e used rocket measurementsoithe ratio ofscattered and the trailing-limb data (forlvard-scattering). As the light radiances at two wavelengths and the degree of layer becomes brighter the ratio becomes larger. linear polarization (WITT, 1969: T~ZEK and BEISON. varying from unity for the weak clouds up to about ten

822

GARY

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THO\l-\S 005

0 05

OrbIt

951

I

Orblt

9527

r 004

004 Day 178

Day I79

. .

t 0.03

003

I

.

t 0 02

002

k’ .*

005-

005 Orbit

9542

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0 045 a :

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Day I80

9557

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0.04 Day

.

181

Aa

003.

:: Y- 002-

.': _ I

0" oot-

.

O.i6r

0 14

Orbrt

0 05

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9572

0 12

r

Orblt

9587

u 1

Day 182 0 IO 008

.

. . .

I

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.

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.

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.

Spun number Fig. 3. Channel 1 peak slant optical depths (cloud apparent emlsslon rate - solar lrradlsncel versus spin numberforsixSMEorbits.Shownare boththeleadlng-limbdara(sca~rermgangle 2 130 at 80 latItudeland thetrailIng-hmbdata(scarteringangle z 50 ),thelatterdatatakcnabout IOmin after therormer.The~raillnghmb data haVe been shifted in order that the tw’o sets ofdata apply to approximateI) the same spatial points Each orb11 isseparated In time by approximately24 handarefor thedays I78 -183 lY83 ThsIar~tude
for the brightest clouds. This observation is the key to determining the mean particle size. In Fig. 4 are shown the ratios of the trailing- to the leading-limb radiances for channel 1plotted against the line-of-sight optical depths as seen in the trailing-limb mode (forward scattering). The ‘optical depth’ in the observed direction was defined by DONAHUE cr al. (1972) as the ratio of the apparent emission rate to the

incident solar flus at the same wavelength. .4 more descriptive term for this quantity is’directional albedo’ (HIANEL. 1975). The scatter of the points reelects real variations in the obser\atlons. both from point-topoint in the same cloud and also Ii-om cloud-to-cloud. These data show clearly the ratio is directly dependent upon the PMC brightness. The Mie theory predicts that for a single particle size the forward/backward

Measurements of polar Troilmg

Io,-

0

tt

l

I 002

/leading

t 0.04

Optical

mesospherlc

ratio

I

1

0.06

0.06

depth (CH

I 01

I) TR

Fig. 4. Ratios of peak trailing-limb radiances to leading-limb radiancesinchannet I versusslant opticaldepthasobservedin the trai~ln~iimb mode The data are for those spins for which the horizontal distance between the two viewing directions was less than 35 km (one field of view). The smooth curve is calculated from Mie theory assuming a monodlspersion of particle sizes. The curve is calculated with the constraint that as the particle radius vanes, the equivalent column mass of water remains fixed at the value of 100 ~c,ern- ‘.

ratio increases rapIdly with sizeparameters. from unity for\- 5 I uptotenf0r.x z 1.5.The theory indicates that the largest particle radius for the clouds observed was about (I = 0.07 pm. A distribution of sizes (a ‘polydispersion’) would require somewhat smaller values for themean radius(THOMASand MCKAY. 19841. The Mie theory atso predicts a very rapid11 increasing scattering cross section with increasing n. However. if the particles are formed as a result of freezing of atmospheric water vapor. one would expect the column Integrated water content to be limited by the amount of avaitable water. regardless ofparticle size. The smooth cur\e tn Fig. 4 was calculated on this basis: that the water column mass is constant at 100 icg m ’ (IIcgm ‘= 1 x IO- ‘Ugmcm-2).Theagreementofthe theoretlcal curve with the louver envelope of the points suggests that this indeed may represent an upper limit to the tiater content of the ice particles. For 1OO~igm ’ and n = 0.05 mtcrons the column densit) is 19 x IO-cm-‘. , for a 1 km thick laler the average particle density is 190 cm j. The above column mass corresponds to the mass of water vapor above 80 km if the volume mixing ratio is constant above that height and equal to 1.2 parts per million by volume (ppmv). It should be repeated here that the above theoretical calculations are based on the simplifying assumption of a monodispersion. It can be shown that a more realistic assumption of a po!ydispersion cause5 the column mass to be larger. We are now considiring this case (THOMASand MCKAS. 1984).

clouds (noctllucent clouds)

823

Turning briefly to our knowledge of water vapor concentrations at these heights, microwave measurements have recently become available (BEVILACOUA el al., 1983). They indicate. at least at mid-latitudes. mixing ratios in the range OS-- 1 ppmv. SOLOMOS ff a/. (1983) have modelled the water vapor distribution at high latitudes in summer and predict zonaily-averaged values between 2 and 3 ppmv at 80 km. Therefore. the present results are in good agreement with the expected amount of water vapor in the upper mesosphere. provided that the monidisperse assumption is approximately valid. Fortunately, the inferred column mass is not \ ery sensitive to the width ofthe si7e distributidn [in contrast to the column densIt> (THOMAS and MCKAY, 19X4)]. The inferred column mass ofwater in the form of ice is aiso in agreement with the range (17 160 icg rn-‘) predicted by the model of TLtRC-O (‘/ (I/. (1987). We noa’ compare our results with the OGO-6 observations and anal!& of DOSAHUE ef ul. (lc)??). The> calculated the maximum ice density by using the brightest values for the PMC apparent emission rate of Xl kRayleighs # ’ at 0.55?7 pm. Adopting a radius of 0.13 itrn the! scaled their brightness Lalues to those 01 Fr~;~~:and RI I s(197_7~~vhoperforrneda Miescattering calculation to dcrile cross-sections for ice particles in the visible spectrum 1 h\c recenti! learned that a factor-of-ten error hdz brc‘n found in this calculation (Do\.~Hc.I:. IYX1t. Instead oftheuidelli-quotedvaltleol‘ 40 cm ‘. thz corrected value is 4 cm ’ for an assumed vertical thickncsb of 1 Lm. .Afurther correction should noa be made to account for the present results. that the particle5 are .smaller than assumed by Donahue CI [I/ .A more appropriate value for the brightest clouds it 0.06 itm. The results of our Mie scattering code pro\ idc a ne\\ \ alue of 2Ul cm ’ for the maximum densit? obscr\ed b! OGO-6. This is quite consistent with our typical ~alur of IY)ocm ’ for 0.05 Itm particles. The concluaionb from the above are: (I) \\hen the correct value?; for the Mie scattering parameters arc applied to the OGO-Sdata. thcderived column mass i> quite consi
.4DDITIO\

\ t. S\lE: S-I-L-DIESOf THE PhlC

it isclear that theabo\ep~\‘esonl) aroughideaofthe potential scientific cOntent of the SME data. The climatolog! of the cloud layer- was onl! sketched. Correlations of the appcarunce of the PMC xvith other factors have not >et begun. For example. are the! anti-

GARY E THOMAS

824

correlated with aurora1 activity, as suggested by ground-based data analyzed by D’ANGELO and UNGSTRUP (1976)? Preliminary indications are that there are no dramatic etfects of either geomagnetic activity or of the solar proton event of 13 July 1982. Are they controlled by phenomena in the stratosphere or troposphere? Are they connected with the local ozone concentration, as suggested by MCKAY (1982)? What are the dinerences. if any, between the Arctic and the Antarctic PMC? Are there significant interannual variations? Are meteor showers or sporadic meteors of importance in the nucleation of new particles? Studies attempting to answer some of the above questions are in progress. In view of the fact that the spacecraft is still operational, it is also important to

consider coordinated observations, both from th< ground and from other satellites. There are reasons to be optimistic that the nexl few years will provide answers to some of the decade-long questions in the field.

~c&no~,~edge~l~tl~.~ The author gratefully acknouledpt\ fruitful discusslons with C. P. MCKAY, J. OLIVERO. D i!‘ RIJSCH,G.M. MouNT.J.Noxo~~~~S.SOI.OMO~. R. ECKV~L assisted me in making available the Mie scattering code. originally devised by HANSEN and TRAVIS (1974). The special observing sequence thal allowed the as~mrnelr~ factor of the PMC tobemeasured was the work ofalarge number ofSME personnel. In particular I wish 10 thank C. BARTH. R. THOMXS and P. WILLIS for lending their special expertise in making t hl> operation a success.

REFERENCES

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1984

Private communicatiorl

unpublished moreriul