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Indian Journal of Radio & Space PhysicsVol. 17, Decemher 1988, pp. 252-263

Optical Studies of Thermospheric Structure & Dynamics-RecentDevelopments

RSRlDHARAN

Physical Research Laboratory, Ahmedabad 380 009

A brief outline of the thermospheric structUre and dynamics and the possible interaction of the same with theF-region of the ionosphere is given. It points out to the need that these two regions should be treated as a closelycoupled system. Some of the most recent developments in optical measurements of neutral temperature and windsin the thermosphere by photometry as well as by high resolution Fabry-Perot spectrometers are outlined.

Fig. 1-Altitude variation of the global mean contribution ofthe various sources of energy to the thermosphere 11

thermal conduction, and by low energy electronprecipitation which sometimes manifests as auror­al arcs7•

The contribution due to hydromagnetic waves,getting generated at the earth's bow shock, be­comes significant only during magnetically dis­tmbed periods and is a highly variable source li­mited mainly to high latitudes.

Precipitating energetic neutrals, producedthrough resonant charge exchange reactions withions in the ring current, are another possible.source of heat8. The global average heating of thethermosphere due to this source is not yet quanti­fied though attempts were made by Rohrborughet al.l) and Tinsley et al.lo to investigate energydeposition rates for several large magnetic stormperiods.

The altitude distribution of some of the globalaverage heating and cooling rates from some ofthe most recent studies are shown in Fig. I, which

GLOBAL-MEAN THERMOSPHERE

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1 IntroductionA full understanding of the earth's atmosphere

requires a knowledge of its mean compositional,dynamical and thermodynamical state as a func­tion of position, time, season, solar cycle andgeomagnetic activity. Not only the structure of thethermosphere is to be understood but also themajor physical and chemical processes responsi­ble for the same are to be delineated. Also, therole of the thermosphere as a modifier of the en­ergy input, mainly solar energy, and the possiblemeans by which the lower atmosphere and themagnetosphere are coupled through it, needs tobe understood.

The basic sources of energy for the thermos­phere are: (i) the sun, mainly by its input in theEUV range, (ii) solar wind through its interactionwith the magnetosphere, (iii) energetic particleprecipitation from the magnetosphere, (iv) loweratmosphere through the atmospheric waves andtides,. planetary scale waves, acoustic gravitywaves, etc., (v) hydromagnetic waves, and (iv)meteorites.

Of the above sources of energy, the contribu­tion from the solar EUV and the tidal and waveenergies are believed to be comparable 1-3. Therecent llROS/NOAA satellite measurements4-6

have revealed that the stream of energetic protonsof solar wind origin deposit - 0.3 mWm-2. Theenergetic particles from the magnetosphere are ahighly variable energy input, increasing by at leasttwo orders of magnitude from magnetically quiettimes to disturbed times, i.e. from 0.1 to 10mWm -2. Though the atmosphere gets heated upby Joule heating only in the auroral latitudes, en­ergy can also get transmitted to subauroral lati­tude from the magnetospheric ring currents by

252

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SRIDHARAN :THERMOSPHERIC STRUCTURE & DYNAMICS

3 Effect of Winds on the F-region Ionization andVice Versa

The neutral winds, especially -the meridionalwinds, do have a significant control on the F-re­gion ionization. In turn, the various F-regionphenomena do alter the wind system, thus esta­blishing a close coupling between the neutral andionized constituents of the upper atmosphere.

A horizonal wind U blowing in a directionreckoned east from geographic north has a com­ponent in magnetic merdian giveq by U cos (8-D).The vertical ion drift (W) produced by this windis given by simple geometrica1 consideration as

... (1)W= - U cos (8-D) sin I cos I

where D is the magnetic declination and I the dip.A poleward wind (8 in the northern and 8-180°

in the southern hemisphere) as normally observedby day produces a downward drift of ionization.An equatorward wind (8-180° in the northern and8 in the sourthern) as is seen by night producesan upward drift.

Whenever there is a downward drift, the maxi­mum density of the F-region NlIf2 gets reducedas the effective recombination coefficient of theF-region, ~, increases downward. Thus windswould modulate NlIf2. However, these effectsare, to some extent, longitude-dependent as D, Iand therefore Ware longitude-dependent.

Herrero and MayrZ5 performed tidal decompo­sition of the WATE data from DE2 and madecomparisons with a similar decomposition of theion drift measurements in the F-regio'n obtainedfrom Jicamarca. From the similarity of the results,once again, the strong coupling between the diur­nal components in the neutral atmosphere and theionospheric F-region is demonstrated.

4 Wind Effects at Low LatitudesThough electric fields are the ones that are pri­

marily responsible for equatorial anomaly, neutralwinds do influence them and, in turn, get influ­enced by the electric fields. In the absence ofwinds, the equatorial anomaly would have result­ed in the distribution of ionization symmetncabout the dip equator. A meridional wind blowingacross the equator would transport ionizationfrom one hemisphere to the other, thereby mak­ing the distribution asymmetrical about the equa­torZ6.

The second aspect of the equatorial anomaly isits post-sunset enhancement. It is believed to bedue to the 'renewal' of the fountain effect as a re­sult of enhancement in the post-sunset equatorial

2 Winds and Energy BalanceAs the winds act as carriers of thermal energy

they play a crucial role in the energy balance ofthe atmospheric system. The prevailing winds, su­per-rotation effects and the midnight pressurebulge are three different aspects of the wind sys­tem. The summer-to-winter hemispheric prevail.ing wind lZ forms a part of the meridional circula­tion cell which has an upward (heat absorbing), adownward (heat releasing) and a return flow atlower altitude completing the cell, thtls makingthe flow of air nondivergent.

The mean west-east zonal win<1s in the ther­

mosphere are termed as the super-rotationeffect 13 • At low latitudes the effect is mainly dueto the electrically driven windsl4, at mid-latitudesthere is still an uncertainty for it to be presentl5,and at ,high latitudes super-rotation due to ther­mal effects is possiblel6. Recent results from theWind and Temperature Experiment (WATE) onDE2 satellite have revealed that super-rotation ef­fects are considerably smaller than those reportedoriginally 17.18.

For the study of equatorial thermospheric dy­namics, the midnight pressure bulge and its effectsare importantI9-Z4. This phenomenon is observedon most nights with different shapes in the latit­udinal distribution. It is generally believed to begenerated due to the variability in the tidal excita­tion source and the prevailing wind system~.

essentially brings out the relative importance ofthe high latitude EUV and tidal inputs and the ra­diational cooling by the 5.3 ~ni band of nitric ox­ide (NO) both during solar minimum and solarmaximum periods. These aspects are comprehens­ively dealt with by Killeen II.

Whether there are any other sources of energy,especially at low latitudes, is a question not yetfully answered. Some of the recent observationsfrom equatorial latitudes suggest that there maybe some, and this aspect will be discussed later.

It is well known that the thermosphere is a mul­ti-constituent medium with the major species be­ing 0, Oz and Nz. These neutral species interactwith a weakly ionized plasma consisting of var­ious ions, the major opes among them being 0 +

and NO+. The dynamical, thermodynamical andcompositional states of the thermosphere are de­pendent on the various dynamical forcings andneutral-plasma interactions. In fact, in recenttimes, the thermosphere and the ionosphere arebeing looked into as a coupled system closely in­teracting with each other.

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INDIAN J RADIO & SPACE PHYS, VOL. 17, DECEMBER 1988

electric field. The thermospheric winds act as aprimary driving force for the F-region dynamo(build up of polarization electric fields due to themarked decrease in E-region electron density andits reflection in the conductivities at sunset).

Another aspect where the significant role ofneutral winds has been shown recently is with re­gard to the equatorial spread-F\ESF). The ESPrefers to the generation of irregularities in elec­tron and ion densities as well as in electric fields

with scale sizes covering orders bf magnitudes.From a rocket experiment carried out just at theonset time of ESF (Fig. 2), the significant role ofvertical winds in enabling/inhibiting the Rayleigh­Taylor instability was demonstrated27 and sup­ported by theoretical studies28•

From the above discussions it be~omes certain

that the neutral winds do have significant controlon the various thermospheric/ionospheric pheno­mena.

The question that how far the neutral windsthemselves get altered is equally important as itprovides a physical insight into the various coup­ling processes. Some of the important effects are:

(i) When the F-region is raised to high altitudes·as a result of a strong zonal electric field, the neu­tral gas would get accelerated as a result of re­duced 'ion drag' that acts on the gas. Subsequent­ly, as the F-Iayer is pushed down, the neutral gasgets decelerated29.

(ii) The importance of ion drag is once again

brought out in an anomalous feature in the latit­udinal distribution of neutral densities at 450 km.The. feature known as neutral anomaly is due tothe formation of two crests of neutral densities on

either side of the dip equator similar to the ioni­zation crests. It has been explained as due to thesignificant ion drag in the zonal direction offeredby the ionization crests30 (the neutrals move wes­twards during forenoon and eastwards during af­ternoon while the ionization moves westward

throughout the day).(iii) A phenomenon closely related to the neu­

tral anomaly is the formation of 'cusps' or 'ledges'of ionization in the topside ionosphere. Theseledge formations have been explained as due tothe inhibition of plasma flow along the magneticfield lines by the field aligned enhancements ofneutral densities, i.e. the neutral anomaly3l.

Having higWighted the various interactions/couplings between the thermosphere and the F-re­tion plasma, it becomes clear that any investiga­tion on the structure and dynamics of the upperatmosphere would be incomplete if it is confinedeither to the thermosphere or to the ionosphere.The problems should be looked upon as one ofthe mainfestations of the thermosphere/ionos­phere system. Systematic investigations areneeded in order to obtain a better understandingof this complex system. This calls for regular andsimultaneous measurements of winds, tempera­ture, density, layer heights and their response to

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km and 40 km), with respect to altitude [Sum total is shown by y' (Ref. 27).1

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SRIDHARAN :THERMOSPHERIC STRUCTURE & DYNAMICS

various energy inputs. The various aspects dis­cussed above are comprehensively dealt with inone of the overview articles by Raghavarao et aCe

Measurements of airglow emission play an im­portant role in the investigatgion of thermasphericstructure and dynamics. In the beginning the stud­ies were morphological in nature and were con­centrated in understanding the various airglowemission processes and in inferring the altitude ofemission and the number densities of the emittingspecies. Once rockets and satellites became avail­able, correlative studies between airglow andother ionospheric phenomena began. With a bet­ter understanding of the mechanism of airglow,investigations pertaining to atmospheric dynamics,namely, kinetic temperatures, line-of-sight neutralwinds and also inference on wave propagationfrom the intensity variations, are made.

The airglow emission, most relevant for ther­mospheric investigations, are 630.0 and 777.4 nmcentred around 250 km and at Nmax altitudes, re­spectively. The former during nighttime is due tothe dissociative recombination of O2 + ions withelectrons, resulting in excited '0' (OlD) atoms.The transitition to ground state gives out a 630.0mm photon, the lifetime of the excited species be­ing - 110 s. The 777.4 nm emission is due to thedirect radiative recombination of the dominant

0+ ions in the F-region. The radiative recombina­tion process is much slower and hence the intens­ity is correspondingly low.

5 Photometric Studies

Apart from regular measurements of intensitiesof various emissions in recent times, simultaneousphotometric measurements of 630.0 and 777.4nm emissions have been carried out using imageintensifiers and field widening optics. Very inter­esting results have been obtained with respect to'plasma depletions' or 'plasma holes' that are as­sociated with equatorial spread-F (ESF )33.34. It isgenerally believed that these depletions originatearound 250 km. The earlier measurements have

shown the field-aligned nature of these depletions,and simultaneous presence of these depletions intwo wavelengths yield information on the altitudeextent of the structures (Fig. 3). Further,Carman}) made photometric observations of630.0 nm emission from Vanimo (Iat., 2042'S;long., 141°1W E; dip lat., 14SS) which revealeddepictions of several minutes' duration in intens­ity. These depletions appear to be due to thepassing of plasma bubbles through the emissionlayer (Fig. 4).

Another phenomenon reported by Herrero and

15 DECEMBER 1979

Ale POSITIONOLAf -4.7GLON 1S.7W

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0.3 3.6 7.0 10.4 13.8 17.320.7 24.2 27.8 31.334.4 GLON(W10034 0005 2336 2306 2236 2208 LT(H:MI

Fig. 3-AII sky images on 630.0 and 777.4 nm emission linesalong with simultaneous AE-E satellite measurements depict­

ing the 'bite out' or 'plasma hole' formation""

Meriwether36 from the equatorial observatory,Arequippa, Peru (dip lat., 3.2°S) is the naturallyoccurring airglow depletions of a few hours' dura­tion. These depletions were correlated with largeincrease of equatorward wind and temperature,especially during magnetically disturbed periods.These were interpreted to be due to the move­ment of the F2 layer along the inclined magneticfield lines in response to both the equatorwardwind generated due to the energy input at aurorallatitudes and the poleward wind generated due tothe midnight pressure bulge caused by the tidalforces at the equatorial latitudes. These results area case in point for the complexities involved inthe circulation patterns at low and equatorial lati­tudes.

From the background of information available,it is clear that ground based airglow intensitymeasurements can be used as a tool for monitor­

ing the formation of 'plasma holes', and theireventual manifestation as ESE The airgiow tech­niques would become important when rocket ex­periments are conducted at the onset of ESEHowever, when active experiments like the artifi­cial cloud releases are to be undertaken, certainspecial requirements from the ground-support ex­periments are called for. The height region of thecloud releases is to be illuminated by the sun toenable ground photography. This implies that theregion of airglow would also be illuminated by thesun. Under such circumstances simple photometry

255

INDIAN J RADIO & SPACE PHYS, VOL. 17, DECEMBER 1988

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fails and highly specialized techniques like dayg­low/twilight glow photometry are needed.

5.1 DaygIow Photometry

The detection of airglow line emission byground-based photometers during day in the pres­ence of a bright background continuum, which isseveral orders more intense than the line emis­sion, is indeed complicated. Very narrow band­pass interference filters and very high resolutiondispersing elements like Fabry-Perot etalons areneeded to filter out the contribution due to thebackground. Multi-etalon systems have also beentried out37 apart from the ingenious technique ofutilizing the polarization properties of the atmos-

pheric scattered radiation (polarized) and the air­glow line emission (unpolarized) by earlier work­ers3H• However, as the bufk of the atmospherethat causes the scattering is confined to a few ki­lometres altitude only, it becomes relatively easyto take measurements on the dayglow intensityfrom a balloon plateform or from rockets andhigh altitude aircrafts. In addition to these, in re­cent times, satellites and space platform havecome to stay providing global coverage over pro­longed periods. The pioneering efforts of Blamontand Luton3,! in this regard, wherein an interfer­ometer mounted on OGO VI satellite yieldedvery useful results, deserve special mention.

In spite of all the added sophistications of the

256

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SRIDHARAN :THERMOSPHERIC STRUCTURE & DYNAMICS

0630 0700 0730 0800 0830TIME, 1ST

Fig. 5-Typical day glow intensity variations (QI 630.0 nm) inthe evening and morning hours over Ahmedabad. The contin­uous line represents the variations in the background continu-

um42•

One more interesting application, as far as thedayglow measurements are concerned, is in itspotential utility in the prediction of the occurr­ence of ESF, the hitherto unpredictable pheno­menon. The recent results by Raghavarao et al.43,based on ground-based ionograms from threestations, have revealed that the ratio in the elec­tron densities in the altitude region of 270-300km over a station under the crest of the equatori­al anomaly (e.g. Ahmedabad: lat., 23°01' N; long.,72°36' E; dip lat." 18.6°N) and from a station be­tween the crest and the trough of the anomaly(e.g. Waltair: lat., 1r 43'N; long., 83°18'E; diplat., 10.6°N) shows a steep increase by a factor of8 to 30 on days of ESF as early as 1800 hrs LT.No such increase is observed on non-ESF days.Such a behaviour is not noticed at N max altitudes.As a significant portion of 01 630.0 run airglowemission orginates in the same altitude region of..•

18001700 1730TlME,IST

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spaceborne instruments, regular measurementswith ground-based experiments are indispensableto provide the much needed 'base level' informa­

tion. So far photometric measurements of dayg­low have not been successful below 95° solar zen­ith angles.4o.

A new daytime photometer on 01 630.0 runhas become operational recently at the PhysicalResearch Laboratory, Ahmedabad. A low resolu­tion pressure-tuned Fabry-Perot etalon (with afree spectral range of 4A), a narrow band (3A)temperature-tuned interference filter; followed bya unique chopping device which radially chopsthe intensity in specific time intervals, constitutethe heart of the system. A unique up/downphoton counting system with EMI 9863A photo­multiplier constitutes the detection stage. It is aninherent characteristic of the Fabry-Perot fringesystem that as one moves away from the centre ofthe concentric fringes, one goes down in wave­length. The sampled areas of the central zone andthe adjoining annular zone are adjusted to beequal. When the Fabry-Perot(FP) etalon is tunedto the wavelength of interest, the central zonecontains the line emission and the background thecontinuum. The adjacent zone contains only thebackground. The assumption being that the con­tribution due to background in this wavelength in­terval, typically 0.05 run, away from 630.0 runline centre, is nearly identical. The photometercan be adjusted to a null by electronic gating inthe absence of airglow, thus overcoming the im­balance, if any, due to dissimilar areas. As thechopping frequency is around 100 Hz, samplingof both the zones are accomplished within shorttime intervals. Th,e up/down counting processsubtracts the background contribution and en­ables the signal integration as long as one desires.

The technique is an imporved version of theone suggested by Desai et al41 with suitable adap­tation for the requirements of dayglow investig­ations. The technical details and calibration meth­ods are discussed elsewhere42• Fig. 5 depicts thesample measurements at Ahmedabad on 01630.0 run airglow emission. The uniqlle way of el­iminating the background contribution withino.sA gap from the line centre and the gating tech­nique eliminate the problem of the Fraunhofer ab­sorption feature in the vicinity of the line emis­sion. The present photometer. is capable of de­tecting airglow emission intensities that are - 0.1%of the background intensities. Along with the day­glow measurements, simultaneous measurementsof ionospheric parameters from equatorial lati­tudes are being planned by our group .

257

INDIAN J RADIO & SPACE PHYS, VOL. 17, DECEMBER 1988

1000 •.• TEMP ( KI

plished either by changing the separation betweenthe plates or by changing the refractive index ofthe gas between them, i.e. by changing the pres­sure in the FP chamber (for one atmosphericpressure change, OA = 0.00029 A). Scanning couldalso be effected by changing the temperature ofthe etalon. Extensive treatment of the FP interfer­ometers is available in literature44•

7 Recent Significant ResultsRajaraman et al.45 made hourly measurements

of temperature from Mt. Abu (lat. 24.6°N; long.,n.r E) during 1977-78. This was the first at­tempt from the Indian side. They showed that thenighttime thermo spheric temperature follows thestatic models like Jacchia'71 only on days whenthere was no equatorial spread-F(ESF) as ob­served in Thumba. On ESF days the temperatureswert( larger by 100-300K than what were expect­ed by the model (Fig. 6). It was interpreted bythem that these larger temperatures were prob-

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6 Interferometric Observations onThermospheric DynamicsHigh resolution Fabry-Perot spectrometers are

currently being operated from a number ofstations all over the globe for regular measure­ments of the emission line profiles. The Dopplerwidth of the emission line is related to the neutral

temperature while the Doppler shift in the linecentre corresponds to the line-of-sight neutralwinds. Very high resolution ( - 105) is called for toobtain the emission line profile. As the airglowemission is usually very weak, the spectral ele­ments must have the largest possible light gather­ing power on 'luminosity'. Interference spectrome­ters, like Fabry-Perot and Michaelson's two beamFourier transform spectrometers, are greatly supe­rior to the prism or grating instruments in theirflux collecting power. The luminosity advantageapplies only to situations where the source over­fills the entrance aperture of the spectrometer andthe phenomenon of airglow provides an ideal situ­ation to this effect. Out of these two interferome­

ters, the Fabry-Perot system enjoys a wider patro­nage because of the inherent simplicity in obtain­ing the line profiles.

The temperature is obtained from the relation

250-300 km, the day/twilight glow intensity ratiosthat are partly related to the electron densitiesmight reflect a similar feature. Alternate samplesfrom two elevations from a photometer locatedsuitably would yield information about the plasmaparameters in the regions of emission separated inlatitude in real time, which might help in predict­ing the occurrence of ESF before hand. This pre­diction would enable rocket experiments to beconducted at the onset time of the ESF pheno­menon.

OA J"f- = 7.16xlO-7x- ... (2)A M

where OA is the full width at half maximum

(FWHM), A the operating wavelength, T the neu­tral temperature and M the mass of the emittingspecies in a.m.u. The basic assumption that T rep­resents the bulk neutral temperature implies ther­mal equilibrium between the emitting species andthe dominant neutral species.

The line-of-sight velocity is given by the expres­sIOn

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same period4'

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SRIDHARAN :THERMOSPHERIC STRUCTURE & DYNAMICS

ably due to the Joule heating associated with theirregular electric fields engendered by the plasmainstabilities associated with ESF along the magne­tic field lines passing through the region of air­glow emission.

Low latitude observations elsewhere by Biondiand Simpler46 as a part of the Brazillian Ionos­phere Modification Experiment (BIME) duringAug.-Sept. 1982, and also regular observationsfrom Marshal Islands have revealed overall agree­ment with the Thermospheric Global CirculationModels (TGCM). Several interesting and as yetunexplained features were also noted occasionally.These included persistent convergence in the hor­izontal meridional flow accompanied by adownward wind and an increase in temperatureduring 2100-2300 hrs UT (Fig. 7). On othernights, oscillations in zonal and vertical velocities

with periodicities of 40-45 min were observedand ascribed to zonally propagating gravity waves.A new station presently operatonal at Arequippa,Peru (lat., 16.2°S; long., 71.4°W) has revealedoverall agreement with TGCM predictions47• Asregard to the temperature measurements for thesame period, large enhancements in neutral tem­perature were reported during geomagneticallydisturbed periods48. These enhancements wereclassified under three categories: (i) prompt in­crease ( ~ 2 hr) in Tn' (ii) 6-7 hr delay in increaseof T,,, and (iii) - 24 hr delay. A fresh look at thereported enhan~ments has revealed certain er­rors in the estimation of the Doppler width andhence in the inferred neutral temperatures (Bion­di, personal communication, 1987).

One of the results obtained from a rocket va­pour release experiment conducted from SHAR

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ies during a geomagnetic storm46

259

INDIAN 1 RADIO & SPACE PHYS, VOL. 17, DECEMBER 1988

TEMPERATURE, K

Fig. 8-Spectroscopically measured neutral temperatures at147 ± 5, 167 ± 5 and 270 ± 5 km along with the model pre­dicted profile just two hours after the occurrence of a stormsudden commencement. The dashed line represents the mod-

el profile for an exospheric temperature of 2000K (Ref 49).

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Ahmedabad depends on the changes in the peakposition of the 01 630.0 nm emission line whichcorresponds to the line-of-sight winds. Anotherfeature that was first noticed and later confirmedto be present on many occasions is the suddennarrowing down of the line profile within 10 min.Whether the narrowing of the line profile wa:; dueto dynamical effects was not known by then.

Systematic temperature measurements are pres­ently being carried out from Mt. Abu by a 100mm diam. optically connected etalon operated ina central aperature scanning mode (pressurescanned, programmable). A narrow band (3 A)temperature-tuned interference filter and photoncounting system with EMI 9863A photomultiplier(thermoelectrically cooled) constitute the spec­trometer. The etalon is housed in an air-tight andtemperature-controlled chamber. Because of thelarge size of the etalon and the eventual increasein the light gathered, scans of 10 min/line profilebecame feasible. The results of temperature mea­surements reveal fluctuations, showing up oscilla­tory feature (of - 30 min period, Fig. 10). Theseresults support the earlier observations by Rajara­man53 about the sudden changes in the width ofthe line profiles. Correlative studies along withmeasurements on the F-region using ionosondelocated at Ahmedabad indicate the validity of the'Servo' model' I.

400

Fig. 9-Near simultaneous measurement of neutral tempera­tures, winds and h' F from Australian latitudes, depicting the

role of neutral dynamics50

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(dip lat., 15S) showed that the Doppler widthand hence the neutral temperature obtained fromthe resonantly scattered sodium D 1 line by thereleased sodium trails revealed excess tempera­tures (exceeding by 500 K) as compared to thosepredicted by the model in the lower thermosphere(Fig. 8). These measurements, the first of theirkinds from an equatorial station, happened to bejust - 2 hr after a storm sudden commencement(SSe). The inference from the temperatutre pro­file is that the source of the additional heatingmust be localized49. The exact mechanism thatcould result in such large temperatures is still notknown. All the hitherto known sources discussed

in the beginning could not explain these measure­ments, and hence the question whether there areany other sources of energy remains to beanswered.

The concept of the thermosphere and the ion­osphere to be looked into as a closely coupledsystem is relatively new and only a few attemptshave been made so far to carry out simultaneousmeasurements on the thermospheric and ionos­pheric parameters.

Yagi and Dyson50 from Australia comparedtheir measurements with ionospheric measure­ments from nearby stations. Also their comparis­on with the 'Sef~o' model of Rishbeth et a1.51,52

yielded positive results (Fig.9). They showed thatthe 'night stationary lever of the F-Iayer dependson temperature with its height changing by 13 ± 6km/100 K.

Further Rajaraman53, from the exploratory in­vestigations from Mt. Abu, showed that h' F over

260

I I II "!'!'I'I'I' 'I ' iillll·11 ,Ii-lllill ,III.,,,,,,,,,,

SRIDHARAN :THERMOSPHERIC STRUCTURE & DYNAMICS

TIME,IST

Fig. 10- Typical thermal oscillations inferred from 01 630.0nm line profiles over Mt. Abu, indicating peri<1dicities of the

order of 30 min

8.1 Enhancement of Luminosity

As the night airglow emission itself is very weakand depends on the solar activity and also on theambient conditions, the flux gathering power hasto be increased by some means to enable fasterscanning; especially during solar minimum periodswhen the intensity often falls below the detectablelevel. The Twin Etalon Scanning Spectrometer(TESS) was first developed by Hernandez et ai.55wherein a light gain by a factor of > 20 isachieved. In a TESS, two identical etalons werekept in tandem. While one etalon acted as an ac­tive aperture, scanning was done in the other etal­on~ Light could be collected simultaneously frommore than 20 fringes. Though a novel idea, insuch systems often one faces problems due to thedissimilarity in the etalons and also in the optical

alignment. Such critical elements make it unsuit­able for field operations which, many times, haveto be unattended.

Another simpler technique was adopted by Bi­ondi et aL56 A physical aperture with annularrings was. fabricated exactly identical to the FP in­terference fringes, and the single aperture re­placed. Though tricky, it is still possible to. fabric­ate such apertures. In fact, three FP spectrometerspresently being operated by Biondi and his grouphave such Multiple Aperture Exit Plates (MAPE).In the FP spectrometer presently being operatedfrom Mt. Abu, one such aperture plate coveringtwo zones has been installed.

Attempts made by Meriwether (personal com­munication, 1987) to fabricate photomultiplierswith annular ring cathodes identical to the inter­ference fringes produced by FP etalon have alsomet with success in enhancing the luminosity.

The instrument that was flown onboard Dy­namic Explorer satellite for the investigation ofthermospheric dynamics has an annular anodesystem similar to the interference pattern of theetalon preceded by a multichannel plate and anS-20 photocathode57•

8.2 Rotational Temperature MeasurementsSo far, the discussions have been pertaining to

the high resolution FP spectrometers that havebeen used to measure the emission line profilefrom which Doppler temperature and line-of-sightwinds could be inferred. Normally in the mea­surements of rotational temperature of molecularbands in the atmosphere, low resolution spec­trometers are used. One of the recent investig­ations by Skinner and Hays58 has revealed thepossibility of using high resolution instruments forinferring temperatures, in particular, at low tem­perature ranges. If the effective line spacing of themolecular band to the free spectral range (FSR) isnearly a rational number, the result4lg signal isfound to be highly temperature dependent. Theonly constraint is not the finesse but a thoroughknowledge of the instrumentalprofile.

Imaging detectors. such as charge coupled de­vices (CCDs), imaging photon counting systerm,etc. figure very often in present day literatureThese imaging devices, which have pixel sizes ofthe order of - 20 f..l.m,have got tremendous lZapa­bility and signal processing. power. A few systemsare already operational in the western countriesby making use of these detectors59 and they prob­ably represent the sort of thirigs that are going tobe used in the ivestigations of thermosphericstructure and dynamicScin the near future.

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brication of optical components and in the fieldof detectors, high resolution spectrometry iscrossing news bounds. The commercial availabil­ity of piezo scanned servo controlled FP, wherefaster scans within seconds are possible while atthe same time maintaining the parallelism betweenthe plates, is a major breakthrough. With theavailability of such etalons, multiple etalorr spec­trometers with very high rejection of the brightcontinuum background are possible, enabling oneto attempt daytime winds and temperature mea­surements.

In fact, one of the attempts by the Australiangroup did indeed yield the fir,st systematic observ­ations on daytime temperatures and winds54• Ex­cept for the iItitial reports nothing much has beenheard. It is learnt that a new triple etalon spec­twmeter has just been completed and commis­sioned at the Scandinavian range, Kiruna (D.Rees, private cOIlllIUlnication, 1988).

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261

Acknowledgement

The author wishes to thank Prof. B H Subbar­

aya for his kind invitation on behalf of the Organ­izing Committee to give this talk in the NationalSpace Sciences Symposium held at Ahmedabadduring December 1987. The constant encourage­ment from Professors R Raghavarao and B HSubbaraya is duly acknowledged. Thanks are dueto the referee for his very useful suggestions. Thiswork is supported by the Department of Space,Government of India.

From the above discussions it is amply clearthat the ground-based optical measurements haveenormous potential in the investigations of theupper atmosphere, especially those pertaining toatmospheric dynamics. Though, a fairly good ne­twork of stations are operational in the high lati­tude northern hemisphere, measurements fromlow and equatorial latitudes are sparse. With theincreased awareness about the need for looking atthe thermosphere/ionosphere system as a whole,and also about the complexities involved in thelow/equatorial latitudes, isolated measurementshave only limited value in spite of the degree ofsophistication of the instrumentation. A networkof stations with coordinated ionospheric measure­ments are called for in order to provide a com­plete understanding of the various phenomenapertaining to these regions.

INDIAN J RADIO & SPACE PHYS, VOL. 17, DECEMBER 1988

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