Optical Studies of Thermospheric Structure Dynamics-Recent...
Transcript of Optical Studies of Thermospheric Structure Dynamics-Recent...
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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
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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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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
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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
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INDIAN J RADIO & SPACE PHYS, VOL. 17, DECEMBER 1988
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Fig. 4-Photometric measurements on 630.0 nm depicting the equatorial depletions traversingthrough the emission region"
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
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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..•
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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 .
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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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Fig. 6-Measured neutral temperature depicting the enhancedvalues from Mt. Abu along with the model estimates for the
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
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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
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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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8 Present State of ArtWith more and more sophistications in the fa
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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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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