fast variations of ground irradiance in auroral regions

6
Fast variations of ground irradiance in auroral regions Hans K. Myrabo and Atle Honne Variations in ground irradiance at frequencies higher than 0.1 Hz may severely degrade the performance of automatic imaging systems. For this reason measurements and analysis of fast variations in ground irra- diance in the auroral region due to variable auroral emissions have been performed. The largest amplitudes and fastest variations are found to occur during the active phase of geomagnetic substorms and during pul- sating auroral conditions, respectively. The power spectra of the variations show a rapid decrease toward higher frequencies. Pulsating auroral conditions contain more power at higher frequencies than do the ac- tive phase. On the average -10% of the total power in the spectra is at frequencies higher than 0.3 Hz. 1. Introduction Fast variations of the irradiance conditions at the ground (i.e., frequencies of 0.1 Hz and higher) may se- verely degrade the performance of automatic imaging systems for surveillance and detection. The aurora is a source that might be expected to give such effects. To our knowledge, measurements of fast ground irradiance variations caused by the aurora have not been pub- lished. From space the aurora appears as a 4-80 wide oval- shaped ring around the polar cap, normally with max- imum brightness near 65-670 geomagnetic latitude. 1 ' 2 In the auroral regions, the aurora is the dominant source of the night sky radiance 3 ' 4 (except for the moon). Fluctuations in the total overhead auroral emissions are thus almost directly transferred to total ground irra- diance variations. During quiet geomagnetic conditions the oval-shaped ring contracts, while during more active periods it brightens and expands toward the equator. 1 ' 2 Normally the aurora is spread out over a large part of the sky. The effect of a general brightening in the source is then a simultaneous and overall brightening at the ground; i.e., there are no significant shadows as would be the case from an angular small source. However, when the aurora appears as a bright rapidly moving source, it might give rise to rapidly moving shadows on the ground. Measurements and analysis The authors are with Norwegian Defence Research Establishment, P.O. Box 25, N-2007 Kjeller, Norway. Received 16 March 1984. 0003-6935/84/152583-06$02.00/0. ©1984 Optical Society of America. of shadow effects in connection with aurora have not been performed so far but are planned. Variations in the ground irradiance due to aurora with periods from minutes and up (frequencies of <0,01 Hz) have been reported by Myrabvo. 5 These types of variation are mainly connected to the large scale de- velopment of auroral substorms. Due to the relatively long periods they are of minor importance in this con- text. II. Considerations Based on Auroral and Pulsing Auroral Morphology In auroral regions there is a typical development of the auroral display as seen by an observer stationed at a fixed point on the earth. The display usually starts with quiet homogeneous arcs of modest intensity elongated approximately in the geomagnetic EW di- rection. After some time, the aurora may develop a ray structure and take the form of band(s) at the same time increasing in intensity. Simultaneously it moves rap- idly with changes in form and intensity. Then, after a few minutes, the aurora may spread over a large part of the sky and decrease in brightness. This development naturally divides the display into three major phases: the buildup, the active, and the breakup. The transition between the buildup and the active phase may be taken as the point when the aurora develops band(s) and brightens, between the active and the breakup phases, the point when the structures be- come irregular and spread out over the sky. In addition we may define postbreakup as a possible fourth state. Outside geomagnetically active periods, the sky is lit up by relatively stable and not very bright auroral fea- tures. 6 During the early buildup phase of a substorm, development occurs at slow to moderate speeds. 7 - 9 Typically 1-2 h or more are spent on the buildup. This leaves little room for significant changes on the time scale of a few minutes and down. Velocities of indi- 1 August 1984 / Vol. 23, No. 15 / APPLIED OPTICS 2583

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Page 1: Fast variations of ground irradiance in auroral regions

Fast variations of ground irradiance in auroral regions

Hans K. Myrabo and Atle Honne

Variations in ground irradiance at frequencies higher than 0.1 Hz may severely degrade the performance ofautomatic imaging systems. For this reason measurements and analysis of fast variations in ground irra-diance in the auroral region due to variable auroral emissions have been performed. The largest amplitudes

and fastest variations are found to occur during the active phase of geomagnetic substorms and during pul-

sating auroral conditions, respectively. The power spectra of the variations show a rapid decrease towardhigher frequencies. Pulsating auroral conditions contain more power at higher frequencies than do the ac-tive phase. On the average -10% of the total power in the spectra is at frequencies higher than 0.3 Hz.

1. Introduction

Fast variations of the irradiance conditions at theground (i.e., frequencies of 0.1 Hz and higher) may se-verely degrade the performance of automatic imagingsystems for surveillance and detection. The aurora isa source that might be expected to give such effects. Toour knowledge, measurements of fast ground irradiancevariations caused by the aurora have not been pub-lished.

From space the aurora appears as a 4-80 wide oval-shaped ring around the polar cap, normally with max-imum brightness near 65-670 geomagnetic latitude.1 '2In the auroral regions, the aurora is the dominant sourceof the night sky radiance 3'4 (except for the moon).Fluctuations in the total overhead auroral emissions arethus almost directly transferred to total ground irra-diance variations. During quiet geomagnetic conditionsthe oval-shaped ring contracts, while during more activeperiods it brightens and expands toward theequator. 1'2

Normally the aurora is spread out over a large partof the sky. The effect of a general brightening in thesource is then a simultaneous and overall brighteningat the ground; i.e., there are no significant shadows aswould be the case from an angular small source.However, when the aurora appears as a bright rapidlymoving source, it might give rise to rapidly movingshadows on the ground. Measurements and analysis

The authors are with Norwegian Defence Research Establishment,P.O. Box 25, N-2007 Kjeller, Norway.

Received 16 March 1984.0003-6935/84/152583-06$02.00/0.© 1984 Optical Society of America.

of shadow effects in connection with aurora have notbeen performed so far but are planned.

Variations in the ground irradiance due to aurorawith periods from minutes and up (frequencies of <0,01Hz) have been reported by Myrabvo.5 These types ofvariation are mainly connected to the large scale de-velopment of auroral substorms. Due to the relativelylong periods they are of minor importance in this con-text.

II. Considerations Based on Auroral and PulsingAuroral Morphology

In auroral regions there is a typical development ofthe auroral display as seen by an observer stationed ata fixed point on the earth. The display usually startswith quiet homogeneous arcs of modest intensityelongated approximately in the geomagnetic EW di-rection. After some time, the aurora may develop a raystructure and take the form of band(s) at the same timeincreasing in intensity. Simultaneously it moves rap-idly with changes in form and intensity. Then, after afew minutes, the aurora may spread over a large part ofthe sky and decrease in brightness.

This development naturally divides the display intothree major phases: the buildup, the active, and thebreakup. The transition between the buildup and theactive phase may be taken as the point when the auroradevelops band(s) and brightens, between the active andthe breakup phases, the point when the structures be-come irregular and spread out over the sky. In additionwe may define postbreakup as a possible fourth state.

Outside geomagnetically active periods, the sky is litup by relatively stable and not very bright auroral fea-tures.6 During the early buildup phase of a substorm,development occurs at slow to moderate speeds.7-9

Typically 1-2 h or more are spent on the buildup. Thisleaves little room for significant changes on the timescale of a few minutes and down. Velocities of indi-

1 August 1984 / Vol. 23, No. 15 / APPLIED OPTICS 2583

Page 2: Fast variations of ground irradiance in auroral regions

vidual structures (bands, arcs) during quiet conditionsand during the first buildup phase are 1 km/sec orless.10'11 Taking 1 km/sec for the velocity and 130km6 "12 for the altitude of the aurora, a structure willneed more than 5 min to pass over the observer's sky.Therefore, movements of individual features should notbe expected to contribute to fast irradiance varia-tions.

During the active phase, characterized by fastmovements, the forming of new structures6 and rapidbrightening from a few kR up to several hundreds of kRmay occur within a fraction of a minute. 13' 14 Althougha single structure covers only a small part of the totalsky, it might give rise to significant and fast variationsin the ground irradiance. Assume an auroral bandsystem, EW elongated, having a 5 NS extension,passing through the zenith from the eastern to westernhorizon. If the band system undergoes a 10X bright-ening from a level of 10 kR, a ground irradiance increaseof -150% would be expected. During the active phasethe bright structure may move at velocities from a fewtens of km/sec up to -100 km/sec.6 At 50 km/sec, abright structure would be able to travel almost fromhorizon to horizon within 10 sec. Motion, brightening,and forming of new features take place continuously,6sometimes working together and sometimes working inthe opposite direction with regard to total ground ir-radiance. A reasonable estimate indicates that varia-tions of the order of 100% may take place within 10 sec.From the nature of the active phase, it is expected thatthe variations are irregular.

During breakup and after breakup pulsating struc-tures often occur. Pulsating describes a condition ofrapid, often rhythmical, fluctuations in brightness, timescales ranging from <1 sec to a few minutes.6 Theclassical case of pulsating aurora is caused by strongelectron precipitation before or around magnetic mid-night slowly developing and changing into a pulsatingand patchy aurora in the postmidnight and morninghours.7 An example of the brightness variation of apulsating auroral patch is in Fig. 1. Pulsating auroramay also occur at other times, for example, beforemidnight as pulsating arcs.15 Observations with anarrow field of view, i.e., a few degrees, of single auroralstructures show that on the average 98% of the poweris at modulation frequencies of <1 Hz.6 Occasionally,however, one has observed short-lived intervals with amuch larger fraction of the power above 1 Hz.1 "15

The variation in the ground irradiance due to a pul-sating arc is estimated to be up to 50%. Contrary to anarc, a pulsating auroral patch usually contains a verysmall fraction of the total light emission of the sky. Areason for this is the size of patches, mostly in the 5-50-km range.'5-'7 A considerable number of pulsatingpatches are normally spread out over a large fraction ofthe sky superimposed on a nonpulsating auroral back-ground.16 The pulsations of the patches over the skyare usually reported to be incoherent.16 "18 With a fewtens of patches, variation in each patch of 25-50%16should result in <5% variation in the ground irradiance.However, there are occasions when a large fraction or

30 3cn

I-z 20-

-a

go0 do 60 10 1,50 180RELATIVE TIME (sec)

Fig. 1. Example of pulsating auroral brightness variations. The datawere taken in the N2 1 Neg 4278-A emission with a field of view

of 4O.

I--a

o0~

LU

C00

18 2l I 3GEOMAGNETIC TIME (hrs)

Fig. 2. Occurrence probability of pulsating aurora as deduced froma smoothing of the results by Oguto et al. The data were obtained

within the geomagnetic latitude range of 64-68°N.

the whole sky has been reported to pulsate coherent-ly.19'20 During such conditions, which are believed tobe rather rare, a 50% modulation or more in the groundirradiance could be expected.

Pulsating aurora is generally observed equatorwardthe proton precipitation region.21 Figure 2 shows thelocal time occurrence between 64 and 68°N geomagneticlatitude as obtained during one of the latest pulsatingauroral campaigns.22 As the geomagnetic activity in-creases, the region of pulsating aurora tends to expandtoward the equator and also toward the evening sector.22

In Fig. 2, the probability of pulsating aurora rises to-100% 3 h after magnetic midnight; i.e., when there isaurora it is pulsating.

In summary fast variations of ground irradiance maybe expected to occur only in the most active phase of thesubstorms and under pulsating auroral conditions.

Ill. Measurements

The experimental data utilized here were gatheredat the University of Tromso's field station in Skibotn,Northern Norway (geographic latitude 6904 N, longi-tude 2003 E, geomagnetic latitude 6609 N, longitude

2584 APPLIED OPTICS / Vol. 23, No. 15 / 1 August 1984

60-

20

.MAGN. . MIDNIGHT...

Page 3: Fast variations of ground irradiance in auroral regions

119°8 E) during a few nights in the 1981/1982 observingseason. The data analyzed are broadband measure-ments in the visual spectral region. However, becausethe aurora dominates the irradiance at the ground fromthe UV to at least 1-,um wavelength,3 the observed timevariations are representative for the whole spectral re-gion of 0.3-1 /um.

A. Instrumentation and Analysis

Two low-light-level photometers (a Gamma-Scien-tific model 2400 digital display together with a Lam-bertian response photometer head and a specially builtphotometer 2 3 ) have been used to collect 27r illuminancedata. The photometer head of the Gamma-Scientificinstrument was equipped with an RCA C21031 GaAsphotomultiplier tube, while the other photometercontained an EMI 9558R (S-20) tube. The photome-ters were set up with adequate filters to match the visualscotopic response curve. Figure 3 shows the responsecurve of the photometers superimposed on a typicalemission distribution of the spectral ground irradiance.The N 1 Neg and the forbidden oxygen emissions at5577 and 6300/64 A are the most prominent emissionfeatures seen.

The data from the Gamma-Scientific photometerwere recorded on a Watanabe 4 channel and later on aPhilips 8010 single-channel paper recorder. Other dataobtained every 5 and 15 sec were sampled directly by amicrocomputer data acquisition system.23 In additionto the above instrumentation, a 40 field of view auroralphotometer equipped with a 15-A HW filter centeredat 4278 A (N' 1 Neg) was employed to record the radi-ance change in the zenith area. This instrument wasonly in use part of the time and then only as a check onthe auroral conditions.

The few data recorded by the microcomputer systemwere stored on diskettes. Those from the paper re-corders was found to be <40 msec. The instrumentaland photon noise have been established by measuring thedark current and a constant light source in the samecorded data. The total time constant of theGamma-Scientific photometer and the respective re-corders was found to be <40 msec. The instrumentaland photo noise have been established by measuring thedark current and a constant light source in the sameintensity range as the actual signal from the varyingaurora. Both noise components were found to be <0.1%of the average signal. The relative measuring accuracyfor the data from the paper recorders is estimated to0.5%, mainly limited by the digitizing accuracy. Thephotometers were calibrated in the field using a pre-calibrated photometer traceable to a National Bureauof Standard (NBS) source. The uncertainty in theabsolute calibration is estimated to 20%.

After digitizing the data and storing on diskettes,Fourier analysis has been applied to the data to revealthe power present at different frequencies of modula-tion. Approximately thirty separated time periodsbelieved to be representative of pulsating aurora and theactive phase of the substorm have been analyzed. Thelength of each time period was typically 30-40 min.

- 80.zLU

z 60LU

P 40,

= 20-

500WAVELENGTH (nm)

Fig. 3. Scotopic (visual) response curve together with a typicalspectral distribution of the ground irradiance in auroral regions.

20

15-x

U

r 10Mi=

J3

_j

5-

30010 200RELATIVE TIME (sec)

Fig. 4. Illuminance variations at the ground during the active phaseof an auroral substorm.

Each time period was divided into 10-15 segmentscontaining from 128 to 512 data points. Hanningwindow and running average with 50% overlap wereused to obtain the average spectrum for each period.Spectra for the different periods have then been furtheraveraged to give mean spectra for pulsating and activeperiods, respectively.

B. Results

Measurements from quiet periods and from the earlybuildup phase of a substorm showed very slow andmoderate variations. Changes ranging up to 20-30%normally took place on time scales of tens of minutes.Faster variations superimposed on the longer devel-opment with amplitudes larger than 1% were normallynot seen. Such data were, therefore, not submitted toFourier analysis. Only slow and moderate variation wasseen for periods after the most active phase of thebreakup except when pulsing occurred. These exper-imental findings are in overall agreement with the es-timates in Sec. II.

During the most active phase of the substorm, how-ever, variations with amplitudes up to 100% could occurwithin tens of seconds. An example of a typical timeinterval with moderate variations is given in Fig. 4.

1 August 1984 / Vol. 23, No. 15 / APPLIED OPTICS 2585

.

n

Page 4: Fast variations of ground irradiance in auroral regions

io0. 'o lo_:LU o

C.'Ca

,o-,

0 005 0.1 015 020 025 030FREQUENCY (Hz)

Fig. 5. Power spectral density of the variations in the ground illu-minance during the active phase of auroral substorms.

Fourier analysis of the variations in the ground illu-minance during the most active phase of the substormsresulted in a power spectrum as presented in Fig. 5.Data from seven different substorms have been applied.Most of the power in the Fourier spectrum is concen-trated at frequencies corresponding to periods of 0.5-2min, falling off sharply toward higher frequencies.

During pulsating auroral conditions, fast and con-siderable variations were found in the ground illumi-nance. An example of fairly periodic variations ispresented in Fig. 6. Individual power spectra deducedfrom a limited time period often showed strong peaks.However, averaging a large number of separated eventsresulted in smoothing, so that no particular peak ap-pears. As for active periods, the power in the spectrumdecreased relatively monotonously toward higherfrequencies. For pulsating conditions, the decrease wasnot as strong, but the spectrum had a much higher dccomponent as compared with active periods. Most ofthe time, the pulsations showed periods less than a fewtens of seconds, often superimposed on a more slowlyvarying pattern. Figure 7 shows the mean powerspectrum from 18 separated periods with pulsatingaurora. Some of the power in the lowest frequency in-terval (i.e., 0.02 Hz) is due to the slower varying back-ground pattern on which the real pulsations were su-perimposed.

To see the difference in power between the frequen-cies present in active periods and during pulsating au-roral conditions and to see the amount of power leftabove a certain frequency, we have calculated

ni

,- pnn1 =0,, (1)11=0

'CLJ

0f30 60 90 120

RELATIVE TIME (sac)

Fig. 6. Illuminance variations at the ground during pulsatingaurora.

100

10-

I-SLU0 -10'

I-LUCL'

0 0.2 0.4 06 0 8 1.0 1.2FREQUENCY (Hz)

Fig. 7. Power spectral density of the variations in the ground illu-minance during pulsating auroral conditions.

i.e., one minus the cumulative values. Here p, is thepower in the nth frequency interval and

noE pn =.

n=0The graphs are presented in Fig. 8. As an example

to find how much power is left at frequencies higherthan 0.3 Hz for pulsating aurora, one finds 0.3 Hz on theabscissa and reads off 10-1 on the ordinate; i.e., 10% ofthe power is at frequencies higher than 0.3 Hz. Therapid falloff at the end of each graph is probably moredue to the limited bandwidth of the analysis than to areal effect in the data.

IV. Discussion

As expected and outlined in Sec. II it was only duringthe active phase of substorms and during pulsatingconditions that significant fast ground illuminancevariations were observed. The variations measured

2586 APPLIED OPTICS / Vol. 23, No. 15 / 1 August 1984

10-4j

Page 5: Fast variations of ground irradiance in auroral regions

C

0a 10-2

C)~

LUJ

LU

-4

0 02 04 06 08 1.0 12

FREQUENCY (Hz)Fig. 8. One minus the cumulative value of the power spectral densityfor the active phase of the substorm and for pulsating conditions,

respectively.

during the active phase of the substorm reveal a sur-prisingly fast decrease in power with frequency. In fact,Fig. 5 shows that there is only 10-4 of the power leftbeyond 0.3 Hz. This is far less than would be expectedfrom the estimates based on auroral morphology of theactive phase of substorms. The reason for this is dif-ficult to pinpoint without a more extended set of data.One should, however, notice that for shorter time peri-ods (i.e., individual spectra) amplitudes in the 10% rangewere found out to 0.15 Hz. The probability of findingsuch variations at much higher frequencies than 0.04 Hzseems small, a fact that is seen from the very fast de-crease of the average spectrum. The peak at 0.15 Hzin the average spectrum is probably due to such a caseand to the fact that the amount of data is limited.There is no obvious physical reason for such a peak toexist, strengthening the above assumption. A definiteanswer can only be given by more extended data.

Another feature of the power spectra during the ac-tive phase is the small dc component. This is a directand logical consequence of the total dominance of theaurora in the ground illuminance3 and the absence ofa nonvarying auroral background at this phase of thesubstorm. This is somewhat contrary to what is nor-mally seen during pulsating conditions. The largestpart of the power is here found in the dc component andin a slowly varying background on which the fastervariations are superimposed. This is in agreement withthe existence of an auroral background on which thepulsating features are often superimposed.

On the average 10% of the total power in the spectrais found at frequencies higher than 0.3 Hz. This is notto be expected from the considerations in Sec. II.Whether it is due to the dominance of one or a fewpulsating features or a larger degree of coherence thanexpected is difficult to judge from photometric dataalone.

Figure 8 clearly shows that for the highest frequency

range, pulsating auroral conditions show the highestamplitudes. Pulsating aurora also lasts for a muchlonger fraction of the total time than does the activephase of substorms.6 In most cases, it is, therefore,reasonable to expect time intervals with pulsating au-roral conditions to impose the largest problems for theuse of automatic imaging systems.

The modulation frequency range covered by thesemeasurements normally contains more than 98% of thepower in a single pulsating auroral structure. 6 Groundirradiance variations at frequencies higher than -1 Hzshould, therefore, on the average contain very littleenergy and thus not be critical for most applications ofautomatic imaging systems. This conclusion has alsobeen strengthened by a few measurements above 1 Hzas recorded with a 25-MHz digital storage oscilloscope.However, to check this thoroughly, more extensivemeasurements with high sampling frequencies have tobe performed. A larger set of data for the frequenciesbelow 1 Hz should also be obtained to estimate thestatistical probability of power vs frequency for thedifferent types of aurora.

We wish to acknowledge 0. E. Harang and J. E. Sol-heim of the University of Tromso for making the opticalfield station available for these measurements.

References1. A. V. Jones, Aurora (Reidel, Dordrecht, 1974).2. Y. I. Feldstein, "Auroral Oval," J. Geophys. Res. 78, 1210

(1974).3. H. K. Myrabo, "Nocturnal Levels of Ground Illuminance in Au-

roral Regions," Appl. Opt. 21, 1881 (1982).4. H. K. Myrab, Nattlys i nordomridene, FFI/RAPPORT-82/3017,

Norwegian Defence Research Establishment (1982) (in Norwe-gian).

5. H. K. Myrabo, "Typical Durations of Nightly Ground IlluminanceEnhancement in the Auroral Zone," FFI/RAPPORT-81/3010,Norwegian Defence Research Establishment (1981).

6. A. Omholt, The Optical Aurora (Springer, Berlin, 1971).7. S. I. Afasofu, Physics of Magnetospheric Substorms (Reidel,

Dordrecht, 1977).8. J. W. Chamberlain, Physics of Aurora and Airglow (Academic,

New York, 1961).9. W. Stoffregen, "East-West Drift of Auroral Forms Determined

from All-Sky Camera Films," J. Atmos. Terr. Phys. 21, 257(1961).

10. J. S. Kim and B. W. Currie, "Further Observations of HorizontalMovements of Aurora," Can. J. Phys. 38, 1366 (1960).

11. D. J. McEwen and C. N. Duncan, "A Campaign to Study Pul-sating Aurora," Can. J. Phys. 59, 1029 (1981).

12. J. S. Boyel, Thesis, U. Alaska, College (1969).13. A. Omholt, "Auroral Morphology," in Cosmical Geophysics, A.

Egeland, 0. Holtet, and A. Omholt, Eds. (Universitetsforlaget,1973).

14. V. G. Vorobjev, G. V. Starkov, and Y. I. Feldstein, "The AuroralOval During the Substorm Development," Planet. Space Sci. 24,955 (1976).

15. K. V. Paulson and G. G. Shepherd, "Fluctuations in BrightnessForm Quiet Form Auroras," Can. J. Phys. 44, 837 (1966).

16. 0. Royrvik and T. N. Davis, "Pulsating Aurora: Local and GlobalMorphology," J. Geophys. Res. 82, 4720 (1977).

17. H. C. Stenbaek-Nielsen and T. J. Hallinan, "Pulsating Aurora:Evidence of Noncollisional Thermalization of PrecipitatingElectrons," J. Geophys. Res. 84, 3257 (1979).

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18. A. D. Johnstone, "Pulsating Aurora," Nature London 274, 119(1978).

19. W. B. Murgray, "Some Properties of the Luminous Aurora asMeasured by a Photoelectric Photometer," J. Geophys. Res. 64,955 (1959).

20. R. W. Gowell and S. I. Akasofu, "Irregular Pulsation of theMorning Sky Brightness," Planet. Space Sci. 17, 289 (1969).

21. D. J. McEwen and C. N. Duncan, "A Campaign to Study Pul-sating Auroras," Can. J. Phys. 59, 1029 (1981).

22. T. Oguti, S. Kokubun, K. Hayashi, K. Tsuruda, S. Machida, T.Kitamura, 0. Saka, and T. Watanabe, "Statistics of PulsatingAuroras on the Basis of All-Sky TV Data from Five Stations. I.Occurrence Frequency," Can. J. Phys. 59, 1150 (1981).

23. H. K. MyraboA, "An Automatic Data Acquisition System," FFI/RAPPORT-82/3011, Norwegian Defence Research Establish-ment (1982).

18-23 29th Ann. Int. Tech. Symp. on Optical & Electro-OpticalEng., San Diego SPIE, P.O. Box 10, Bellingham,Wash. 98227

September

? Gradient-Index Optical Imaging Systems (Grin VI), ItalyD. Moore, Inst. Optics, U. Rochester, Rochester, N. Y.14627

15-20 Optical & Electro-Optical Eng. Symp., CambridgeSPIE, P.O. Box 10, Bellingham, Wash. 98227

17-19 Mathematics in Signal Processing Conf., Bath TheDeputy Secretary, Inst. of Mathematics & Its Appli-cations, Maitland House, Warrior Square, South-end-on-Sea, Essex SS1 2JY, England

22-25 Picture Archiving & Communications Systems forMedical Applications Mtg., Kansas City SPIE, P.O.Box 10, Bellingham, Wash. 98227

October

Meetings Calendar continued from page 2544 15-18 OSA Ann. Mtg., Wash., D.C. OSA Mtgs. Dept., 1816Jefferson Pl., N. W., Wash., D.C. 20036

1985

November

June

9-13 Soc. for Experimental Stress Analysis Spring Mtg., LasVegas SESA, 14 Fairfield Dr., Brookfield Ctr., Conn.06805

11-14 4th Int. Congr. on Applications of Lasers & Electro-Optics, San Francisco Laser Inst. of Am., 5151 Mon-roe St., Suite 118W, Toledo, Ohio 43623

December

10-13 Int. Lens Design, OSA Tech. Mtg., Cherry Hill OSAMtgs. Dept., 1816 Jefferson P., N.W., Wash., D.C.20036

10-14 OSA Spring Conf., Cherry Hill OSA Mtgs. Dept., 1816Jefferson Pl., N. W., Wash., D.C. 20036

12-14 Workshop on Optical Fabrication & Testing, OSATech. Mtg., Cherry Hill OSA Mtgs. Dept, 1816 Jef-ferson Pl., N. W., Wash., D.C. 20036

10-14 Image Science & Technology ICO Conf., Helsinki P.Oittinen, Helsinki U. Technology, Lab. of GraphicArts Technology, Tekniikantie 3, 02150 Espoo 15,Finland

17-19 Int. Conf. on Chemical Kinetics, Gaithersburg J. Her-ron, A147 Chem. Bldg., NBS, Wash., D.C. 20234

24-28 7th Int. Conf. on Laser Spectroscopy, Maui T. Hansch,Physics Dept., Stanford U., Stanford, Calif. 94305

24-29 Int. Conf. on Fourier & Computerized Infrared Spec-troscopy, Ottawa L. Baignee, Conf. Services Office,Ottawa, Ontario KIA OR6, Canada

8-13 10th Ann. Int. Conf. on Infrared & Millimeter Waves,Lake Buena Vista, Fla. K. Button, MIT Natl. MagnetLab., Bldg. NW14, Cambridge, Mass. 02139

1986January

19-24 Optical & Electro-Optical Eng. Symp., Los AngelesSPIE, P.O. Box 10, Bellingham, Wash. 98227

February

9-15 Astronomical Instrumentation Confs., Tucson SPIE,P.O. Box 10, Bellingham, Wash. 98227

13-14 Optical Fiber Sensors, OSA Top. Mtg., San DiegoOSA Mtgs. Dept., 1816 Jefferson Pl., N. W., Wash.,D.C. 20036

March

9-14 Microlithography Confs., Santa Clara SPIE, P.O. Box10, Bellingham, Wash. 98227

August

4-8 Photoacoustic, Thermal & Related Sciences mtg., Que-bec L. Bertrand, Departement de genie physique,Ecole Polytechnique, Campus de 'Universite deMontreal, P.O. Box 6079, Succursale A, Montreal H3C3A7, Canada

June

9-13 Quantum Electronics Int. Conf., Phoenix OSA Mtgs.Dept., 1816 Jefferson Pl., N. W., Wash., D.C. 20036

continued on page 2659

2588 APPLIED OPTICS / Vol. 23, No. 15 / 1 August 1984