zenith sky brightness and airglow emissions during the equatorial solar eclipse of 30 june 1973

Zenith sky brightness and airglow emissions during theequatorial solar eclipse of 30 June 1973

E. H. Carman, N. J. Skinner, and M. P. Heeran

Experimental and calibration procedures used for photometric zenith measurements at Loiyengalani(2.75 0 N, 36.61E) during the total solar eclipse of 30 June 1973 are described briefly. Comparison was made

between sky brightness at wavelengths in the 3914-6300-A range during totality and morning twilight at Dares Salaam. The twilight 3914- and 6300-A sky background ratio is sometimes close to the expected valuefor a pure Rayleigh molecular scattering process, but the corresponding ratio at eclipse mid-totality is aboutone-third this, indicating a shift toward the red consistent with an atmosphere containing aerosols. The ze-nith sky brightness at 5600 A was <1 kR/A, somewhat lower than has been observed at low latitudes duringthe past 50 years. Comparison of the 6300-A line emission rate of 1.4 + 0.6 kR with the Jan. 1974 Atmo-

sphere Explorer satellite dayglow analysis suggests the main source of O(1 D) atoms during totality is by exci-

tation of 0(3p) atoms in the lower thermosphere. About 1 kR of 5577 A was also observed, but the 5200-Aemission was too low for reliable detection. An unexpectedly high 3914-A emission rate of at least 1 kR wasapparent, the high rate being supported by observation at 4278 A.

I. Introduction

Ground-based photometers 1 were used to observe thecontinuum sky brightness and zenith emission rates at6300, 5577, 5200,4278, and 3914 A during the total solareclipse of 30 June 1973. The observations were made-3-km south of Loiyengalani, Kenya (latitude 2.750 N,longitude 36.60 E). Totality was centered on 12-h59.9-min UT with a duration of 4 min 56.2 sec when thesolar altitude was 37°. Other details of the local cir-cumstances and sky and ground conditions at Loiyen-galani have been reported elsewhere. 2 3 The airglowemissions come from a sky partly illuminated by the sunat the emission heights. They are observed against abackground illuminated by sunlight multiple scatteredinto the lower atmosphere, but which is sufficiently darkto have the visual appearance of civil twilight corre-sponding to a solar depression angle of 6.70. The solareclipse thus provides an opportunity to observe the dayairglow emissions under conditions of greatly atten-uated background emission.

When this work was done E. H. Carman was with University ofPapua New Guinea, Physics Department, P.O. Box 4820, Papua NewGuinea; he is now with Ionospheric Prediction Station, P.O. Box 165,Vanimo, West Sepik Province, Papua New Guinea. N. J. Skinneris with University of the South Pacific, Suva, Fiji, and M. P. Heeranis with University of Nigeria, Physics Department, Nsukka, AnambraState, Nigeria.

Received 4 June 1980.0003-6935/81/050778-08$00.50/0.© 1981 Optical Society of America.

Shaw et al. 3 4 investigated sky radiance during the 30June 1973 event at eight wavelengths in the 4000-7000-A range. Much of this carefully executed programoverlaps the observations reported in this paper, andnew information is derived from a comparison betweenthe two sets of observations.

From the measurements of the (3P-1D) transition at6300 A from the Atmosphere Explorer-C satellite, Hayset al.5 investigated the chemistry of thermosphericO('D) atoms. They deduced the relative contributionsto the measured volume emission rate coming fromthermal electrons, dissociative recombination, photo-electrons, and the Schumann-Runge continuum. Thisanalysis is discussed in relation to our observed 6300-Aemission rate during mid-totality.

II. Experimental Method and Calibration Procedures

The measurements were made with three filter-wheelphotometers each having four filter positions. Filter.arrangements together with the respective half-powerwidths are shown in Table I. Filters 6300 (1) and 6300(2) were a matched pair and were installed in separatephotometers as a control to test instrumental differ-ences. A two-filter method has been used for elimi-nating sky background from the observed spectral lineemission rates. To obtain maximum use of the twelveavailable filter positions a background filter for the redline was installed in photometer 2 only. A similarprocedure was used for the two 5577-A filters; The4000-A filter served for background elimination at 3914and 4278 A, but differences in Fraunhofer absorptionat these three wavelengths are significant and will bediscussed in Sec. III.

778 APPLIED OPTICS / Vol. 20, No. 5 / 1 March 1981

Table 1. Interference Filter Details

Photometer 1 Photometer 2 Photometer 3

6300 (1) (10.5 A) 6300 (2) (10.2 A) 3914 (10.7 A)5577 (1) (7.5 A) 5577 (2) (7.8 A) 5160 (5.8 A)6300 (3) (8.5 A) 6250 (7.5 A) 4000 (14.4 A)5200 (6.0 A) 5525 (7.9 A) 4278 (6.0 A)

Table 11. Absorption and Scattering Errors

Atmospheric BackgroundWavelength Fraunhofer scattering error

(A) absorption I0/IB (%)

6300 0.95 0.99 (0.98) 1 (1)6250 0.955577 0.96 0.98 (0.98) 0 (0)5525 0.945200 0.91 0.99 (0.99) 2 (2)5160 0.924278 0.72 0.96 (0.91) 9 (14)4000 0.763914 0.66 1.01 (1.03) 12 (11)4000 0.76

2-0

1-2

KR/A

0-8

0-4 -

3600 4400 5200 6000WAYELENGTH (A)

6800

Fig. 1. Zenith sky brightness at mid-totality (dashed line from Ref.3).

Use of a two-filter method to detect emission linesagainst the strong background present during theeclipse led to serious calibration problems because thefield calibration was carried out with a 14C mixedphosphor source designed for nightglow measurements.The nighttime continuum is of the order of 1 RA-1.This is comparable with the source which ranges from0.1 to 7 RA-1. On the other hand the continuum duringa solar eclipse emits in the approximate 0.5-2-kRA-1range, leading to a sky-to-source ratio of almost 100:1for 6300 A. With the multirange dc amplifier used, anerror of 10% could be expected in converting betweenadjacent ranges leading to quite large errors when sev-eral ranges were involved.

To overcome this problem scale factors for each filterwere measured directly by observing the twilight withthe photometer through a rotating sectored disk. Thedisk had an -10° aperture and was alternately rotatedat high speed for the more sensitive range and then heldstationary with the aperture over the photometer en-trance for the less sensitive range. By carrying out al-

ternate measurements over a period of some 30 min itwas possible to produce two curves showing instrumentresponse for twilight sources differing by a known in-tensity ratio, 35.3:1 in the present case.

The effect of temperature changes on the interferencefilter passband characteristics and on the response ofthe instruments to the 14C source was also investigated.For instrument temperatures in the 20-37°C rangethere was negligible variation with the 6300- and 6250-Afilters, but a 30% increase occurred for 5577 A and a 64%increase for 5525 A. The effect of the latter sensitivityto temperature would not be serious since 14C calibra-tions were made at the time of the eclipse observa-tions.

Corroborative calibrations were subsequently carriedout in the laboratory using a 9 Sr activated phosphorwith surface brightness of the same order as the eclipsesky brightness. This source was calibrated against a 14Csource of comparable brightness at the Max-PlanckInstitut fur Aeronomie, Landau-Harz. The resultingvalues of surface brightness were 3630 RA- 1 at 6300 A,3850 RA- 1 at 6250 A, 3125 RA- 1 at 5577 A, and 4037RA- 1 at 5525 A.

Ill. Fraunhofer Absorption and AtmosphericScattering

Since elimination of the strong multiple scatteredsunlight was based on a two-filter method, it is impor-tant to consider the influence of Fraunhofer absorptionand atmospheric scattering. A weakness in the two-filter method is that measurement of the sky back-ground at a wavelength different from the emissionwavelength could conceivably result in inaccuracyproportional to the ratio of the inverse fourth power ofthe two respective wavelengths. Differences inFraunhofer absorption at the two wavelengths lead tofurther somewhat greater error. To estimate Fraun-hofer absorption, the photospheric spectrum observedby Delbouille et al. 6 7 was integrated over the respectiveinterference filter passbands.

The second column in Table II shows the calculatedreduction in instrument response to be expected fromthe absorption. For the N2 emission at 3914 and 4278A the background brightness observed at 4000 A led toerrors of 10% and 4%, respectively. The measurementsof Shaw3 taken during the solar eclipse and at solar noonon 30 June 1973 have been used as a basis for estimatingerror due to molecular scattering. Shaw's results showthat the noon zenith sky radiance does not vary withwavelength according to a pure molecular (Rayleigh)atmosphere, but it is influenced by the presence ofaerosols. The ratio of intensities at 4000 and 7000 A,I4000/I7000, for both observed and theoretical noon datais only between one and two-tenths of the variation fora pure Rayleigh scattering by air molecules alone.During mid-totality the observed I4000/o700o ratio isabout twice the noon variation observed by Shaw andabout one-third the ratio expected from Rayleighscattering alone. The significance of this in relation tothe present results will be discussed further in Sec.IV.

1 March 1981 / Vol. 20, No. 5 / APPLIED OPTICS 779

I I I I I I I

I I I I I I I

1-6

5-0

KR/A

1.0

0.5

1258 1259 1300U T

1301 1302

Fig. 2. Smoothed curves showing the variation of zenith skybrightness during totality.

Table Ill. Sky Brightness Observed During Low Latitude Total Solar Eclipses

Site details Sky conditions(i) Location (i) Solar altitude

(ii) Approx. lat. (ii) Cloud condition(iii) Observer's (iii) Equiv. brightness

Date elevation at 5600 A (kR/A) Ref.

14 Jan. 1926 (i) Benkoelen, (i) - Stetson et al."Sumatra

(ii) 3.8°S(iii) Sea level

(i) Alor Star,Malaya

(ii) 6.10 N(iii) Sea level

(i) Niuafo'ouTonga Is.

(ii) 15.1-S(iii) Sea level

(i) Patos,Brazil

(ii) 180 S(iii) 762 m (2500 ft)

(i) Bocaiuva,Brazil

(ii) 17.20 S(iii) 670 m (2200 ft)

(i) Quehua,Bolivia

(ii) 20.120 S(iii) 4206 m (13,800 ft)

(i) Lake RudolfKenya

(ii) 2.75 0N(iii) 396 m (1300 ft)

(ii) Moving haze andthin cirrus cloud

(iii) 7.7

(i) -(ii) Veil of thin

cirrus cloud(iii) 8.3

(i) -(ii) Not perfectly

clear(iii) 21.1

(i)-(ii)-(iii) 10.6

(i) 40.20(ii) Very thin

patchy cirruscloud

(iii) 2.1

(i) 510(ii) Clear and

cloudless(iii) 8.4

(i) 370(ii) 10% cover,

mainly smallcumulus patches

(iii) (a) 0.8 at6000 A

(b) 1.0

Stetson et al.12

Stetson et al.12

Hulbert1 3

Richardson andHulbert 14

D'andekar' 0

(a) Presentwork

(b) Shaw et al.4


I I I . . I .. I . . I

9 May 1929

21 Oct. 1930

1 Oct. 1940

20 May 1947

12 Nov. 1966

30 June 1973

. . . . . . . .

L

I l

1000

500

100

R/A

50

10

5

SOLAR DEPRESSION ANGLE12 10 8

(W)

0240 0250 0300U T

Fig.3. Morning twilight at Dares Salaam for the night of 22,23 Aug.1974.

I I I I

1259 1300 1301 1302U T (SOLAR ECLIPSE)

1-20 _-i

1;10

lit

1 00 a 0 C

- I(at | Is | 2 | is | )

0-90' I I I I I I I 7. 716'9B_ S'7S3 7-1 6A9 6-7 6E5

SOLAR DEPRESSION ANGLE ( )

Fig. 4. Correlation between sky brightness and wavelength. Thecurves show how the respective correlation coefficients (r) relativeto the 1% critical value (t) vary with time for (a) morning twilight atDar es Salaam; (b) eclipse values throughout totality; (c) twilight

combined with eclipse values.

Ratios of the expected sky brightness at the wave-lengths corresponding to the five pairs of spectralemission (IO) and background (IB) filters are shown inthe third column of Table II. Values for these ratioswere taken from the zenith sky intensity distributioncalculated by Shaw3 for an atmosphere where theRayleigh scattering was increased by the presence ofaerosols. Corresponding ratios taken from Shaw's in-tensity distribution at mid-totality are shown in pa-rentheses, but except for I4278/I4000 these values do notlead to significant error.

The final estimate of errors in background observa-tion at the five spectral emission wavelengths is listedin column four of Table II. The percentages shown areestimated from the Fraunhofer absorption and atmo-spheric scattering values and represent the amounts bywhich the observed background emission rates shouldbe reduced. The corrections are applied to the presentspectral emission rates in Sec. VI but are small com-pared with instrument errors.

IV. Sky Brightness During Totality

Zenith sky brightness at 3914, 4278, 5200, 5577, and6300 A was deduced from measurements with the ap-propriate background filters listed in Table I. Thevalues so obtained at mid-totality are plotted in Fig. 1.The spectral distribution obtained by Shaw3 is drawnon the same figure. In Shaw's paper the results wereexpressed in absolute units of solar flux, and these havebeen converted into units of apparent emission rates.The agreement between the two sets is very good, thediscrepancy being -12% at 4000 A, 0% at 5000 A, and25% at 6000 A. The data in Fig. 1 are expressed as aregression line fitted to the observed values. The cor-relation coefficient (r = 0.90) is less than the 1% criticalvalue (0.96) indicating random scatter from instru-mental and observational causes. Figure 2 shows thevariation of sky brightness throughout totality as theobserved wavelengths. The curves show smoothedvalues of intensity normalized to mid-totality valuestaken from the regression line in Fig. 1.

Plots of the raw data from which Fig. 2 was derivedshow that, for photometers (1), (2), and (3), the averageminimum response occurs, respectively, 12, 14, and 11sec after mid-totality. Looked at another way the rel-ative intensities are, respectively, 20%, 18%, and 17%higher 1.5 min before mid-totality than they are 1.5 minafter mid-totality. The displacement in time of theminima of the twelve curves is well outside the esti-mated accuracy of +2 sec during totality, and the ob-served displacement must be regarded as a real effect.It has been observed by Deehr and Rees,8 Sharp et al., 9

and from the present event by Shaw.3 These authorsdescribe it as a skewing or asymmetry. They suggestit results from noncircular lunar shape, topographicirregularities in the moon's surface, and systematicvariation in atmospheric scattering as the umbra movesacross the photometer's field of view.

Dandekar10 summarized the sky brightness duringabout twenty solar eclipses between 1918 and 1963.Table III shows the low latitude events excluding two


very high semiqualitative values but including twoevents since 1963 together with additional site and skydetails from the original references. The data of TableIII agree within about an order of magnitude. Ourvalue and that of Shaw are lower than the others.Dandekar notes that, since there are differences in solarelevation and observer's altitude at mid-totality, the skybrightness values of Table III will be affected in acomplex way by differences in air mass and henceRayleigh scattering. The sky condition, ground albedo,and method of observation add to the complexity.Shaw establishes that atmospheric scattering has adominant influence in relation to sky brightness; thescattering itself is markedly affected by the presence ofaerosols. If the concentration of aerosols in the sunlitsky were to decrease, the intensity ratio Iblue/Ired wouldincrease, and the zenith intensities would shift towardthe blue-the zenith sky would appear bluer. Perhapsthe low mid-totality values at Lake Rudolf can be ac-counted for by a relatively high concentration of aero-sols in the umbra region progressively falling off athigher altitudes. This is consistent with the observedincrease in I4000/o7000 by a factor of 2 as the umbrapasses over the cone of observation in the zenith.

V. Comparison of Eclipse Sky Brightness andTwilight Intensity

The eclipse continuum was compared with morningtwilight at Dar es Salaam (latitude 6.90S, longitude39.20 E) for 20, 21 Aug. and 22,23 Aug.1974. Figure 3shows plots of the latter twilight data using the samefilters as for the eclipse but with the photomultipliertube cooled to -200C. Correlation coefficients betweensky brightness and wavelength were used first to test theself-consistency of the twilight data and then to find thetime that the zenith sky brightness during the twilightwas equivalent to that of the eclipse at mid-totality.The twilight values appear near the upper part of Fig.3 between 0300 and 0304 UT. Values at eight equallyspaced times or solar depression angles (/) were readfrom each of the five curves along lines parallel to they axis. Scatter diagrams similar to those in Fig. 1 andcorresponding correlation coefficients (r) relative to the1% critical values (t) were produced for each set ofpoints. The resulting plots of rt against :3 appearingin Fig. 4(a) show that rt > 1 throughout the chosenrange. Hence for all values of : in this range plots oftwilight sky brightness against wavelength are not sig-nificantly affected by random scatter due to instru-mental or observational causes. Compare this with thecorresponding curve [Fig. 4(b)] for the eclipse totalityplotted on the same graph for convenience. Here rtis always <1 and has a minimum at mid-totality. Thisshows that, as mid-totality is approached, plots of skybrightness against wavelength are increasingly affectedby random scatter due to instrument noise. Zenith skybrightness during the twilight and eclipse mid-totalitymay be regarded as equivalent when the respectiveemission rates vary in the same manner with wave-length. Figure 5(c) is the composite plot of both setsof data nearest the twilight time (0303 UT) when cor-

2 0 ( (b)

'.5.

KR/A'.0.

0*5'

0

2-0 (e) (d)

t'5 , _< oTiI~ghl - TII,h,~b

KR/A

1.0

0.5

03600 4400 5200 6000 360 4400 5200 6000

WAVELENGTH (A)

Fig. 5. Corrections for atmospheric scattering and Fraunhofer ab-sorption: (a) uncorrected twilight at Dar es Salaam when the solardepression angle is 6°32' (0303 UT); (b) the data of (a) after correction;(c) composite plots of uncorrected twilight and eclipse; (d) the data

of (c) after correction.

Table IV. Absorption and Scattering Corrections at Mid-Totality

r r/t

Eclipse mid- Uncorrected 0.90 0.94totality

Corrected 0.86 0.90

Twilight Uncorrected 0.97 1.01(0303 UT)22, 23 Aug. 1974 Corrected 0.99 1.03

Eclipse mid- Uncorrected 0.93 1.21totalitywith twilight Corrected 0.93 1.21

relation between the combined points is greatest. Tofind this time, values of rt from plots similar to thosein Fig. 5(c) were found for the eight times shown in Fig.4(c). The maximum of this curve represents maximumcorrelation between the combined points and shows thatthe equivalent sky brightness for 22,23 Aug. 1974 occurswhen /3 6.5°. A similar analysis for 20, 21 Aug. 1974produces /3 6.9°. In terms of time this is -1.5 minearlier while sunrise was almost 0.5 min later than on22, 23 Aug. There is thus a considerable uncertaintyin the time of equivalence from one twilight to an-other.

Attempts to correct the eclipse mid-totality data ofFig. 1 for Fraunhofer absorption and atmosphericscattering failed. The overall observational error wastoo large for useful correction to be made and indeed thecorrected values show more random scatter than theuncorrected values of Fig. 1. The first two rows ofTable IV show the correlation coefficients before andafter correction. Twilight values fared better.

Figures 5(a) and (b) show plots of the twilight dataequivalent to the eclipse mid-totality data before andafter the corrections have been applied. Figure 5(c)contains the data of Fig. 5(a) combined with the eclipse


10

a

00 6

4

2

SOLAR DEPRESSION ANGLE'12 11 10 9 8

0240 0248U T

0256

(B')7

02

0304

Fig. 6. Variation of the UV/red sky brightness ratio in the twilightas the solar depression angle decreases for (a) Dar es Salaam, 22, 23Aug. 1974 (KP = 5+); (b) Dar es Salaam, 20, 21 Aug. 1974 (K, = 6-);

(c) solar eclipse (Kp = 5-).

Table V. Oxygen Emission Rates at Mid-Totality

Cali- Photometer 1 Photometer 2bration 6300 (1) 5577 (1) 6300 (2) 5577 (2)source (kR) (kR) (kR) (kR)

C14 2.4 1.0 1.6 1.4Sr 90 0.7 - 0.6 1.0 0.8

Average 6300 A Average 5577 A1.4 + 0.6 0.9 + 0.3

mid-totality data; Fig. 5(d) represents the latter pointsafter correction for absorption and scattering. Thecorrelation coefficients for all four graphs of Fig. 4 aregiven in Table IV. The net effect of applying the cor-rections is to reduce slightly the spread of points for thetwilight data, and, as expected, there is no improvementin the combined data.

Quite apart from any small discrepancies that mightarise in sky background elimination with the two-filtermethod it is interesting to compare scattering undertwilight and eclipse conditions. If atmospheric scat-tering were to increase, the shorter wavelength com-ponents in the zenith would become more dominant,and the ratio of continuum intensities I3914/I6300 wouldbe expected to increase. Figure 6 compares the varia-tion of I3914/I6300 during morning twilight and eclipsetotality. In the figure the intensity axis is common toall curves, but the eclipse time axis has been expandedfor convenience. At the solar depression angle when thetwilight conditions are closest to the conditions duringeclipse totality ( = 6.7° average), both twilight curveshave blue-to-red ratios considerably greater than theeclipse ratio. In Sec. III the ratio of blue-to-red in-tensity at mid-totality was seen to be about one-thirdthat to be expected from multiple scattering from airmolecules alone in the normal day atmosphere assumedfree from aerosols. The two twilight curves in Fig. 6

show that conditions approaching a Rayleigh atmo-sphere can be achieved in varying degrees during thetropical morning twilight. But the considerable dif-ferences in blue shift evident in the morning twilight ontwo days so close to each other show how difficult it isto make generalizations when comparing events asdissimilar as twilight and solar eclipses. On the otherhand the zenith twilight sky would be expected to bebluer than the eclipse sky in view of the appearance ofthe tropical sky prior to sunrise and after sunset. Lowclouds take on a vivid pink color-at any position in thesky, while the cloudless sky is distinctly blue overheadbecoming white toward the horizon. The pink color inthe clouds comes from illumination by near horizontalrays from which blue light has been singly scattered-once illuminated by the blue depleted light the cloudsappear red from any direction even if observed in thezenith. As the solar depression angle decreases towardthe sunrise, multiple scattering increases, and the bluelight will be scattered back into the horizontal rayscausing the cloud color to shift toward the blue. Theresults and conclusions of this section differ from thoseof another observer. Dandekar10 finds an effectiveblackbody temperature during mid-totality which isgreater than during twilight. This corresponds to abluer zenith sky during totality than during twilight.There is also an unresolved problem concerning therelative brightness of eclipse skies when compared withtwilight. In the present work the solar depression anglegiving twilight brightness values closest to the eclipsevalue at mid-totality is between 6.5° and 6.60 on 22, 23Aug. 1974 and -6.9° on 20,21 Aug. 1974. We comparethese values with the Deehr and Rees8 value of between7.50 and 7.80 and observe that the two eclipse eventsrelate to twilight intensities some 4 or 5 min apart, withour event corresponding to the brighter twilight sky.The magnitude of the apparent shift is significant andindicates an eclipse sky some 3 or 4 times brighter in ourcase. This question has not been resolved here, but wenote that the present twilight and eclipse events wereall observed under magnetically disturbed condi-tions.

VI. Zenith Atomic and Molecular Emission Rates atMid-Totality

Values for zenith optical emissions at 6300 and 5577A obtained with the filter arrangements shown in TableI are summarized in Table V. The two 6300-A filtersin photometer 1 yielded the same values so that only oneof these values has been included. The results in TableV are somewhat tentative because the errors given onlyindicate the spread in the various observed values.Estimation of error due to measuring a very small signalagainst a very large noisy background is extremely dif-ficult and has not been attempted here. The fully il-luminated atmosphere produces emission rates for 6300A between 2 and 20 kR depending on magnetic activi-ty.8"151 6 The 2-kR value corresponds to magneticallyquiet conditions, while the June 1973 event occurredduring a period of magnetic activity (Kp = 5-). Solareclipses at mid- and high latitudes have been used to


I I I I I I I I I

(a)

(b) 91256 1300 13

(C) -

Morning TwilightI I I I I

00

ELECTRONS S

DISSOCIATIVE - - tRECOMBINATION TOA

ELECSR 3s SCAMANN,RUNGE

10011 10 100 Soo

VOLUME EMISSION RATE (ph ci' d')

Fig. 7. Dayglow volume emission rates deduced from compositionsof neutral and ionic species measured from the Atmosphere Explo-

rer-C satellite on 24 Jan. 1974 (from Ref. 5).

estimate the contribution to the normal dayglow of UVphotodissociation of 02 in the Schumann-Runge con-tinuum. The maximum contribution from this mech-anism occurs near 150 km,'7 but within the field of viewof our photometers only -6% of the sun's disk would bevisible at that altitude. Noxon and Markham' 5 de-duced that the dissociation mechanism will operateduring totality at the same proportion of its normaldaytime efficiency as the proportion of visible solar disk.Thus under disturbed conditions we would not expectto observe more than 0.5-1 kR if photodissociation of02 were the principal mechanism. But the contributionof other processes such as dissociative recombinationof molecular oxygen and excitation of ground stateO(3P) atomic oxygen with energetic thermal electronsand photoelectrons must also be considered. While itis true that the cross section for dissociation in theSchumann-Runge continuum is well known, the liter-ature contains many uncertainties concerning the crosssections and rates for production and loss of the meta-stable O(1D) oxygen atom by other mechanisms. Thisis because laboratory observations of long lifetimemetastable species do not relate well to conditions in theatmosphere. A major advance toward the solution ofthe problem has come from simultaneous measurementof O('D) atoms and atmospheric composition from theAtmosphere Explorer-C satellite (AE-C). Directmeasurements of concentrations of 0, N2, and 02 weremade by Nier et al. 18 from the AE-C mass spectrometerdata. The recombination rate of °2 was calculatedfrom AE-C measurements of O+, 02, and °2 concen-trations, electron densities, and electron temperatureby Torr et al.19 Then Hays et al.5 made a detailedstudy of the (3P - D) 6300-A transition from the AE-Cresults. They calculated new rate coefficients for dis-sociative recombination, O(1D) quenching by molecularnitrogen, photodissociation, and photoelectron impact.Calculations of the respective volume emission rates of01 (6300-A) airglow resulting from the new rate coef-

ficients were made. Of particular interest here are theresults from a single daytime AE-C orbit on 24 Jan.1974. Figure 7 shows the results for that orbit takenfrom Hays et al. 5 The curve labeled total correspondsto the direct measurement of the volume emission ratewith the spacecraft airglow photometer.2 0 By inte-grating the various curves in Fig. 7 estimates have beenproduced of the relative contributions from the re-spective processes to the zenith airglow intensity anobserver on the ground might expect to find in the ab-sence of atmospheric extinction. The values are shownin Table VI. These results were obtained on a mag-netically very quiet day (YKp = 7+) and so agree verywell with observed quiet dayglow values reported byearlier workers. Hays et al.5 states that the dissociativerecombination process is of little significance in thedaytime, and their results show that only -12% of thequiet dayglow from a fully illuminated sky comes fromthis process. Much less can be expected at mid-totality.Of the other processes under conditions prevailing atmid-totality, photoelectrons and hot thermal electronsare active in producing O(1D) atoms in the lower ther-mosphere at the top of the ionosphere. At these alti-tudes illumination of the atmosphere during mid-to-tality is relatively high, and we conclude the mid-totality01 (6300-A) emission comes mainly from direct excita-tion of O(3 P) atoms by thermal electrons and photo-electrons-especially the latter.

Table VII summarizes the zenith dayglow opticalemissions resulting from atomic and molecular pro-cesses observed by us during eclipse mid-totality to-gether with comparison with other observers. Thesame background noise problem applies to the molec-ular emissions as mentioned above regarding atomicemission rates. Discussion of numerical values is con-fined to establishing limits rather than precise values.The two-filter method fails when an extremely weak lineemission has to be detected against a very strongbackground. This is the case with our attempt to detect


Table VI. Integrated O('D) Emission Rates for Atomic and MolecularProcesses

Dayglowintensity

Mechanism (kR)

Schumann-Runge 0.45Dissociative recombination 0.21Photoelectrons 1.05Thermal electrons 0.08

Total 1.79

Table VII. Emission Rates Observed During Recent Solar Eclipses

Zenith emission rate (kR)Eclipse 6300 5577 5200 4278 3914 Ref.

Oct. 1950 6 - - - - Tanabe andTomhatsu"

20 July 1963 4 - - - - Noxon andMarkham'5

20 July 1963 0.6 0.8 - - - Deehr and Rees8

20 July 1963 1 1 - - - Zipf and Fastie 2 2

12 Nov. 1966 1 1 - - - Dandekar' 0

30 June 1973 1.4 0.9 0.03 4 3 Present work

the nitrogen emission at 5200 A. At this wavelength thebackground emission rate is -3.8 kR, while only -0.1kR of NI 5200 A is to be expected from an altitude of-200 km.16 We have not been able to find publishedreports of the emission rate of the nitrogen line duringa solar eclipse, and we can only conclude that thestrongly attenuated emission rate to be expected duringtotality would be a few Rayleighs-too low to be reliablydetected with our instruments.

Excitation of N' to the B2 2+ state by resonancescattering and fluorescence should yield -2 kR of 3914from an altitude of 150 km in the fully illuminated at-mosphere. The data of Table VII for 3914 and 4278 Aindicate observational error since the observed ratio13914/I4278 is too low by a factor of 4. Correction to thebackground for Fraunhofer absorption and atmosphericscattering from Table II increases the ratio by <20% andso becomes lost in the overall error. The tabulatedvalues therefore indicate the order of magnitude of theN' emission. But the emission rate at 3914 A is 25times the value of -120 R to be expected from the 6%illuminated 150-km region and cannot be accounted forfrom instrument error alone. The high value suggestsmultiple scattering into the umbra of N' emission re-sulting from resonance scattering and fluorescence inthe penumbral region.

We would like to record our appreciation to G. Rolandof the Institut d'Astrophysique, Universite de Liege forproviding prepublication data from the photosphericspectrum; to Hans Lauche of the Max-Plank Institfltfur Aeronomie for calibrating our 90Sr source; and to R.N. Srivastava for helpful discussions.

References1. E. H. Carman, M. P. Heeran, and V. Wiltshire, Planet. Space Sci.

17, 1073 (1969).2. R. R. La Count, NSF Solar Eclipse 1973 Bulletin 1-5 (1972-

1974).3. G. E. Shaw, Appl. Opt. 14, 388 (1975).4. G. E. Shaw, C. S. Deehr, and L. R. Sweet, NSF Solar Eclipse 1973

Bulletin Final Report 5, 94 (1974).5. P. B. Hays, D. W. Rusch, R. G. Roble, and J. C. G. Walker, Rev.

Geophys. Space Phys. 16, 225 (1978).6. L. Delbouille, G. Roland, and L. Neven, Photometric Atlas of the

Solar Spectrum from X3000 to X10000 (Institut d'Astrophysiquede l'Universite de Liege, 1973).

7. L. Delbouille, G. Roland, and L. Neven, Universit6 de Liege;private communication.

8. C. S. Deehr and M. H. Rees, Planet. Space Sci. 12, 875 (1964).9. W. E. Sharp, S. M. Silverman, and J. W. F. Lloyd, Appl. Opt. 10,

1207 (1971).10. B. S. Dandekar, Appl. Opt. 7, 705 (1968).11. H. T. Stetson, W. W. Coblentz, W. Arnold, and W. A. Spurr, As-

trophys. J. 66, 65 (1927).12. H. T. Stetson, W. Arnold, and J. Johnson, Astrophys. J. 74, 122

(1931).13. E. 0. Hulbert, Natl. Geophys. Soc. Contrib. Tech. Paper Solar

Eclipse Ser. 2, 78 (1942).14. R. A. Richardson and E. 0. Hulbert, J. Geophys. Res. 54, 229

(1949).15. J. F. Noxon and T. P. Markham, J. Geophys. Res. 68, 6059

(1963).16. D. M. Hunten, The Radiating Atmosphere (Reidel, Dordrecht,

Holland, 1971), p. 3.17. J. Brandt, Astrophys. J. 128, 718 (1958).18. A. 0. Nier, W. E. Potter, and D. C. Kayser, J. Geophys. Res. 81,

17 (1976).19. D. G. Torr, M. R. Torr, J. C. G. Walker, A. 0. Nier, L. H. Brace,

and H. C. Brinton, J. Geophys. Res. 81, 5578 (1976).20. P. B. Hays, G. Carignan, B. C. Kennedy, G. G. Shepherd, and J.

C. G. Walker, Radio Sci. 8, 369 (1973).21. H. Tanabe and T. Tomhatsu, Rep. Ionos. Space Res. Jpn. 13,290

(1950).22. E. C. Zipf and W. G. Fastie, J. Geophys. Res. 68, 6208 (1963).


zenith sky brightness and airglow emissions during the equatorial solar eclipse of 30 june 1973

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