Global atomic oxygen density derived from OGO-6 1304 Å airglow measurements

P&net Space Sci., Vol. 24. pp. 313 to 326.

Pergamon Press. 1976.

Printed in Northern Ireland

GLOBAL ATOMIC OXYGEN DENSITY DERIVED OGO-6 1304A AIRGLOW MEASUREMENTS Laboratory

for Atmospheric

FROM

D. J. STEICELAND* and Space Physics, University of Colorado, Boulder, CO 80302, U.S.A. and G. E. THOMAS

Department

of Astro-Geophysics and Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80302, U.S.A. (Received 27 October 1975)

Abstract-The OGO-6 UV photometer experiment measured the atomic oxygen 011304 A triplet in the Barth’s dayglow between 400 and 1100 km. We have analyzed the data for the period 15 September-25 October 1969 by obtaining best-fit models in which the 1304 A emission is excited by solar resonance scattering and photoelectron excitation. Provided the excitation processes are specified, we find a unique relationship between the vertical column density of atomic oxygen and the zenith 1304 A intensity. This is essentially independent of the atmospheric temperature. Because of the large numerical uncertainties, the excitation sources are determined from the 1304A data and quiet-time in situ measurements of atomic oxygen density. They are found to be in good agreement with recent solar measurements of the 1304 A lines and with calculations of the photoelectron excitation source. The deduced variations of atomic oxygen column densities over the daytime atmosphere are found to agree well with the Jacchia 1971 models. During the geomagnetic storm, the column density generally increased above a fixed altitude. However, the latitudinal dependence is complex. Following the strong geomagnetic activity between 15 September and 1 October, depletions in atomic oxygen are observed. At times, there is evidence of high-altitude transport of atomic oxygen from high latitude to low latitude.

1. INTRODUCPION

The ultraviolet photometer experiment on the OGO-6 spacecraft measured the hydrogen Lyman-a (1216 & and the atomic oxygen zenith emissions at 1302.2& 1304.9 A and 1306.0 A (hereafter referred to as 1304 A) in the Earth’s ultraviolet airglow. The results from the Lyman-b: channel are reported in a companion paper (Thomas and Anderson, 1975). In this paper, we report an analysis of 1304 A data measured during the period 15 September-25 October 1969, which includes both quiet and disturbed conditions. The quantity deduced from the data is the column density of atomic oxygen above the satellite altitude. Several measurements of the 1304 A emission in the dayglow have been made from rockets (Chubb et al., 1958; Barth, 1963; Fastie et al., 1964; Fastie and Crosswhite, 1964; Kaplan et al., 1965; Fastie, 1968; Heath, 1968; Pearce, 1971; Rottman, et al., 1973; Takacs, 1974) and satellites (Thomas,

1971; Sheffer, 1971). Aurora1 1304 A emission has also been observed (see Strickland and Rees, 1974, for references). The dominant source of the 1304 A dayglow is photoelectron excitation of atomic oxygen (0). The following theoretical analyses of the dayglow rocket data have aided in a quantitative estimate of this source: Donahue and Fastie, 1964; Tohmatsu, 1964; Donahue, 1965; Strickland and Donahue, 1970. The other source of importance is resonance scattering by 0 of the solar 1304 A flux. In the present paper, we describe briefly the experiment and the orbital characteristics during the time period under study. Examples of the 1304 A intensity data are then shown over a single orbit and at 8xed altitudes throughout the 40 day period. The theoretical approach by which we obtain the column density of atomic oxygen is then described followed by a description of the excitation processes and the least-squares fitting procedure. The results

1970; Barth and Schaffner, 1970; Meier and Prim,

are presented for oxygen column densities during both quiet and geomagnetically disturbed times. We then compare our results for the excitation sources

* Present address: VA, U.S.A.

with theoretical investigators.

Science Applications,

Inc., McLean, 313

calculations

and with those of other

314

D. J. STRICKLAND and G. E. THOMAS 2. INSTRUMENTAL AND ORBITAL CHARACTERISTICS

LOCAL TIME

Thomas and Krassa (1971) have given a basic description of the OGO-5 photometer which is identical to the OGO-6 instrument. Briefly, the 01(sP-3S) 1304 A emission is measured in the local zenith by the long wavelength (B-channel) detector, responsive to U.V. emission in the spectral range 1225-1800 A. In this spectral range only oxygen emission is present in the airglow at satellite altitudes. 01(3P-5S) emission at 1356 A has an intensity less than 0.02 kR at 400 km. (1 kR equals log photons cm2/sec/4n sr.) “Spikes” due to continuum emission from early-type stars can be easily removed from the smoothly-varying airglow signal. This is a consequence of the small (2.4” full width at half maximum) field of view. A large dynamic range in sensitivity was made possible by an electronic feed-back circuit which controlled the photomultiplier voltage. This provided nearly uniform accuracy at all signal levels from the maximum airglow signal (--4 kR) to LONGITUDE RELATIVE TO SUBSOLAR POINT the minimum threshold sensitivity of -20 R. FIG. 1. SATELLITE POSITIONS BETWEEN ORBITS 1500 AND The satellite was launched into a near-polar orbit 2000 RELATIVE TO LATITUDE AND LONGITUDE OP THE of inclination 82’. Perigee and apogee were at SUBSOLAR POINT. altitudes of 400 and 1100 km, respectively. The The motion of the satellite is south for that portion of motion of the line of apsides caused the orbit to each orbit shown. The dashed curves are isotherms from J71 and are included to indicate how the satellite crosses precess in latitude at ~2.7’ per day. The orbital isotherms during geomagnetically quiet times. plane precessed in inertial space at ~1’ per day, or relative to the Sun at ~2’ per day. An illustration of these orbital perturbations is provided in Fig. 1, observed. However, the agreement was improved by where the subsatellite point is plotted in a coordinate assuming that the OGO-6 B-channel degraded in system relative to the sub-solar point. Since the sensitivity with time in parallel with that found in orbits under study occurred near autumnal equinox, the A-channel (Thomas and Anderson, 1976). This was found to be roughly linear at about 5% loss in the vertical scale is nearly equivalent to geographical latitude. Perigee moved from below the equator at sensitivity per month. When we attempted to relate orbit 1500 to high northern latitudes by orbit 2000. the peak signal for 17UM for both OGO-5 and OGOand their relative sensitivity to At the same time, the orbital precession carried the 6 photometers, plane of the orbit from near-twilight toward a Lyman alpha airglow, we found inconsistencies that could only be explained by the fact that our noon-midnight orientation. The time scale indicates that the pertinent data were recorded during the knowledge of the spectral response (particularly at long wavelengths) of the two photometers is not afternoon hours. The dashed lines are isotherms sufficiently well-known. It appears that the adopted from Jacchia (1971; hereafter J71) and are included to illustrate possible temperature gradients over the spectral response curve (given by Thomas and Krassa, 1971) over-estimated the OGO-5 signal and orbits during geomagnetically quiet conditions. under-estimated the OGO-6 signal in both channels Since the instrument views in a radially outward with respect to the stellar continuum sources. Even direction, it is possible, in principle, to perform though they both responded equally to 1216A inflight calibrations using selected star observations. and probably 1304 A airglow signals in their This was attempted using the peak signals recorded for the star 7 Ursa Majoris (q UM) during the respective channels, their response to stars was unequal. This uncertainty prevents our being able period around 1 June 1970. Using the same method described by Keller and Thomas (1975) for the to use stars as accurate calibration sources and implies that to within a factor of ~2, the quantities OGO-5 photometer, we found that the theoretical quoted in this paper that rely upon the absolute peak signals predicted from the absolute spectral calibration are similarly uncertain. data of Bless et al. (1974) were nearly twice that

Global atomic oxygen density 3. DATA The data were originally recorded every 0.144 set and were later averaged in one-second intervals by the initial data processing. For the data reported here, we have removed the effects of stars, instrumental ‘drop-cuts,” and radiation belt interference, and further averaged in one-minute intervals. A sample of one orbit of smoothed 1304 A data is shown in Fig. 2. This data sample is typical of the quiet period preceding the late September geomagnetic storm. The smoothness of the profile indicates a smaff statistical un~rtainty (~10 R). The “‘spikes” occurring at 05: 57 and 06 :OOGMT were caused by stars. Altitude and solar zenith angle (SZA) are shown on a linear scale below the data plot. The two theoretical profiles and their sum are discussed later. Figures 3 and 4 show plots at fixed altitudes between orbits 1450 and 2050, spanning 40 days from 15 September to 25 October 1969. In each figure, intensities are plotted for five ahitudes410, 450, 500, 550 and 600 km. The altitudes in Figs. 3 L d 0006

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AND SZA. Stellar U.V. fluxes are responsible for the observed enhancements between 5 fir and 57 min and S hr and01 min. The theoretica intensities were calculated with an 1100 IC 371 model atmoqbere.

AND

THE

GEOMETRICAL

PARAMETERS

ALTITUDE

315

and 4 occurred before and after perigee respectively. In each data panel, SZA and geomagnetic latitude of the subspacecraft point are included. The 24&r os~~lfat~ons in the Iatter quantity are caused by the offset of the geomagnetic pole from the geographic pole. In each figure are shown the 3-hr geoma~etic index I$, and the daily solar 10.7 cm radio flux F,., The airglow is seen to correhzte with both of the intensity at 410 km indices An e~~~rnent during the geomagnetic activity on day 273 is particularly apparent in Fig. 4. Enhancements are also evident at higher altitudes after orbit 1900 at which time F 10.7was strongly perusing. Refore we describe the corresponding variations in 0, it is necessary to first consider the excitation processes and the theory which accounts for the multiple scattering of the 1304 A photons. 4. DEI%XMINATION OF OXYGEN COLUMN DENSITY FROM THE 1304A ZENITH INTENSITY We assume that the only sources of t304A are solar resonance scattering (SRS) of the 1304 A lines and photoelectron excitation (PE) of 0. The volume production rate for SRS, Sgfs) (photons/ems/ set) is

The symbols are detined below: P = cosine of SZA = resonance cross section at iine centre 3 nFol = soiar flux in jth line above the atmosphere (photons~~m2~s~) AI0 = terrestria1 Doppler width at a specified temperature AA, = effective width of thejth solar line T(Tjjp) = transmission function for the slant optical depth T~I,u between the Sun and altitude z at the given SZA Pj =i: Boltzmarm term giving relative pop~ation Of the “Pj level. The Cfimction includes the effects of pure absorption by 0% (see Strickland and Rees, 1974). The index values j = 2, 1 and 0 are for the three lines 3P,-3S,(1302 A), 3P$‘r(1305 A), end 3P0-3&(1306 A). At 1100 K, rs,, = 8.4 x lo-l3 cm% based on the measured oscillator strength f = 0.046 by Lawrence (1970). The effective widths AA, were taken to be Ail, = 0.26 A, AS = 0.22 A, and Ait, = 0.16A from high resolution measurements by Pruner et al. (1970).

316

Global atomic oxygen density

311

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is assigned by fitting theoretical intensities to OGO-6 data. Examples of So@) and So(“) are shown in Fig. 5. Values of ?fFoj are arbitrarily chosen to be four times the measured values by Hall and Hinteregger (1970, hereafter HH). For the solar 1304 8, fluxes arriving at a given altitude, resonance absorption

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within the scattering volume has been treated us&g a Doppler frequency line profile. The model atmospheres in this section are based on temperatures and densities from 571. Table 1 gives the 1100 K model. The vertical optical depths ts, TVand r. are line-centre values at 1302, 1305 and 1306 A, respectively.

The vertica1 optical depth t refers to

318

D. J. STRICKLAND and G. E. THOMAS

DOPPLER SOLUTIONS

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(KR

FIG. 6. 1304A

INTENSITIESBASED UPON THE RATES IN FIG. 5. Complete frequency redistributionis assumed with a pure Doppler frequency profile.

The rates were calculated with an 1100 K 571 model atmosphere. The values of the solar fluxes are (arbitrarily) taken to be four times those measured by Hall follow the work of Hummer (1969) who showed and Hinteregger (1970) and g&V, = 0) is 3.5 x 1O-8. that CFR solutions based on a Doppler profile are Each profile is specified by an SZA. in better agreement with more exact solutions than those based on Voigt profiles. This is discussed pure absorption by 0, and is based on a cross further by Strickland et al. (1972) and Strickland section of 5 X lo-19 cm2 for all three lines. and Rees (1974). As will be seen later, the choice is For fixed values of rFsi and the solar EUV flux, the zenith intensity as recorded by the OGO-6 UV not critical if one is considering the variation of the photometer can be expressed in terms of (1) the 0 density; however, it is important in determining the absolute magnitude of the excitation sources. 0 column density N(O) above the observation Before presenting the intensity-cohmm density height z and (2) the SZA. Because of multiple relationship, we ilhrstrate the various properties of scattering of the 13048L photons, the intensity the emission in the optically thick medium in the versus c~hnn density relationship is not a simpIe proportionality, as would be the case for an next three figures. Figure 6 shows zenith intensities based on a Doppler prose and the volume producoptic~ly-Tim emission. The theoretical relations~p tion rates of Fig. 5. Figure 7 shows a similar set of is established by solving the equation of radiative intensities for a Voigt profZe with the damping transfer beginning with volume production rates parameter a = 6.0 x 10-s. These results indicate shown in Fig. 5. The technique has been described the pronounced differences between the SRS and by Strickland and Rees (1974). An important PE components of the intensity and show how each assumption made is that of complete frequency component varies with the SZA. For either comporedistribution (CFR) within the line. To minimize nent a comparison between Figs. 6 and 7 also shows the errors that arise from this approximation, we TABLET. MODELATMOSPHERE. DENSITIES AND TEMPERATURESARE FROM JACCHIA(~~~~),Q,T~ANDT,,AREOPTICAL DEPTHS IN 0 FOR 1302-5-6A, AND t IS THE OPTICAL DEPTH FOR PURE ABSORPTION BY oa

100 110

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FIG. 7. INTENSITIESSIMILAR TO THOSE IN FIG. 5 WITH A VOIGT PROFILE (a = 6 x lo-*) REPLACING THEDOPPLER PROFTLE (U = 0).

319

Global atomic oxygen density COLUMN DENSITY 10’3

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Fro. 8. INTENSITIES FORJ71 MODELS FROM700 TO 1300 K IN EXOSPHERIC TEMPERATURE AT THE FIXED SZA OF 60”. The column densities of atomic oxygen from the models are shown on the left side of the figure.

FIG. 10. COMPARISON OF INITIALAND TOTALPE

there to be significant differences arising from the The use of the different frequency profiles. intensities based on a Doppler profile are larger than those for a Voigt profile since, with a Doppler profile, photons escape the medium less readily and thus contribute to a greater buildup of imprisoned radiation. Figure 8 shows Doppler intensities at SZA = 60’ for models from 571 with exospheric temperatures ranging from 700 to 1300 K. The 0 column densities for the various models are given on the left side of the figure to illustrate the similarity in behaviour with temperature exhibited by the two intensity components. It is apparent from this figure that the intensity for each component as a

function of N(0) is almost independent of the particular model. Figure 9 shows the adopted relationship between the intensity components and N(0) for various values of the SZA from 0” to 90”. Not shown is a slight effect on the SRS component due to exospheric temperature which arises from the temperature dependence of AL,,. The PE component shows no variation due to temperature. It will be affected, however, by changes in the 0 to 0, and 0 to N, concentrations since all species compete for the energy of the photoelectrons. Figure 10 shows how the total source function S(T~) for the PE component changes by reducing the 571 model O-density a factor of three. At optical depths less than 1000 (above -300 km), only 20% change occurs in S. This difference, in turn, is exhibited in the zenith intensity. Thus, at satellite altitudes, the zenith PE component of the intensity as a function of N(0) varies weakly with relative composition changes and consequently such changes that may take place do not affect the interpretation of the data.

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The intensities are labelled by SZA. 2

10’6

(CW2) AGAINST

We have shown that the 0 column denisty can be determined uniquely from the measured 1304 A intensity, provided the excitation processes can be quantitatively described. Unfortunately, there are large uncertainties in the parameters describing these processes. Furthermore, there is the question of whether the absolute calibration of the photometer is consistent with values of the solar 1304A flux and the actual values of photoelectron flux measured at different periods of the solar cycle, etc.

320

D. J. &WXLAND

To overcome these di&ulties, the values of the excitation parameters have been determined directly from the data. Such an approach was used earlier by Strickland ef al. (1972, iG73) to analyze Mariners 6, 7 and 9 1304 A data from Mars. Theoretical intensities have been fitted to data for individual orbits in a least squares fashion by minimizing the quantity

wk is the kth weight, dk is the kth data value, and c, and cg are least-squares coefficients. The intensities 4&k’ and 4%&’ are theoretical intensities for SRS and PE, respectively. They are evaluated at the kth point in the orbit at known values of altitude and SZA. They apply to a particular 571 0 model defined by the exospheric temperature T,. The values of 4&’ are based on the solar 13O4 A fluxes of HH. The values of 4~1~B are based on an arbitrary value of g,(Np = 0) of 1.0 x lO-s/sec. Therefore, c, x HH fluxes are the solar fluxes derived from the OGO-6 data and c, x IF8 is the corresponding value of gp(NT = 0). Weights wk of both unity and the inverse of dk were used. The derived values of (c,, c,& were not sensitive to the choice. Returning to Fig. 2, an example of a least-squares fit is shown. A model atmosphere with 1100 K was chosen for this case, for which the values of (cs, c~) were (4.2,3.6). We found that the effects of the expected temperature gradient along the orbit were small. To establish best values of c, and c,,, the data between orbits 1570 and 1590 were used. During this period (day 265.27-266.6s) the Kp index was less than 1. The average F& flux was 150. We obtained the best-fit values of (c,, c%) for three different models where T, = 1lOO” and the oxygen density was arbitrarily reduced by a factor of three (Fig. 11). The coefficient c, is quite sensitive to the model, whereas cp is rather insensitive. Excellent fits were obtained for all modeIs chosen, thus ruling out the possibility of determining both the ccoefficients and the best model atmosphere. The choice of model was made by using the temperature data of Blamont and Luton (1972) and the measurements of 0 density from the OGO-6 mass spectrometer (Hedin, private communication, 1972). Blamont and Luton reported temperatures of -llOO” on day 267 at 275 km. The altitude correction to obtain T, is about 50K. Thus, Tal - 1150 K. 0 densities obtained from OGO-6 were found to be generally consistent with 571

and G. E. THOMAS

‘O”

CP Fm, 11. LEASTSQUARES COEFFICIENTS c, AND c,, OBTAINED BY !XSTINQ ~~~~AL 1304 A IOKIDATA BETWEEN ORBITS 1570 AND 1%). The dependence on model atmospheres is illustrated using 571 models.

values for 1150 K. Selecting this model allows us to fix the values of (c,, c~) at (4.0, ~3.0) as illustrated in Fig. 11. We assume for the remainder of the data that neither of these coefficients changed, and that the best-fit model is found by adjusting the exospheric temperature (actually column density) at each point in the orbit. This procedure is valid to the extent to which the solar 13O4 A flux and the solar EUV flux remained constant. This was probably not the case for the entire period under study, since &., varied from 120 to 200. For this reason, we emphasize relative changes in the 0 density in this work, particularly during the late-September storm period. Deduced changes over the time scale of a geomagnetic storm are probabIy accurate, whereas deduced changes for a 28&y period are probably overestimates. This is because of the parallel changes that are probably occurring in the two excitation sources during a solar rotation period. 6. RESULTS The deduced variation in N(0) at 410 km after perigee is shown in Fig. 12. At this location in the orbit, the signal over the time interval shown is well above threshold. Variations in the data during the storm are particUlarly prominent. The lower two panels appeared in Figs. 3 and 4. The middle panel gives N(0) on an absolute scale based on

321

Global atomic oxygen density

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FIG. 12. COLUMN DENSITIES OF ATOMIC OXYGEN AT 41Okm AFTER PERIGEE. The lower two panels appeared in Fig. 4. The column density in the second panel from the top is on a relative linear scale to emphasize variations. The temperature in the top panel was obtained from the deduced column density using density-exospheric temperature relationships from 571.

c, = 4 and c, = 3. The next panel above gives relative changes in N(0) and is plotted on a linear scale for emphasis. The top panel shows a temperature obtained from N(O), using the N(0) - T, relationship from J71 at the specified altitude. The column density was obtained by an integration over the model density.

Throughout this time period, changes in N(0) correlate well with K,, even for small variations in this index. During the storm period, N(0) varied by a factor of two in a matter of a few hours. Just after the storm, starting at orbit 1700, there appears to be a depression lasting about four days. This will be more evident in further results presented

322

D. J. Sriucx~~i~

below. More variation from orbit to orbit occurred past orbit 1900 as the observation point moved into the northern aurora1 region. At high latitudes, short-term variations are expected as a result of sporadic heating due to aurora1 phenomena. At certain times, the variations may be overestimated due to contamination by aurora1 1304 A emission. For this reason, intensity enhancements recorded above geomagnetic latitudes 60” are not emphasized. The temperature in the top panel should not be interpreted as the actual exospheric temperature. It is to be viewed in the manner Jacchia uses this quantity, i.e. as a parameter in a formula yielding the observed quantity, which is here, N(0). Departures from the actual temperature will occur due to deviations with time of the O-density in the lower thermosphere from the Jacchia model, and with variations in the excitation parameters. The largest difference between the temperatures in Fig. 12 and actual temperatures probably occurs during the storm. Blamont and Luton observed a smooth variation of -70 K in temperature near the equator between days 271 and 275. Knight et al. (1973) obtain a variation of 125 K during this period at low

A

and G. E. THOMAS latitudes. Their results are based on solar EUV attenuation measurements at dusk and dawn. The variation shown in the figure is -200 K. The discrepancy between this value and the lower ones would indicate that 0 is being transported into the equatorial region at high altitudes during the storm. Figure 13 indicates how N(0) varied with geomagnetic latitude from orbit to orbit between orbits 1604 and 1750. The temperature as de8ned above illustrates the variation. The baseline for each orbit is 1150 K with the vertical scale given in the lower right hand comer. The K,, index is included in the plot. At fixed altitudes over this latitude range, N(0) shows a gentle decrease from low to high latitudes before the geomagnetic storm, in agreement with the 571 model. During the storm, there is an overall increase of N(0) with the largest changes occurring at low latitudes. A detailed comparison between the relative variations in Fig. 13 and the relative 0 density variations given by Tauesch et al. (1971) can be made from orbits 1658 to 1668. In their Fig. 3, the orbits shown are 1658, 1660, 1662, 1664, 1667 and 1668. There is good agreement between the two sets of results

1650.

FIG.~~. TEMPERATCJRESASINTRODUCEDINFIG. 12 FORINDIVIDUALORBITSBETWEEN 1604 AND 1750. The K, index is shown to the left of the temperatures and indicates the geomagnetic storm period. The baseline for each plot is 1150 K with 150 K per horizontal line as the scale. The figure represents one scheme for removing the altitude dependence of the deduced column density.

1760.

Global atomic oxygen density depicting relative variations. The largest increase in N(0) occurred on orbit 1684 at low northern latitudes. The temperature increase from just prior to the storm period is 200 K corresponding to a doubling of the column density (see alss Fig. 12). After orbit 1700, a depletion in N(0) is evident at northern mid- and high-latitudes. The signal level was too low at southern latitudes ta yield reliable information in those regions. It should be noted that this high latitude depletion is almost certainly real, since aurora1 1304 w contamination would At always produce an apparent increase in N(0). the equator, N(O) returned to values comparable to pre-storm values.

7.1 1304 A soiar resonance excitation At the time we began analysing the OGO-6 data, the HH 1304A fluxes were the most recently measured values appearing in the literature and were chosen as the reference for the scaling parameter c,. Solutions ta the transport equation and analysis of the OGO-6 data led us to c, values of 4. Analyses of Mariners 6, 7 and 9 1304 A data by Strickland et al. (1972; 1973) also led to a similar scaling value. These results now seem substantiated by other measurements made in the last few years. Table 2 shows observed values of the solar line fiuxes. The values by Dupree et aJ. (1973) are lower limits for the time of observations since they apply to an ideal uniformly bright disc based on measurements from only a small quiet region of the Sun. The recent obsemations and the above mentioned Mariner analyses do fend support to the assumptions TABLE 2. OBSERVED lo* photons cma sec.

SOLAR

WERE

VIEWING

OBTAINED

QUIET-TIME

BY

SUN

AND

THE

1304A

VALUES

BRIGHT

2.0

etal.

(1958)

2.6

Delwuer, et al. (t96l)

0.88

zrall-Hinteregpr

0.54

Dupree,

etal.

(18701

f1973)

1.5

1.8

A

A

AN

SOLAR

1302.2

Byram,

ONLY TO

REFER

FLUXES

BY DUPREE

IN

PORTION IDEAL

UNITS

f?f d.

OF

(1973) OF

THE

UNIFORMLY

DISC

1

!

3um 7.8

10. 4.0 1.5 5.3 7.3 6.7

~

323

made in the transfer theory as applied to the 000 analysis, namely in the use of a Doppler profile to both (1) derive ,Sgr8f and (2) to describe multiple scattering. Had we used a Voigt profile in (2), the resulting c, value would be -10, a value unacceptably high. Within the framework of CFR, there is some question regarding the appropriate frequency profile for &(*). A Voigt profile does more accurately describe the total amount of resonance absorption of the solar photuns. With the wings of the absurption c&cient included, So@)will reach a min%num with decreasing altitude and then begin rising rapidly below -200 km due to the rapid increase in the 0 density (see Fig. 7 in Meier and Prinz, 1971). Without the wings, S, @)behaves as shown in Fig. 5, The steeply rising S, fsl below 200 km refers to photons in the far wings of the emission profile and should not be included in a CFR calculation since few of them will scatter into the core of the line, After a few scatterings, these photons are either absorbed by 0, or escape out the top of the medium. Our procedure has been to consider only those solar photons which are absorbed in the cure of the terrestrial Doppler fine. To determine the importance of the remaining photons, we performed single scattering calculations and found that the contribution to the zenith intensity at OGO-6 aItitudes is insignificant, although for nadir intensities it is important to the total SRScomponent.

In the past, there has been a lack of understanding in the relationship between the 1304 A airglow and the productian rate of OyS> by photoelectrons due to uncertainties in (1) the excitation cross section, (2) photoelectron energy spectrum, (3) dayglow observations and (4) radiative transfer calculations. The present situation regarding uncertainties in the excitation cross section is considerably improved over previous work (Dalgamo et al., 1969; Stewart, 1970; Zipf and Stone, 1971; Rountree and Henry, 1973). Recent measurements by Stone and Zipf (1974) give a maximum value of 5.3 x lo--l9 cm2 at 20 ev for the % cross section. Julienne (private communication, 1975) obtains a similar cross section from theoretical calculations which include cascading contributions from higher states in an optically thick medium. There are still serious problems with item (2) above (see the review by Cicerone, 1974). Large discrepancies often occur between measurements and theory and are probably due mainly to uncertainties in the EUV solar flux rather than to our lack of understanding of the physical processes. There appear to be

D. J. STRICKLAND and G. E. RroMas

324

serious discrepancies in 1304 A dayglow observations (item (3)), especially between nadir and zenith data with the former being much larger than theory would predict. We will restrict this discussion to zenith observations, the type made by the OGO-6 photometer. We believe the largest uncertainty in item (4) involves the choice of frequency profile. Figures 6 and 7 have demonstrated the sensitivity of the intensity to the choice of profile. With this background information, we will now compare our deduced photoelectron source with other independent information discussed above. We start by comparing with the quantityg&V, = 0) which we call “the g-value” and which is directly derivable from the photoelectron flux and the excitation cross section cr: &IV& = 0) =

a(E)@(z + 00, E)dE, (4)

I Its value is 1.2 x lo-8 for (3 given by Stone and Zipf (1974) and the calculated flux of Cicerone et ai. (1973). Ag-value of from 3 to 4.8 x 10-a is obtained if we use the observed photoelectron escape flux which is in the range 5-8 x 108 (Yngvesson and Perkins, 1968; Heikkila, 1970; Cicerone and Bowhill, 1971; Knudsen, 1972; Cicerone, 1974) rather than 2 x 108 as the calculations of Cicerone et al. predicted, and if we assume the shape of the theoretical differential photoelectron flux is correct. The above calculated g-values compare with our deduced value of ~3 x IO-*. We now introduce the available 1304 A zenith data for the remaining comparisons. Figure 14 6001

1

1

500-

1

OGO-6,600

? Y400-

\

ki 2 300s

200 -

‘OO.t

1

1.,lll, 1.0

10

INTENSITY(KR) Flc&

14.

ROCKBT

AND

SATELLITB

NEAR-ZENITH

13ohA

zBNITIi

OR

DATA.

anete Iabeling each curve &es the SZA and the letters, -giving re%rences, are -ident&ied as follows: F-F&e (1968); H-Heath (1968); S-She&r (1971); RFM-Rottman et al. (1973); T-Takacs (1974). The

The

dashed curve is an extension of the OGO-6 data based on the transfer results and excitation parameters discussed in this work. The OGO-6 data plotted are smoothed averaged values for quiet time conditions existing at the time of the late September storm.

TABLE 3. O(%) FROM AVAILABLB TIONS AND OF

S’IONB

COLUMN NBAR

PRODUCTION

ZENITH

PROM Ag-VALUE AND

ZIPP

(19%)

1304 A

RATES

DEDUCED

AIBGLGW

OBSERVA-

BASED ON THJ3 CROSS SECTION AND

OBSERVED

ESCAPINO

PI3OTGELECTBONS

Reference

57%

Column Rate WA)

Fat&

600

0.w

1.6

5&J

1.8

2.9

@SW

Heath (1968) Sheffer

(1971)

Rottman et al. Takacs

(1973)

(1974)

600

1.4

2.3

.se

0.76

3.9

6p

1.5

2.9 2.7-4.3

(4)

Equation

COlUUUl Rate (szA=o*,

This Work

2.s

109cm-2 see-l

Units:

shows the altitude profiks of the 1304A intensity from both rocket and satellite experiments. To compare corresponding excitation rates or g-values appropriate to a given measurement requires us to solve the transfer equation. We have chosen to use the inferred column production rate at 0” SZA as the parameter for comparison given by p = 1) d.2’ =

n(z’)gV s x &fZ’,P = I)1 dz’, (5) since it is a more useful quantity thang, for relating to other types of observed emissions. The comparison in P-values may be made using Table 3 with the results given at both the observing SZA and at 0” SZA. In the transfer calculations, c, was assumed equal to 4, the frequency profile was Doppler, and the 0 densities were from 571. The values at 0” SZA range from 1.6 to 3.9. The spread in values can be attributed to one or more of the following: (1) calibration errors, (2) variations with time of the solar 1304 A fluxes and solar EUV fluxes, and (3) variations with time in the 0 concentration. In general, the results of the calculations indicate reasonable consistency in observations, especially in recent ones. It is not strictly appropriate to say that the agreement in deduced production rates lends support for the magnitude of our assumed g-value since the same transfer theory was applied to all data. Nevertheless, itisencouraging toachievethe consistency indicated in TabIe 3 and the agreement in correspondi g-values with that based on measured photoelectron fluxes and the O(*S) electron impact cross section. The independent estimates of c, and cl, discussed in these last two sections and their correspondence with our deduced values do support the transfer theory applied and indicate that we do now reasonably understand the W(z,

P=

s

Global atomic oxygen density relationship between daytime production and the 1304 8, dayglow.

of O(8s)

8. CONCLUDING RBMABKS In light of the unavoidable uncertainties in the spectication of the excitation rates, which the data themselves cannot completely remove, it would appear that 1304 A airglow observations in the zenith between 400 and 1100 km are of limited value in determining absolute atomic oxygen densities. On the other hand, they are of value in deducing relative density variations over short periods of time (e.g. one or two orbits). A method to place such measurements on an absolute basis would be to monitor simultaneously one or more optically-thin emissions (such as [01] 1356 A) which are excited primarily by photoelectrons. In this way one could independently determine the necessary information concerning the photoelectron flux. Another way would be to carry out measurements in a spinning mode in order to view the airglow in many directions. This would provide more altitude information on the total source and could possibly determine both sources independently. It of course can be argued that in situ measurements are of much greater value and require little if any analysis. However, the great advantage of airglow measurements is that they can be carried out from remote observation platforms, such as planetary fly-by and orbiter missions. This technique has already been proven for the Mariner ultraviolet observations of the atmosphere of Mars. Similar observations of Venus were made from the Mariner 10 fly-by in 1974, and are planned for an orbiting mission in 1978. It is of obvious importance that we understand the terrestrial airglow mechanisms before we can make full use of the planetary observations. OGO project was supported by the National Aeronautics and Space Administration under contract NGR 06-003-127, and the data analysis by NASA Grant NGR 06-003-052. Acknowledgements-The

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