euv (800-1400 Å) observations of the tropical airglow

GEOPHYSICAL RESEARCH LE•'•ERS, VOL. 11, NO. 6, PAGES 565-568, JUNE 1984

EUV (800-1400 •) OBSERVATIONS OF THE TROPICAL AIRGLOW

Supriya Chakrabarti

Space Sciences Laboratory, University of California, Berkeley

Abstract. We have obtained the first spectrum of the tropical airglow in the 800-1400 • range. The data were obtained at 8 • resolution by the EUV spectrometer on board the STP78-1 satellite. The time-averaged downlooking spectrum within the arc shows strong 0I features at 1304 and 1356 •. We have detected the oxygen recombination continuum at 911 •, which provides the first conclusive evidence that radiative recombination

is the primary excitation mechanism for the tropical ultraviolet emissions. We have also dis-

that the ion neutralization process does not contribute significantly to the observed emissions. In order to conclusively demonstrate that radiative recombination is the dominant

excitation mechanism, one needs to observe the continuum edge at 911 • rising from direct recombination into the ground state of the oxygen atom. Such a feature should show the same

latitude profile as do the OI 1304 and 1356 % features.

In this paper, we report on the first covered a weak OI 989 • emission from the tropical moderate resolution (< 10 •) spectroscopic arc region, which is not expected in ordinary radiative recombination spectrum. Such observation indicates that other processes, such as dielectronic recombination or mutual neutraliza-

tion, might be present in these regions.

Introduction

The tropical ultraviolet arcs in the nightglow were first observed from the OGO-4 satellite

[Hicks and Chubb, 1970; Barth and Schaffner, 1970] and later from the moon [Carruthers and Page, 1976]. With the help of the 20 • resolution of the OGO-4 spectrometer, Barth and Schaffner [1970] identified the emission to be 0I transitions at 1304 and 1356 •. These authors observed bright 0I signals (> 1 kR) which were later revised downward by Gerard et al., 1977 based on 0G0-5,6 measurements. Anderson et al. [1976] have reported observations of emission in this band using a 860-1050 • photometer from the STP72-1 satellite, which the authors tentatively attributed to the oxygen recombination continuum. Hanson [1969] proposed that these emission features are excited by radiative recombination of ionospheric 0+:

0 + + e + O* + hv c (1)

O* + 0 + Zhvl (2)

where hv c and hv 1 indicate continuum and line emissions, respectively. Knudsen [1970] proposed ion-ion neutralization as the primary candidate for this excitation through the following processes:

0 + e + O- + h (3) v

0 + + O- + O* + 0 (4)

O* + 0 + ZhVl (5) It was theoretically shown [see, for example, Tinsley et al., 1973 and references therein]

Copyright 1984 by the American Geophysical Union.

Paper number 4L6078. 0094-8276/84/004L-6078503.00

observations of the tropical airglow in the 800-1400 % pass band. The data were obtained by the EUV spectrometer on board the STP78-1 satellite launched in February 1979. Several previously unobserved atomic oxygen features are discovered in the spectrum. No atomic nitrogen emissions could be positively identified in the tropical EUV spectrum.

Instrument Description and Observations

The spectrometer (described in detail by Bowyer et al. [1981]) consists of a 0.5-mm wide rectan- gular slit, a concave reflection grating, and two position-sensitive EUV detectors, lying on the Rowland cylinder. The inside first order spectrum is used, yielding a reciprocal disper- sion of 10.4 •/mm and a resolution of 8 •. The combination of 17.4-mm slit height and grating diameter of 48 mm yields an intrinsic field of view of full width, 7 ø by 9 ø that, in combination with the spin and telemetry rate of the spacecraft, provides a triangular working field of view of full width, 18 ø by 9 ø . The spectrometer was found to have a peak sensitivity of about

1 1 , 10 -1 count s- Rayleigh- in the range 500-700 % decreasing •slowly to •bout 10 -2 c s -• R -1 at 350 • and 1350 A. The absolute accuracy of the calibration is estimated to be + 20%.

The satellite was placed in a 600-km altitude circular orbit, with an inclination of 97.7 ø and

an orbital period of 96 min. The orbit is sun- synchronous, precessing at 1 ø per day with the spacecraft orbit lying essentially in the noon- midnight plane. The spectrometer is housed in the spinning wheel of the spacecraft with the spin axis of the wheel perpendicular to the orbital plane. The spectrometer's line of sight is oriented 120 ø from the spin axis and, as the wheel rotates at 11 RPM, the instrument's line of sight sweeps out a cone, alternately viewing earth and space, never looking closer than 30 ø to the sun. The observations persented in this paper were made near local midnight between March 20 and March 25, 1979.

Results

In order to identify the spectral features in the nightglow emissions we have added 20320 individual spectra obtained while the line of sight

565

566 Chakrabarti: Tropical EUV Airglow Observation

I

o

750 850

o 5 Ld

I-- 4 z

o

o 3

0• R T O

CO edC)

• •C) • Go o O ,--,

I I I I

950 1050 1150 1250

WAVELENGTH (.•)

1350

Fig. 1. Average downlooking EUV nightglow spectrum obtained from 600 km when the spacecraft was within ñ 30 ø magnetic latitude range and outside the South Atlantic Anomaly region.

(LOS) of the spectrometer was pointed near nadir In order to obtain an "average" spectrum in- (LOS zenith angle 120-150 ø and the spacecraft was side the tropical arcs, we have added 4500 within ñ 30 ø in magnetic latitude). We have ex- individual spectra recorded in twelve individual cluded spectra obtained while the satellite was orbits. These spectra were taken while the in the South Atlantic Anomaly region. The average spacecraft was passing over the brightest region

of the arc and were selected in such a way that the spectrometer's line of sight was pointed no

downlooking nightglow spectrum obtained in this way is shown in Figure 1. The thr•e prominent features in the EUV nightglow spectrum besides the hydrpgen Lyman-• emission are the 1304, 1356 and 911 • features, which are due to atomic oxygen. Of these, the last feature extends over several spectral bins and is much broader than a line. This is a continuum caused by direct recombination to the ground state. The width of this continuum is proportional to the temperature and since the emissions are caused by thermal 0 + ions, it looks more like a line than a typical continuum feature (e.g., the Birge-Hopfield feature in the dayglow: see Chakrabarti et al., 1983a).

Figure 2 shows the latitudinal profiles of the intensities of the above spectral features observed simultaneously as the spacecraft moved from the north hemisphere to the south. The data are binned in 50 s intervals which correspond to approximately 3 ø latitude bins. One • count statistics asso- ciated with the measurements are shown for each

bin. All three emission features show two inten-

sity peaks on either side of the dip equator and a minimum at the equator. The peak intensities for the 1304, 1356 and 911 • features are 200, 200 and 180 Rayleighs, respectively. The intensities at the equator are 30, 20 and 15 Rayleighs. The absolute intensities are similar to the ex-

pected intensities for nominal ionospheric conditions using recombination coefficients of Julienne et al. [1976]. The absolute intensities and the latitude distributions of these observed

features thus confirm the longstanding predic- tion [Hanson, 1969] that radiative recombination of 0 + is the primary recombination mechanism for the tropical ultraviolet emissions.

LATITUDE PROFILES OF SELECTED NIGHTGLOW FEATURES

ORBIT 373, DAY 80, 1979

300

250 X: 1304• -

.

150 r -

o3 100-

E• 50

• 250 : 1356• -

•_ 2oo- - g 150 -

g • 0 0 0 • 200- : - g

0 150

i00

50

0

TIME 26400 26800 27200 27600 28000 MAG. LAI 63. 40. 18. - 5. -28. -50.

Fig. 2. Latitude profiles of selected nightglow features for near nadir oh- s etvat 1on.

Chakrabarti: Tropical EUV Airglow Observation 56?

more than 60 ø from the nadir. The resultant

spectrum is shown in Figure 3 in counts s -1 as a function of wavelength in angstroms in the 800-1400 • range. Not all spectral features shown in Figure 3 display the doubly.peaked latitudinal profile. The flat background of about 0.1 c s -1 bin -• throughout the spectrum is due primarily to Lyman alpha scattering from the grating and inherent detector noise. Table 835 1 lists the intensities of the observed feature•. 850 The calibration accuracy for these measurements 910 is 20%. The quoted intensities for the weak or 950 blended features could be uncertain by as much 977 as a factor of two. The intensities of the weak 989 features blended into strong ones (for example, 1027 NI 1200 • and H Lyman •) were obtained by sub- 1040 tracting the intensities of the strong line using 1085 in-flight line profiles, obtained while the weak 1134 features were not expected to be present. The 1168 absolute intensities of the features shown in 1200 Figure 3 and listed in Table 1 undergo considerable 1216 pas to pass and peak to peak variations. There- 1304 fore, the displayed intensities should be consi- 1356 dered averages taken over this time period.

TABLE 1. Tropical airglow features (800-1400 •) as observed from 600 km looking at zenith

angle between 120-150 ø

t (•) Intensity (Rayleighs) Transition

2.35 OII 834 < 6.4 NI recomb. continuum? 83.18 OI recomb. continuum

2.13 0I 949-953, 0II952-955 2.60 0I 973, 0I 978 4.15 0I 989

3.39 0I 1027, HI 1026 <_ 1.33 0I 10407 -< 6.5 Nil 1085 -< 10.7 NI 1134 2.64 Hei584, 2nd order -< 21 NI 1200 1670. HI 1216

106.40 0I 1304 86.88 0I 1356

Discussion

The most intense feature in the spectrum is HI 1216 • line, which is excited primarily by multiple scattering of the solar hydrogen Lyman alpha line by geocoronal hydrogen atoms. The HI 1216 feature is excited in a similar manner

[Chakrabarti et al., unpublished manuscript, 1984]. Although there should be a contribution from the 0I 1027 line to the emission near 1026

•, this could not be separated without recourse to detailed calculations, which is beyond the scope of this paper.

This would indicate that radiative recombination

is the primary excitation mechanism responsible for the 911 and 1356 • features in the tropical airglow. The 1304 • feature is optically, thick, hence in order to calculate its expected intensity, one needs first to remove the effects of multiple scattering. We have performed a detailed calculation to obtain the theoretical

intensities and found that the 1304 • intensities can be explained with the radiative recombination scenario.

The next prominent features in the spectrum Recombination coefficients are not available are the 1356, 1304 and 911 • emissions. Of these, for the remaining 0I features listed in Table the 911 and 1356 • features are optically thin and the observed intensities are in agreement with those theoretically expected for our ob- serving conditions using the recombination coefficients reported by Julienne et al. [1976].

0.7

0.6

0.5

z 0.5

o

0.2

o.I

o.o

DOWNLOOKING EUV SPECTRUM IN THE TROPICAL ARC

ACCUMULATION TIME -- 921 SEC

• I•,/•,- • - = = •

•1 I II •'•._

800 I000 1200

WAVELENGTH (.•)

I 14oo

Fig. 3. Downlooking EUV spectrum in the tropical arc. Accumulation time = 921 sec. The instrument response function for each wavelength bin in counts/sec/5 Rayleighs is shown as the continuous line marked 5 R. Represen- tative 1 o error in the count rate is shown.

1. These transitions are most likely excited by a radiative recombination mechanism. Using Julienne et al.'s recombination coefficients and

our observation of 87 Rayleighs for the 1356 • feature, we expect 17 Rayleighs of 1027 • feature. We only measure a signal of 3 Rayleighs for this feature. Such discrepancy may be attributed to absorption effects. It could also be due to the fact that these recombination coef-

ficients are only poorly known. A_possible candidate for the feature near

989 • is the N 2, BH(1,0) band at 986 •. This however implies that there should be other BH features in the spectrum, especially the (1,1), (1,2) and (1,3) bands near 1009, 1033 and 1058 • respectively. Using the laboratory cross- sections of Zipf and Gorman [1980], the expected intensities for these bands are 8, 8 and 5 Rayleighs respectively. The absence of these bands would argue against N 2 emission as a possible candidate for the 989 • feature.

The observations of the 989 • feature are particularly interesting. In order to observe these photons from the radiative recombination mechanism, the 0 + ions must be in the 2D state. The lifetime of this state is about two hours and

hence there should not be many such ions near local midnight, when our measurements were taken. It might be possible to obtain the 0I 989 • emissions by dielectronic recombination mechanism. Although dielectronic recombination is most effective at high temperature (• 104ø K), a recent

568 Chakrabarti: Tropical EUV Airglow Observation

calculation shows that it could be effective

at ionospheric temperature regimes [Nuss- baumer and Storey, 1983]. The expected 0I 989 intensities for such mechanism agrees with our measurements. There might be some contribution of mutual neutralization to all

of these features.

The 0II 834 features does not show the

doubly peaked latitude distribution and hence it is unlikely that this feature is excited by any recombination mechanism. Most likely, solar or auroral 834 photons are being multi- ply scattered by 0 + ions in the ionosphere. One needs to perform detailed calculations to verify such a hypothesis, which is beyond the scope of this paper.

We have looked for NI emission in the ob-

served spectrum of tropical airglow. One might expect a recombination continuum near 853 • due to atomic nitrogen, similar to the 911 • feature. Two other possible NI features are the NI 1200 • and NI 1134 • lines. We could not make a positive 3o detection of either of these features in our spectrum at • 2 R level and have placed a 2o upper limit of 21 and 11 R, respectively. These upper limits are consistent with the expected intensities for these emissions.

Summary and Conclusion

We have made the frist spectroscopic observa-

tions of the •ropical airglow emissions between 800 and 1400 A. We have found that a strong continuum feature near 911 • shows the same latitudinal profile as the 1304 and 1356 • features. Their intensities correlate well

with the recombination coefficients of Julienne

et al. [1976]. This provides the first direct proof that radiative recombination (proposed by Hanson [1969]) is the primary excitation mechanism responsible for these emissions. We have also observed several previously unobserved and weak 0I features in 'the spectrum. Among these, the presence of the 0I 989 A feature might be indicative of other processes responsible for some emissions in the tropical arcs. We did not detect any NI emissions at the 2 Rayleigh level in the downlooking tropical EUV spectrum.

ultraviolet OI emissions in tropical ionosphere, Planet. Space Sci., 24, 945, 1976.

Barth, C.A. and S. Schaffner, OGO-4 spectrometer measurements of the tropical ultraviolet airglow, J. Geophys. Res., 75, 4299, 1970.

Bowyer, S., R. Kimble, F. Paresce, M. Lampton, and G. Penegor, Continuous readout EUV airglow spectrometer, Appl. Opt., 20, 477, 1981.

Carruthers, G.R. and T. Page, Apollo 16 far ultraviolet spectra of the terrestial airglow, J. Geophys. Res., 80, 1683, 1976.

Chakrabarti, S., R.P. McCoy, D.E. Anderson, R.R. Meier and S. Bowyer, Satellite observations of tropical ultraviolet airglow, EOS, 64, 788, 1983b.

Chakrabarti, S., F. Paresce, R. Kimble and S. Bowyer, The EUV day airglow," J. Geophys. Res., 88, 4908, 1983a.

Gerard, J-C., D.N. Anderson, and S. Matsushita, Magnetic storm effects on the tropical ultraviolet airglow, J. Geophys. Res ø , 82, 1126, 1977.

Hanson, W.B., Radiative recombination of atomic

oxygen ions in the nighttime F region, J. Geophys. Res., 74, 3720, 1969.

Hicks, G.T. and T.A. Chubb, Equatorial aurora/ airglow in the far ultraviolet, J. Geophys. Res., 75, 6233, 1970.

Julienne, P.S., J. Davis, and E. Oran, Oxygen recombination in the tropical nightglow, J. Geophys. Res., 79, 2540, 1976.

Knudsen, W.C., Tropical ultraviolet nightglow from oxygen ion-ion neutralization, J. Geophys. Res., 75, 3862, 1970.

Nussbaumer, H., and P.J. Storey, Dielectronic recombination at low temperatures, Astron. Astrophys., 126, 75, 1983.

Tinsley, B.A., A.B. Christensen, J. Bittencourt, H. Gouveia, P.D. Angreji and H. Takahashi, Excitation of oxygen permitted line emissions in the tropical nightglow, J. Geophys. Res., 78, 1174, 9173.

Zipf, E.C]--and M.R. Gorman, Electron-impact

excitation of the singlet states of N 2. •. The Birge-Hopfield system (bleu - X1Z •) J. Chem. Phys., 73(2), 813, 1980.

S. Chakrabarti, Space Sciences Laboratory, Acknowledgements. The author acknowledges helpful University of California, Berkeley, CA 94720 discussions with Stuart Bowyer and Robert Meier. This work was carried out with the aid of NSF

grant ATM-8305758.

References

Anderson, D.E., Jr., R.R. Meier, and C.S. Weller, Observations of far and extreme

(Received February 15, 1984; accepted March 20, 1984.)

euv (800-1400 Å) observations of the tropical airglow

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