A first look at the ASSI ultraviolet results

Adv. Space Res. Vol. 13, No. 1, pp. (1)247—(1)254, 1993

0273-1177/93 $15.00 Copyright © 1992 COSPAR

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A FIRST LOOK AT THE ASSI ULTRAVIOLET RESULTS S. Chakrabarti,” G. R. Gladstone,* W. K. Tobiska,* G. Schmidtke,** H. Doll*** and J.-C. Gerardt *Spgj~eSciences Laboratory, Universily of California, Berkeley, CA 94720, U. S. A. ** Fraunhofer-Institutfür Physikalische MeJ3technik, Heidenhofstrafte 8, D-7800 Freiburg, Ger,nany “,~‘ Physikalisch—TechnischeStudien GmbH, Leinenweberstrafie 16, D-7800 Freiburg, Germany t Instit ut d’Astrophysique, Universite de Liege, Av. de Cointe 5, 4000 Liege, Belgium

ABSTRACT The Airgiow and Solar Spectrometer Instrument (ASS!) on the San Marco D satellite has obtained near-simultaneous measurements of solar irradiances and airgiow emissions in the 200—7000 A spectral region. The satellite was placed in an equatorial, elliptical orbit on 25 March 1988, which permitted observations of airglow emissions in the 280—600-km altitude range at various local times. The instrument complement on the satellite provides an opportunity both for self-consistent examination of the excitation mechanisms of various airgiow features and for constraining model parameters. An overview of the data obtained by ASS! will be presented along with preliminary modelling results of the ultraviolet airglow.

INTRODUCTION Characterizing the extreme and far ultraviolet (EUV and FUV) airglow in terms of the physical parameters that govern interaction processes (thermospheric composition, temperature, ion density, and solar EUV flux) will allow remote sensing in the EUV to be routinely employed as a quantitative diagnostic of the terrestrial neutral atmosphere and plasma environment, which is one of the goals of ultraviolet remote sensing. The complex interplay between density, composition, temperature, and airglow as a function of solar EUV irradiance (sometimes represented by 10.7-cm radio flux) is a crucial component of this research. The San Marco D/L satellite program was designed to conduct a comprehensive study of the equatorial thermosphere. Its primary scientific goal is the understanding of the complex physical and chemical processes resulting from the solar electromagnetic energy input into the thermosphere. Accordingly, a number of instruments were selected to measure simultaneously the solar irradiance, thermospheric density, neutral wind and temperature, ion velocity, and electric field. Of these, the Airgiow and Solar Spectrometer Instrument (ASS!) obtained, for the first time, simultaneous solar irradiajice and airglow emissions measurements /1/. Such measurements provide the exciting opportunity to eliminate one key uncertainty that plagued most previous airglow modelling. No previous measurement of EUV airglow was accompanied by solar EUV or photoelectron flux measurements, so the excitation source for the airglow emissions had to be inferred from the airglow data itself. This introduced some uncertainties into all previous interpretations of EUV airglow data. In this paper, we describe briefly the EUV and FUV emissions observed by ASS!, as well as the present status of data reduction and analysis. INSTRUMENT DESCRIPTION AND OPERATIONS On 25 March 1988, the San Marco D/L satellite was launched into an elliptical orbit having an apogee of 600 km and a perigee of 280 km. The satellite spins at a rate of six revolutions per (1 Y)A’7

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minute, with the spin axis parallel to that of the Earth. The orbit is near-equatorial, with an inclination of 2.5°. Briefly, the ASS! package consists of two instruments, each containing two spectrometers utilizing the Rowland circle mount. Each spectrometer consists of a holographically ruled concave diffraction grating on a toroidal substrate which minimizes optical aberration. At preselected locations along the Rowland circle, individual photomultiplier (open window channelirons as well as conventional sealed photomultiplier tubes) detectors were placed to record different spectral regions simultaneously. The field of view of the instrument is approximately 10°x 10°. A total of eighteen detectors was used to cover the 200—7000 A spectral region. The coverage of individual channels is shown in Figure 1. The complete wavelength coverage is obtained by stepwise rotation of the grating (a total of 112 steps) from its nominal position by ±3°. During the flight, a complete spectrum was obtained approximately every twenty minutes by stepping the grating once per sate!lite spin.

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Wavelength/nm Fig. 1. The wavelength coverage of the four ASSI spectrometers. The desire to measure solar flux and airglow with the same instrument necessitated an instrument with a very high dynamic range. ASSI achieved this through a combination of automatic aperture modification, automatic gain modification for the photomultiplier tubes (squint), and the use of detectors having a dynamic range of approximately 106. With its large dynamic range, ASSI provided simultaneous measurements of the airglow and solar EUV excitation source. ASSI was operated in several modes. In mode I, the grating was stationary at its nominal position, and only selected emissions features were monitored. In mode II, eight or sixteen steps out of a total 112 steps were used to scan the spectrum. Mode III obtained the complete spectrum with a scan of all 112 grating positions. Last, for in-flight calibration, the entrance apertures were closed in mode IV, and the instrument’s response to a radioactive Ni63 source was monitored. Although ASSI was originally calibrated in the laboratory, the long delay in launch necessitated a recalibration, which was performed onsite in the days preceding launch. Calibration lamps were used for this purpose, to examine the spectral region from the FUV to the near infrared. The EUV calibration was achieved through comparison with a sounding rocket that measured solar EUV flux

ASS! UltravioletResults

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EUV SPECTRAL IRRADIANCE: 1~1—11—88 1010

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Fig. 2. Solar EUV irradiance observed by ASS! (dashed line), LASP sounding rocket /2/ (solid line), and the SERF2 model (dotted line). from White Sands, New Mexico, on 10 November 1988 /2/. Figure 2 shows a solar spectrum observed by these two instruments and the prediction of the Solar Electromagnetic Radiation Model (SERF2) /3/. The general agreement between the solar flux values inferred from ASSI preflight calibration, rocket measurements, and the SERF2 model provides confidence in the instrument calibration. The reduction of ASSI data in terms of physical values has just begun. The final calibration values for all channels are not available yet. Data from some of the orbits have been made available to the ASS! co-experimenters. However, a preliminary look at the data qualitatively indicates that the instrument operation was nominal. In this paper, we present the first results from ASSI in mode III. The redundancy presented by overlapping wavelength coverage of ASSI’s eighteen detectors is shown in the two panels of Figure 3. Down-looking (zenith angle ranging from 150°to 180°)dayglow emission intensities in the 400—2200 A wavelength range are shown for orbit 600. The top panel shows spectra obtained by channels 12, 16, and 17, while the bottom panel shows the same spectral region covered by channels 8, 15, and 18. This example vividly demonstrates that, while the inferred intensities should be identical for all six channels, the preflight calibration values produce significantly different results. It is the task of the ASSI instrument team to reconcile these and other similar differences using various in-flight calibration data. DATA DESCRIPTION Figure 4 shows dayglow spectra recorded by the channel 18 detector of ASSI on 5 May 1988 in the up- and down-looking directions. Preflight calibration data were used in computing the emission

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wavelength (nm) Fig. 3. EUV and FUV down-looking airglow spectra observed by various channels on 5 May 1988. Top panel shows channels 8 (solid line), 15 (dashed line), and 18 (dotted line). Bottom panel shows channels 17 (solid line), 12 (dashed line), and 16 (dotted line).

ASS! UltravioletResults

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Fig. 4. EUV dayglow observed by channel 18 of ASSI on 5 May 1988. Top panel shows the up-looking data. The down-looking data is shown in the bottom panel. intensities. Since such a spectrum requires approximately twenty minutes for a complete wavelength scan from long to short wavelengths, the orbital parameters underwent a substantial change. The analysis of an individual feature must consider the observation conditions at the time the feature was measured. The important parameters are summarized in Table 1, and the intensities of the bright emission features are listed in Table 2. The spectroscopy of the EUV/FUV dayglow has now been conducted at moderate to high spectral resolution (Eastes et al. /4/ and references therein). In general, the identification of the majority of the transitions reflects the primary emission feature. Since the spectral resolution of the instrument in the EUV and FUV region is of the order of 10 A, more that one transition is present in some of these features. The intensity values of all of the individual features have not been

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TABLE 1 Geophysical and Orbital Parameters for 5 May 1988 Parameter

Range

Universal Time

13:01—13:28 10:40—12:56

Local Time F107 F107 Ap

121 126 20

Altitude

444—534 km

Longitude Solar Zenith Angle MSIS T..

351—321° 17—33° 957—1033 K (at the equator)

TABLE 2 Intensity of Selected Dayglow Features Observed on 5 May 1988 Feature Hel 584 A On 617 A 011718 A On 834 A NI, Nil 916 A Ni 953 A Oi 989 A 011027, Hi 1025 Nfl 1084 A Ni 1134 A Ni 1168, 011172 Hi 1216 A N2 1273 A? Oi 1304 A N2 1325 A 011356 A

Up Intensity (Rayleigh)

Down Intensity (Rayleigh)

88.5±12.1 355.5±9.8 24.7±1.6 7.2±7.5 109.0±15.3 82.9±13.7 21.4±8.2 —30.8±7.9

61.6±11.1 16.8±6.9 11.6±5.2 539.4±24.4 156.0±19.2 67.5±18.7 571.9±34.6 121.6±20.5 197.7±20.7 134.7±19.8

541.4±27.8

534.7±29.8

11897.2±111.0 609.8±26.9 18.8±7.4

11317.9±108.6 17.8±11.1 3654.0±63.0 56.9±14.4

28.0±9.6

315.2±19.5

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compared against theoretical models. However, some of the intensity values are surprisingly different from other observations. We emphasize that the calibration data used here are preliminary. Once the final calibration values are obtained, these intensity values may vary substantially. The sensitivity of the instrument is not sufficiently high to study faint emissions, such as night airglow features. Only the prominent EUV and FUV features in nightglow are appropriate for detailed analysis. At the present time, only the hydrogen Ly a appears to be a good candidate. We have made a preliminary theoretical evaluation of the ASS! dayglow data presented here. The modelling effort follows the same procedure as that outlined in Gladstone er a!. /5/. We have examined three prominent atomic oxygen emissions (01 989, 1304, and 1356 A). We chose the data from 5 May 1988, shown above. Although these emissions can be observed by different channels, we have chosen data from channel 18 only (Figure 4). As explained earlier, these spectral features are not observed simultaneously. Care was taken to use input parameters appropriate for the observing condition for the line in question. The MSIS-83 model of the neutral atmosphere was used. Other parameters are as described in /6/. As with all our previous analysis of these emissions, none of the input parameters were scaled. The results are shown in Table 3.

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TABLE 3 ASSI Preliminary Modelling Results Line

Data

Model

Ratio (Model/Data)

Up-Looking Data 989 1304 1356

109 610 28

291 1683 5.4

2.7 2.8 0.19

Down-Looking Data 989 1304 1356

572 3660 315

1048 15139 2185

1.8 4.1 6.9

Data from One Complete Spin 989 1304 1356

353 2270 200

643 7590 1072

1.8 3.3 5.4

Down/Up Ratio 989 1304 1356

5.2 6.0 5.5

3.6 9.0 4.02

The difference between the data and model results can be attributed to several possible factors. First, as shown in Figure 3, the calibration of the instrument drifted significantly from the preflight value for channel 18. Second, we have not taken proper care to eliminate the contribution of other sources to the above lines. For example, the 1356 A line has a contribution from the Lyman Birge Hopfield (LBH) band (3,0). We have not investigated these instrumental and data reduction effects to determine the cause of the discrepancy. SUMMARY We have presented the EUV/FUV data from the Airglow and Solar Spectrometer Instrument (ASS!). A qualitative look at the data confirms that they contain the well-known features of EUV/FUV dayglow. The intensities of these features should not be taken as definitive at this point, since the final calibration values are not available. However, preliminary modelling results indicate that the intensities observed and their dependence on the view directions are generally consistent with model predictions. ACKNOWLEDGEMENT We are grateful to Amit Bhauacharyya for his computational help. We thank Peter Seidl and Claus Wita for making the ASS! data available to us. This work was supported by NSF grant FD89—00072, U.S. Army DAALO3—89K0057, and NATO travel grant CRG. 870511 at Berkeley and FRFC CRG—910500 at Liege. REFERENCES 1.

G. Schmidtke, P. Seidl, and C. Wits, Airglow-solar spectrometer instrument (20—700 nm) aboard the San Marco D/L satellite, App!. Opt., 24, 3206 (1985).

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2.

T. N. Woods and G. J. Rottman, Solar and EUV irradiance derived from a sounding rocket

3.

experiment on November 10, 1988, J. Geophys. Res., 95, 6227 (1990). W. K. Tobiska and C. A. Barth, A solar EUV flux model, J. Geophys. Res., 95, 8243 (1990).

4.

R. W. Eastes, P. D. Feldman, E. P. Gentieu, and A. B. Christensen, The ultraviolet dayglow at solar maximum. I. FUV spectroscopy at 3.5 A resolution, J. Geophys. Res., 90, 6594 (1985).

5.

G. R. Gladstone, R. Link, S. Chakrabarti, and J. C. McConnell, Modelling of the 1173-A ratio in the terrestrial dayglow, I. Geophys. Res., 92, 12,445 (1987).

6.

R. Link, S. Chakrabarti, G. R. Gladstone, and I. C. McConnell, An analysis of satellite observations of the EUV dayglow, J. Geophys. Res., 93, 2693 (1988).

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