Solar EUV/UV and equatorial airglow measurements from San Marco-5
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Journolo/Atmosphrri~ and Terrrsrrial Physics, Vol. 53. No. 8. pp. 781-785, 1991. CCU-9169/91 $3.00+ .OO Printed in Great Britain. Pergamon Press plc Solar EUV/UV and equatorial airglow measurements from San Marco-5 G. SCHMIDTKE,* H. DOLL,~ C. WITA? and S. CHAKRABARTI~ * Fraunhofer-Institut fiir Physikalische MeDtechnik, Heidenhofstr. 8, D-7800 Freiburg, F.R.G. ; t Physikalisch-Technische Studien GmbH, Leinenweberstr. 16, D-7800 Freiburg, F.R.G. ; 1 Space Science Laboratory, University of California, Berkeley, CA 94720, U.S.A. (Received in jinal form I 1 February 199 1) Abstract-Equatorial airglow and solar radiation measurements from 20 to 700 nm have been conducted with two spectrophotometric instruments aboard the San Marco-5 satellite from March to December 1988. The in-flight performance of the experiment and its in-flight calibration aspects are described. Preliminary results of the solar flux measurements and the planned scheme of the airglow modelling are presented. 1. INTRODUCTION In the thermosphere the solar Extreme Ultraviolet (EUV)/Ultraviolet (UV) radiation is the most impor- tant energy source. The radiation itself and its effects in the upper atmosphere can be observed best in equa- torial regions. Though the escaping airglow radiation in the EUV spectral region amounts to only about l- 2% of the incoming solar flux, the emissions reflect the different atmospheric processes such as photo- electron impact excitation, resonance and fluor- escence scattering, radiative recombination, etc. The Airglow-Solar Spectrometer Instrument (ASSI) aboard San Marco-5 measured the solar energy input to the equatorial thermosphere. To understand the complex Sun-thermosphere coupled system, ASS1 monitored reactions of the upper atmo- sphere to solar EUVjUV variations during the same period. The in-flight performance of ASSI, its in-flight cali- bration and first results from the mission are reported in this paper. The planned scheme of the airglow model is also discussed. 2. AIRGLOW-SOLAR SPECTROMETER INSTRUMENT (ASS) PERFORMANCE The ASS1 experiment was conducted aboard the San Marco-5 satellite from 25 March to 6 December 1988 in a near-equatorial orbit (2.9 inclination, 262 km perigee and 619 km apogee at the beginning of the mission). The instrument is described in detail by SCHMIDTKE et al. (1985). ASS1 consisted of 4 different scanning spec- trometers each containing 4 or 5 detectors, for a total of 18 detectors. Due to volume constraints, the optical set-up of the Rowland type spectrometers with gra- tings of 115.5 mm radius were chosen such that two pairs of the gratings shared one motor drive in a common housing making up two independent exper- imental units, ASSI-A and ASSI-B. The units were mounted on separate solar pointing devices. They viewed 180 away from each other from the equatorial plane of the spacecraft. Since the satellite was spinning at 6 r.p.m., the spectrometers sensed all atmospheric directions from zenith to nadir every 10 s. This is of special interest for the aeronomy of the lower ther- mospheric height regimes. The wavelength range from 20 to 700 nm was covered with spectral resolutions from about 1 to 3 nm. Some of the spectra1 regions were recorded in up to six different channels thereby providing high redun- dancy. Data were recorded in all of the 18 channels from 25 March 1988 until the end of the mission (6 December 1988). One complete set of spectra was obtained by the 112~step scan (with simultaneous measurements in all of the 18 channels). Since one of the horizon sensors of the spacecraft controlled the movement of the gra- tings, the 10s per step amounted to about 20min for a complete wavelength scan. During this time, the geographical parameters such as longitude, latitude, observational height, solar zenith angle, etc. changed according to the satellite orbit. Due to power restric- tions, only about 60 min of data per day were acquired. A large instrumental dynamic range of up to 1 x IO was necessary for the recording of strong 781 782 G. SCHMIDTKE et al. solar and faint airglow emissions by the same instru- . . . . ISII,Chwl 16 File: t113378.6R = ll_IIowB ment. - A8C,lltOl-spct?al-h1b?atio~ bcbt= le-w-88 3. CALIBRATION For EUV space instrumentation, calibration is one of the most difficult tasks. Although the ASS1 instru- ments underwent rigorous laboratory calibration in 1983, the calibration parameters such as photoelectric and electron multiplication yield of the detectors and grating efficiency vary strongly with time in an unpre- dictable manner [see REEVES and PARKINSON (1969) and SCHMIDTKE ef al. (1975)]. Since these surface properties are influenced by the interaction with the solar EUV radiation in the space environment, lab- oratory simulation experiments are almost imposs- ible. The most critical calibration parameter is the efficiency of the photomultipliers. In addition to the long-term sensitivity degradation, some instrument parameters showed dramatic changes with tem- perature drifts of only a few degrees (SCHMIDTKE et al., 1975). I I 60 70 80 90 100 wavelength Inm ---+ Fig. I. Intercomparison of ASS1 and SCR solar spectra recorded on 11 and 10 November 1988, respectively. The SCR spectrum as shown in Fig. 1 was corrected for atmospheric absorption, thus representing the solar fluxes outside the atmosphere. Ideally, the efficiency changes during the mission should be traced directly. For technical reasons, this has not been possible up to now. Consequently, no true in-flight calibration has ever been applied to satellite spectrometers. Pre-flight calibration as obtained up to five years prior to the satellite launch in most cases does not reflect any calibration changes during the mission itself. Cross-calibration by rocket instrumentation as performed by HEROUX and HIN- TEREGGER (1978) provides just one point in the com- plex time-dependent data set, from which the short-, medium- and long-term variability of the solar flux has to be extracted by distinguishing between real and fictitious flux changes. An on-board radioactive Ni63 source was included in the ASS1 instrument to provide an estimate of the overall sensitivity change during the mission. The ASS1 data in Fig. 1 are based on pre-flight calibration. The determination of the spectrometric efficiencies at various times during the mission has not been completed. Since the change of the radiometric parameters during the mission is expected to be wavelength-dependent, no correction of the ASS1 data could yet be taken into account. However, there are good estimates available : (1) (2) For ASSI, different calibration techniques had to be applied in the EUV, UV and VIS spectral regions. In the EUV, pre-flight calibration was performed about 5 years prior to flight, whereas in the UVjVIS ASS1 was recalibrated some days before launch. In addition, a Spectral Calibration Rocket (SCR) was launched on 10 November 1988 as described by WOODS and ROTTMAN (1990) to provide an in-flight sensitivity calibration. Data from the internal calibration (Mode 4) show a degradation of the channeltron in chan- nel 16 from 100% (pre-flight calibration) down to 32% on 5 November 1988, which is rep- resentative for the short wavelength part of the spectrum (below 80 nm). While the spectrum shown in Fig. 1 was recorded, the altitude at the satellite in its equa- torial orbit changed from 263 km at the longest wavelength to 228 km at the shortest. During the same period the solar zenith angle increased from 30.8 to 65.2 as Local Time changed from 10.06 to 15.09 h. Using the MSIS-86 model (HEDIN, 1987) for the appropriate condition and the absorption cross-sections as published by SCHMIDTKE (1979), atmospheric absorption was found to be less than 1% for the emission Cl11 at 97.7nm, about 17% at 89-86nm (Lyman continuum) and about 63% at Hel 58.4nm. The SCR was launched from White Sands Missile Range, New Mexico (latitude = 32.42N, long- itude = 106.32W), at local noon time (FlO., = 147.7). At this time the solar zenith angle was SO. The rocket reached a peak height (apogee) of 237 km. In view of these complex effects, the agreement of the solar fluxes as recorded in channel 16 of ASS1 on 11 November 1988 (F,0,7 = 153.8) with the results from SCR on the previous day is encouraging. How- ever, this comparison must be considered preliminary Equatorial airglow measurements 783 due to incomplete calibration analysis of the ASS1 data. The efficiency changes during the San Marco-5 mis- sion are being evaluated according to the following procedures : (a) At the beginning of the mission, pre-flight cali- bration is used. (b) Evaluation of the in-flight calibration based on the exposure of the four EUV channeltrons to radioactive beta-emission (Ni63) during the mission (Mode 4). (c) At longer wavelengths (above 280 nm) the Sun itself is used as a stable calibration source during the mission. (d) Since a large degree of redundancy exists, inter- comparison of the recorded fluxes from the 18 chan- nels during the mission is used to ensure self-con- sistency. Since there are more relevant calibration data avail- able for this experiment than for any other spec- trometer flown so far, we expect to derive absolute fluxes with a radiometric accuracy of better than 20%. 4. ASS1 MEASUREMENTS During the San Marco-S mission, ASS1 was re- cording complete solar (and dayglow) spectra almost every second day. During other periods, solar emis- sions were measured in smaller wavelength intervals at 16 out of the 112 steps. In this sequence, the solar variability has been monitored with a time resolution down to one day (due to spacecraft manoeuvres there are some weeks without data acquisition) that is adequate for most of the aeronomic applications. Also the data statistics and the spectral resolution are adapted to aeronomic requirements (see Figs 2 and SOIAR SPECTRUM SO600 (05%MAY-1988) 351 km WANNLL 12 5 llll 9770 km 30 50 70 90 wavelength /nm - Fig. 2. Solar spectrum recorded in channel 15 on 5 May 1988. SOCAR SPECTRUM SO600 (05~MAY-I 988) 351km CWNHEL I, 512km I 105 115 125 135 115 wavelength / nm - Fig. 3. Solar spectrum recorded in channel 12 on 5 May 1988. 3). The final data for the San Marco-5 mission will be presented in solar flux tabulations similar to the ones from the AEROS-EUV spectrometer by SCHMIDTKE (1976a). As derived from the first data set available, the solar EUV flux revealed changes between 20 and 30% for the days 27 April-21 May 1988. Similar to the ASS1 data in Fig. 1, the attitude of the spacecraft changed for the complete spectral scans shown in Figs 2-6, obtained during a single pass. From the long- to the short-wavelengths the altitude decreased from 542 to 353 km, while the solar zenith angle increased from 28.9 to 47.1, respectively. Since ASS1 has measured solar and airglow emis- sions quasi-simultaneously (within 10s of one spin revolution) by the same channels at the same wave- length positions, for the first time solar flux changes and the corresponding airglow intensity changes have AIRGLOW SPECTRUM A50600 (05~MAY-1988) 351km CIUNNEL II 512km h&00/ I IHI B z 300 m : 250 2 200 s 150 100 li5 f, 135 115 wavelength Inm - Fig. 4. Airglow spectrum recorded in channel 15 on 5 May 1988 (each step is averaged for one spin revolution). 784 G. SCHMIDTKE et al. AIRGLOW SPECTRUM Al 0600 (05%MAY- 1988) 70 90 110 130 wavelength / nm - Fig. 5. Airglow spectrum recorded in channel 18 on 5 May 1988 (each step is averaged for about 2 s in zenith direction). been monitored by the same instrument. Because of different orbit conditions during the mission, a variety of in-situ data are available now, reflecting atmo- spheric and solar conditions not yet sensed. For exam- ple, the various thermospheric emissions are char- acterized by different optical thickness thus showing very specific intensity distributions in different ther- mospheric height regimes as recorded during each of the spin revolutions. The spatial resolution achieved in these measurements allows for up to 40 separated measurements per spin revolution. Measurements of this type will also be important for testing airglow models. As seen in Figs 4-6, the dominant airglow emissions originate from atomic hydrogen, helium, oxygen and nitrogen. As to the latter, the resonance emission at 120.0nm is clearly separated from the dominating hydrogen Lyman-alpha line. Its spatial distribution is AIRGLOW SPECTRUM A10600 (05~MAY-1988) 351km CMNEL 18 Mfw ww*Fa, 5 t 500 :m wavelength I nm - Fig. 6. Airglow spectrum recorded in channel 18 on 5 May 1988 (each step is averaged for about 2 s in nadir direction). of special interest to the physics in this height regime. The Lyman-Birge-Hopfield bands from molecular nitrogen contribute less to the escaping airglow flux than the emissions from the atomic species (see Fig. 4 with flux values averaged for one spin revolution). A lot of information on the state of the ther- mosphere is contained in the airglow spectra looking upward or downward from the position of the space- craft (LINK et al., 1988). Data, similar to those shown in Figs 5 and 6, and others will be used in a detailed analysis of airglow emissions to infer the physical state of the ther- mosphere and its morphology. 5. DATA EVALUATION The detailed study of airglow and aurora1 emissions includes computer models of non-thermal electron energy degradation, optical excitation processes, photochemistry and radiative transfer of optically thick resonance lines. In the case of dayglow emissions, such as those observed by ASS1 (see Figs &6), the analysis begins with the use of appropriate models of atmospheric composition and solar EUV flux for the observing condition. The effects of solar and geomagnetic activities are included in these models, as appropriate. The geographic location and hour angles are computed. The interaction of the solar EUV flux on the atmosphere is described by the prod- ucts of photoionization, photodissociation, photo- absorption and photoelectron production. In the case of visible emission, both ion and neutral photo- chemistry plays an important role in the deter- mination of the final optical products ; however, since the emission thresholds of the EUV and UV emissions are high (approximately, 1OeV or larger), photo- chemistry can be ignored. For emissions of ionic spec- ies, one needs to invoke a model ionosphere. The effect of the geomagnetic field on the distribution of ions and electrons also needs to be included in the model. The model computes the ionization and excitation rates of various emission features of the most abun- dant thermospheric species. Subsequently, a radiative transfer model computes the expected intensities for the observing geometry. The model results are then compared with the ASS1 observations. At Berkeley, the following theoretical models have been developed for : (a) atmospheric photoionization, photodis- sociation, photoabsorption, and photoelectron pro- duction ; aurora1 and photoelectron transport and energy degradation processes with energy-dependent pitch angle redistribution (multi-stream, 2-stream, and local equilibrium versions) ; (b) one- and two-dimensional radiative transfer of optically thick resonance lines with complete or par- tial frequency redistribution including temperature gradients, with external or internal sources ; (c) thermospheric ion and neutral photochemistry. Other models that have been coded and/or implemented are : Equatorial airglow measurements 785 LINK et al., 1988). The application to the specific equatorial airglow conditions as encountered by ASSI/San Marco-5 using the solar input as measured by the same instrument is underway. 6. CONCLUSIONS (a) thermospheric composition [JACCHIA, 1977 ; MSIS-76, MSIS-83, and MSIS-86 : HEDIN (1987) and references therein] ; The good performance of the ASS1 experiment shows that it is possible to measure solar and airglow fluxes with one and the same instrument over a broad wavelength range. (b) ionospheric structure [CHIU, 1975; Inter- national Reference Ionosphere (IRI)] ; (c) the Semi-empirical Low-latitude Ionospheric Model (SLIM) (ANDERSON et al., 1987) ; (d) the Fully Analytic, Low- and Middle-latitude Ionospheric Model (FAIM) (ANDERSON et al., 1989) ; (e) the geomagnetic field (International Geo- magnetic Reference Field) and field-line tracing models. In the EUV spectral region the strongest variability of the solar flux coincides with the strongest changes of the calibration parameters in space instru- mentation. In order to distinguish between both effects and to provide flux data of higher radiometric accuracy, no future satellite mission should be under- taken without true in-flight calibration. To achieve the latter, a method has been proposed by SCHMIDTKE (1976b). A versatile, comprehensive modelling capability is now at hand which enables us to model complex exci- tation processes such as those that occur in the sunlit dayside aurora, where photodissociative excitation, aurora1 electron impact, photoelectron impact, photo- chemistry, and resonance scattering of sunlight may all play a role. These models have been used in the analysis of specific problems (GAULT et al., 1980; LINK and COGGER, 1990; GLADSTONE et al., 1987; The development of airglow models relevant to ASSI/San Marco-5 data is at a stage to be applied to the new type of in-situ measurements. Acknowledgements-We greatly acknowledge the competent support provided by the Bundesministerium fiir Forschung und Technologie, by the Deutsche Forschungsanstalt fiir Luft- und Raumfahrt and by the Deutsche Agentur fiir Raumfahrt-Angelegenheiten, represented by Herrn Otter- bein and by Dr Schneppe. Work at U.C. Berkeley was sup- ported by NASA grant NAG5446 and U.S. Army grant DAAL03-89-K0057. REFERENCES ANDERSON D. N., FORBES J. M. and CODRFXU M. ANDERSON D. N., MENDILLO D. N. and HERNITER A. CHIU Y. T. GALJLT W. A., KOEHLER R. A., LINK R. and SHEPHERD G. G. GLADSTONE G. R., LINK R., CHAKRABARTI S. and MCCONNEL J. C. HEDIN A. E. HEROUX L. and HINTEREGGER H. E. JACCHIA L. G. LINK R. and COGGER L. L. LINK R., GLADSTONE G. R., CHAKRABARTI S. and MCCONNEL J. C. REEVES E. M. and PARKINSON W. H. SCHMIDTKE G. SCHMIDTKEY G. SCHMIDTKE G. SCHMIDTKE G., KNOTHE M. and HEIDINGER F SCHMIDTKE G., SEIDL P. and WITA C. WEEDS T. N. and ROITMAN G. J. 1989 J. geophys. 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Temperature determination in the lower equatorial thermosphere from occultation measurements, made aboard the San Marco 5 satellite