Preliminary Results from the SOVA Experiment on ...

Preliminary Results from the SOVA Experiment on Board the European Retrievable Carrier (EURECA)

D.Crommelynck, V. Domingo, A . Fichot, C. Frohlich, B. Penelle, J . Romero and Ch. Wehrli

Abstract. The experiment SOVA (Solar VAriability) on board EURECA (European REtrievable CArrier) contains two types of radiometers sunphotometers for measuring the total and spectral irradiance at five wavelengths between 335 nm and 862 nm. Besides the direct observation of variations of the solar irradiance, these measurements will also be used for the observation of solar oscillations. Preliminary results are presented from the observations of total and spectral irradiance during the first five months of the mission (August to December 1992). 1. Introduction

The SOVA (Solar VAriability) investigation is the result of a collaboration between the Institut Royal Miteorologique de Belgique (IRMB) in Brussels (Belgium), the Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center (PMOD/WRC) in Davos (Switzerland) and the Space Science Department (SSD) of the European Space Agency (ESA) at ESTEC in Noordwijk (the Netherlands). The experiment measures the total and selected spectral solar irradiances at five wavelengths between 335 nm and 862 nm and was launched on board EURECA (European REtrievable CArrier) 3 1 July 1992 by the shuttle Atlantis. The total irradiance of the Sun measured at a distance of one astronomical unit is traditionally called the solar constant, S, although it is known to vary. Incident on the Earth it is its prime source of energy, any change in S affects the energy budget of the Earth and, if prevailing over long periods of time, may influence the Earth’s climate. Knowledge of the absolute value of S and its temporal variations is important for evaluation of anticipated global change D. Crommelynck, A. Fichot and B. Penelle: Institut Royal Meteorologique de Belgique, B-1180 Bruxelles, Belgium. V. Domingo: Space Science Department of the European Space Agency, NL-2200 Noordwijk, the Netherlands. C. Frohlich, J. Romero and Ch. Wehrli: PhysikalischMeteorologisches Observatorium, World Radiation Center, CH-7260 Davos-Dorf, Switzerland.

and the distinction between anthropogenic and natural forcing of the Earth’s climate. On the other hand an improved understanding of the physical mechanisms producing the variations, and their link to other solar phenomena such as activity, is a necessary prerequisite for understanding possible past and future solar forcing of the Earth’s climate. The observation of solar oscillations also has to be put in this context. The importance of such oscillations for the understanding of the interior of the Sun is widely accepted and in the recent past the connection between changes in solar oscillation frequencies and solar irradiance variations has been recognized as an important contribution to the investigation of the influence of changing magnetic activity on both - the basis for the understanding of the solar cycle. The main science objective of SOVA is to obtain time series of solar irradiance of very high precision and accuracy which will be used to: (a) obtain continuous high quality measurements of variations in the solar irradiance; (b) provide accurate total and spectral irradiance measurements for input to climate modelling; (c) search for the periodicities or quasi-periodicities (up to periods of about 150 days) found in other solar parameters; (d) determine the frequencies, amplitudes and phases for solar oscillation modes in the frequency range 1 pHz to 10 mHz and, if they exist, detect and classify low-degree g-modes of solar oscillations;

(e) study the relation between solar oscillation frequency changes and luminosity variations; ( f ) study the influence of active regions and other large-scale solar structures on total irradiance;

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(g) determine the amount of spectral redistribution of the solar output during changes of total irradiance; (h) investigate the energy storage in the convection zone in connection with the energy blocking by sunspots and enhanced emission by other magnetic structures in active regions and the quiet parts. A further important objective is to contribute to the metrology of radiation through materialization of the World Radiometric Reference (WRR) in space by the simultaneous operation of two different types of absolute radiometer which are traceable to the WRR realized by the World Standard Group maintained at the PMOD/WRC. With the retrieval of EURECA and SOVA in spring 1993 possible degradation mechanisms, especially of the spectral instruments, can be studied. 2. Instrumentation

The experiment comprises three parts, designed and manufactured by the IRMB for SOVA 1, the PMOD/WRC for SOVA 2 and the SSD for SOVA 3 (Figure 1). SOVA 1 contains a dual-channel differential absolute radiometer of type CROM [l] and a sunphotometer with two channels at 480 nm and 546 nm (HPBW 5 nm). SOVA2 contains a set of two redundant absolute radiometers of type PMO-6 [2], which have a device to monitor the reflectance of the cavity during flight (hence PMO-6R), and two redundant sunphotometers of the type used in the IPHIR experiment (Interplanetary Helioseismology with IRradiance measurements, [3]) flown on the Russian PHOBOS mission in 1988/89. They have three channels centred at 335 nm, 500 nm and 862 nm (HPBW 5 nm). The selection of wavelengths was guided by interest in covering a wide range of the spectrum without being too much influenced by solar lines or by the requirement that tests on the ground, with the Sun as source, should be possible. Furthermore, a high resolution radiometer RELOS (RELative OScillation radiometer) designed to measure oscillations in total solar irradiance is included. Both SOVA 1 and SOVA 2 have their own data acquisition systems based on voltage-to-frequency converters with a resolution of 22 bits in 8,2 s. Several housekeeping parameters, such as temperatures, currents and voltages, necessary to monitor the health of the experiment are measured, and these are used, if required, to correct the acquisition of data. Moreover, fine and coarse Sun-pointing monitors are im-

Figure 1. The SOVA 1 (B), SOVA 2 (A) and SOVA 3 (C) packages with RELOS (D), triple sunphotometers (E), PMO-6 radiometers (F), fine Sun sensor TASS (G), CROM radiometer (right channel, H), dual photometer (I) and the coarse Sun sensor (J).

plemented (in SOVA 2 and SOVA 1, respectively) to measure the attitude and relative pointing of the satellite with respect to the Sun’s centre. From this pointing information, corrections for the measured irradiances are deduced. SOVA 3 contains the interface to the spacecraft’s data handling and power system, and control software for SOVA 1 and SOVA 2 that handles telecommands and packetized telemetry. It also provides the control of SOVA 1 and SOVA2 as well as their digital data acquisition. All these functions are performed with one of the two redundant CPUs in SOVA3. The dual-channel differential absolute radiometer CROM is based on two cylindrical cavities, with flat bottoms, which are coated with diffuse black paint and mounted side-by-side on a common heat sink. The cavities are identical, each is fixed to a heat flux sensor on top of which is mounted a flat heater. The symmetrical arrangement of the sensors allows for sixteen different radiometric modes of operation. These modes allow for internal tests and for redundancy in the operation of the radiometer. In the active

mode normally used, the shuttered cavity is heated with a constant power. The difference of the two thermopiles sensing the temperature difference across the heat flow meter is used to control the heater of the alternatively shaded and irradiated cavity in such a way that the heat fluxes in the cavities remain constant. The shutters are actuated on a timing cycle which corresponds to periods of 99 s closed and 99 s open. Thus an irradiance value is obtained every 198 s. In the PMO-6R radiometer the radiation is absorbed in a cavity with an inverted cone painted with a specular black paint, which ensures high absorptivity over the spectral range of interest for solar radiometry. The detector arrangement of PMO6 consists of two sensors mounted back-to-back; the front one is used for radiation measurement and the back one for compensation of rate of change variations of the heat sink (differential heat flow meter). The instrument is also used in active mode, which means that the heat flux is held constant by suitable control of the power fed to the front cavity heater during the shaded and the irradiated cycle. The irradiance is calculated from the difference between the shaded and irradiated electrical powers taking into account a constant C which is the inverse of the aperture area times a correction factor for deviations from ideal behaviour, which are determined experimentally [2]. The back cavity is heated at constant power from a d-a converter, which can be adjusted by commands from the ground. The timing is based on cycles of 41 s closed and 58 s open, and an irradiance value is obtained every 99 s. The sunphotometers consist of DFlO series filtered silicon diodes made by EG&G with custommade filters by Barr Associates, Inc. The main advantage of this detector assembly is the fact that it has only one exposed optical surface which is the glass window of the case. The detectors are used in unbiased mode in order to minimize l/f noise. Ultralow bias current, low-noise electrometer amplifiers are used to convert the detector current to voltage. A precision of < I ppm is reached with these instruments for an integration time of 8,25 s.

of the orbit. This has a small, but important, effect on the behaviour and specifications of the radiometers which is not yet completely accounted for in the evaluation of the data. Hence we only present relative values of the solar total irradiance. During the period analysed to date (end of 1992) several other temperature excursions have been experienced due to operation of the satellite's cooling system on which the base of the experiment is mounted. The pointing of the EURECA spacecraft towards the Sun was specified to be within f 1". These rather large limits necessitated the addition of the coarse and fine Sun sensors in order to enable corrections to be made for changes in the angular sensitivity of the sensors. The first period of observation showed a much better performance of the attitude control system: the offset between the SOVA optical axis and the Sun-satellite line never exceeded 0,4" and stayed well within 0,2" most of the time. For the radiometers, the correction for offset pointing is straightforward: the cosine law is used. The SOVA 2 SPMs, however, show a high sensitivity to offset pointing as found during the PHOBOS mission, so the correction algorithm is much more complicated and has not yet been completed.

4. Preliminary Results The sunphotometers showed an important sensitivity degradation right from the beginning of the operation. This behaviour was expected as it had already been experienced by IPHIR during the PHOBOS mission. It is shown in Figure 2 for the SOVA2 continuously operated SPM and for its backup. This degradation is due partly to the detectors and partly

3. Operation The operation of SOVA started on 10 August 1993

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