The following instruments were placed on Picard microsatellite:
- SOVAP measures the total solar irradiance.
- PREMOS measures the spectral irradiance in four spectral domains as well as the total solar irradiance,
- Measures the solar diameter and the limb shape at three wavelength in the photospheric continuum according to the heliographic latitude. The chosen wavelengths are 535, 607, 782 nm in the spectral domains excluding the Fraunhofer lines.
- Detects the active regions (faculae and sunspots) that might corrupt the diameter measurements. The chosen wavelength is the CaII emission at 393 nm which will furthermore be used to measure the differential rotation.
- Studies in relation with PREMOS the effets of the solar activity on the spectral irradiance and the Sun radiance image at 215 nm.
- Studies the influence of the active regions on the diameter.
- Achieves a deep sounding of the Sun.
- The instruments were operated by on-board electronics contained in a box named PGCU (Picard Gestion Charge Utile), which includes all functions necessary to operate the three instruments: formatting of telemetry, receipt of commands, thermal regulation system, image compression, measurements sequencing, power supply, safety management.
The Picard instruments and measurements general characteristics are:
- SOVAP is constituted of 2 instruments: the first is a DIARAD differential absolute radiometer (Crommelynck and Domingo, 1984; Dewitte et al., 2001 et 2004) which measures the total solar irradiance (with a measurement accuracy of ± 0.1%). This instrument has been flown on several space platforms on board the Nasa's shuttle, the EURECA carrier and the Soho satellite. The principle of this instrument consists in two cavities: one is directly heated by the solar flux, and the other is heated by Joule effect, but shielded from the Sun by a shutter. To ensure the thermal equilibrium both cavities are linked to a thermocouple. When the temperatures inside both cavities are equal, the energy recieved from the Sun can be deduced from the measurement of the heating current of the shielded cavity. Corrective terms linked to thermal leakages and to the surface of each entrance pupil must be accurately measured. Each cavity has a shutter to enable alternatively exposing each cavity to the Sun radiation. The second instrument is the Bolometric Sensor BOS (BOlometric Sensor) which measures the variations of the total solar irradiance with a measurement every 10 seconds (while DIARAD realise an absolute measurement every 3 mn). The 2 measurements combined enable to measure the total solar irradiance with an improved temporal resolution.
- PREMOS also measures the total solar irradiance. The instrument uses the same principle, but differs by several details, particularly its operating mode for which only one cavity is exposed to the Sun. This radiometer has been flown with several space missions in particular on board Soho (Fröhlich et al., 1995 ; Fröhlich et al., 1997). PREMOS also measures the solar spectral irradiance in 5 channels: two in UV corresponding to the spectral domains specific of the ozone photochemistry, one in the visible and two in the near infrared domain. These measurements will also be used for helioseismologic measurements and will be related to those from SODISM at the same wavelengths. Since 1978, two radiometers or more have always been in orbit, and as the missions have had overlap periods to ensure continuity and comparison of the measurements. Picard used the radiometers as in the configuration of the VIRGO instrument on board Soho.
- SODISM is an 11 cm diameter telescope with a 2048x2048 usable pixels CCD. The relative accuracy of the measurements to reach the scientific goals is a few milliarcseconds (3 ) per image. The uncertainty is essentially due to the pixellisation of the observed limb. Its effect will be decreased by integration (by orbit, by day,...) due to the time constant as expected from the development of the solar activity, enabling to reach our objective of one milliarcsecond. The expected accuracy of the measurements is based on the very great dimensional stability, which is ensured by use of stable materials (Invar and carbon-carbon for the structure, Zerodur for the mirrors) and an accurate thermal regulation (1°C) of the whole instrument. The detector will also be thermally regulated (0.1°C) to keep constant the size of the pixels. Nevertheless, to alleviate any evolutions of the metric characteristics of the instrument, an angular reference has been included in the instrument. Four prisms are used generating four auxiliary images placed in each corner of the CCD. The distance between a point of the limb of the central solar image and the corresponding point of the auxiliary image only depends on the angle of the prism and of the temperature which will be measured with the appropriate accuracy. These measurements enable to check the relationship between the angular distance of two points on the Sun and the distance of their images on the CCD (see optical diagram). The solar diameter will be referenced to the angular distances of doublets of stars so that the measurements which will be achieved in the next decades referenced to the same doublets, enable to evaluate the long term evolution of our sun. These doublets have been identified thanks to the Hipparcos catalog. Their positions should be corrected of their own proper movement by the use of future astrometric missions.
Some modern measurements performed from the ground show contradictory variations of the diameter according to the solar activity as detailed in the "Science" page. To avoid the influence of the atmosphere, measurements will be made in orbit. However, it is necessary to understand and interpret the measurements from the ground, which constitute the longest series currently available. That is why an important program of measurements from the ground is associated to the space operation before, during and after the Picard mission. For that, we already have developed a radiation transfer model through the atmosphere of the Earth which will be validated by comparison between simultaneous measurements in orbit and from the ground.
The ground measurements will be achieved by the following instruments placed on the Plateau de Calern (South of France) which is the observing site of the Observatoire de la Côte d'Azur (OCA):
- the Danjon astrolabe, DORAYSOL (Définition et Observation du RAYon Solaire, Delmas, 1999) and the replica of the space instrument (SODISM II) for the measurement of the solar diameter,
- a telescope to characterise and monitor the atmosphere turbulence (MISOLFA).
This set of instruments named Picard-SOL will enable understanding the modification of the solar limb shape induced by the solar photons travelling through the atmosphere, by comparing the limb shape and diameter measured in orbit with the ground based measurements. It will enable after the end of Picard mission to continue the measurements from the ground with the possibility to interpret them, in principle without ambiguity.