The Picard satellite carried three instruments:
- SOVAP to measure total solar irradiance
- PREMOS to measure spectral solar irradiance in four spectral domains and total solar irradiance
- SODISM to:
- Measure the Sun’s diameter and the shape of its limb at three wavelengths in the solar continuum as a function of latitude. The wavelengths were 535, 607 and 782 nm in spectral domains that exclude Fraunhofer lines.
- Detect active regions (faculae and sunspots) likely to affect the quality of measurements at the limb. The wavelength was the Ca II emission wavelength at 393 nm, which was also used to measure differential rotation.
- Study in tandem with PREMOS the effects of solar activity on spectral irradiance and Sun images at 215 nm.
- Study the influence of active regions on the diameter.
- Probe the Sun’s deep interior.
- The instruments were managed by an electronic unit called PGCU (for Picard Gestion Charge Utile, or payload management unit) providing all the functions needed to operate the three instruments: telemetry formatting, reception of commands, thermal control, image compression, measurement sequencing, low-voltage power, safe-mode management, etc.
General instrument and measurement characteristics:
- SOVAP in fact consisted of two instruments. The first was an absolute differential radiometer called DIARAD (Crommelynck and Domingo, 1984; Dewitte et al., 2001 et 2004) that measures total solar irradiance (accurate to ± 0.1%). This instrument had already flown missions on the U.S. space shuttle, the EURECA platform and the SOHO satellite. The instrument comprised two cavities, one heated directly by the solar flux and the other by Joule effect but shielded from the Sun by a shutter. When the temperatures inside the two cavities are equal, the received solar energy is deduced by measuring the current needed to reheat the masked cavity. Corrective terms for thermal leaks and leaks at the surface of each front lens must be measured precisely. Each cavity has a shutter to expose it alternately with the other to the Sun. The second instrument was BOS (BOlometric Sensor), which measured variations in total solar irradiance every 10 seconds (whereas DIARAD acquires absolute measurements every 3 minutes). The two measurements were combined to gauge the total solar irradiance with improved temporal resolution.
- PREMOS also measured total solar irradiance, using the same principle as SOVAP but with a few differences, notably its operating mode for which only one cavity was exposed to the Sun. This radiometer had flown several missions previously, including on SOHO (Fröhlich et al., 1995; Fröhlich et al., 1997). PREMOS additionally measured solar flux in five channels: two UV channels corresponding to the spectral domains characteristic of ozone chemistry, one in the visible and two in the infrared. These measurements were used for helioseismology and matched to those from SODISM acquired at the same wavelengths. Since 1978, two radiometers or more have constantly been in orbit, thus providing overlap to assure data continuity and comparison. Picard’s radiometers were operated in the same configuration as the VIRGO instrument on SOHO.
- SODISM was an 11-cm-diameter telescope with a 2048x2048-pixel CCD. The relative precision of measurements needed to accomplish the mission’s science goals was a few milli-arcseconds (3) per image. The main uncertainty concerned pixellation, which was reduced by integration (per orbit, per day) due to the expected time constants as a result of solar activity, thus making it possible to reach the goal of 1 milli-arcsecond.
Certain recent measurements from the ground show contradictory variations in the Sun’s diameter as a result of solar activity, as detailed in the Science section. Measurements from orbit avoid the effects of Earth’s atmosphere. However, ground measurements—which constitute the longest time-series currently available—need to be clearly understood and interpreted. A major programme to collect ground measurements in tandem with space measurements was therefore planned before, during and after the Picard mission. A programme modelling radiative transfer through Earth’s atmosphere was developed for this purpose, to be validated by comparing simultaneous measurements from orbit and from the ground.
Ground measurements were performed by instruments on the Calern plateau in the South of France, attached to the Côte d'Azur Observatory (OCA):
- DORAYSOL (Définition et Observation du RAYon Solaire, Delmas, 1999) Danjon astrolabe and the replica of the space instrument (SODISM II) to measure the Sun’s diameter
- a telescope to characterize atmospheric turbulence (MISOLFA)
This set of Picard ground instruments helped to understand how the solar limb is altered by photons coming through the atmosphere, by comparison with measurements from space. Ground measurements will be pursued after the Picard mission, and we should thus be able to interpret them unambiguously.
View of the Calern site where the Picard mission’s
ground instruments are located.