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Astron. Astrophys. 346, 626-632 (1999)

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2. Instrument and observational data

The SOHO mission consists of a three-axis stabilized spacecraft with eleven scientific instruments that studies the Sun (Domingo & Poland 1988). It offers an unprecedented opportunity to study its deep interior through helioseismology under ideal conditions at the Lagrange [FORMULA] point. The GOLF instrument measures the disk integrated radial velocity of the Sun over a wide frequency range and the longitudinal component of the line-of-sight global magnetic field with an accuracy better than 1 mG in the same frequency range.

GOLF is a disk integrated sunlight resonant scattering spectrophotometer that measures the Doppler shift of the solar sodium doublet ([FORMULA] at [FORMULA] 5896 and [FORMULA] at [FORMULA] 5890 Å) due to a non zero relative Sun-instrument line-of-sight velocity. This technique has been successfully used in solar physics by several researchers (Snider 1970; Fossat & Roddier 1971; Brookes et al. 1978).

The solar absorption line (half-width [FORMULA]500 mÅ) enters a sodium vapor cell, placed in a longitudinal magnetic field ([FORMULA]) of [FORMULA] 5000 G, which has an intrinsic (thermal) absorption line-width of the order of 25 mÅ, where it is absorbed and re-emitted in all directions. This scattered light is symmetrically split into its Zeeman components displaced [FORMULA]106 mÅ ([FORMULA] case) from the rest wavelength, allowing a measurement on either side of the wings of the solar absorption profile. Switching between both wings, by appropriately moving a quarter-wave plate and a linear polarizer, alternates the measurement of the intensity, therefore measuring its Doppler shift. The observed velocity will be proportional to a normalized difference of intensities that measures the shift.

By using a modulated magnetic field ([FORMULA]), the solar absorption profile can be sampled at four different points and an instantaneous calibration of the instrument sensitivity can be made (Isaak & Jones 1988; Boumier 1991; García 1996). Adding a fixed quarter wave plate in front of the other two polarizing elements and moving them appropriately, an alternate selection of the two circular polarized components of the sodium doublet can be made, allowing the measure of the longitudinal comp onent of the line-of-sight global magnetic field of the Sun and leading to a total of 8 different observables with an integrating time of 5 seconds each (see Fig. A1). A full description of the instrument can be found in Gabriel et al. (1995, 1997).

These 8 measurements of the nominal GOLF operating mode began on January 18, 1996. However, due to occasional malfunctions of the mechanism that turns the quarter wave plate, it was stopped on February 12 to ensure the continuity of the velocity data acquisition, with a duty cycle near [FORMULA]. Therefore, only 26 days of SMMF measurements are available. It should also be noted that this period of time corresponded to the instrument commissioning when the conditions of subsystem temperatures, mechanism and pointing were not always stable, introducing some extra noise into our data set.

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© European Southern Observatory (ESO) 1999

Online publication: May 21, 1999