GLOBAL OSCILLATIONS AT LOW-FREQUENCY FROM THE SOHO MISSION (GOLF)

The GOLF experiment on the SOHO mission aims to study the internal str ucture of the sun by measuring the spectrum of global oscillations in the frequency range 10(-7) to 10(-2) Hz. Both p and g mode oscillation s will be investigated, with the emphasis on the low order long period waves which penetrate the solar core. The instrument employs an exten sion to space of the proven ground-based technique for measuring the m ean line-of-sight velocity of the viewed solar surface. By avoiding th e atmospheric disturbances experienced from the ground, and choosing a non-eclipsing orbit, GOLF aims to improve the instrumental sensitivit y limit by an order of magnitude to 1 mm s(-1) over 20 days for freque ncies higher than 2.10(-4) Hz. A sodium vapour resonance cell is used in a longitudinal magnetic field to sample the two wings of the solar absorption line. The addition of a small modulating field component en ables the slope of the wings to be measured. This provides not only an internal calibration of the instrument sensitivity, but also offers a further possibility to recognise, and correct for, the solar backgrou nd signal produced by the effects of solar magnetically active regions . The use of an additional rotating polariser enables measurement of t he mean solar line-of-sight magnetic field, as a secondary objective.