Studies of the solar mean magnetic field with the Birmingham Solar-Oscillations Network (BiSON)

The first analysis of 10 yr of solar mean magnetic field data from the Birmingham Solar-Oscillations Network (BiSON) instrument at Sutherland, SA is presented. Long-period (>1 d) variations correlate well with those observed by the Stanford group, but our daily mean values have a statistical precision over 10 times better than previous work. Differences between the absolute values determined from Sutherland and Stanford are discussed, as are systematic annual variations of the order of 15 per cent. At high frequencies (>0.5 mHz), the mean noise spectral density for our decade of observations is 0.53 G2 Hz−1, setting a lower limit to the noise level than that obtained from brief observations by GOLF on board the ESA/NASA satellite SOHO in 1996. The high-frequency mean noise spectral density is different in different years. The lowest annual value, obtained for 1997, is 0.22 G2 Hz−1, whilst the highest, for 1995, is 1.46 G2 Hz−1. It is not yet clear whether these variations are related to solar activity.