We have shown that the NSPA, which varys with the magnetic cycle, corrupts the low degree mode spectrum impeding a precise seismic sounding of the Sun's core. In years of low activity, the effect is well within the observers' quoted errors. One could evade the problem by discarding data from years of high activity. However, such an approach would not take advantage of data from long timebase observational programs like BISON, IRIS, GONG or SOHO. Another way to evade is to keep only low frequency modes which are much less affected by the NSPA, but it is the high frequency modes, as Figs. 1 and 2 show, which contain most of the information about the inner core. We must realize that there is abundant information about the NSPA in the high-l modes, and the NSPA may be precisely calculated from these modes. The requisite information base is vast and is contained in the even-a coefficients. This information may be straightforwardly converted into corrections to low-l mode frequencies.
© European Southern Observatory (ESO) 1997
Online publication: July 8, 1998