The fine-structure spacing d _ { \ell } ( n ) = \nu _ { \ell,n } - \nu _ { \ell + 2 ,n - 1 } for low-degree solar p modes of angular degree \ell and radial order n is sensitive to conditions in the deep radiative interior of the Sun .
Here we present fine-structure spacings derived from the analysis of nearly five years of helioseismological data collected between 1991 July and 1996 February by the Birmingham Solar-Oscillations Network ( BiSON ) .
These data cover 9 \leq n \leq 28 for d _ { 0 } ( n ) , and 11 \leq n \leq 27 for d _ { 1 } ( n ) .
The measured spacings are much more precise and cover a greater range , than earlier measurements from BiSON data ( Elsworth et al .
1990a ) .
The predicted fine-structure spacings for a “ standard ” solar model are clearly excluded by the BiSON data ( at \approx 10 \sigma ) ; models that include helium and heavy element settling provide a much better match to the observed spacings ( see also Elsworth et al .
1995 ) .
Since the inclusion of core settling in solar models will tend to slightly increase the predicted neutrino flux , the BiSON fine-structure data appear to reinforce previous conclusions , i.e. , an astrophysical solution to the solar neutrino problem seems unlikely .