This discussion covers two aspects of abundances/processes in the photosphere. One regards only those nuclei which are easily destroyed by thermonuclear processes, which can provide information about the development and mixing history at the base of the convection zone with the hotter material below it. The second part of the discussion relates to the more general issue of gravitational settling in the sun.
Constraints based on thermonuclear destruction are interpretationally intertwined with that discussed in Document E. The concept here is to observe consequences of solar thermonuclear processes (as opposed to nuclear reactions due to accelerated particles), either during an early completely convective stage in solar evolution or at the base of the surface convection zone throughout the age of the solar system. Only time integrated isotopic variations would be observed; consequently the results should be viewed as a constraint on theories for the structure and evolution of the surface convection zone including the time, temperature history of the development of the convection zone in the early stages of solar evolution and a constraint on early main sequence mass loss (e.g., Boothroyd et al., 1991). Like the analogous situation discussed in Document H, the effects of charged particle and thermonuclear reactions can be sorted out from the data themselves, although in this case, obtaining the required measurements for a unique interpretation is more difficult. The nuclei sensitive to thermonuclear processes are D, 3He, and the isotopes of Li, Be, and B (measurement objective 11 in Table 1-2). As a specific example, the photospheric abundance Li/Si is about 50 times less than the corresponding chondritic value. This is normally interpreted as thermonuclear destruction of Li, either in the early totally convective phase or continuously over the age of the solar system at the base of the presentday surface convection zone. From these assumptions all solar wind Li should be 7Li due to preferential destruction of 6Li. If the solar Li abundance is controlled by solar flare spallation reactions, the 7Li/6Li would be about 2. By comparison, the terrestrial and meteoritic 7Li/6Li is 12.5.
Other than Li and 3He, and perhaps Be and B, it is
widely assumed that the average elemental composition of the
solar system is preserved by the material on the surface of the
Sun (photosphere). This is because the present surface mixing
zone (SMZ) and radiative interior of the Sun formed very
early, before p-p and CNO cycle thermonuclear burning
could produce abundance changes. Thus, the original solar
composition should be preserved in the photosphere, which
is the top of the SMZ. However, an inhomogeneous solar
model, with the solar core depleted in heavy elements, was
recently suggested as a possible solution to the solar neutrino
deficit (Levy and Ruzmaikina, 1994). Recent solar models
(e.g. Bahcall and Pinsonneault, 1992; Proffitt, 1994) allow
for gravitational settling, thermal gradient diffusion, and
differential radiation pressure which collectively produce
compositional gradients beneath a well-mixed SMZ.
In the radiative interior, heavy elements which are not fully
ionized have a slight tendancy to settle towards the core. Diffusion,
or "Settling out", at the base of the SMZ can occur in principle, but
the turbulent nature of the mixing, (e.g. convective overshoots) counters
the settling. Theoretical differences (Proffitt, 1994) between
photospheric and initial abundances are predicted to be small
(<10% for elements other than He), but it is important
to make observational tests to see if differences have been
underestimated. One needs real data to make specific
interpretations, but if we pretend that the relative solar wind
abundances of a few light elements (X,Y,Z) closely match
those of CI chondrites, then a close comparison of elemental
abundance ratios, e.g. A/X, where A is a heavy and X a light
element, with the A/X ratio for CI chondrites can test for heavy
element depletions from the SMZ. The best test is to select
elements, e.g. Ni and Ir, differing greatly in mass but only in
mass, i.e. with similar first ionization potentials and chemical properties.