Equation of State Studies Using SOHO Data

W. Dappen, Univ. of Southern Calif.

This is the continuation of a task to use the SOHO data to 
obtain constraints on the equation of state.  While pre-helioseismic solar
models were quite adequately using simple ideal-gas equations of
state, it turned out that non-ideal corrections had to be included in 
order to match the constraints from solar oscillation frequencies.  In
particular, the effect of the Coulomb pressure correction in the
solar plasma became clearly evident.  Finer non-ideal effects are
emerging in analyses of current observational data.  The finer effects
in the euqation of state are subject of current controversial
physical models of plasmas.  A helioseismic analysis effectively
allows experiments to be carried out on the physics of the plasma t
this level of detail.  This is a severe test of the theory.  Although
the non-ideal effects are relatively small corrections (typically
1-10% of the pressure), the extraordinary observational accuracy 
makes the helioseismic data sensitive to them, imposing stringent
constraints on their theoretical computation to the required level.
In this way, the subtle quantum many-body effects that contribute to
the small, but theoreticall challenging non-ideal corrections in the
equation of state, can be studied in the Sun.  Knowledge about these
non-ideal corrections will have astrophysical implications for higher 
densities plasmas of low-mass stars, brown dwarfs, and giant planets.
In this way, SOHO data will made a contribution to solar physics,
stellar astrophysics, and basic physics.  So far, the PI has been
carrying  out solar modeling and inversion tasks in collaboration with
either Jorgen Christensen-Dalsgaard (Aarhus) or Sarbani Basu 
(Princeton).  To enable the higher caadence required by the flurry of 
the SOI/MDI data, solar modeling and inversion capabilities are being
developed at USC.  They are funded by two previous SOHO GI grants.
Since funding for the first one only started in August 97, work is 
still in its first year.  The GI support allowed one of the PI's 
graduate students (A. Nayfonov) to finish his PhD work and to publish
two equation-of-state related papers (Arndt et al., ApJ, 1998; Nayfonov
& Dappen, ApJ,. 1998).  However, the development of the numerical inversion
tools, and the heavy educational involvement of several graduate
students, require a longer-term effort.  Therefore, this proposal
requests an extension of support to a third year.