The stellar evolution code YREC is outlined with emphasis on its applications to helio-and asteroseismology. The procedure for calculating calibrated solar and stellar models is described. Other features of the code such as a non-local treatment of convective core overshoot, and the implementation of a parametrized description of turbulence in stellar models, are considered in some detail. The code has been extensively used for other astrophysical applications, some of which are briefly mentioned at the end of the paper.

We construct models of the structure and evolution of the Sun which include variable magnetic fields and turbulence. The magnetic effects are (1) magnetic pressure, (2) magnetic energy, and (3) magnetic modulation to turbulence. The effects of turbulence are (1) turbulent pressure, (2) turbulent kinetic energy, and (3) turbulent inhibition of the radiative energy loss of a convective eddy, and (4) turbulent generation of magnetic fields. Using these ingredients we construct five types of solar variability models (including the standard solar model) with magnetic effects. These models are in part based on three-dimensional numerical simulations of the superadiabatic layers near the surface of the Sun. The models are tested with several sets of observational data, namely, the changes of (1) the total solar irradiance, (2) the photospheric temperature, (3) radius, (4) the position of the convection zone base, and (5) low-and medium-degree solar oscillation frequencies. We find that turbulence plays a major role in solar variability, and only a model that includes a magnetically modulated turbulent mechanism can agree with all the current available observational data. We find that because of the somewhat poor quality of all observations (other than the helioseismological ones), we need all data sets in order to restrict the range of models.

This paper describes three‐dimensional (3D) large eddy simulations of stellar surface convection using realistic model physics. The simulations include the present Sun, a subgiant of one solar mass and a lower‐gravity subgiant, also of one solar mass. We examine the thermal structure (superadiabaticity) after modification by 3D turbulence, the overshoot of convective motions into the radiative atmosphere and the range of convection cell sizes. Differences between models based on the mixing length theory (MLT) and the simulations are found to increase significantly in the more evolved stages as the surface gravity decreases. We find that the full width at half maximum (FWHM) of the turbulent vertical velocity correlation provides a good objective measure of the vertical size of the convective cells. Just below the convection surface, the FWHM is close to the mean vertical size of the granules and 2 × FWHM is close to the mean horizontal diameter of the granules. For the Sun, 2 × FWHM = 1200 km, a value close to the observed mean granule size. For all the simulations, the mean horizontal diameter is close to 10 times the pressure scaleheight at the photospheric surface, in agreement with previous work.

We argue that a variety of solar data suggest that the activity-cycle timescale variability of the total irradiance, is produced by structural adjustments of the solar interior. Assuming these adjustments are induced by variations of internal magnetic fields, we use measurements of the total irradiance and effective temperature over the period from 1978 to 1992, to infer the magnitude and location of the magnetic field. Using an updated stellar evolution model, which includes magnetic fields, we find that the observations can be explained by fields whose peak values range from 120k to 2.3k gauss, located in the convection zone between 0.959R ⊙ and 0.997R ⊙ , respectively. The corresponding maximal radius changes, are 17 km when the magnetic field is located at 0.959R ⊙ and 3 km when it is located at 0.997R ⊙ . At these depths, the W parameter (defined by ∆ ln R/∆ ln L, where R and L are the radius and luminosity) ranges from 0.02 to 0.006. All these predictions are consistent with helioseismology and recent measurements carried out by the MDI experiment on SOHO.