  Type Ia Supernovae are thought to be thermonuclear explosions of massive
white dwarfs (WD). We present the first study of multi-dimensional effects
during the final hours prior to the thermonuclear runaway which leads to the
explosion. The calculations utilize an implicit, 2-D hydro code.Mixing and the
ignition process are studied in detail. We find that the initial chemical
structure of the WD is changed but the material is not fully homogenized. The
exploding WD sustains a central region with a low C/O ratio. This implies that
the explosive nuclear burning will begin in a partially C-depleted environment.
The thermonuclear runaway happens in a well defined region close to the center.
It is induced by compressional heat when matter is brought inwards by
convective flows. We find no evidence for multiple spot or strong off-center
ignition. Convective velocities are of the order of 100 km/sec which is well
above the effective burning speeds in SNe Ia previously expected right after
the runaway. For about 0.5 to 1 sec, the speed of the burning front will
neither be determined by the laminar speed nor the Rayleigh-Taylor
instabilities but by convective flows produced prior to the runaway. The
consequences are discussed for our under- standing of the detailed physics of
the flame propagation, the deflagration detonation transition, and the
nucleosynthesis in the central layers. Our results strongly suggest the
pre-conditioning of the progenitor as a key-factor for our understanding of the
diversity in SNeIa.
