  Sub-Chandrasekhar CO white dwarfs accreting helium have been considered as
candidates for Type Ia supernova(SNIa) progenitors since the early 1980s
(helium shell mass $> 0.1 M_\odot $). These models, once detonated did not fit
the observed spectra and light curve of typical SNIa observations. New
theoretical work examined detonations on much less massive ($< 0.05 M_\odot $)
envelopes. They find stable detonations that lead to light curves, spectra and
abundances that compare relatively well with the observational data. The exact
mechanism leading to the ignition of helium detonation is a key issue, since it
is a mandatory first step for the whole scenario. As the flow of the accreted
envelope is unstable to convection long before any hydrodynamic phenomena
develops, a multidimensional approach is needed in order to study the ignition
process. The complex convective reactive flow is challenging to any
hydrodynamical solver. To the best of our knowledge, all previous 2D studies
ignited the detonation artificially. We present here, for the first time, fully
consistent results from two hydrodynamical 2D solvers that adopt two
independent accurate schemes. For both solvers an effort was made to overcome
the problematics raised by the finite resolution and numerical diffusion by the
advective terms. Our best models lead to the ignition of a detonation in a
convective cell. Our results are robust and the agreement between the two
different numerical approaches is very good.
