  First results of our 3D numerical simulations of thermohaline convection
driven by 3He burning in a low-mass RGB star at the bump luminosity are
presented. They confirm our previous conclusion that this convection has a
mixing rate which is a factor of 50 lower than the observationally constrained
rate of RGB extra-mixing. It is also shown that the large-scale instabilities
of salt-fingering mean field (those of the Boussinesq and advection-diffusion
equations averaged over length and time scales of many salt fingers), which
have been observed to increase the rate of oceanic thermohaline mixing up to
one order of magnitude, do not enhance the RGB thermohaline mixing. We
speculate on possible alternative solutions of the problem of RGB extra-mixing,
among which the most promising one that is related to thermohaline mixing is
going to take advantage of the shifting of salt-finger spectrum towards larger
diameters by toroidal magnetic field.
