  For realistic geodynamo simulations, one must solve the magnetohydrodynamic
equations to follow time development of thermal convection motion of
electrically conducting fluid in a rotating spherical shell. We have developed
a new geodynamo simulation code by combining the finite difference method with
the recently proposed spherical overset grid called Yin-Yang grid. We achieved
performance of 15.2 Tflops (46% of theoretical peak performance) on 4096
processors of the Earth Simulator.
