Effect of rotation on convection and solidification of a binary eutectic system: Evidence for the Earth inner core

In the cooling Earth the fluid outer core consists of a molten alloy over the solidifying inner core. Here, compositional convection due to the preferential solidification of the heavy phase releases a low density residual liquid. The buoyancy flux generated by this process is thought to be the main driving force sustaining the geodynamo.The effect of rotation on compositional convection and channel formation during unidirectional solidification of a binary eutectic system is investigated experimentally. A vertically oriented cylindrical annulus filled with an aqueous ammonium chloride solution is cooled from the bottom and can be rotated about its axis at rates of up to 10.5 rad/s, corresponding to Ekman numbers down to 7.5E-6. The solidifying layer forms a crystal mush consisting of a network of solid dendrites with fluid in the interstices. Compositional stratification in the mushy layer causes the interstitial fluid to be buoyantly unstable. In non-rotating cases the upwelling in the mushy layer takes the form of narrow, crystal free cylindrical channels or of chimneys resulting in plumes that rise into the fluid above the mush-liquid interface. Return flow into the mush is diffuse.The main features of convection described above are retained if the experiment is rotated. The experiments show that, for the rotation rates presented, chimney formation and convection in the mushy layer are unaltered. Accordingly, the pattern of channels is found to be a result of the initial solidification conditions and not primaryly caused by the convection in the porous layer. However, rotation has a strong effect on the form of convection in the fluid above the mush-liquid interface.In the fluid over the mushy layer rotational shear due to the Coriolis force causes an instability characterized by secondary plume formation from sub-horizontal primary plume conduits. This new instability has the form of small fluid parcels or blobs. The height at which this instability sets in is proportional to the Ekman number. Thus, the flow pattern becomes increasingly complex on smaller length scales for growing rotation rates. At Ekman numbers less than 9.5E-6 the plume conduits are completely unstable when leaving the porous layer