Magnetic Reynolds Number of the Dynamo Region in Uranus and Neptune

This paper examines whether the ionic water layer of each ice giant can sustain dynamo action. Using only published input values and transparent formulae, the calculation addresses what magnetic Reynolds number follows from published electrical conductivities for ionic water, a stated convective flow speed, and the thickness of the conducting shell, and by what margin it exceeds the usual dynamo threshold. The headline results are: conductivity_S_m_ionic_water = 2000; magnetic_diffusivity_m2_s = 397.887; magnetic_reynolds_number_uranus = 1116. The method, inputs and limitations are stated in full so that any reader can reproduce or rescale every number from the deposited calculation script. The central finding is that even at a flow speed of centimetres per second the Reynolds number exceeds the threshold by a factor of nearly thirty, so conductivity is not the bottleneck for ice-giant dynamos; geometry and flow organisation are. Agreement and disagreement with independent published estimates are reported at their actual size, and the conclusions are limited to what the budget arithmetic supports.