We modify the corotating convection model usually applied to the Jovian magnetosphere by including a term that simulates the effects of microdiffusion. By applying a technique of Fourier analysis followed by three-mode truncation, we recover precisely the well known Lorenz equations of meteorology. The implications are that under certain restrictions plasma motion in the Jovian magnetosphere, which is modulated by ionospheric ohmic drag through the Pedersen ionospheric conductivity &Sgr;, can be expected to be chaotic if &Sgr; is less than a critical value &Sgr;c; if &Sgr; is greater than &Sgr;c the plasma motion is steady-state convection. The value of &Sgr;c depends on a parameter q which characterizes an assumed stationary radial profile (∝L-q) in plasma mass per unit magnetic flux. ¿ American Geophysical Union 1992