A simplified model of collision-driven dynamo action in small bodies

•The spin-up process can drive the dynamo action.•There exists a dynamo window of collision frequency.•Dynamo mechanisms are different at high and low Ekman numbers, the former is laminar flow and the latter is instability.•There exists intermittency of magnetic field.

We investigate numerically the self-sustained dynamo action in a spinning sphere whose sense of rotation reverses periodically. This system serves as a simple model of a dynamo in small bodies powered by frequent collisions. It is found that dynamo action is possible in some intervals of collision rates. At high Ekman numbers the laminar spin-up flow is helical in the boundary layers and the Ekman circulation together with the azimuthal shear powers the dynamo action. At low Ekman number a non-axisymmetric instability helps the dynamo action. The intermittency of magnetic field occurs at low Ekman number.