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.