  Evidence from numerical simulations suggest that the nature of dynamo
transition changes from supercritical to subcritical as the magnetic Prandtl
number is decreased. To explore this interesting crossover we first use direct
numerical simulations to investigate the hysteresis zone of a subcritical
Taylor-Green dynamo. We establish that a well defined boundary exists in this
hysteresis region which separates dynamo states from the purely hydrodynamic
solution. We then propose simple dynamo models which show similar crossover
from supercritical to subcritical dynamo transition as a function of the
magnetic Prandtl number. Our models show that the change in the nature of
dynamo transition is connected to the stabilizing or de-stabilizing influence
of governing non-linearities.
