Speaker
Dr
Petri Käpylä
(University of Helsinki)
Description
The results for the α-effect as a function of rotation rate
are consistent with earlier numerical studies, i.e.
increasing magnitude as rotation increases and approximately
cos θ latitude profile for moderate rotation. Turbulent
diffusivity, η_t, is proportional to the square of the
turbulent vertical velocity in all cases. Whereas ηt
decreases approximately inversely proportional to the
wavenumber of the field, the α-effect and turbulent pumping
show a more complex behaviour with partial or full sign
changes and the magnitude staying roughly constant. In the
presence of shear and no rotation, a weak α-effect is
induced which does not seem to show any consistent trend as
a function of shear rate. Provided that the shear is large
enough, this small α-effect is able to excite a dynamo in
the mean-field model. The coefficient responsible for
driving the shear-current effect shows several sign changes
as a function of depth but is also able to contribute to
dynamo action in the mean-field model. The growth rates in
these cases are, however, well below those in direct
simulations, suggesting that an incoherent α-shear dynamo
may also act in the simulations. If both rotation and shear
are present, the α-effect is more pronounced. At the same
time, the combination of the shear-current and Ω×{
J}-effects is also stronger than in the case of shear alone,
but subdominant to the α-shear dynamo. The results of direct
simulations are consistent with mean-field models where all
of these effects are taken into account without the need to
invoke incoherent effects.
