8 research outputs found
Screw dynamo in a time-dependent pipe flow
The kinematic dynamo problem is investigated for the flow of a conducting
fluid in a cylindrical, periodic tube with conducting walls. The methods used
are an eigenvalue analysis of the steady regime, and the three-dimensional
solution of the time-dependent induction equation. The configuration and
parameters considered here are close to those of a dynamo experiment planned in
Perm, which will use a torus-shaped channel. We find growth of an initial
magnetic field by more than 3 orders of magnitude. Marked field growth can be
obtained if the braking time is less than 0.2 s and only one diverter is used
in the channel. The structure of the seed field has a strong impact on the
field amplification factor. The generation properties can be improved by adding
ferromagnetic particles to the fluid in order to increase its relative
permeability,but this will not be necessary for the success of the dynamo
experiment. For higher magnetic Reynolds numbers, the nontrivial evolution of
different magnetic modes limits the value of simple `optimistic' and
`pessimistic' estimates.Comment: 10 pages, 12 figure
Local Helioseismology of Sunspots: Current Status and Perspectives (Invited Review)
Mechanisms of the formation and stability of sunspots are among the
longest-standing and intriguing puzzles of solar physics and astrophysics.
Sunspots are controlled by subsurface dynamics hidden from direct observations.
Recently, substantial progress in our understanding of the physics of the
turbulent magnetized plasma in strong-field regions has been made by using
numerical simulations and local helioseismology. Both the simulations and
helioseismic measurements are extremely challenging, but it becomes clear that
the key to understanding the enigma of sunspots is a synergy between models and
observations. Recent observations and radiative MHD numerical models have
provided a convincing explanation to the Evershed flows in sunspot penumbrae.
Also, they lead to the understanding of sunspots as self-organized magnetic
structures in the turbulent plasma of the upper convection zone, which are
maintained by a large-scale dynamics. Local helioseismic diagnostics of
sunspots still have many uncertainties, some of which are discussed in this
review. However, there have been significant achievements in resolving these
uncertainties, verifying the basic results by new high-resolution observations,
testing the helioseismic techniques by numerical simulations, and comparing
results obtained by different methods. For instance, a recent analysis of
helioseismology data from the Hinode space mission has successfully resolved
several uncertainties and concerns (such as the inclined-field and phase-speed
filtering effects) that might affect the inferences of the subsurface
wave-speed structure of sunspots and the flow pattern. It becomes clear that
for the understanding of the phenomenon of sunspots it is important to further
improve the helioseismology methods and investigate the whole life cycle of
active regions, from magnetic-flux emergence to dissipation.Comment: 34 pages, 18 figures, submitted to Solar Physic