Is the small-scale magnetic field correlated with the dynamo cycle?
Nordita Astrophysics seminars
Wednesday 27 May 2015
to 14:15 at
Bidya Binay Karak (Nordita)
The small-scale magnetic field is ubiquitous at the solar surface---even
at high latitudes.
From observations we know that this field is uncorrelated (or perhaps
even weakly anticorrelated) with the global sunspot cycle.
Our aim is to explore the origin, and particularly the cycle dependence
of this field using three-dimensional dynamo simulations.
We use a simple model of a turbulent dynamo in a shearing box driven by helically
forced turbulence. Depending on the dynamo parameters, large-scale (global)
and small-scale (local) dynamos can be excited independently in this model.
Based on simulations in different parameter regimes, we find
that, when only the large-scale dynamo is operating in the system,
the small-scale magnetic field generated through shredding and tangling
of the large-scale magnetic field is positively correlated with the global
However, when both dynamos are operating, the small-scale field is produced
from both the small-scale dynamo and the tangling of the large-scale field.
In this situation, when the large-scale field is weaker than the
equipartition value of the turbulence, the small-scale field is almost uncorrelated with the
large-scale magnetic cycle.
On the other hand, when the large-scale field is stronger than
the equipartition value, we observe a clear anticorrelation between
the small-scale field and the large-scale magnetic cycle.
This anticorrelation can be interpreted as a suppression of both the
small-scale dynamo and the tangling of the large-scale field.
Based on our studies we conclude that the observed small-scale
magnetic field in the Sun is generated by the combined mechanisms of
small-scale dynamo and tangling of the large-scale field.
The observed cyclic variation of the small-scale field is produced
by the interaction between the large-scale field and the flow.