A. R. Yeates, G. Hornig, B. T. Welsch
Aims. We show how the build-up of magnetic gradients in the Sun's corona may
be inferred directly from photospheric velocity data. This enables computation
of magnetic connectivity measures such as the squashing factor without recourse
to magnetic field extrapolation.
Methods. Assuming an ideal evolution in the corona, and an arbitrary initial
connectivity, the magnetic field line mapping is computed by integrating
trajectories of the (time-dependent) horizontal photospheric velocity field.
The method is applied to a 12 hour high-resolution sequence of photospheric
flows derived from Hinode/SOT magnetograms.
Results. We find the generation of a network of quasi-separatrix layers in
the magnetic field, which correspond to Lagrangian coherent structures in the
photospheric velocity. The visual pattern of these structures arises primarily
from the diverging part of the photospheric flow, hiding the effect of the
rotational flow component: this is demonstrated by a simple analytical model of
photospheric convection. We extract the rotational component from the observed
flow and show qualitative agreement with a purely rotational model. Increasing
the flow speeds in the model suggests that our observational results are likely
to give a lower bound for the rate at which magnetic gradients are built up by
real photospheric flows. Finally, we show how to construct a magnetic field
with the inferred topology, that can be used for future investigations of
reconnection and energy release.
View original:
http://arxiv.org/abs/1110.3957
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