Surface appearence of locally produced flux concentrations
by
Axel Brandenburg(Nordita), Illa R. Losada(Nordita and Astronomy Department)
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Europe/Stockholm
FA31
FA31
Description
Recent work carried out in the framework of the AstroDyn project has
provided further evidence that coronal mass ejections play a major role
in shedding small-scale magnetic helicity from the dynamo to alleviate
an otherwise catastrophic quenching of the dynamo (see BS05 for a review).
At the same time, such models have made contact with unexpected phenomena
taking place in the solar wind.
A striking example is the sign reversal of small-scale magnetic helicity
away from the Sun (BSBG11).
This surprising result was first obtained by analyzing data from the
Ulysses spacecraft, but the interpretation was greatly aided by similar
results from simulations of (WBM11+WBM12).
Two recent papers on the subject of quenching are Hubbard & Brandenburg
(2012, ApJ) and Del Sordo et al. (2013, MNRAS).
While the work of WBM11 and WKMB11 has focussed on parameter studies
exploring the conditions for plasmoid ejections from helically
forced turbulence as well as rotating convection, the physical
realism of the model remained poor.
The density contrast between dynamo and corona is much bigger in
reality, for example (Pinto et al. 2012).
Significant improvements are possible with only modest increase of
numerical resolution, as has been shown by BP11 using
Pencil Code simulations with a realistic setup.
Their model is based on early work by Gudiksen & Nordlund (2005).
Reconstruction of coronal fields from a dynamo-generated field
and comparison with the self-consistently produced coronal fields.
One possibility is NEMPI; another one is turbu-thermomagnetic instability
(Kitchatinov & Mazur 2000), and yet another one is just the buoyant rise
of a pre-existing flux tube. We can easily do better models than in the
past (WBM11,WKMB11) by including radiation (master's thesis of Atefeh).
What is still neglected in all those models is partial ionization, which
produces a strong density contrast near the surface, which may strongly
enhance NEMPI and also turbu-thermomagnetic instability. For technical
aspects, the relevant paper on radiation is Heinemann et al. (2006).
A goal would be to produce synthetic X-ray (and XUV) images from
self-consistent simulations of flux-tube emergence, using a setup
simular to that of WB10, but then with radiation instead of the
old so-called force-free model.
