Magnetic jam in the corona of the Sun |
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Feng Chen Submitted: 2015-08-25 03:46
The outer solar atmosphere, the corona, contains plasma at temperatures of more than a million kelvin-more than 100 times hotter than the solar surface. How this gas is heated is a fundamental question tightly interwoven with the structure of the magnetic field. Together this governs the evolution of coronal loops, the basic building block prominently seen in X-rays and extreme ultraviolet (EUV) images. Here we present numerical experiments accounting for both the evolving three-dimensional structure of the magnetic field and its complex interaction with the plasma. Although the magnetic field continuously expands as new magnetic flux emerges through the solar surface, plasma on successive field lines is heated in succession, giving the illusion that an EUV loop remains roughly at the same place. For each snapshot the EUV images outline the magnetic field. However, in contrast to the traditional view, the temporal evolution of the magnetic field and the EUV loops can be quite different. This indicates that the thermal and the magnetic evolution in the outer atmosphere of a cool star should be treated together, and should not be simply separated as predominantly done so far.
Authors: Feng Chen, Hardi Peter, Sven Bingert, and Mark Cheung
Projects: None
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Publication Status: Published in Nature Physics, 2015, 11, 492
Last Modified: 2015-08-25 16:46
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A model for the formation of the active region corona driven by magnetic flux emergence |
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Feng Chen Submitted: 2014-02-24 10:35
We present the first model that couples the formation of the corona of a solar active region to a model of the emergence of a sunspot pair. This allows us to study when, where, and why active region loops form, and how they evolve. We use a 3D radiation MHD simulation of the emergence of an active region through the upper convection zone and the photosphere as a lower boundary for a 3D MHD coronal model. The latter accounts for the braiding of the magnetic fieldlines, which induces currents in the corona heating up the plasma. We
synthesize the coronal emission for a direct comparison to observations. Starting with a basically field-free atmosphere we follow the filling of the corona with magnetic field and plasma. Numerous individually identifiable hot coronal loops form, and reach temperatures well above 1 MK with densities comparable to observations. The footpoints of these loops are found where small patches of magnetic flux concentrations move into the sunspots. The loop formation is triggered by an increase of upwards-directed Poynting flux at their footpoints in the photosphere. In the synthesized EUV emission these loops develop within a few minutes. The first EUV loop appears as a thin tube, then rises and expands significantly in the horizontal direction. Later, the spatially inhomogeneous heat input leads to a fragmented system of multiple loops or strands in a growing envelope.
Authors: F. Chen, H. Peter, S. Bingert, M. C. M. Cheung
Projects: None
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Publication Status: accepted for publication in A&A
Last Modified: 2014-02-24 13:04
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