Journal Article

Modelling stellar coronae from surface magnetograms: the role of missing magnetic flux

C. Johnstone, M. Jardine and D. H. Mackay

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 404, issue 1, pages 101-109
Published in print May 2010 | ISSN: 0035-8711
Published online April 2010 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2010.16298.x
Modelling stellar coronae from surface magnetograms: the role of missing magnetic flux

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Recent advances in spectropolarimetry have allowed the reconstruction of stellar coronal magnetic fields. This uses Zeeman–Doppler magnetograms (ZDI) of the surface magnetic field as a lower boundary condition. The ZDI maps, however, suffer from the absence of information about the magnetic field over regions of the surface due to the presence of dark starspots and portions of the surface out of view due to a tilt in the rotation axis. They also suffer from finite resolution which leads to small-scale field structures being neglected. This paper explores the effects of this loss of information on the extrapolated coronal fields. For this, we use simulated stellar surface magnetic maps for two hypothetical stars. Using the potential field approximation, the coronal fields and emission measures are calculated. This is repeated for the cases of missing information due to, (i) starspots, (ii) a large area of the stellar surface out of view and (iii) a finite resolution. The largest effect on the magnetic field structure arises when a significant portion of the stellar surface remains out of view. This changes the nature of the field lines that connect to this obscured hemisphere. None the less, the field structure in the visible hemisphere is reliably reproduced. Thus, the calculation of the locations and surface filling factors of accretion funnels is reasonably well reproduced for the observed hemisphere. The decrease with height of the magnetic pressure, which is important in calculating disc truncation radii for accreting stars, is also largely unaffected in the equatorial plane. The fraction of surface flux that is open and therefore able to supply angular momentum loss in a wind, however, is often overestimated in the presence of missing flux. The magnitude and rotational modulation of the calculated emission measures is consistently decreased by the loss of magnetic flux in dark starspots. For very inactive stars, this may make it impossible to recover a magnetic cycle in the coronal emission. Finite resolution has little effect on those quantities, such as the emission measure and the average coronal electron density, that can currently be observed.

Keywords: stars: coronae; stars: magnetic field; stars: starspots

Journal Article.  6003 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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