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What you see might not be what you get

posted May 30, 2013, 8:04 PM by Jeremy Drake   [ updated Jun 2, 2013, 7:21 AM ]
One of the powerful tools we are using to investigate the outer atmospheres of stars and their winds is a magnetohydrodynamic code that enables us to compute the magnetic and gas structure of a star's magnetosphere and wind based on a surface magnetic field map.  The computer model determines how the magnetic field should be structured in altitude for each point on the surface of the star, then works out how the wind would be driven based on the field strength and topology. The outflowing gas in the wind influences the magnetic field structure, so the model has to be iterated to arrive at a solution. This all works fairly well for the Sun - high resolution surface magnetic field maps are readily available.  

For stars the situation is very different. Magnetic field maps are derived using the technique of "Zeeman-Doppler Imaging" - essentially using the Doppler effect to separate out the spectral signatures of magnetic Zeeman splitting of spectral lines that occurs in sunspots, and by extension, starspots.  While the technique appears to work, it has limited spatial resolution - we cannot see small spots, or distinguish a group of small spots from a single much larger spot. How does this lack of resolution affect the interpretation of the outer atmosphere? We investigated this by computing models of the solar wind and magnetosphere for solar magnetic maps with different degrees of spatial resolution and compared them with observations. The figure shows the X-ray sun as viewed by the Yohkoh satellite in 2000 (top left), together with the model X-ray image for a high resolution solar magnetogram (top left), a low resolution magnetogram (bottom left) and the same magnetogram artificially enhanced with fine magnetic structure.  The image based on low resolution data is quite poor at matching the observations, while the enhanced one - essentially with made up data included - fares significantly better. The warning is that X-ray images for stars based on low-resolution magnetic maps might not be very accurate. Artificially enhancing the data might help, but is no replacement for a higher resolution magnetogram.  This work was lead by postdoc Cecilia Garraffo and was published in the 2013 February 10 edition of the Astrophysical Journal.