A Cross-Comparison of Cotemporal Magnetograms Obtained with MDI/SOHO and SP/ Hinode |
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Mei Zhang Submitted: 2010-01-11 22:41
We compared a set of cotemporal magnetograms of active regions obtained with the Michelson Doppler Imager (MDI) aboard SOHO and the Spectro-Polarimeter (SP) of the Solar Optical Telescope (SOT) on board Hinode. The comparison shows that even with the recent calibration of level-1.8 data, the magnetic flux density derived from the MDI data is still lower than that obtained with SP. The average ratio between current version 2008 MDI level-1.8 data and SP magnetograms is 0.71, and is 0.82 for version 2007 MDI level-1.8 data. The comparison also shows that the most recent version 2008 calibration of MDI level-1.8 data has successfully removed the center-to-limb variation, while version 2007 level-1.8 data did not, as estimated by Ulrich et al.
Authors: Wang, D.; Zhang, M.; Li, H.; Zhang, H. Q.
Projects: None
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Publication Status: Solar Physics, 2009, Volume 260, Issue 1, pp.233-244
Last Modified: 2010-01-12 08:51
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Magnetic Field Confinement in the Corona: The Role of Magnetic Helicity Accumulation |
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Mei Zhang Submitted: 2006-02-14 20:11
A loss of magnetic field confinement is believed to be the cause of coronal mass ejections (CMEs), a major form of solar activity in the corona. The mechanisms for magnetic energy storage are crucial in understanding how a field may possess enough free energy to overcome the Aly limit and open up. Previously, we have pointed out that the accumulation of magnetic helicity in the corona plays a significant role in storing magnetic energy. In this paper, we investigate another hydromagnetic consequence of magnetic-helicity accumulation. We propose a conjecture that there is an upper bound on the total magnetic helicity that a force-free field can contain. This is directly related to the hydromagnetic property that force-free fields in unbounded space have to be self-confining. Although a mathematical proof of this conjecture for any field configuration is formidable, its plausibility can be demonstrated with the properties of several families of power-law, axisymmetric force-free fields. We put forth mathematical evidence, as well as numerical, indicating that an upper bound on the magnetic helicity may exist for such fields. Thus, the accumulation of magnetic helicity in excess of this upper bound would initiate a non-equilibrium situation, resulting in a CME expulsion as a natural product of coronal evolution.
Authors: Mei Zhang, Natasha Flyer & Boon Chye Low
Projects:
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Publication Status: ApJ, 644, 575
Last Modified: 2006-06-09 07:27
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