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A Cross-Comparison of Cotemporal Magnetograms Obtained with MDI/SOHO and SP/ Hinode  

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

Publication Status: Solar Physics, 2009, Volume 260, Issue 1, pp.233-244
Last Modified: 2010-01-12 08:51
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Subject will be restored when possible  

Mei Zhang   Submitted: 2008-05-13 00:00

This paper follows up on a previous study showing that in an open atmosphere such as the solar corona the total magnetic helicity of a force-free field must be bounded and the accumulation of magnetic helicity in excess of its upper bound would initiate a non-equilibrium situation resulting in an expulsion such as a coronal mass ejection (CME). In the current paper, we investigate the dependence of the helicity bound on the boundary condition for several families of nonlinear force-free fields. Our calculation shows that the magnitude of the helicity upper bound of force-free fields is non-trivially dependent on the boundary condition. Fields with a multipolar boundary condition can have a helicity upper bound ten times smaller than those with a dipolar boundary condition when helicity values are normalized by the square of their respective surface poloidal fluxes. This suggests that a coronal magnetic field may erupt into a CME when the applicable helicity bound falls below the already accumulated helicity as the result of a slowly changing boundary condition. Our calculation also shows that a monotonic accumulation of magnetic helicity can lead to the formation of a magnetic flux rope applicable to kink instability. This suggests that CME initiations by exceeding helicity bound and by kink instability can both be the consequences of helicity accumulation in the corona. Our study gives insights into the observed associations of CMEs with the magnetic features at their solar surface origins.

Authors: Mei Zhang and Natasha Flyer
Projects: None

Publication Status: accepted by ApJ
Last Modified: 2008-09-23 21:01
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Vector Magnetic Field Measurement at Huairou and Helicity of Active Regions  

Mei Zhang   Submitted: 2007-03-18 19:26

The Solar Magnetic Field Telescope (SMFT) at the Huairou Solar Observing Station of the National Astronomical Observatory has been obtaining photospheric vector magnetograms on a daily basis for more than 20 years. In this paper we first introduce our magnetograph and our data reduction routine. Then we focus on what we can learn from these vector magnetic field measurements about the magnetic non-potentiality of active regions. Derived physical quantities, such as shear angle, α value and current helicity, are good indicators of magnetic helicity. By studying values and evolutions of these physical quantities, we get a glimpse of the dynamo process in the solar interior. By examining the evolution of these quantities we can also learn what are the favorable conditions for coronal eruptions such as flares and coronal mass ejections.

Authors: M. Zhang, Y. Y. Deng and H. Q. Zhang
Projects: None

Publication Status: To appear in Advances in Space Research
Last Modified: 2007-03-18 20:26
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Helicity Observation of Weak and Strong Fields  

Mei Zhang   Submitted: 2006-06-09 07:38

We report in this letter our analysis of a large sample of photospheric vector magnetic field measurements. Our sample consists of 17200 vector magnetograms obtained from January 1997 to August 2004 by Huairou Solar Observing Station of the Chinese National Astronomical Observatory. Two physical quantities, α and current helicity, are calculated and their signs and amplitudes are studied in a search for solar cycle variations. Different from other studies of the same type, we calculate these quantities for weak (100G<|B_z|<500G) and strong (|B_z|>1000G) fields separately. For weak fields, we find that the signs of both α and current helicity are consistent with the established hemispheric rule during most years of the solar cycle and their magnitudes show a rough tendency of decreasing with the development of solar cycle. Analysis of strong fields gives an interesting result: Both α and current helicity present a sign opposite to that of weak fields. Implications of these observations on dynamo theory and helicity production are also briefly discussed.

Authors: Mei Zhang
Projects: None

Publication Status: accepted for publication in ApJ Letter
Last Modified: 2006-09-18 17:53
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Magnetic Field Confinement in the Corona: The Role of Magnetic Helicity Accumulation  

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:

Publication Status: ApJ, 644, 575
Last Modified: 2006-06-09 07:27
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The Hydromagnetic Nature of Solar Coronal Mass Ejections  

Mei Zhang   Submitted: 2005-02-21 01:34

Solar coronal mass ejections (CMEs) are a major form of activity on the Sun. A CME takes 1015-16 g of plasma from the low corona into the solar wind, to disturb the near-Earth space if the CME direction is favorable. We summarize current observations and ideas of CME physics to provide a hydromagnetic view of the CMEs as the products of continual magnetic flux emergence and an interplay between magnetic reconnection and approximate magnetic-helicity conservation in the corona. Each flux emergence brings helicity to accumulate additively in a coronal structure while excess magnetic energy is flared away by reconnection. Self-confinement eventually fails with a CME shedding the accumulated helicity out of the low corona to enable the field to reach the minimum-energy state. Similar evolutionary processes may occur in other magnetic stars and galaxies.

Authors: Mei Zhang & Boon Chye Low
Projects: None

Publication Status: Annual Reviews of Astronomy and Astrophysics
Last Modified: 2005-02-21 01:34
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Abstracts by Author
A Cross-Comparison of Cotemporal Magnetograms Obtained with MDI/SOHO and SP/ Hinode
Subject will be restored when possible
Vector Magnetic Field Measurement at Huairou and Helicity of Active Regions
Helicity Observation of Weak and Strong Fields
Magnetic Field Confinement in the Corona: The Role of Magnetic Helicity Accumulation
The Hydromagnetic Nature of Solar Coronal Mass Ejections

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