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On Asymmetry of Magnetic Helicity in Emerging Active Regions: High Resolution Observations  

Lirong Tian   Submitted: 2010-10-13 09:20

We employ the DAVE (differential affine velocity estimator, Schuck 2005; 2006) tracking technique on a time series of MDI/1m high spatial resolution line-of-sight manetograms to measure the photospheric flow velocity for three newly emerging bipolar active regions. We separately calculate the magnetic helicity injection rate of the leading and following polarities to confirm or refute the magnetic helicity asymmetry, found by Tian & Alexander (2009) using MDI/96m low spatial resolution magnetograms. Our results demonstrate that the magnetic helicity asymmetry is robust being present in the three active regions studied, two of which have an observed balance of the magnetic flux. The magnetic helicity injection rate measured is found to depend little on the window size selected, but does depend on the time interval used between the two successive magnetograms tracked. It is found that the measurement of the magnetic helicity injection rate performs well for a window size between 12X10 and 18X15 pixels, and at a time interval ∆t=10 minutes. Moreover, the short-lived magnetic structures, 10-60 minutes, are found to contribute 30-50% of the magnetic helicity injection rate. Comparing with the results calculated by MDI/96m data, we find that the MDI/96m data, in general, can outline the main trend of the magnetic properties, but they significantly underestimate the magnetic flux in strong field region and are not appropriate for quantitative tracking studies, so provide a poor estimate of the amount of magnetic helicity injected into the corona.

Authors: Lirong Tian, Pascal Demoulin, David Alexander, Chunming Zhu
Projects:

Publication Status: ApJ (accepted)
Last Modified: 2010-10-13 18:30
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Asymmetry of helicity injection flux in emerging active regions  

Lirong Tian   Submitted: 2009-01-14 13:57

Observational and modeling results indicate that typically the leading magnetic field of bipolar active regions is often spatially more compact, while more dispersed and fragmented in following polarity. In this paper, we address the origin of this morphological asymmetry, which is not well understood. Although it may be assumed that, in an emerging Omega-shaped flux tube, those portions of the flux tube in which the magnetic field has a higher twist may maintain its coherence more readily, this has not been tested observationally. To assess this possibility, it is important to characterize the nature of the fragmentation and asymmetry in solar active regions and this provides the motivation for this paper. We separately calculate the distribution of the injected helicity flux injected in the leading and following polarities of fifteen emerging bipolar active regions, using the Michelson Doppler Image (MDI) 96 minute line-of-sight magnetograms and a local correlation tracking technique. We find from this statistical study that the leading (compact) polarity injects several times more helicity flux than the following (fragmented) one (typically 3-10 times). This result suggests that the leading polarity of the Omega-shaped flux tube possesses a much larger amount of twist than the following field prior to emergence. We argue that the helicity asymmetry between the leading and following magnetic field for the active regions studied here results in the observed magnetic field asymmetry of the two polarities due to an imbalance in the magnetic tension of the emerging flux tube. We suggest that the observed imbalance in the helicity distribution results from a difference in the speed of following legs of an inclined Omega-shaped flux tube. In addition, there is also the effect of magnetic flux imbalance between the two polarities with the fragmented following polarity displaying spatial fluctuation in both the magnitude and sign of helicity measured.

Authors: Lirong Tian and David Alexander
Projects: None

Publication Status: ApJ (accepted in Jan., 2009)
Last Modified: 2009-01-15 07:04
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Subject will be restored when possible  

Lirong Tian   Submitted: 2008-04-08 11:20

Exploring the origins of coronal helicity and energy, as well as determining the mechanisms that lead to coronal energy release, are fundamental topics in solar physics. Using MDI/96 minute line-of-sight and HSOS vector magnetograms in conjunction with TRACE white light and UV (1600 AA) images and BBSO/ Hα and SOHO/EIT (195 AA), we find in active region NOAA 10030 that a large positive polarity sunspot, located in the center of the region, exhibited significant counter-clockwise rotation, which continued for six days during the period, July 12-18, 2002. This rotating sunspot was related to the formation of inverse-S-shaped filaments, left-handed twist of the vector magnetic fields, and the production of strong negative vertical current, but exhibited little emergence of magnetic flux. In all, five M-class and two X-class flares were produced around this rotating sunspot over the six day period. The observed characteristics of the strongly rotating sunspot suggest that sunspots can undergo strong intrinsic rotation, the source of which may originate below the photosphere and can play a significant role in helicity production and injection and energy buildup in the corona. A sunspot with negative magnetic polarity showed fast and significant emergence in the eastern portion of the active region, and moved north-eastward over several days, but exhibited little rotation. The moving sunspot also exhibited the formation of inverse-S-shaped filaments, left-handed twist of vector magnetic fields and coronal structure, and the production of stronger positive current. The observed characteristics of the emerging sunspot suggest that significant emergence of twisted magnetic fields may not always result in the rotation of the associated sunspots, but do play also a very important role in the coronal helicity accumulation and free-energy build-up.

Authors: Lirong Tain, David Alexander, and Richard Nightingale
Projects: None

Publication Status: accepted by ApJ
Last Modified: 2008-04-09 19:15
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Subject will be restored when possible  

Lirong Tian   Submitted: 2008-04-08 11:20

Exploring the origins of coronal helicity and energy, as well as determining the mechanisms that lead to coronal energy release, are fundamental topics in solar physics. Using MDI/96 minute line-of-sight and HSOS vector magnetograms in conjunction with TRACE white light and UV (1600 AA) images and BBSO/ Hα and SOHO/EIT (195 AA), we find in active region NOAA 10030 that a large positive polarity sunspot, located in the center of the region, exhibited significant counter-clockwise rotation, which continued for six days during the period, July 12-18, 2002. This rotating sunspot was related to the formation of inverse-S-shaped filaments, left-handed twist of the vector magnetic fields, and the production of strong negative vertical current, but exhibited little emergence of magnetic flux. In all, five M-class and two X-class flares were produced around this rotating sunspot over the six day period. The observed characteristics of the strongly rotating sunspot suggest that sunspots can undergo strong intrinsic rotation, the source of which may originate below the photosphere and can play a significant role in helicity production and injection and energy buildup in the corona. A sunspot with negative magnetic polarity showed fast and significant emergence in the eastern portion of the active region, and moved north-eastward over several days, but exhibited little rotation. The moving sunspot also exhibited the formation of inverse-S-shaped filaments, left-handed twist of vector magnetic fields and coronal structure, and the production of stronger positive current. The observed characteristics of the emerging sunspot suggest that significant emergence of twisted magnetic fields may not always result in the rotation of the associated sunspots, but do play also a very important role in the coronal helicity accumulation and free-energy build-up.

Authors: Lirong Tian, David Alexander, and Richard Nightingale
Projects:

Publication Status: accepted by ApJ
Last Modified: 2008-09-23 21:10
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Lirong Tian   Submitted: 2008-04-08 11:16

23 active regions (ARs) with well-defined SXR sigmoids are selected to study what is the most important origin of the coronal helicity-the emergence of magnetic fields or the photospheric horizontal motions. The redial magnetic flux of each polarity, the helicity injection rate, and total helicity flux (Delta Hlct) are calculated using local correlation tracking technique and MDI 96m line-of-sight magnetograms, and the helicity budget of the differential rotation (Delta Hrot) is also extimated. It is found that six ARs inject helicity flux greater than 1043 Mx^2,(Delta H =Delta Hlct-Delta Hrot), in a sample of seven ARs with emerging magnetic flux greater than 1022 Mx. On the other hand, in sixteen ARs with emerging magnetic flux less than 1022 Mx, only 4 ARs injected the helicity flux greater than 1043 Mx^2, which denotes the main comtribution of the horizontal motions. The statitical results suggest that the horizontal motions are not important to the coronal helicity injection when there is little magnetic field emergence.

Authors: Lirong Tian
Projects: None

Publication Status: published in ASP Conference Series, Vol 383, P213
Last Modified: 2008-09-23 21:10
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Subject will be restored when possible  

Lirong Tian   Submitted: 2008-04-08 11:15

23 active regions (ARs) with well-defined SXR sigmoids are selected to study what is the most important origin of the coronal helicity-the emergence of magnetic fields or the photospheric horizontal motions. The redial magnetic flux of each polarity, the helicity injection rate, and total helicity flux (Delta Hlct) are calculated using local correlation tracking technique and MDI 96m line-of-sight magnetograms, and the helicity budget of the differential rotation (Delta Hrot) is also extimated. It is found that six ARs inject helicity flux greater than 1043 Mx^2,(Delta H =Delta Hlct-Delta Hrot), in a sample of severn ARs with emerging magnetic flux greater than 1022 Mx. On the other hand, in sixteen ARs with emerging magnetic flux less than 1022 Mx, only 4 ARs injected the helicity flux greater than 1043 Mx^2, which denotes the main comtribution of the horizontal motions. The statitical results suggest that the horizontal motions are not important to the coronal helicity injection when there is little magnetic field emergence.

Authors: Lirong Tian
Projects: None

Publication Status: published in ASP Conference Series, Vol 383, P213
Last Modified: 2008-09-23 21:10
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Subject will be restored when possible  

Lirong Tian   Submitted: 2008-04-08 11:14

23 active regions (ARs) with well-defined SXR sigmoids are selected to study what is the most important origin of the coronal helicity-the emergence of magnetic fields or the photospheric horizontal motions. The redial magnetic flux of each polarity, the helicity injection rate, and total helicity flux (Delta Hlct) are calculated using local correlation tracking technique MDI 96m line-of-sight magnetograms, and the helicity budget of the differential rotation (Delta Hrot) is also extimated. It is found that six ARs inject helicity flux greater than 1043 Mx^2,(Delta H =Delta Hlct-Delta Hrot), in a sample of severn ARs with emerging magnetic flux greater than 1022 Mx. On the other hand, in sixteen ARs with emerging magnetic flux less than 1022 Mx, only 4 ARs injected the helicity flux greater than 1043 Mx^2, which denotes the main comtribution of the horizontal motions. The statitical results suggest that the horizontal motions are not important to the coronal helicity injection when there is little magnetic field emergence.

Authors: Lirong Tian
Projects: None

Publication Status: published in ASP Conference Series, Vol 383, P213
Last Modified: 2008-09-23 21:10
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Subject will be restored when possible  

Lirong Tian   Submitted: 2008-04-08 10:37

Exploring the origins of coronal helicity and energy, as well as determining the mechanisms that lead to coronal energy release, are fundamental topics in solar physics. Using MDI/96 minute line-of-sight and HSOS vector magnetograms in conjunction with TRACE white light and UV (1600 A) images and BBSO/Ha and SOHO/EIT (195 A), we find in active region NOAA 10030 that a large positive polarity sunspot, located in the center of the region, exhibited significant counter-clockwise rotation, which continued for six days during the period, July 12-18, 2002. This rotating sunspot was related to the formation of inverse-S-shaped filaments, left-handed twist of the vector magnetic fields, and the production of strong negative vertical current, but exhibited little emergence of magnetic flux. In all, five M-class and two X-class flares were produced around this rotating sunspot over the six day period. The observed characteristics of the strongly rotating sunspot suggest that sunspots can undergo strong intrinsic rotation, the source of which may originate below the photosphere and can play a significant role in helicity production and injection and energy buildup in the corona. A sunspot with negative magnetic polarity showed fast and significant emergence in the eastern portion of the active region, and moved north-eastward over several days, but exhibited little rotation. The moving sunspot also exhibited the formation of inverse-S-shaped filaments, left-handed twist of vector magnetic fields and coronal structure, and the production of stronger positive current. The observed characteristics of the emerging sunspot suggest that significant emergence of twisted magnetic fields may not always result in the rotation of the associated sunspots, but do play also a very important role in the coronal helicity accumulation and free-energy build-up.

Authors: Lirong Tian and David Alexander
Projects: None

Publication Status: accepted by ApJ
Last Modified: 2008-09-23 21:11
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Subject will be restored when possible  

Lirong Tian   Submitted: 2007-10-02 12:09

Twenty-three active regions associated with pronounced sigmoidal structure in Yohkoh soft X-ray observations are selected to investigate the origin of magnetic helicity in the solar corona. We calculate the radial magnetic flux of each polarity, the rate of magnetic helicity injection, and total flux of the helicity injection (Delta Hlct) over 4-5 days using MDI/96m line-of-sight magnetograms and a local correlation tracking technique. We also estimate the contribution from differential rotation to the overall helicity budget (Delta Hrot). It is found that of the seven active regions for which the flux emergence exceeds 1.0x1022 Mx, six exhibited a helicity flux injection exceeding 1.0x1043 Mx^2 (i.e. Delta H=Delta Hlct-Delta Hrot). Moreover, the rate of helicity injection and the total helicity flux are larger (smaller) during periods of more (less) increase of magnetic flux. Of the remaining sixteen active regions, with flux emergence less than 1022 Mx, only four had significant injection of helicity, exceeding 1043 Mx^2. Typical contributions from differential rotation over the same period were two-three times smaller than that of the strong magnetic field emergence. These statistical results signify that the strong emergence of magnetic field is the most important origin of the coronal helicity, while horizontal motions and differential rotation are insufficient to explain the measured helicity injection flux. Furthermore, the study of the helicity injection in nineteen newly emerging active regions confirms the result on the important role played by strong magnetic flux emergence in controlling the injection of magnetic helicity into the solar corona.

Authors: Lirong Tian and David Alexander
Projects: None

Publication Status: Accepted by ApJ
Last Modified: 2007-10-03 07:44
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Lirong Tian   Submitted: 2007-10-02 11:41

Twenty-three active regions associated with pronounced sigmoidal structure in Yohkoh soft X-ray observations are selected to investigate the origin of magnetic helicity in the solar corona. We calculate the radial magnetic flux of each polarity, the rate of magnetic helicity injection, and total flux of the helicity injection (Delta H_lct) over 4-5 days using MDI/96m line-of-sight magnetograms and a local correlation tracking technique. We also estimate the contribution from differential rotation to the overall helicity budget (Delta H_rot). It is found that of the seven active regions for which the flux emergence exceeds 1.0 x 1022 Mx, six exhibited a helicity flux injection exceeding 1.0 x 1043 Mx^2 (i.e. Delta H=Delta H_lct-Delta H_rot). Moreover, the rate of helicity injection and the total helicity flux are larger (smaller) during periods of more (less) increase of magnetic flux. Of the remaining sixteen active regions, with flux emergence less than 1022 Mx, only four had significant injection of helicity, exceeding 1043 Mx^2. Typical contributions from differential rotation over the same period were two-three times smaller than that of the strong magnetic field emergence. These statistical results signify that the strong emergence of magnetic field is the most important origin of the coronal helicity, while horizontal motions and differential rotation are insufficient to explain the measured helicity injection flux. Furthermore, the study of the helicity injection in nineteen newly emerging active regions confirms the result on the important role played by strong magnetic flux emergence in controlling the injection of magnetic helicity into the solar corona.

Authors: Lirong Tian and David Alexander
Projects: None

Publication Status: Accepted (ApJ)
Last Modified: 2007-10-02 11:41
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Role of Sunspot and Sunspot-group Rotation in Driving Sigmoidal Active Region Eruptions  

Lirong Tian   Submitted: 2005-08-30 13:15

We study active region NOAA 9684 (N06L285) which produced an X1.0/3B flare on Nov. 04, 2001 associated with a fast CME (1810 km s-1) and the largest proton event (31700 pfu) in cycle 23. SOHO/MDI continuum image data show that a large leading sunspot rotated counter-clockwise around its umbral center for at least four days prior to the flare. Moreover, it is found from SOHO/MDI 96 m line-of-sight magnetograms that the systematic tilt angle of the bipolar active region, a proxy for writhe of magnetic fluxtubes, changed from a positive value to a negative one. This signifies a counter-clockwise rotation of the spot-group as a whole. Using vector magnetograms from Huairou Solar Observing Station, we find that the twist of the active region magnetic fields is dominantly left-handed α _best=-0.03), and that the vertical current and current helicity are predominantly negative, and mostly distributed within the positive rotating sunspot. The active region exhibits a narrow inverse S-shaped Hα filament and soft X-ray sigmoid distributed along the magnetic neutral line. The portion of the filament which is most closely associated with the rotating sunspot disappeared on Nov. 4, and the corresponding portion of the sigmoid was observed to erupt, producing the flare and initiating the fast CME and proton event. These results imply that the sunspot rotation is a primary driver of helicity production and injection into the corona. We suggest that the observed active region dynamics and subsequent filament and sigmoid eruption are driven by a kink instability which occurred due to a large amount of the helicity injection.

Authors: L. Tian and D. Alexander
Projects: None

Publication Status: Solar Physics (accepted)
Last Modified: 2005-09-26 12:21
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The Role of the Kink Instability of a Long-lived Active Region AR 9604  

Lirong Tian   Submitted: 2005-04-26 15:58

We have traced the long-term evolution of a non-Hale active region composed of NOAA 9604-9632-9672-9704-9738, which displayed strong transient activity with associated geomagnetic effects from September to December, 2001. By studying the development of spot-group and line-of-sight magnetic field together with the evolution of H_ α filaments, the EUV and X-ray corona (TRACE 171 Å, Yohkoh/SXT), we have found that the magnetic structure of the active region exhibited a continuous clockwise rotation throughout its entire life. Vector magnetic data obtained from Huairou Solar Observing Station (HSOS) and full disk line-of-sight magnetograms from SOHO/MDI allowed the determination of the best-fit force-free parameter (proxy of twist), α best, and the systematic tilt angle (proxy of writhe) which were both found to take positive values. Soft X-ray coronal loops from Yohkoh/SXT displayed a pronounced forward-sigmoid structure in period of NOAA 9704. These observations imply that the magnetic fluxtube (loops) with the same handedness (right) of the writhe and the twist rotated clockwise in the solar atmosphere for a long time. We argue that the continuous clockwise rotation of the long-lived active region may be a manifestation that a highly right-hand twisted and kinked fluxtube was emerging through the photosphere and chromosphere into the corona.

Authors: L. Tian, Y. Liu, J. Yang and D. Alexander
Projects: None

Publication Status: Solar Physics (in press)
Last Modified: 2005-06-24 14:11
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Abstracts by Author
On Asymmetry of Magnetic Helicity in Emerging Active Regions: High Resolution Observations
Asymmetry of helicity injection flux in emerging active regions
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Role of Sunspot and Sunspot-group Rotation in Driving Sigmoidal Active Region Eruptions
The Role of the Kink Instability of a Long-lived Active Region AR 9604

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