Lirong Tian

On the Origin of Magnetic Helicity in the Solar Corona

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 ( -->? HLCT) over 4-5 days using MDI 96 minute line-of-sight magnetograms and a local correlation tracking technique. We also estimate the contribution from differential rotation to the overall helicity budget ( -->? Hrot). It is found that of the seven active regions for which the flux emergence exceeds -->1.0 ? 1022 Mx, six exhibited a helicity flux injection exceeding -->1.0 ? 1043 Mx2 (i.e., -->? H = ? HLCT ? ? 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 16 active regions, with flux emergence less than 1022 Mx, only 4 had significant injection of helicity, exceeding 1043 Mx2. Typical contributions from differential rotation over the same period were 2-3 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.

Origins of Coronal Energy and Helicity in NOAA 10030

Exploring the origins of coronal helicity and energy, as well as determining the mechanisms that lead to coronal energy release, is a fundamental topic in solar physics. Using MDI 96 minute line-of-sight and HSOS vector magnetograms in conjunction with TRACE white-light and UV (1600 ?) images and BBSO H? and SOHO EIT (195 ?) images, we find in active region NOAA 10030 that a large positive polarity sunspot, located in the center of the region, exhibited significant counterclockwise rotation, which continued for 6 days during the period 2002 July 12-18. This rotating sunspot was related to the formation of inverse-

ASYMMETRY OF HELICITY INJECTION FLUX IN EMERGING ACTIVE REGIONS

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On the Origin of the Asymmetric Helicity Injection in Emerging Active Regions

-->-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 6 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 northeastward over several days, but exhibited little rotation. The moving sunspot also exhibited the formation of inverse-

Role of Sunspot and Sunspot-Group Rotation in Driving Sigmoidal Active Region Eruptions

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Magnetic Twist and Writhe of δ Active Regions

-->-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 they do play a very important role in the coronal helicity accumulation and free-energy build-up.

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

Observational and modeling results indicate that typically the leading magnetic field of bipolar active regions (ARs) 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 Ω-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 ARs and this provides the motivation for this paper. We separately calculate the distribution of the helicity flux injected in the leading and following polarities of 15 emerging bipolar ARs, using the Michelson Doppler Image 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 Ω-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 ARs 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 emergence between the leading and following legs of an inclined Ω-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.

The Role of the Kink Instability of a Long-Lived Active Region AR 9604

To explore the possible causes of the observed asymmetric helicity flux in emerging active regions between the leading and following polarities reported in a recent study by Tian & Alexander, we examine the subsurface evolution of buoyantly rising ?-shaped flux tubes using three-dimensional, spherical-shell anelastic MHD simulations. We find that due to the asymmetric stretching of the ?-shaped tube by the Coriolis force, the leading side of the emerging tube has a greater field strength, is more buoyant, and remains more cohesive compared to the following side. As a result, the magnetic field lines in the leading leg show more coherent values of local twist ? ? (? ? B) ? B/B 2, whereas the values in the following leg show large fluctuations and are of mixed sign. On average, however, the field lines in the leading leg do not show a systematically greater mean twist compared to the following leg. Due to the higher rise velocity of the leading leg, the upward helicity flux through a horizontal cross section at each depth in the upper half of the convection zone is significantly greater in the leading polarity region than that in the following leg. This may contribute to the observed asymmetric helicity flux in emerging active regions. Furthermore, based on a simplified model of active region flux emergence into the corona by Longcope & Welsch, we show that a stronger field strength in the leading tube can result in a faster rotation of the leading polarity sunspot driven by torsional Alfv?n waves during flux emergence into the corona, contributing to a greater helicity injection rate in the leading polarity of an emerging active region.