Xiaoli Yan

Rapid rotation of a sunspot associated with flares

Context. Active region NOAA 10424 observed on August 5, 2003 is studied in detail by using TRACE, SOHO/MDI, BBSO H alpha monograph, and GOES data. This investigation focuses on the sunspot rotation and its relation with the eruptive phenomena by analyzing the magnetic configuration that the rotation results in. Aims. It is shown that there is a close relationship between the sunspot rotation and the emerging kinked magnetic Omega-loops, where the flares occur. Methods. Through tracing the traceable features motion by using the TRACE white-light images, one can get the rotation velocities of the umbra, the penumbra, the area near the penumbra, and the area far from the penumbra. Furthermore, the evolution of the emerging kinked magnetic Omega-loops and magnetic fields were studied. Results. For the sunspot with positive polarity, the umbra, the penumbra, and the area near the penumbra exhibit a conspicuous counterclockwise rotation. Moreover, the velocities decrease from the umbra through the penumbra to the area near the penumbra. It is interesting that the rotation of the umbra, the penumbra, and the area near the penumbra are opposite to that of the area far from penumbra. The rotation velocities of the umbra, the penumbra, polarity separation, and total magnetic flux increase with time. During the largest event (M1.7/Sn flare), emerging kinked magnetic Omega-loops are observed from TRACE 171 angstrom images. Conclusions. The different rotation speeds of the different parts of the sunspot cause twist, and then the twist is injected through the chromosphere into the corona to trigger the flares.

A Statistical Study on Rotating Sunspots: Polarities, Rotation Directions, and Helicities

Observations of white-light images from SOHO MDI, TRACE, Hinode, SOHO MDI magnetograms, and Hinode SP vector magnetograms show sufficient samples of rotating sunspots on the solar photosphere for a statistical study. During the period from 1996 December to 2007 December, after individually checking the active regions in detail, we found 182 measurable rotating sunspots. According to the statistics, the number of rotating sunspots in the northern hemisphere is 12% greater than the number in the southern hemisphere. The number of rotating sunspots with clockwise rotation is approximately the same as the number with counterclockwise rotation in both hemispheres. Furthermore, in the northern hemisphere the number of rotating sunspots with positive polarity is nearly twice the number of those with negative polarity, while in the southern hemisphere the situation is almost reversed. During the maximum of solar cycle 23, there were more rotating sunspots than during any other period observed. Finally, the current helicity imbalance shows a very weak hemispheric tendency. These findings may be instructive and place further constrains on the dynamo and flux emergence theories.

UNWINDING MOTION OF A TWISTED ACTIVE REGION FILAMENT

To better understand the structures of active-region filaments and the eruption process, we study an active-region filament eruption in active region NOAA 11082 in detail on June 22, 2010. Before the filament eruption, the opposite unidirectional material flows appeared in succession along the spine of the filament. The rising of the filament triggered two B-class flares at the upper part of the filament. As the bright material was injected into the filament from the sites of the flares, the filament exhibited a rapid uplift accompanying the counterclockwise rotation of the filament body. From the expansion of the filament, we can see that the filament is consisted of twisted magnetic field lines. The total twist of the filament is at least 5

Phase analysis of sunspot group numbers on both solar hemispheres

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The causality between the rapid rotation of a sunspot and an X3.4 flare

obtained by using time slice method. According to the morphology change during the filament eruption, it is found that the active-region filament was a twisted flux rope and its unwinding motion was like a solar tornado. We also find that there was a continuous magnetic helicity injection before and during the filament eruption. It is confirmed that magnetic helicity can be transferred from the photosphere to the filament. Using the extrapolated potential fields, the average decay index of the background magnetic fields over the filament is 0.91. Consequently, these findings imply that the mechanism of solar filament eruption could be due to the kink instability and magnetic helicity accumulation.

Observing the release of twist by magnetic reconnection in a solar filament eruption

Cross-correlation analysis and wavelet transform methods are proposed to investigate the phase relationship between the monthly sunspot group numbers in the solar northern and southern hemispheres. It is found that (1) the monthly sunspot group numbers in the northern hemisphere begin two months earlier than those in the southern one, which should lead to phase asynchrony between them but with a slight effect; (2) the Schwabe cycle length for the monthly sunspot group numbers in the two hemispheres obviously differs from each other, and the mean Schwabe cycle length of the monthly sunspot group numbers in the northern hemisphere is slightly larger than that in the southern one; (3) the monthly sunspot group numbers in the northern hemisphere precede those in the southern hemisphere during the years of about 1874-1927, after which, the southern hemisphere leads the northern hemisphere in the years 1928-1964, and then the northern hemisphere leads in time till the present.

THE CONTRACTION OF OVERLYING CORONAL LOOP AND THE ROTATING MOTION OF A SIGMOID FILAMENT DURING ITS ERUPTION

Using multi-wavelength data of Hinode, the rapid rotation of a sunspot in active region NOAA 10930 is studied in detail. We found extraordinary counterclockwise rotation of the sunspot with positive polarity before an X3.4 flare. From a series of vector magnetograms, it is found that magnetic force lines are highly sheared along the neutral line accompanying the sunspot rotation. Furthermore, it is also found that sheared loops and an inverse S-shaped magnetic loop in the corona formed gradually after the sunspot rotation. The X3.4 flare can be reasonably regarded as a result of this movement. A detailed analysis provides evidence that sunspot rotation leads to magnetic field lines twisting in the photosphere. The twist is then transported into the corona and triggers flares.

Relationship between eruptions of active‐region filaments and associated flares and coronal mass ejections

Magnetic reconnection is a fundamental process of topology change and energy release, taking place in plasmas on the Sun, in space, in astrophysical objects and in the laboratory. However, observational evidence has been relatively rare and typically only partial. Here we present evidence of fast reconnection in a solar filament eruption using high-resolution H-alpha images from the New Vacuum Solar Telescope, supplemented by extreme ultraviolet observations. The reconnection is seen to occur between a set of ambient chromospheric fibrils and the filament itself. This allows for the relaxation of magnetic tension in the filament by an untwisting motion, demonstrating a flux rope structure. The topology change and untwisting are also found through nonlinear force-free field modelling of the active region in combination with magnetohydrodynamic simulation. These results demonstrate a new role for reconnection in solar eruptions: the release of magnetic twist.

SUCCESSIVE SOLAR ERUPTIONS TRIGGERED BY THE COLLISION OF TWO SMALL SUNSPOTS WITH OPPOSITE POLARITIES AND MOTIONAL DIRECTIONS

We present an observation of overlying coronal loop contraction and rotating motion of the sigmoid filament during its eruption on 2012 May 22 observed by the Solar Dynamics Observatory (SDO). Our results show that the twist can be transported into the filament from the lower atmosphere to the higher atmosphere. The successive contraction of the coronal loops was due to a suddenly reduced magnetic pressure underneath the filament, which was caused by the rising of the filament. Before the sigmoid filament eruption, there was a counterclockwise flow in the photosphere at the right feet of the filament and the contraction loops and a convergence flow at the left foot of the filament. The hot and cool materials have inverse motion along the filament before the filament eruption. Moreover, two coronal loops overlying the filament first experienced brightening, expansion, and contraction successively. At the beginning of the rising and rotation of the left part of the filament, the second coronal loop exhibited rapid contraction. The top of the second coronal loop also showed counterclockwise rotation during the contraction process. After the contraction of the second loop, the left part of the filament rotated counterclockwise and expanded toward the right of NOAA AR 11485. During the filament expansion, the right part of the filament also exhibited counterclockwise rotation like a tornado.

Polarimeter with two ferroelectric liquid-crystal modulators attached to the Yunnan solar tower

To better understand the dynamical process of active-region filament eruptions and associated flares and coronal mass ejections (CMEs), we carried out a statistical study of 120 events observed by Big Bear Solar Observatory (BBSO), Transition Region and Coronal Explorer (TRACE) and the Extreme-ultraviolet Imaging Telescope (EIT) on board Solar and Heliospheric Observatory (SOHO) from 1998 to 2007. We combined filament observations with the NOAAs flare reports, Michelson Doppler Imager (MDI) magnetograms and Large Angle and Spectrometric Coronagraph (LASCO) data, to investigate the relationship between active-region filament eruptions and other solar activities. We found that 115 out of 120 (about 96 per cent) filament eruptions are associated with flares. 56 out of 105 (about 53 per cent) filament eruptions are found to be associated with CMEs except for 15 events without corresponding LASCO data. We note the limitation of coronagraphs duo to geometry or sensitivity, leading to many smaller CMEs that are Earth-directed or well out of the plane of sky not being detected by near-Earth spacecraft. Excluding those without corresponding LASCO data, the CME association rate of active-region filament eruptions clearly increases with X-ray flare class from about 32 per cent for C-class flares to 100 per cent for X-class flares. We also found that the eruptions of active-region filaments associated with halo CMEs are often accompanied by large flares (18 out of 20 events; >= M1.0). About 92 per cent events (11 out of 12) associated with X-class flare are associated with halo CMEs. Such a result is due to the fact that the Earth-directed CMEs detected as halo CMEs are often the larger CMEs and many of the smaller ones are not detected because of the geometry and low intensity. The average speed of the associated CMEs of filament eruptions increases with X-ray flare size from 563.7 kms(-1) for C-class flares to 1506.6 kms(-1) for X-class flares. Excluding the active region located in the area more than 50. from the solar centre and five without corresponding MDI data, the beta magnetic field configuration (about 47 per cent; 36 out of 77) is more likely to form eruptive filaments than the other ones and there are 33 filament eruptions associated with magnetic flux cancellation, 42 events associated with magnetic flux emergence and two events without variation of magnetic field. The average area of emergence regions is 855.9 arcsec(2). These findings may be instructive not only in respect to the modelling of active-region filament eruptions but also in predicting flares and CMEs.