Z. Q. Qu

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.

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

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.

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

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.

Deformation and deceleration of coronal wave

Aims. We studied the kinematics and morphology of two coronal waves to better understand the nature and origin of coronal waves. Methods. Using multi-wavelength observations of the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) and the Extreme Ultraviolet Imager (EUVI) on board the twin spacecraft Solar-TErrestrial RElations Observatory (STEREO), we present morphological and dynamic characteristics of consecutive coronal waves on 2011 March 24. We also show the coronal magnetic field based on the potential field source surface model. Results. This event contains several interesting aspects. The first coronal wave initially appeared after a surge-like eruption. Its front was changed and deformed significantly from a convex shape to a line-shaped appearance, and then to a concave configuration during its propagation to the northwest. The initial speeds ranged from 947 km s(-1) to 560 km s(-1). The first wave decelerated significantly after it passed through a filament channel. After the deceleration, the final propagation speeds of the wave were from 430 km s(-1) to 312 km s(-1). The second wave was found to appear after the first wave in the northwest side of the filament channel. Its wave front was more diffused and the speed was around 250 km s(-1), much slower than that of the first wave. Conclusions. The deformation of the first coronal wave was caused by the different speeds along different paths. The sudden deceleration implies that the refraction of the first wave took place at the boundary of the filament channel. The event provides evidence that the first coronal wave may be a coronal MHD shock wave, and the second wave may be the apparent propagation of the brightenings caused by successive stretching of the magnetic field lines.

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

WepresentastudyofthetwosuccessiveM-classflaresassociatedwithtwocoronalmassejections(CMEs)triggered by the collision of two small sunspots with opposite magnetic polarities and motional directions in NOAA active region (AR) 10484 on 2003 October 22. From the evolution of this AR in the TRACE white-light images and 96 minute line-of-sight magnetograms observed by the Michelson Doppler Imager on board SOHO, a large sunspot and a small sunspot with negative polarity rotated clockwise about 33 ◦ and 18 ◦ , respectively, from the northeast of a quiescent sunspot with negative polarity to the southeast from 15:00 UT on October 21 to 16:24 UT on October 23. During the process of their motion, the small sunspot with negative polarity collided with the small sunspot with positive polarity and opposite motional direction. In the collision, this AR produced two successive M-class flares and CMEs according to the observations of GOES and the Large Angle and Spectrometric Coronagraph. By analyzing the magnetic fields at polarity inversion lines (PILs) between the two small sunspot, it is found that a sudden squeeze occurred near the onset of the two M-class flares and then recovered itself after the flares. We ruled out the emergence of the magnetic fields near the PIL. According to the brightenings in TRACE 1600 A and the hard X-ray sources of the RHESSI of two M-class flares, we found that the locations of the two flares are almost situated in the same location at the PIL between the two small sunspots. We suggest that the sudden squeeze between the opposite magnetic polarities is caused by the pressure of the collision of the two small sunspots and resulted in the magnetic reconnection. These results could contribute to understanding the mechanism of flares and CMEs.

Key Properties of Solar Chromospheric Line Formation Process

The distribution or wavelength-dependence of the formation regions of frequently used solar lines, Halpha, Hbeta, CaIIH and CaII8542, in quiet Sun, faint and bright flares is explored in the unpolarized case. We stress four aspects characterising the property of line formation process: 1) width of line formation core; 2) line formation region; 3) influence of the temperature minimum region; and 4) wavelength ranges within which one can obtain pure chromospheric and photospheric filtergrams. It is shown that the above four aspects depend strongly on the atmospheric physical condition and the lines used. The formation regions of all the wavelength points within a line may be continuously distributed over one depth domain or discretely distributed because of no contribution coming from the temperature minimum region, an important domain in the solar atmosphere that determines the distribution pattern of escape photons. On the other hand, the formation region of one wavelength point may cover only one height range or spread over two domains which are separated again by the temperature minimum region. Different lines may form in different regions in the quiet Sun. However, these line formation regions become closer in solar flaring regions. Finally, though the stratification of line-of-sight velocity can alter the position of the line formation core within the line band and result in the asymmetry of the line formation core about the shifted line center, it can only lead to negligible changes in the line formation region or the line formation core width. All these results can be instructive to solar filtering observations.

LINEAR POLARIZATION OF FLASH SPECTRUM OBSERVED FROM A TOTAL SOLAR ECLIPSE IN 2008

We measured linear polarization signals of the flash spectrum ranging from 502.5 nm to 528.5 nm after second contact of a total solar eclipse which occurred on 2008 August 1 in China. A large group of spectral lines (especially those lines produced by neutral iron, neutral copper, and as carbon molecules) are found with very high polarization degrees relative to the continuum polarization, and the linear polarization spectrum is more abundantly structured than the flash spectrum itself. According to the observational result, we conclude that coherent scattering and scattering geometry as well as other mechanisms may together play a very important role in producing the high polarization amplitudes. This will help us deepen our understanding of the physical conditions of the solar upper atmospheres and the physical processes occurring there.

A solar Stokes spectrum telescope

A solar Stokes spectrum telescope has been built and is routinely used to measure full Stokes spectra of solar surface features. Its optical and electronic systems are outlined while the polarization analyzer is described in detail. The observations carried out by this telescope are sampled and the performance parameters are given. The measured Stokes spectra during solar cycle 23 show that the measurement accuracy of Stokes polarization components can reach an r.m.s. noise level of less than 9.0×10−4 with short time integration for an active region and 5.0×10−4 with long time integration for a weak field region without smoothing.

ON THE COMBINATION OF IMAGING-POLARIMETRY WITH SPECTROPOLARIMETRY OF UPPER SOLAR ATMOSPHERES DURING SOLAR ECLIPSES

We present results from imaging polarimetry (IP) of upper solar atmospheres during a total solar eclipse on 2012 November 13 and spectropolarimetry of an annular solar eclipse on 2010 January 15. This combination of techniques provides both the synoptic spatial distribution of polarization above the solar limb and spectral information on the physical mechanism producing the polarization. Using these techniques together we demonstrate that even in the transition region, the linear polarization increases with height and can exceed 20%. IP shows a relatively smooth background distribution in terms of the amplitude and direction modified by solar structures above the limb. A map of a new quantity that reflects direction departure from the background polarization supplies an effective technique to improve the contrast of this fine structure. Spectral polarimetry shows that the relative contribution to the integrated polarization over the observed passband from the spectral lines decreases with height while the contribution from the continuum increases as a general trend. We conclude that both imaging and spectral polarimetry obtained simultaneously over matched spatial and spectral domains will be fruitful for future eclipse observations.

Case study of a complex active-region filament eruption

Context. We investigated a solar active-region filament eruption associated with a C6.6 class flare and a coronal mass ejection (CME) in NOAA active region 08858 on 2000 February 9. Aims. We aim to better understand the relationship between filament eruptions and the associated flares and CMEs. Methods. Using BBSO, SOHO/EIT, and TRACE observational data, we analyzed the process of the active-region filament eruption in the chromosphere and the corona. Using the SOHO/MDI magnetograms, we investigated the change of the magnetic fields in the photosphere. Using the GOES soft X-ray flux and the SOHO/LASCO images, we identified the flare and CME, which were associated with this active-region filament eruption. Results. The brightenings in the chromosphere are a precursor of the filament expansion. The eruption itself can be divided into four phases: In the initial phase, the intertwined bright and dark strands of the filament expand. Then, the bright strands are divided into three parts with different expansion velocity. Next, the erupting filament-carrying flux rope expands rapidly and combines with the lower part of the expanding bright strands. Finally, the filament erupts accompanied by other dark strands overlying the filament. The overlying magnetic loops and the expansion of the filament strands can change the direction of the eruption. Conclusions. The time delay between the velocity peaks of the filament and that of the two parts of the bright strands clearly demonstrates that the breakup of the bright loops tying on the filament into individual strands is important for its eruption. The eruption is a collection of multiple processes that are physically coupled rather than a single process.