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Collective Study of Polar Crown Filaments in the Past Four Solar Cycles  

Yan Xu   Submitted: 2018-07-31 19:00

Polar Crown Filaments (PCFs) form above the magnetic polarity inversion line, which separates the unipolar polar fields and the nearest dispersed fields from trailing part of active regions with opposite polarity. The statistical properties of PCFs are correlated with the solar cycle. Therefore, study of PCFs plays an important role in understanding the variation of solar cycle, especially the prolonged cycle 23 and the current "abnormal" solar cycle 24. In this study, we investigate PCFs using full disk Hα data from 1973 to early 2018, recorded by Kanzelhöhe Solar Observatory (KSO) and Big Bear Solar Observatory (BBSO), in digital form from 1997 to 2018 and in 35 mm film (digitized) from 1973 to 1996. PCFs are identified manually because their segmented shape and close-to-limb location were not handled well by automatical detections in several previous studies. Our results show that the PCFs start to move poleward at the beginning of each solar cycle. When the PCFs approach to the maximum latitude, the polar field strength reduces to zero followed by a reversal. The migration rates are about 0.4 to 0.7 degree per Carrington rotation, with clear N-S asymmetric pattern. In cycles 21 and 23, the PCFs in the northern hemisphere migrate faster than those in the southern hemisphere. However, in the "abnormal" cycle 24, the southern PCFs migrate faster, which is consistent with other observations of magnetic fields and radio emission. In addition, there are more days in cycle 23 and 24 without PCFs than in the previous cycles.

Authors: Yan Xu, Werner Potzi, Hewei Zhang, Nengyi Huang, Ju Jing, and Haimin Wang
Projects: Other

Publication Status: in press
Last Modified: 2018-07-31 19:06
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Transient rotation of photospheric vector magnetic fields associated with a solar flare  

Yan Xu   Submitted: 2018-01-09 16:03

As one of the most violent eruptions on the Sun, flares are believed to be powered by magnetic reconnection. The fundamental physics involving the release, transfer and deposition of energy have been studied extensively. Taking advantage of the unprecedented resolution provided by the 1.6-m Goode Solar Telescope, here we show a sudden rotation of vector magnetic fields, about 12-20 degrees counterclockwise, associated with a flare. Unlike the permanent changes reported previously, the azimuth-angle change is transient and co-spatial/temporal with Hα emission. The measured azimuth angle becomes closer to that in potential fields suggesting untwist of flare loops. The magnetograms were obtained in the near infrared at 1560 nm, which is minimally affected by flare emission and no intensity profile change was detected. We believe that these transient changes are real and discuss the possible explanations in which the high energy electron beams or Alfvén waves play a crucial role.

Authors: Yan Xu, Wenda Cao, Kwangsu Ahn, Ju Jing, Chang Liu, Jongchul Chae, Nengyi Huang, Na Deng, Dale E. Gary and Haimin Wang
Projects: BBSO/NST

Publication Status: Published
Last Modified: 2018-01-10 11:25
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Ultra-narrow Negative Flare Front Observed in Helium-10830 Å using the 1.6 m New Solar Telescope  

Yan Xu   Submitted: 2016-01-21 17:20

Solar flares are sudden flashes of brightness on the Sun and are often associated with coronal mass ejections and solar energetic particles which have adverse effects in the near Earth environment. By definition, flares are usually referred to bright features resulting from excess emission. Using the newly commissioned 1.6 m New Solar Telescope at Big Bear Solar Observatory, here we show a striking ``negative'' flare with a narrow, but unambiguous ``dark'' moving front observed in \ion{He}{1} 10830 Å, which is as narrow as 340 km and is associated with distinct spectral characteristics in Hα and Mg II lines. Theoretically, such negative contrast in \ion{He}{1} 10830 Å can be produced under special circumstances, by nonthermal-electron collisions, or photoionization followed by recombination. Our discovery, made possible due to unprecedented spatial resolution, confirms the presence of the required plasma conditions and provides unique information in understanding the energy release and radiative transfer in astronomical objects.

Authors: Yan Xu, Wenda Cao, Mingde Ding, Lucia Kleint, Jiangtao Su, Chang Liu, Haisheng Ji, Jongchul Chae, Ju Jing, Kyuhyoun Cho, Kyungsuk Cho, Dale Gary, and Haimin Wang
Projects: IRIS,Other,SDO-AIA,SDO-HMI

Publication Status: accepted by ApJ
Last Modified: 2016-01-26 21:32
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Comparison of Emission Properties of two Homologous Flares in AR 11283  

Yan Xu   Submitted: 2014-04-28 18:33

Large, complex, active regions may produce multiple flares within a certain period of one or two days. These flares could occur in the same location with similar morphologies, commonly referred to as homologous flares. In 2011 September, active region NOAA 11283 produced a pair of homologous flares on the 6th and 7th, respectively. Both of them were white-light (WL) flares, as captured by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory in visible continuum at 617.3 nm which is believed to originate from the deep solar atmosphere.We investigate the WL emission of these X-class flares with HMIs seeing-free imaging spectroscopy. The durations of impulsive peaks in the continuum are about 4 minutes. We compare the WL with hard X-ray (HXR) observations for the September 6 flare and find a good correlation between the continuum and HXR both spatially and temporally. In absence of RHESSI data during the second flare on September 7, the derivative of the GOES soft X-ray is used and also found to be well correlated temporally with the continuum. We measure the contrast enhancements, characteristic sizes, and HXR fluxes of the twin flares, which are similar for both flares, indicating analogous triggering and heating processes. However, the September 7 flare was associated with conspicuous sunquake signals whereas no seismic wave was detected during the flare on September 6. Therefore, this comparison suggests that the particle bombardment may not play a dominant role in producing the sunquake events studied in this paper.

Authors: Yan Xu, Ju Jing, Shuo Wang, Haimin Wang
Projects: None

Publication Status: accepted to ApJ
Last Modified: 2014-04-29 12:04
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On the Relationship Between Coronal Magnetic Decay Index and CME Speed  

Yan Xu   Submitted: 2012-10-17 05:15

Numerical simulations suggest that kink and torus instabilities are two potential contributors to the initiation and prorogation of eruptive events. A magnetic parameter named decay index (i.e., the coronal magnetic gradient of the overlying fields above the eruptive flux ropes) could play an important role in controlling kinematics of eruptions. Previous studies have identified a threshold range of the decay index that distinguishes between eruptive and confined configurations. Here we advance the study by investigating if there is a clear correlation between the decay index and CME speed. 38 CMEs associated with filament eruptions and/or two-ribbon flares are selected using the H data from the Global H Network. The filaments and flare ribbons observed in H associated with the CMEs help to locate the magnetic polarity inversion line, along which the decay index is calculated based on the potential field extrapolation using MDI magnetograms as boundary conditions. The speeds of CMEs are obtained from the LASCO C2 CME catalog available online. We find that the mean decay index increases with CME speed for those CMEs with a speed below 1000 km s-1, and stays flat around 2.2 for the CMEs with higher speeds. In addition, we present a case study of a partial filament eruption, in which the decay indexes show different values above the erupted/non-erupted part.

Authors: Yan Xu, Chang Liu, Ju Jing, & Haimin Wang
Projects: Other,SoHO-MDI

Publication Status: ApJ, accepted
Last Modified: 2012-10-19 10:00
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High resolution observations of white-Light emissions \ from the opacity minimum during an X-class flare  

Yan Xu   Submitted: 2010-03-04 11:13

Using high cadence, high resolution near infrared (NIR) observations of the X10 white-light flare (WLF) on 2003 October 29, we investigated the evolution of the core-halo structure of white-light emission during the two-second period flare peak. We found that size and intensity of the halo remained almost constant in the range of 10~Mm2. However, the core area was very compact and expanded rapidly from about 1~Mm2 to 4~Mm2.At the same time, the total emission of the core increased nearly twenty times. This distinct behavior indicates that different heating mechanisms might be responsible for core and halo emissions. In addition to the temporal analysis, we compared the intensity enhancements of the flare core and halo. The result shows that the halo contrast increased by about 8% compared to the flare-quiet region, which could be explained by a combination of direct-heating and backwarming models.

Authors: Yan Xu, Wenda Cao, Ju Jing, and Haimin Wang
Projects: None

Publication Status: Accepted for Publication in Astron. Nachr.
Last Modified: 2010-03-05 07:55
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Abstracts by Author
Collective Study of Polar Crown Filaments in the Past Four Solar Cycles
Transient rotation of photospheric vector magnetic fields associated with a solar flare
Ultra-narrow Negative Flare Front Observed in Helium-10830~\AA\ using the 1.6 m New Solar Telescope
Comparison of Emission Properties of two Homologous Flares in AR 11283
On the Relationship Between Coronal Magnetic Decay Index and CME Speed
High resolution observations of white-Light emissions \ from the opacity minimum during an X-class flare

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