Yan Xu

New Jersey Institute of Technology, Newark, New Jersey, United States

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Publications (40)176.09 Total impact

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    ABSTRACT: We present the first part of a project on the global energetics of solar flares and coronal mass ejections (CMEs) that includes about 400 M- and X-class flares observed with AIA and HMI onboard SDO. We calculate the potential energy, free energy, and the flare-dissipated magnetic energy. We calculate these magnetic parameters using two different NLFFF codes: The COR-NLFFF code uses the line-of-sight magnetic field component $B_z$ from HMI to define the potential field, and the 2D coordinates of automatically detected coronal loops in 6 coronal wavelengths from AIA to measure the helical twist of coronal loops caused by vertical currents, while the PHOT-NLFFF code extrapolates the photospheric 3D vector fields. We find agreement between the two codes in the measurement of free energies and dissipated energies within a factor of $ \approx 3$. The size distributions of magnetic parameters exhibit powerlaw slopes that are approximately consistent with the fractal-diffusive self-organized criticality model. The magnetic parameters exhibit scaling laws for the nonpotential energy, $E_{np} \propto E_p^{1.02}$, for the free energy, $E_{free} \propto E_p^{1.7}$ and $E_{free} \propto B_{\varphi}^{1.0} L^{1.5}$, for the dissipated energy, $E_{diss} \propto E_p^{1.6}$ and $E_{diss} \propto E_{free}^{0.9}$, and the energy dissipation volume, $V \propto E_{diss}^{1.2}$. The potential energies vary in the range of $E_p = 1 \times 10^{31} - 4 \times 10^{33}$ erg, while the free energy has a ratio of $E_{free}/E_p \approx 1%-25%$. The Poynting flux amounts to $F_{flare} \approx 5 \times 10^{8} - 10^{10}$ erg cm$^{-2}$ s$^{-1}$ during flares, which averages to $F_{AR} \approx 6 \times 10^6$ erg cm$^{-2}$ s$^{-1}$ during the entire observation period and is comparable with the coronal heating rate requirement in active regions.
    10/2014;
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    ABSTRACT: 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.
    04/2014; 787(1).
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    ABSTRACT: Dynamic phenomena indicative of slipping reconnection and magnetic implosion were found in a time series of nonlinear force-free field (NLFFF) extrapolations for the active region 11515, which underwent significant changes in the photospheric fields and produced five C-class flares and one M-class flare over five hours on 2012 July 2. NLFFF extrapolation was performed for the uninterrupted 5 hour period from the 12 minute cadence vector magnetograms of the Helioseismic and Magnetic Imager on board the Solar Dynamic Observatory. According to the time-dependent NLFFF model, there was an elongated, highly sheared magnetic flux rope structure that aligns well with an Hα filament. This long filament splits sideways into two shorter segments, which further separate from each other over time at a speed of 1-4 km s–1, much faster than that of the footpoint motion of the magnetic field. During the separation, the magnetic arcade arching over the initial flux rope significantly decreases in height from ~4.5 Mm to less than 0.5 Mm. We discuss the reality of this modeled magnetic restructuring by relating it to the observations of the magnetic cancellation, flares, a filament eruption, a penumbra formation, and magnetic flows around the magnetic polarity inversion line.
    The Astrophysical Journal Letters 03/2014; 784(1):L13. · 6.35 Impact Factor
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    ABSTRACT: This Letter reports two rarely observed three-ribbon flares (M1.9 and C9.2) on 2012 July 6 in NOAA AR 11515, which we found with Halpha observations of 0.1" resolution from the New Solar Telescope and CaII H images from Hinode. The flaring site is characterized with an intriguing "fish-bone-like" morphology evidenced by both Halpha images and a nonlinear force-free field (NLFFF) extrapolation, where two semi-parallel rows of low-lying, sheared loops connect an elongated, parasitic negative field with the sandwiching positive fields. The NLFFF model also shows that the two rows of loops are asymmetric in height and have opposite twists, and are enveloped by large-scale field lines including open fields. The two flares occurred in succession in half an hour and are located at the two ends of the flaring region. The three ribbons of each flare run parallel to the PIL, with the outer two lying in the positive field and the central one in the negative field. Both flares show surge-like flows in Halpha apparently toward the remote region, while the C9.2 flare is also accompanied by EUV jets possibly along the open field lines. Interestingly, the 12-25 keV hard X-ray sources of the C9.2 flare first line up with the central ribbon then shift to concentrate on the top of the higher branch of loops. These results are discussed in favor of reconnection along the coronal null-line producing the three flare ribbons and the associated ejections.
    The Astrophysical Journal Letters 01/2014; 781(1):L23. · 6.35 Impact Factor
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    ABSTRACT: Rapid, irreversible changes of magnetic topology and sunspot structure associated with flares have been systematically observed in recent years. The most striking features include the increase of horizontal field at the polarity inversion line (PIL) and the co-spatial penumbral darkening. A likely explanation of the above phenomenon is the back reaction to the coronal restructuring after eruptions: a coronal mass ejection carries the upward momentum while the downward momentum compresses the field lines near the PIL. Previous studies could only use low resolution (above 1") magnetograms and white-light images. Therefore, the changes are mostly observed for X-class flares. Taking advantage of the 0.1" spatial resolution and 15s temporal cadence of the New Solar Telescope at Big Bear Solar Observatory, we report in detail the rapid formation of sunspot penumbra at the PIL associated with the C7.4 flare on 2012 July 2. It is unambiguously shown that the solar granulation pattern evolves to alternating dark and bright fibril structure, the typical pattern of penumbra. Interestingly, the appearance of such a penumbra creates a new delta sunspot. The penumbral formation is also accompanied by the enhancement of horizontal field observed using vector magnetograms from the Helioseismic and Magnetic Imager. We explain our observations as due to the eruption of a flux rope following magnetic cancellation at the PIL. Subsequently the re-closed arcade fields are pushed down towards the surface to form the new penumbra. NLFFF extrapolation clearly shows both the flux rope close to the surface and the overlying fields.
    The Astrophysical Journal Letters 08/2013; 774(2). · 6.35 Impact Factor
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    ABSTRACT: Using Big Bear Solar Observatory film data recently digitized at NJIT, we investigate a Moreton wave associated with an X9 flare on 1990 May 24, as well as its interactions with four filaments F1-F4 located close to the flaring region. The interaction yields interesting insight into physical properties of both the wave and the filaments. The first clear Moreton wavefront appears at the flaring-region periphery at approximately the same time as the peak of a microwave burst and the first of two γ-ray peaks. The wavefront propagates at different speeds ranging from 1500-2600 km s–1 in different directions, reaching as far as 600 Mm away from the flaring site. Sequential chromospheric brightenings are observed ahead of the Moreton wavefront. A slower diffuse front at 300-600 km s–1 is observed to trail the fast Moreton wavefront about one minute after the onset. The Moreton wave decelerates to ~550 km s–1 as it sweeps through F1. The wave passage results in F1's oscillation which is featured by ~1 mHz signals with coherent Fourier phases over the filament, the activation of F3 and F4 followed by gradual recovery, but no disturbance in F2. Different height and magnetic environment together may account for the distinct responses of the filaments to the wave passage. The wavefront bulges at F4, whose spine is oriented perpendicular to the upcoming wavefront. The deformation of the wavefront is suggested to be due to both the forward inclination of the wavefront and the enhancement of the local Alfvén speed within the filament channel.
    The Astrophysical Journal 08/2013; 773(2):166. · 6.73 Impact Factor
  • Yan Xu, J. Jing, S. Wang, H. Wang
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    ABSTRACT: In this study, we present the imaging spectroscopy of an X-class flare with white-light emission on September 06, 2011, observed with Helioseismic Magnetic Imager (HMI) on board Solar Dynamics Observatory (SDO). The HMI provides seeing-free images at 6173 \AA continuum with a 45s cadence and six-point spectrograms centered at 6173.34 \AA with 0.172 \AA steps. Taking advantage of the 0.5\arcsecond image scale, the flare kernels are fully resolved and fine structures, including the core and halo, are able to be identified. We analyzed the line-profile, constructed from six spectral positions, of the flare core and halo pixels, respectively. We studied the morphology of the continuum flare kernel comparing with previous white-light observations. The resemblance and the discrepancy of the two kinds of spectra, which could be related to different heating mechanisms, are then discussed.
    07/2013;
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    ABSTRACT: He I D3 line has a unique response to the flare impact on the low solar atmosphere and can be a powerful diagnostic tool for energy transport processes. Using images obtained from the recently digitized films of Big Bear Solar Observatory, we report D3 observation of the M6.3 flare on 1984 May 22, which occurred in an active region with a circular magnetic polarity inversion line (PIL). The impulsive phase of the flare starts with a main elongated source that darkens in D3, inside of which bright emission kernels appear at the time of the initial small peak in hard X-rays (HXRs). These flare cores subsequently evolve into a sharp emission strand lying within the dark halo simultaneously with the main peak in HXRs, reversing the overall source contrast from -5% to 5%. The radiated energy in D3 during the main peak is estimated to be about 10^30 ergs, which is comparable to that carried by nonthermal electrons above 20 keV. Afterwards the flare proceeds along the circular PIL in the counterclockwise direction to form a dark circular ribbon in D3, which apparently mirrors the bright ribbons in Halpha and He I 10830 A. All these ribbons last for over one hour in the late gradual phase. We suggest that the present event resembles the so-called black-light flare that is proposed based on continuum images, and that D3 darkening and brightening features herein may be due to, respectively, the thermal conduction heating and the direct precipitation of high-energy electrons.
    The Astrophysical Journal 06/2013; 774(1). · 6.73 Impact Factor
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    ABSTRACT: We present an unprecedented high-resolution \ha\ imaging spectroscopic observation of a C4.1 flare taken with IBIS on 2011 October 22. The flare consists of a main circular ribbon that occurred in a parasitic magnetic configuration and a remote ribbon that was observed by the IBIS. Such a circular-ribbon flare with a remote brightening is predicted in 3D fan-spine reconnection but so far has been rarely observed. During the flare impulsive phase, we define "core" and "halo" structures in the observed ribbon. Examining the \ha\ emission spectra averaged in the flare core and halo areas, we find that only those from the flare cores show typical nonthermal electron beam heating characteristics. These characteristics include: broad and centrally reversed emission spectra, excess emission in the red wing with regard to the blue wing (i.e., red asymmetry), and redshifted bisectors of the emission spectra. We also observe rather quick timescales for the heating (30 s) and cooling (14--33 s) in the flare core locations. Therefore, we suggest that the flare cores revealed by IBIS track the sites of electron beam precipitation with exceptional spatial and temporal resolution. The flare cores show two-stage motion (a parallel motion along the ribbon followed by an expansion motion perpendicular to the ribbon) during the two impulsive phases of the flare. Some cores jump quickly (30 \kms) between discrete magnetic elements implying reconnection involving different flux tubes. We observe a very high temporal correlation ($\gtrsim0.9$) between the integrated \ha\ and HXR emission during the flare impulsive phase. A short time delay (4.6 s) is also found in the \ha\ emission spikes relative to HXR bursts. The ionization timescale of the cool chromosphere and the extra time taken for the electrons to travel to the remote ribbon site may contribute to this delay.
    The Astrophysical Journal 04/2013; 769(2). · 6.73 Impact Factor
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    ABSTRACT: 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 Halpha data from the Global Halpha Network. The filaments and flare ribbons observed in Halpha 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, 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.
    The Astrophysical Journal 10/2012; 761(1). · 6.73 Impact Factor
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    ABSTRACT: Both magnetic and current helicities are crucial ingredients for describing the complexity of active-region magnetic structure. In this Letter, we present the temporal evolution of these helicities contained in NOAA active region 11158 during five days from 2011 February 12 to 16. The photospheric vector magnetograms of the Helioseismic and Magnetic Imager on board the Solar Dynamic Observatory were used as the boundary conditions for the coronal field extrapolation under the assumption of nonlinear force-free field, from which we calculated both relative magnetic helicity and current helicity. We construct a time-altitude diagram in which altitude distribution of the magnitude of current helicity density is displayed as a function of time. This diagram clearly shows a pattern of upwardly propagating current helicity density over two days prior to the X2.2 flare on February 15 with an average propagation speed of ~36 m s–1. The propagation is synchronous with the emergence of magnetic flux into the photosphere, and indicative of a gradual energy buildup for the X2.2 flare. The time profile of the relative magnetic helicity shows a monotonically increasing trend most of the time, but a pattern of increasing and decreasing magnetic helicity above the monotonic variation appears prior to each of two major flares, M6.6 and X2.2, respectively. The physics underlying this bump pattern is not fully understood. However, the fact that this pattern is apparent in the magnetic helicity evolution but not in the magnetic flux evolution makes it a useful indicator in forecasting major flares.
    The Astrophysical Journal Letters 05/2012; 752(1):L9. · 6.35 Impact Factor
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    ABSTRACT: In this Letter, we present a new approach to estimate the formation height of visible and near-infrared emission of an X10 flare. The sizes of flare emission cores in three wavelengths are accurately measured during the peak of the flare. The source size is the largest in the G band at 4308 Å and shrinks toward longer wavelengths, namely the green continuum at 5200 Å and NIR at 15600 Å, where the emission is believed to originate from the deeper atmosphere. This size-wavelength variation is likely explained by the direct heating model as electrons need to move along converging field lines from the corona to the photosphere. Therefore, one can observe the smallest source, which in our case is 065 ± 002 in the bottom layer (represented by NIR), and observe relatively larger kernels in upper layers of 103 ± 014 and 196 ± 027, using the green continuum and G band, respectively. We then compare the source sizes with a simple magnetic geometry to derive the formation height of the white-light sources and magnetic pressure in different layers inside the flare loop.
    The Astrophysical Journal Letters 04/2012; 750(1):L7. · 6.35 Impact Factor
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    ABSTRACT: The rapid, irreversible change of the photospheric magnetic field has been recognized as an important element of the solar flare process. This Letter reports such a rapid change of magnetic fields during the 2011 February 13 M6.6 flare in NOAA AR 11158 that we found from the vector magnetograms of the Helioseismic and Magnetic Imager (HMI) with 12 minute cadence. High-resolution magnetograms of Hinode that are available at ~–5.5, –1.5, 1.5, and 4 hr relative to the flare maximum are used to reconstruct a three-dimensional coronal magnetic field under the nonlinear force-free field (NLFFF) assumption. UV and hard X-ray images are also used to illuminate the magnetic field evolution and energy release. The rapid change is mainly detected by HMI in a compact region lying in the center of the magnetic sigmoid, where the mean horizontal field strength exhibited a significant increase of 28%. The region lies between the initial strong UV and hard X-ray sources in the chromosphere, which are cospatial with the central feet of the sigmoid according to the NLFFF model. The NLFFF model further shows that strong coronal currents are concentrated immediately above the region, and that, more intriguingly, the coronal current system underwent an apparent downward collapse after the sigmoid eruption. These results are discussed in favor of both the tether-cutting reconnection producing the flare and the ensuing implosion of the coronal field resulting from the energy release.
    The Astrophysical Journal Letters 12/2011; 745(1):L4. · 6.35 Impact Factor
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    ABSTRACT: In this paper, we present a method to automatically segment chromospheric fibrils from Hα observations and further identify their orientation. We assume that chromospheric fibrils are aligned with the magnetic field. By comparing the orientation of the fibrils with the azimuth of the embedding chromospheric magnetic field extrapolated from a potential field model, the shear angle, a measure of nonpotentiality, along the fibrils is readily deduced. Following this approach, we make a quantitative assessment of the nonpotentiality of fibrils in two NOAA active regions (ARs): (1) the relatively simple AR 11092, observed with very high resolution by Interferometric Bidimensional Spectrometer, and (2) a β-γ-δ AR 9661, observed with median resolution by Big Bear Solar Observatory before and after an X1.6 flare.
    The Astrophysical Journal 09/2011; 739(2):67. · 6.73 Impact Factor
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    ABSTRACT: We present a spectroscopic study of an X-class flare observed on 2011 Feb. 15, which is confirmed as a white-light flare by HINODE/SOT continuum images. HXR observation of this flare was taken by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and radio observation was taken by the Korean Solar Radio Burst Locator (KSRBL). During the 10 minute lifetime of this flare, four major temporal peaks have been identified in both HXR and radio emissions. Spectra at each individual peak are obtained with a frequency range in microwave from 5 to 18 GHz and an energy range in HXR from 35 to 150 keV, respectively. The high cadence observations in HXR and microwave provide an unique tool to diagnose the properties of energetic electrons and their temporal evolution. In particular, we will present the result of electron power index as derived from these two wavelengths, and its peak-to-peak variation.
    05/2011;
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    ABSTRACT: We present a striking filament pattern in the nonlinear force-free (NLFF) chromospheric magnetic field of the active region NOAA 10956. The NLFF chromospheric field is extrapolated from the Hinode high-resolution photospheric vector magnetogram using the weighted optimization method. The modeled structure is characterized by a highly sheared field with strong horizontal magnetic components and has a virtually identical shape and location as the filament seen in H{alpha}. The modeled field strength agrees with the recent He I 10830 A observations by Kuckein et al.. The unequivocal resemblance between the NLFF extrapolation and the H{alpha} observation not only demonstrates the ability of the NLFF field to reproduce chromospheric features, but also provides a valuable diagnostic tool for the filament magnetic fields.
    The Astrophysical Journal Letters 08/2010; · 6.35 Impact Factor
  • Yan Xu, R. Liu, J. Jing, H. Wang
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    ABSTRACT: Since early this year, more and more activities have occurred on the solar disk indicating that we are entering the ascending phase of the solar cycle 24. The active region NOAA 11045 appeared to be incredibly dynamic and produced more than 40 flares above C-class from 2010 February 06 to 14. The Global H-alpha Network (GHN) observed most of these flares from its nine stations world wide with 1 minute cadence and 1 arcsecond pixel resolution. We present the dynamics of filaments during 3 M-class flares on 2010 February 07 and 08. On February 07, we found that the filament shrank after the flare, which could be one of the forms of the "implosion" due to the reduction of the magnetic pressure. Partial eruptions were observed during the February 08 events. Erupted material was found to be cooled and returned to the surface along a filament channel, which was invisible in H-alpha lines prior to the flare.
    05/2010;
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    ABSTRACT: In this study, the photospheric vector magnetograms, obtained with the Spectro-Polarimeter of the Solar Optical Telescope on board Hinode, are used as the boundary conditions to extrapolate the three-dimensional nonlinear force-free (NLFF) coronal magnetic fields. The observed non-force-free photospheric magnetic fields are preprocessed toward the nearly force-free chromospheric magnetic fields. The performance of the preprocessing procedure is evaluated by comparing with chromospheric magnetic fields obtained by the Vector SpectroMagnetograph instrument located on the Synoptic Optical Long-term Investigations of the Sun Tower. Then, the weighted optimization method is applied to the preprocessed boundary data to extrapolate the NLFF fields with which we are able to estimate the free magnetic energy stored in the active regions. The magnitude scaling correlation between the free magnetic energy and the soft X-ray flare index (FI) of active regions is then studied. The latter quantifies the impending flare production of active regions over the subsequent 1, 2, and 3 day time windows. Based on 75 samples, we find a positive correlation between the free energy and the FI. We also study the temporal variation of free magnetic energy for three active regions, of which two are flare-active and one is flare-quiet during the observation over a period of several days. While the magnitude of free magnetic energy unambiguously differentiates between the flare-active and the flare-quiet regions, the temporal variation of free magnetic energy does not exhibit a clear and consistent pre-flare pattern. This may indicate that the trigger mechanism of flares is as important as the energy storage in active regions.
    The Astrophysical Journal 03/2010; 713(1):440. · 6.73 Impact Factor
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    ABSTRACT: In this Letter, we present hard X-ray (HXR) observation by the Reuven Ramaty High Energy Solar Spectroscopic Imager of the 2003 October 29 X10 flare. Two pairs of HXR conjugate footpoints have been identified during the early impulsive phase. This geometric configuration is very much in the manner predicted by the "tether-cutting" scenario first proposed by Moore & Roumeliotis. The HXR light curves show that the outer pair of footpoints disappeared much faster than the other pair. This temporal behavior further confirms that this event is a good example of the "tether-cutting" model. In addition, we reconstructed a three-dimensional magnetic field based on the nonlinear force-free extrapolation and found that each pair of HXR footpoints were indeed linked by corresponding magnetic field lines.
    The Astrophysical Journal Letters 01/2010; 709(2):L142. · 6.35 Impact Factor
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    Rui Liu, Yan Xu, Haimin Wang
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    ABSTRACT: We present a selected few cases in which the sense of chirality of filament barbs changed within as short as hours. We investigate in detail a quiescent filament on 2003 September 10 and 11. Of its four barbs displaying such changes only one overlay a small polarity inversion line inside the EUV filament channel (EFC). No magnetic elements with magnitude above the noise level were detected at the endpoints of all barbs. In particular, a pair of barbs first approached toward and then departed from each other in H-alpha, with the barb endpoints migrating as far as ~10". We conclude that the evolution of the barbs was driven by flux emergence and cancellation of small bipolar units at the EFC border. Comment: To appear in the Proceedings of the 25th NSO Workshop
    01/2010;

Publication Stats

386 Citations
176.09 Total Impact Points

Institutions

  • 2004–2014
    • New Jersey Institute of Technology
      • • Space Weather Research Laboratory
      • • Department of Physics
      Newark, New Jersey, United States
  • 2013
    • Queen's University Belfast
      • Astrophysics Research Centre (ARC)
      Béal Feirste, N Ireland, United Kingdom
  • 2009–2013
    • Weather Research Center
      Houston, Texas, United States
  • 2012
    • Max Planck Institute for Solar System Research
      Göttingen, Lower Saxony, Germany
  • 2008
    • Oklahoma State University - Stillwater
      • Department of Physics
      Stillwater, OK, United States