Space Weather Research Laboratory

Newark, NJ, United States

Space Weather Research Laboratory

Newark, NJ, United States
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Liu R.,Hefei University of Technology | Liu R.,Space Weather Research Laboratory | Kliem B.,University of Potsdam | Kliem B.,University College London | And 7 more authors.
Astrophysical Journal | Year: 2012

We study an active-region dextral filament that was composed of two branches separated in height by about 13 Mm, as inferred from three-dimensional reconstruction by combining SDO and STEREO-B observations. This "double-decker" configuration sustained for days before the upper branch erupted with a GOES-class M1.0 flare on 2010 August 7. Analyzing this evolution, we obtain the following main results. (1) During the hours before the eruption, filament threads within the lower branch were observed to intermittently brighten up, lift upward, and then merge with the upper branch. The merging process contributed magnetic flux and current to the upper branch, resulting in its quasi-static ascent. (2) This transfer might serve as the key mechanism for the upper branch to lose equilibrium by reaching the limiting flux that can be stably held down by the overlying field or by reaching the threshold of the torus instability. (3) The erupting branch first straightened from a reverse S shape that followed the polarity inversion line and then writhed into a forward S shape. This shows a transfer of left-handed helicity in a sequence of writhe-twist-writhe. The fact that the initial writhe is converted into the twist of the flux rope excludes the helical kink instability as the trigger process of the eruption, but supports the occurrence of the instability in the main phase, which is indeed indicated by the very strong writhing motion. (4) A hard X-ray sigmoid, likely of coronal origin, formed in the gap between the two original filament branches in the impulsive phase of the associated flare. This supports a model of transient sigmoids forming in the vertical flare current sheet. (5) Left-handed magnetic helicity is inferred for both branches of the dextral filament. (6) Two types of force-free magnetic configurations are compatible with the data, a double flux rope equilibrium and a single flux rope situated above a loop arcade. © © 2012. The American Astronomical Society. All rights reserved.


Liu R.,Space Weather Research Laboratory | Lee J.,NJIT | Wang T.,Catholic University of America | Stenborg G.,Interferometrics Inc. | And 2 more authors.
Astrophysical Journal Letters | Year: 2010

Magnetic reconnection changes the magnetic field topology and powers explosive events in astrophysical, space, and laboratory plasmas. For flares and coronal mass ejections (CMEs) in the solar atmosphere, the standard model predicts the presence of a reconnecting current sheet, which has been the subject of considerable theoretical and numerical modeling over the last 50 years, yet direct, unambiguous observational verification has been absent. In this Letter, we show a bright sheet structure of global length (>0.25R ⊙) and macroscopic width ((5-10)×103 km) distinctly above the cusp-shaped flaring loop, imaged during the flare rising phase in EUV. The sheet formed due to the stretch of a transequatorial loop system and was accompanied by various reconnection signatures. This unique event provides a comprehensive view of the reconnection geometry and dynamics in the solar corona. © 2010. The American Astronomical Society. All rights reserved.


Ruan G.,Shandong University | Chen Y.,Shandong University | Wang H.,Space Weather Research Laboratory | Wang H.,Big Bear Solar Observatory
Astrophysical Journal | Year: 2015

In this paper, we present a study of the persistent and gradual penumbral decay and the correlated decline of the photospheric transverse field component 10-20 hr before a major flare (X1.8) eruption on 2011 September 7. This long-term pre-eruption behavior is corroborated by the well-imaged pre-flare filament rising, the consistent expansion of the coronal arcades overlying the filament, and the nonlinear force-free field modeling results in the literature. We suggest that both the long-term pre-flare penumbral decay and the transverse field decline are photospheric manifestations of the gradual rise of the coronal filament-flux rope system. We also suggest that the C3 flare and the subsequent reconnection process preceding the X1.8 flare play an important role in triggering the later major eruption. © 2015. The American Astronomical Society. All rights reserved..


Liu R.,Space Weather Research Laboratory | Liu C.,Space Weather Research Laboratory | Wang S.,Space Weather Research Laboratory | Deng N.,Space Weather Research Laboratory | And 2 more authors.
Astrophysical Journal Letters | Year: 2010

Sigmoids are one of the most important precursor structures for solar eruptions. In this Letter, we study a sigmoid eruption on 2010 August 1 with EUV data obtained by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamic Observatory (SDO). In AIA 94 Å (Fe xviii; 6 MK), topological reconfiguration due to tether-cutting reconnection is unambiguously observed for the first time, i.e., two opposite J-shaped loops reconnect to form a continuous S-shaped loop, whose central portion is dipped and aligned along the magnetic polarity inversion line (PIL), and a compact loop crossing the PIL. A causal relationship between photospheric flows and coronal tethercutting reconnections is evidenced by the detection of persistent converging flows toward the PIL using line-of-sight magnetograms obtained by the Helioseismic and Magnetic Imager on board SDO. The S-shaped loop remains in quasi-equilibrium in the lower corona for about 50 minutes, with the central dipped portion rising slowly at ∼10 km s-1. The speed then increases to ∼60 km s-1 about 10 minutes prior to the onset of a GOES-class C3.2 flare, as the S-shaped loop speeds up its transformation into an arch-shaped loop, which eventually leads to a looplike coronal mass ejection. The AIA observations combined with Hα filtergrams as well as hard X-ray imaging and spectroscopy are consistent with most flare loops being formed by reconnection of the stretched legs of lesssheared J-shaped loops that envelopes the rising flux rope, in agreement with the standard tether-cutting scenario. © 2010. The American Astronomical Society. All rights reserved.


Liu R.,Space Weather Research Laboratory | Wang H.,Space Weather Research Laboratory
Astrophysical Journal Letters | Year: 2010

On 2005 September 8, a coronal loop overlying the active region NOAA 10808 was observed in TRACE 171 to contract at ∼ 100 km s-1 at the peak of an X5.4-2B flare at 21:05 UT. Prior to the fast contraction, the loop underwent a much slower contraction at ∼ 6 km s-1 for about 8 minutes, initiating during the flare preheating phase. The sudden switch to fast contraction is presumably corresponding to the onset of the impulsive phase. The contraction resulted in the oscillation of a group of loops located below, with the period of about 10 minutes. Meanwhile, the contracting loop exhibited a similar oscillatory pattern superimposed on the dominant downward motion. We suggest that the fast contraction reflects a suddenly reduced magnetic pressure underneath due either to (1) the eruption of magnetic structures located at lower altitudes or to (2) the rapid conversion of magnetic free energy in the flare core region. Electrons accelerated in the shrinking trap formed by the contracting loop can theoretically contribute to a late-phase hard X-ray burst, which is associated with Type IV radio emission. To complement the X5.4 flare which was probably confined, a similar event observed in SOHO/EIT 195 on 2004 July 20 in an eruptive, M8.6 flare is briefly described, in which the contraction was followed by the expansion of the same loop leading up to a halo coronal mass ejection. These observations further substantiate the conjecture of coronal implosion and suggest coronal implosion as a new exciter mechanism for coronal loop oscillations. © 2010. The American Astronomical Society.


Jiang Y.,Chinese Academy of Sciences | Yang J.,Chinese Academy of Sciences | Wang H.,Space Weather Research Laboratory | Ji H.,Chinese Academy of Sciences | And 3 more authors.
Astrophysical Journal | Year: 2014

In this paper, we report the interaction and subsequent merging of two sinistral filaments (F1 and F2) occurring at the boundary of AR 9720 on 2001 December 6. The two filaments were close and nearly perpendicular to each other. The interaction occurred after F1 was erupted and the eruption was impeded by a more extended filament channel (FC) standing in the way, in which F2 was embedded. The erupted material ran into FC along its axis, causing F1 and F2 to merge into a single structure that subsequently underwent a large-amplitude to-and-fro motion. A significant plasma heating process was observed in the merging process, making the mixed material largely disappear from the Hα passband, but appear in Extreme Ultraviolet Telescope 195 Å images for a while. These observations can serve as strong evidence of merging reconnection between the two colliding magnetic structures. A new sinistral filament was formed along FC after the cooling of the merged and heated material. No coronal mass ejection was observed to be associated with the event; though, the eruption was accompanied by a two-ribbon flare with a separation motion, indicating that the eruption had failed. This event shows that, in addition to overlying magnetic fields, such an interaction is an effective restraint to make a filament eruption fail in this way. © 2014. The American Astronomical Society. All rights reserved..


Liu R.,Space Weather Research Laboratory | Xu Y.,Space Weather Research Laboratory | Wang H.,Space Weather Research Laboratory
Solar Physics | Year: 2011

We revisit the flare that occurred on 13 January 1992, which is now universally termed the "Masuda flare". The new analysis is motivated not just by its uniqueness despite the increasing number of coronal observations in hard X-rays, but also by the improvement of Yohkoh hard X-ray image processing, which was achieved after the intensive investigations on this celebrated event. Using an uncertainty analysis, we show that the hard X-ray coronal source is located closer to the soft X-ray loop by about 5000 km (or 7 arcsec) in the re-calibrated Hard X-ray Telescope (HXT) images than in the original ones. Specifically, the centroid of the M1-band (23 - 33 keV) coronal source is above the maximum brightness of the Soft X-ray Telescope (SXT) loop by 5000±1000 km (9600 km in the original data) and above the apex of the SXT loop represented by the 30% brightness contour by 2000±1000 km (~ 7000 km in the original data). The change is obviously significant, because most coronal sources are above the thermal loop by less than 6 arcsec. We suggest that this change may account for the discrepancy in the literature, i. e., the spectrum of the coronal emission was reported to be extremely hard below ~ 20 keV in the pre-calibration investigations, whereas it was reported to be considerably softer in the literature after the re-calibration done by Sato, Kosugi, and Makishima (Pub. Astron. Soc. Japan51, 127, 1999). Still, the coronal spectrum is flatter at lower energies than at higher energies, due to the lack of a similar, co-spatial source in the L-band (14 - 23 keV), for which a convincing explanation is absent. © 2010 Springer Science+Business Media B.V.


Liu R.,Hefei University of Technology | Liu R.,Space Weather Research Laboratory | Liu C.,Space Weather Research Laboratory | Torok T.,Predictive Science Inc. | And 2 more authors.
Astrophysical Journal | Year: 2012

It has recently been noted that solar eruptions can be associated with the contraction of coronal loops that are not involved in magnetic reconnection processes. In this paper, we investigate five coronal eruptions originating from four sigmoidal active regions, using high-cadence, high-resolution narrowband EUV images obtained by the Solar Dynamic Observatory (SDO). The magnitudes of the flares associated with the eruptions range from GOES class B to class X. Owing to the high-sensitivity and broad temperature coverage of the Atmospheric Imaging Assembly (AIA) on board SDO, we are able to identify both the contracting and erupting components of the eruptions: the former is observed in cold AIA channels as the contracting coronal loops overlying the elbows of the sigmoid, and the latter is preferentially observed in warm/hot AIA channels as an expanding bubble originating from the center of the sigmoid. The initiation of eruption always precedes the contraction, and in the energetically mild events (B- and C-flares), it also precedes the increase in GOES soft X-ray fluxes. In the more energetic events, the eruption is simultaneous with the impulsive phase of the nonthermal hard X-ray emission. These observations confirm that loop contraction is an integrated process in eruptions with partially opened arcades. The consequence of contraction is a new equilibrium with reduced magnetic energy, as the contracting loops never regain their original positions. The contracting process is a direct consequence of flare energy release, as evidenced by the strong correlation of the maximal contracting speed, and strong anti-correlation of the time delay of contraction relative to expansion, with the peak soft X-ray flux. This is also implied by the relationship between contraction and expansion, i.e., their timing and speed. © 2012. The American Astronomical Society. All rights reserved.


Liu R.,Hefei University of Technology | Titov V.S.,Predictive Science Inc. | Gou T.,Hefei University of Technology | Wang Y.,Hefei University of Technology | And 2 more authors.
Astrophysical Journal | Year: 2014

We report the observation of an X-class long-duration flare which is clearly confined. It appears as a compact-loop flare in the traditional EUV passbands (171 and 195 Å), but in the passbands sensitive to flare plasmas (94 and 131 Å), it exhibits a cusp-shaped structure above an arcade of loops like other long-duration events. Inspecting images in a running difference approach, we find that the seemingly diffuse, quasi-static cusp-shaped structure consists of multiple nested loops that repeatedly rise upward and disappear approaching the cusp edge. Over the gradual phase, we detect numerous episodes of loop rising, each lasting minutes. A differential emission measure analysis reveals that the temperature is highest at the top of the arcade and becomes cooler at higher altitudes within the cusp-shaped structure, contrary to typical long-duration flares. With a nonlinear force-free model, our analysis shows that the event mainly involves two adjacent sheared arcades separated by a T-type hyperbolic flux tube (HFT). One of the arcades harbors a magnetic flux rope, which is identified with a filament that survives the flare owing to the strong confining field. We conclude that a new emergence of magnetic flux in the other arcade triggers the flare, while the preexisting HFT and flux rope dictate the structure and dynamics of the flare loops and ribbons during the long-lasting decay phase, and that a quasi-separatrix layer high above the HFT could account for the cusp-shaped structure. © 2014. The American Astronomical Society. All rights reserved..


Liu R.,Space Weather Research Laboratory | Liu C.,Space Weather Research Laboratory | Park S.-H.,Space Weather Research Laboratory | Wang H.,Space Weather Research Laboratory
Astrophysical Journal | Year: 2010

The pre-coronal mass ejection (pre-CME) structure is of great importance to understanding the origin of CMEs, which, however, has been largely unknown for CMEs originating from active regions. In this paper, we investigate this issue using the wavelet-enhanced EUV Imaging Telescope (EIT) observations combined with the Large Angle and Spectrometric Coronagraph, Michelson Doppler Imager, and GOES soft X-ray observations. Selected for studying are 16 active-region coronal arcades whose gradual inflation lead up to CMEs. Twelve of them clearly build upon post-eruptive arcades resulting from a preceding eruption; the remaining four are located high in the corona in the first place and/or have existed for days. The observed inflation lasts for 8.7 ± 4.1 hr, with the arcade rising from 1.15 ± 0.06 R⊙ to 1.36 ± 0.07 R⊙ within the EIT field of view (FOV). The rising speed is less than 5 km s-1 most of the time. Only at the end of this quasi-static stage does it increase to tens of kilometers per second over tens of minutes. The arcade then erupts out of the EIT FOV as a CME with similar morphology. This pre-CME structure is apparently unaffected by the flares occurring during its quasi-static inflation phase, but is closely coupled with the flare occurring during its acceleration phase. For four events that are observed on the disk, it is found that the gradual inflation of the arcade is accompanied by significant helicity injection from the photosphere. In particular, a swirling structure, which is reminiscent of a magnetic flux rope, was observed in one of the arcades over 4 hr prior to the subsequent CME, and the growth of the arcade is associated with the injection of helicity of opposite sign into the active region via flux emergence.We propose a four-phase evolution paradigm for the observed CMEs, i.e., a quasi-static inflation phase which corresponds to the buildup of magnetic free energy in the corona, followed by the frequently observed three-phase paradigm, including an initial phase, an acceleration phase, and a gradual phase. © 2010. The American Astronomical Society.

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