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Spectroscopic Observations of a Current Sheet in a Solar Flare  

Ying Li   Submitted: 2018-01-12 07:58

Current sheet is believed to be the region of energy dissipation via magnetic reconnection in solar flares. However, its properties, for example, the dynamic process, have not been fully understood. Here we report a current sheet in a solar flare (SOL2017-09-10T16:06) that was clearly observed by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory as well as the EUV Imaging Spectrometer on Hinode. The high-resolution imaging and spectroscopic observations show that the current sheet is mainly visible in high temperature (>10 MK) passbands, particularly in the Fe XXIV 192.03 line with a formation temperature of ~18 MK. The hot Fe XXIV 192.03 line exhibits very large nonthermal velocities up to 200 km s-1 in the current sheet, suggesting that turbulent motions exist there. The largest turbulent velocity occurs at the edge of the current sheet, with some offset with the strongest line intensity. At the central part of the current sheet, the turbulent velocity is negatively correlated with the line intensity. From the line emission and turbulent features we obtain a thickness in the range of 7-11 Mm for the current sheet. These results suggest that the current sheet has internal fine and dynamic structures that may help the magnetic reconnection within it proceeds efficiently.

Authors: Y. Li, J. C. Xue, M. D. Ding, X. Cheng, Y. Su, L. Feng, J. Hong, H. Li, W. Q. Gan
Projects: Hinode/EIS

Publication Status: ApJL accepted
Last Modified: 2018-01-14 00:00
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Spectroscopic Observations of Magnetic Reconnection and Chromospheric Evaporation in an X-shaped Solar Flare  

Ying Li   Submitted: 2017-08-29 20:40

We present observations of distinct UV spectral properties at different locations during an atypical X-shaped flare (SOL2014-11-09T15:32) observed by the Interface Region Imaging Spectrograph (IRIS). In this flare, four chromospheric ribbons appear and converge at an X-point where a separator is anchored. Above the X-point, two sets of non-coplanar coronal loops approach laterally and reconnect at the separator. The IRIS slit was located close to the X-point, cutting across some of the flare ribbons and loops. Near the location of the separator, the Si IV 1402.77 Å line exhibits significantly broadened line wings extending to 200 km s-1 but an unshifted line core. These spectral features suggest the presence of bidirectional flows possibly related to the separator reconnection. While at the flare ribbons, the hot Fe XXI 1354.08 Å line shows blueshifts and the cool Si IV 1402.77 A, C II 1335.71 A, and Mg II 2803.52 A lines show evident redshifts up to a velocity of 80 km s-1, which are consistent with the scenario of chromospheric evaporation/condensation.

Authors: Y. Li, M. Kelly, M. D. Ding, J. Qiu, X. S. Zhu, W. Q. Gan
Projects: IRIS

Publication Status: ApJ accepted.
Last Modified: 2017-08-30 12:29
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Plasma Brightenings in a Failed Solar Filament Eruption  

Ying Li   Submitted: 2017-02-19 19:35

Failed filament eruptions are solar eruptions that are not associated with coronal mass ejections. In a failed filament eruption, the filament materials usually show some ascending and falling motions as well as generate bright EUV emissions. Here we report a failed filament eruption that occurred in a quiet-Sun region observed by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. In this event, the filament spreads out but gets confined by the surrounding magnetic field. When interacting with the ambient magnetic field, the filament material brightens up and flows along the magnetic field lines through the corona to the chromosphere. We find that some materials slide down along the lifting magnetic structure containing the filament and impact the chromosphere to cause two ribbon-like brightenings in a wide temperature range through kinetic energy dissipation. There is evidence suggesting that magnetic reconnection occurs between the filament magnetic structure and the surrounding magnetic fields where filament plasma is heated to coronal temperatures. In addition, thread-like brightenings show up on top of the erupting magnetic fields at low temperatures, which might be produced by an energy imbalance from a fast drop of radiative cooling due to plasma rarefaction. Thus, this single event of failed filament eruption shows existence of a variety of plasma brightenings that may be caused by completely different heating mechanisms.

Authors: Y. Li, M. D. Ding
Projects: SDO-AIA

Publication Status: ApJ accepted.
Last Modified: 2017-02-21 15:43
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Imaging Observations of Magnetic Reconnection in a Solar Eruptive Flare  

Ying Li   Submitted: 2017-01-04 12:43

Solar flares are one of the most energetic events in the solar atmosphere. It is widely accepted that flares are powered by magnetic reconnection in the corona. An eruptive flare is usually accompanied by a coronal mass ejection, both of which are probably driven by the eruption of a magnetic flux rope (MFR). Here we report an eruptive flare on 2016 March 23 observed by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. The extreme-ultraviolet imaging observations exhibit the clear rise and eruption of an MFR. In particular, the observations reveal solid evidence for magnetic reconnection from both the corona and chromosphere during the flare. Moreover, weak reconnection is observed before the start of the flare. We find that the preflare weak reconnection is of tether-cutting type and helps the MFR to rise slowly. Induced by a further rise of the MFR, strong reconnection occurs in the rise phases of the flare, which is temporally related to the MFR eruption. We also find that the magnetic reconnection is more of 3D-type in the early phase, as manifested in a strong-to-weak shear transition in flare loops, and becomes more 2D-like in the later phase, as shown by the apparent rising motion of an arcade of flare loops.

Authors: Y. Li, X. Sun, M. D. Ding, J. Qiu, E. R. Priest
Projects: SDO-AIA

Publication Status: ApJ accepted.
Last Modified: 2017-01-11 12:10
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Observations of an X-shaped Ribbon Flare in the Sun and Its Three-dimensional Magnetic Reconnection  

Ying Li   Submitted: 2016-05-08 18:47

We report evolution of an atypical X-shaped flare ribbon which provides novel observational evidence of three-dimensional (3D) magnetic reconnection at a separator. The flare occurred on 2014 November 9. High-resolution slit-jaw 1330 A images from the Interface Region Imaging Spectrograph reveal four chromospheric flare ribbons that converge and form an X-shape. Flare brightening in the upper chromosphere spreads along the ribbons toward the center of the "X" (the X-point), and then spreads outward in a direction more perpendicular to the ribbons. These four ribbons are located in a quadrupolar magnetic field. Reconstruction of magnetic topology in the active region suggests the presence of a separator connecting to the X-point outlined by the ribbons. The inward motion of flare ribbons in the early stage therefore indicates 3D magnetic reconnection between two sets of non-coplanar loops that approach laterally, and reconnection proceeds downward along a section of vertical current sheet. Coronal loops are also observed by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory confirming the reconnection morphology illustrated by ribbon evolution.

Authors: Y. Li, J. Qiu, D. W. Longcope, M. D. Ding, K. Yang
Projects: IRIS

Publication Status: ApJL accepted.
Last Modified: 2016-05-09 09:11
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Chromospheric Evaporation in an X1.0 Flare on 2014 March 29 Observed with IRIS and EIS  

Ying Li   Submitted: 2015-08-17 23:39

Chromospheric evaporation refers to dynamic mass motions in flare loops as a result of rapid energy deposition in the chromosphere. These have been observed as blueshifts in X-ray and extreme-ultraviolet (EUV) spectral lines corresponding to upward motions at a few tens to a few hundreds of km s-1. Past spectroscopic observations have also revealed a dominant stationary component, in addition to the blueshifted component, in emission lines formed at high temperatures (~10 MK). This is contradictory to evaporation models predicting predominant blueshifts in hot lines. The recently launched Interface Region Imaging Spectrograph (IRIS) provides high resolution imaging and spectroscopic observations that focus on the chromosphere and transition region in the UV passband. Using the new IRIS observations, combined with coordinated observations from the EUV Imaging Spectrometer, we study the chromospheric evaporation process from the upper chromosphere to corona during an X1.0 flare on 2014 March 29. We find evident evaporation signatures, characterized by Doppler shifts and line broadening, at two flare ribbons separating from each other, suggesting that chromospheric evaporation takes place in successively formed flaring loops throughout the flare. More importantly, we detect dominant blueshifts in the high temperature Fe XXI line (~10 MK), in agreement with theoretical predictions. We also find that, in this flare, gentle evaporation occurs at some locations in the rise phase of the flare, while explosive evaporation is detected at some other locations near the peak of the flare. There is a conversion from gentle to explosive evaporation as the flare evolves.

Authors: Y. Li, M. D. Ding, J. Qiu, J. X. Cheng
Projects: IRIS

Publication Status: ApJ in press
Last Modified: 2015-08-18 07:56
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On the Nature of the EUV Late Phase of Solar Flares  

Ying Li   Submitted: 2014-07-23 19:20

The extreme-ultraviolet (EUV) late phase of solar flares is a second peak of warm coronal emissions (e.g., Fe XVI) for many minutes to a few hours after the GOES soft X-ray peak. It was first observed by the EUV Variability Experiment (EVE) on board the Solar Dynamics Observatory (SDO). The late phase emission originates from a second set of longer loops (late phase loops) that are higher than the main flaring loops. It is suggested as being caused by either additional heating or long-lasting cooling. In this paper, we study the role of long-lasting cooling and additional heating in producing the EUV late phase using the "enthalpy-based thermal evolution of loops" (EBTEL) model. We find that a long cooling process in late phase loops can well explain the presence of the EUV late phase emission, but we cannot exclude the possibility of additional heating in the decay phase. Moreover, we provide two preliminary methods based on the UV and EUV emissions from the Atmospheric Imaging Assembly (AIA) on board SDO to determine whether an additional heating plays some role or not in the late phase emission. Using nonlinear force-free field modeling, we study the magnetic configuration of the EUV late phase. It is found that the late phase can be generated either in hot spine field lines associated with a magnetic null point or in large-scale magnetic loops of multipolar magnetic fields. In this paper, we also discuss why the EUV late phase is usually observed in warm coronal emissions and why the majority of flares do not exhibit an EUV late phase.

Authors: Y. Li, M. D. Ding, Y. Guo, Y. Dai
Projects: None

Publication Status: Accepted for publication in ApJ
Last Modified: 2014-07-24 15:57
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Heating and Dynamics of Two Flare Loop Systems Observed by AIA and EIS  

Ying Li   Submitted: 2013-12-18 22:57

We investigate heating and evolution of flare loops in a C4.7 two-ribbon flare on 2011 February 13. From SDO/AIA imaging observations, we can identify two sets of loops. Hinode/EIS spectroscopic observations reveal blueshifts at the feet of both sets of loops. The evolution and dynamics of the two sets are quite different. The first set of loops exhibits blueshifts for about 25 minutes followed by redshifts, while the second set shows stronger blueshifts, which are maintained for about one hour. The UV 1600 observation by AIA also shows that the feet of the second set of loops brighten twice. These suggest that continuous heating may be present in the second set of loops. We use spatially resolved UV light curves to infer heating rates in the few tens of individual loops comprising the two loop systems. With these heating rates, we then compute plasma evolution in these loops with the ''enthalpy-based thermal evolution of loops'' (EBTEL) model. The results show that, for the first set of loops, the synthetic EUV light curves from the model compare favorably with the observed light curves in six AIA channels and eight EIS spectral lines, and the computed mean enthalpy flow velocities also agree with the Doppler shift measurements by EIS. For the second set of loops modeled with twice-heating, there are some discrepancies between modeled and observed EUV light curves in low-temperature bands, and the model does not fully produce the prolonged blueshift signatures as observed. We discuss possible causes for the discrepancies.

Authors: Y. Li, J. Qiu, M. D. Ding
Projects: SDO-AIA

Publication Status: Accepted for publication in ApJ
Last Modified: 2013-12-19 07:43
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Analysis and Modeling of Two Flare Loops Observed by AIA and EIS  

Ying Li   Submitted: 2012-08-27 20:06

We analyze and model an M1.0 flare observed by SDO/AIA and Hinode/EIS to investigate how flare loops are heated and evolve subsequently. The flare is composed of two distinctive loop systems observed in EUV images. The UV 1600 Å emission at the feet of these loops exhibits a rapid rise, followed by enhanced emission in different EUV channels observed by AIA and EIS. Such behavior is indicative of impulsive energy deposit and the subsequent response in overlying coronal loops that evolve through different temperatures. Using the method we recently developed, we infer empirical heating functions from the rapid rise of the UV light curves for the two loop systems, respectively, treating them as two big loops of cross-sectional area 5arcsec by 5arcsec, and compute the plasma evolution in the loops using the EBTEL model (Klimchuk et al. 2008). We compute the synthetic EUV light curves, which, with the limitation of the model, reasonably agree with observed light curves obtained in multiple AIA channels and EIS lines: they show the same evolution trend and their magnitudes are comparable by within a factor of two. Furthermore, we also compare the computed mean enthalpy flow velocity with the Doppler shift measurements by EIS during the decay phase of the two loops. Our results suggest that the two different loops with different heating functions as inferred from their footpoint UV emission, combined with their different lengths as measured from imaging observations, give rise to different coronal plasma evolution patterns captured both in the model and observations.

Authors: Y. Li, J. Qiu, M. D. Ding
Projects: SDO-AIA

Publication Status: Accepted for publication in ApJ
Last Modified: 2012-08-28 12:31
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Interaction and Eruption of Two Filaments Observed by Hinode, SOHO, and STEREO  

Ying Li   Submitted: 2011-11-18 02:30

This abstract was corrupted following database problems and is being recovered. It will be restored as quickly as possible. Any questions, please send them to Alisdair. Sorry for any incovenience.


Authors: Y. Li and M. D. Ding
Projects:

Publication Status: Accepted for publication in RAA
Last Modified: 2011-11-19 00:57
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Different Patterns of Chromospheric Evaporation in a Flaring Region Observed with Hinode/EIS  

Ying Li   Submitted: 2010-11-22 18:39

We investigate the chromospheric evaporation in the flare of 2007 January 16 using line profiles observed by the EUV Imaging Spectrometer (EIS) onboard Hinode. Three points at flare ribbons of different magnetic polarities are analyzed in detail. We find that the three points show different patterns of upflows and downflows in the impulsive phase of the flare. The spectral lines at the first point are mostly blue shifted, with the hotter lines showing a dominant blue-shifted component over the stationary one. At the second point, however, only weak upflows are detected; in stead, notable downflows appear at high temperatures (up to 2.5-5.0 MK). The third point is similar to the second one only that it shows evidence of multi-component downflows. While the evaporated plasma falling back down as warm rain is a possible cause of the redshifts at points 2 and 3, the different patterns of chromospheric evaporation at the three points imply existence of different heating mechanisms in the flaring active region.

Authors: Y. Li & M. D. Ding
Projects: Hinode/EIS

Publication Status: Accepted for publication in ApJ
Last Modified: 2010-11-22 23:55
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Abstracts by Author
Spectroscopic Observations of a Current Sheet in a Solar Flare
Spectroscopic Observations of Magnetic Reconnection and Chromospheric Evaporation in an X-shaped Solar Flare
Plasma Brightenings in a Failed Solar Filament Eruption
Imaging Observations of Magnetic Reconnection in a Solar Eruptive Flare
Observations of an X-shaped Ribbon Flare in the Sun and Its Three-dimensional Magnetic Reconnection
Chromospheric Evaporation in an X1.0 Flare on 2014 March 29 Observed with IRIS and EIS
On the Nature of the EUV Late Phase of Solar Flares
Heating and Dynamics of Two Flare Loop Systems Observed by AIA and EIS
Analysis and Modeling of Two Flare Loops Observed by AIA and EIS
Interaction and Eruption of Two Filaments Observed by Hinode, SOHO, and STEREO
Different Patterns of Chromospheric Evaporation in a Flaring Region Observed with Hinode/EIS

Related Pages
MSU Solar Physics.
Max Millennium Science Mail Archive.
Max Millennium Message of the Day Mail Archive.
Max Millennium Flare Catalog

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