The Lyα Emission in Solar Flares. I. A Statistical Study on Its Relationship with the 1-8 Å Soft X-Ray Emission |
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Ying Li Submitted: 2021-02-24 19:09
We statistically study the relationship between the Lyα and 1-8 Å soft X-ray (SXR) emissions from 658 M- and X-class solar flares observed by the Geostationary Operational Environmental Satellite during 2006-2016. Based on the peak times of the two wave band emissions, we divide the flares into three types. Type I (III) has an earlier (a later) peak time in the Lyα emission than that in the SXR emission, while type II has nearly the same peak time (within the time resolution of 10 s) between the Lyα and SXR emissions. In these 658 flares, we find that there are 505 (76.8%) type I flares, 10 (1.5%) type II flares, and 143 (21.7%) type III flares, and that the three types appear to have no dependence on the flare duration, flare location, or solar cycle. Besides the main peak, the Lyα emission of the three type flares also shows sub-peaks which can appear in the impulsive or gradual phase of the flare. It is found that the main-peak (for type I) and subpeak (for type III) emissions of Lyα that appear in the impulsive phase follow the Neupert effect in general. This indicates that such Lyα emissions are related to the nonthermal electron beam heating. While the main-peak (for type III) and subpeak (for type I) emissions of Lyα that appear in the gradual phase are supposed to be primarily contributed by the thermal plasma that cools down.
Authors: Jing, Zhichen; Pan, Wuqi; Yang, Yukun; Song, Dechao; Tian, Jun; Li, Y.; Cheng, X.; Hong, Jie; Ding, M. D.
Projects: GOES/EUVS
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Publication Status: published in ApJ
Last Modified: 2021-02-24 20:20
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Radiative hydrodynamic simulations of the spectral characteristics of solar white-light flares |
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Ying Li Submitted: 2021-02-24 19:06
As one of the most violent activities in the solar atmosphere, white-light flares (WLFs) are generally known for their enhanced white-light (or continuum) emission, which primarily originates in the solar lower atmosphere. However, we know little about how white-light emission is produced. In this study, we aim to investigate the response of the continua at 3600 Å and 4250 Å and also the Hα and Lyα lines during WLFs modeled using radiative hydrodynamic simulations. We take non-thermal electron beams as the energy source for the WLFs in two different initial atmospheres and vary their parameters. Our results show that the model with non-thermal electron beam heating clearly shows enhancements in the continua at 3600 Å and 4250 Å as well as in the Hα and Lyα lines. A larger electron beam flux, a smaller spectral index, or an initial penumbral atmosphere leads to a stronger emission increase at 3600 Å, 4250 Å and in the Hα line. The Lyα line, however, is more obviously enhanced in a quiet-Sun initial atmosphere with a larger electron beam spectral index. It is also notable that the continua at 3600 Å and 4250 Å and the Hα line exhibit a dimming at the start of heating and reach their peak emissions after the peak time of the heating function, while the Lyα line does not show such behaviors. These results can serve as a reference for the analysis of future WLF observations.
Authors: Yang, Yu-Tong; Hong, Jie; Li, Ying; Ding, Ming-De; Li, Hui
Projects: None
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Publication Status: published in RAA
Last Modified: 2021-02-24 20:20
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Spectroscopic Observations of a Current Sheet in a Solar Flare |
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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
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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 |
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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
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Publication Status: ApJ accepted.
Last Modified: 2017-08-30 12:29
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Imaging Observations of Magnetic Reconnection in a Solar Eruptive Flare |
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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
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Publication Status: ApJ accepted.
Last Modified: 2017-01-11 12:10
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Chromospheric Evaporation in an X1.0 Flare on 2014 March 29 Observed with IRIS and EIS |
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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
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Publication Status: ApJ in press
Last Modified: 2015-08-18 07:56
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Heating and Dynamics of Two Flare Loop Systems Observed by AIA and EIS |
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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
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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 |
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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
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Publication Status: Accepted for publication in ApJ
Last Modified: 2012-08-28 12:31
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