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Two Kinds of Dynamic Behavior in a Quiescent Prominence Observed by the NVST  

Dong Li   Submitted: 2018-07-11 23:51

We present high-resolution observations of two kinds of dynamic behavior in a quiescent prominence using the New Vacuum Solar Telescope, i.e., Kelvin-Helmholtz instabilities (KHIs) and small-scale oscillations. The KHIs were identified as rapidly developed vortex-like structures with counter-clockwise/clockwise rotations in the Ha red-wing images at +0.3 A, which were produced by the strong shear-flows motions on the surface/interface of prominence plumes. The KHI growth rates are estimated to be about 0.0135 ±0.0004 and 0.0138 ± 0.0004. Our observational results further suggest that the shear velocities (i.e, supersonic) of the mass flows are fast enough to produce the strong deformation of the boundary and overcome the restraining surface tension force. This flow-driven instability might play a significant role in the process of plasma transfer in solar prominences. The small-scale oscillations perpendicular to the prominence threads are observed in the Ha line-center images. The oscillatory periods changed non-monotonically and showed two changing patterns, in which one firstly decreased slowly and then it changed to increase, while the other grew fast at the beginning and then it changed to decrease. Both of these two thread oscillations with changing periods were observed to be unstable for an entire cycle, and they were local in nature. All our findings indicate that the small-scale thread oscillations could be magnetohydrodynamic waves in the solar corona.

Authors: Dong Li, Yuandeng Shen, Zongjun Ning, Qingmin Zhang, Tuanhui Zhou
Projects: New Vacuum Solar Telescope (NVST)

Publication Status: accept by the Astrophysical Journal
Last Modified: 2018-07-17 10:51
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Spectroscopic and imaging observations of small-scale reconnection events  

Dong Li   Submitted: 2018-07-07 20:37

We present spectroscopic and imaging observations of small-scale reconnection events on the Sun. Using the Interface Region Imaging Spectrograph (IRIS) observations, one reconnection event is first detected as IRIS jets with fast bi-directional velocities in the chromosphere and transition region, which are identified as non-Gaussian broadenings with two extended wings in the line profiles of Si iv, C ii, and Mg ii k. The magnetograms under the IRIS jets from Helioseismic and Magnetic Images exhibit magnetic flux cancellation simultaneously, supporting that the IRIS jets are driven by magnetic reconnection. The Atmospheric Imaging Assembly images also show an extreme ultraviolet (EUV) brightening which is shortly after the underlying IRIS jets, i.e., in the 131 A, 171 Å, 193 Å, 211 A, and 94 Å channels, implying that the over-lying EUV brightening in the corona is caused by the IRIS jets in the chromosphere and transition region. We also find another three reconnection events which show the same features during this IRIS observation. Our observational results suggest that the small-scale reconnection events might contribute to the coronal heating. The new result is that the process of magnetic reconnection is detected from the photosphere through chromosphere and transition region to the corona.

Authors: Dong Li, Leping Li, Zongjun Ning
Projects: IRIS,SDO-AIA

Publication Status: accept by MNRAS
Last Modified: 2018-07-09 10:18
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Non-damping oscillations at flaring loops  

Dong Li   Submitted: 2018-06-22 08:01

We investigate the properties of non-damping oscillations at flaring loops using observations from the IRIS, SDO, Fermi and GOES. The double-component Gaussian fitting method is used to extract the line pro le of Fe XXI 1354.08 A at the spectral window. The quasi-periodicity of loop oscillations are identified in the Fourier and wavelet spectra. A quasi-period of about 40 s is detected in the line properties of Fe XXI 1354.08 A, hard X-ray emissions in GOES 1-8 A derivative, and Fermi 26-50 keV. The Doppler velocity and line width oscillate in phase, while a phase shift of about Pi/2 is detected between the Doppler velocity and peak intensity. The amplitudes of Doppler velocity and line width oscillation are about 2.2 km s-1 and 1.9 km s-1, respectively, while peak intensity oscillate with amplitude at about 3.6% of the background emission. The observations show that 40 s oscillations are not damped significantly, which could be linked to the global kink modes of flaring loops. The magnetic field strengths at flaring loops are estimated to be about 120-170 G using the MHD seismology diagnostics and the flux rope insertion method. Meanwhile, a quasi-period of about 155 s is identified in the Doppler velocity and peak intensity of the Fe xxi 1354.08 A line emission, and AIA 131 A intensity, and it is likely a signature of recurring downflows after chromospheric evaporation along the flaring loops.

Authors: Li, D.; Yuan, D.; Su, Y. N.; Zhang, Q. M.; Su, W.; Ning, Z. J.
Projects: Fermi/GBM,IRIS

Publication Status: accepted by A&A
Last Modified: 2018-06-22 11:42
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Observations of Electron-driven Evaporation during a Flare Precursor  

Dong Li   Submitted: 2018-01-22 18:34

We investigate the relationship between the blue shifts of a hot emission line and the nonthermal emissions in microwave and hard X-ray (HXR) wavelengths during the precursor of a solar flare on 2014 October 27. The flare precursor is identified as a small but well-developed peak in soft X-ray and extreme-ultraviolet passbands before the GOES flare onset, which is accompanied by a pronounced burst in microwave 17 & 34 GHz and HXR 25-50 keV. The slit of Interface Region Imaging Spectrograph (IRIS) stays at one ribbon-like transient during the flare precursor, where shows visible nonthermal emissions in NoRH and RHESSI images. The IRIS spectroscopic observations show that the hot line of Fe XXI 1354.09 Å (logT ~ 7.05) displays blue shifts, while the cool line of Si IV 1402.77 Å (logT ~ 4.8) exhibits red shifts. The blue shifts and red shifts are well correlated to each other, indicative of an explosive chromospheric evaporation during the flare precursor particularly combining with a high nonthermal energy flux and a short characteristic timescale. In addition, the blue shifts of Fe XXI 1354.09 Å are well correlated with the microwave and HXR emissions, implying that the explosive chromospheric evaporation during the flare precursor is driven by nonthermal electrons.

Authors: Dong Li, Ying Li, Wei Su, Yu Huang, Zongjun Ning
Projects: IRIS

Publication Status: accept by the Astrophysical Journal
Last Modified: 2018-01-23 11:52
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Doppler shift oscillations from a hot line observed by IRIS  

Dong Li   Submitted: 2017-09-28 18:17

We present a detailed investigation of the Doppler shift oscillations in a hot loop during an M7.1 flare on 2014 October 27 observed by the Interface Region Imaging Spectrograph. The periodic oscillations are observed in the Doppler shift of Fe XXI 1354.09 Å (logT~7.05), and the dominant period is about 3.1 minutes. However, such 3.1-min oscillations are not found in the line-integrated intensity of Fe XXI 1354.09 Å, AIA EUV fluxes, or microwave emissions. SDO/AIA and Hinode/XRT imaging observations indicate that the Doppler shift oscillations locate at the hot loop-top region (> 11MK). Moreover, the differential emission measure (DEM) results show that the temperature is increasing rapidly when the Doppler shift oscillates, but the number density does not exhibit the corresponding increases nor oscillations, implying that the flare loop is likely to oscillate in an incompressible mode. All these facts suggest that the Doppler shift oscillations at the shorter period are most likely the standing kink oscillations in a flare loop. Meanwhile, a longer period of about 10 minutes is identified in the time series of Doppler shift and line-integrated intensity, GOES SXR fluxes and AIA EUV light curves, indicating the periodic energy release in this flare, which may be caused by a slow mode wave.

Authors: Dong Li, Zongjun Ning, Yu Huang, N.-H. Chen, Qingmin Zhang, Yingna Su, Wei Su
Projects: IRIS

Publication Status: accept by the Astrophysical Journal
Last Modified: 2017-10-02 12:31
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Quasi-periodic pulsations with multiple periods in hard X-ray emission  

Dong Li   Submitted: 2017-06-06 04:16

We explore the quasi-periodic pulsations (QPPs) with multiple periods in hard X-ray (HXR) emission from Fermi/GBM during the impulsive phase of solar flare (SOL2014-09-10). The completely new observational result is that the shorter periods appear at lower energies of the X-ray photons at the beginning and the longer periods appear at higher energies at the end, with some intersection of the periods at medium energies. We also find the shorter and then the longer periods during the same phase of this flare. Using the wavelet power spectrum and fast Fourier transform (FFT) spectrum, we analyze the normalized rapidly varying signal divided by its slowly varying signal, which is the smoothed original HXR flux. The periods of 27 s and 37 s are derived at lower energy channels between 17:25 UT and 17:29 UT, i.e., 12.0-27.3 keV and 27.3-50.9 keV. Then the periods of 27 s, 46 s and 60 s are observed at medium-energy channel from 17:26 UT to 17:33 UT, such as 50.9-102.3 keV. And the period of 80 s is detected at higher energy channel from 17:28 UT to 17:33 UT, such as 102.3-296.4 keV.

Authors: Li, Dong; Zhang, Qingmin
Projects: Fermi/GBM

Publication Status: MNRAS Letters (in press)
Last Modified: 2017-06-10 19:39
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Explosive chromospheric evaporation driven by nonthermal electrons around one footpoint of a solar flare loop  

Dong Li   Submitted: 2017-05-09 08:32

We explore the temporal relationship between microwave/HXR emission and Doppler velocity during the impulsive phase of a solar flare on 2014 October 27 (SOL2014-10-27), which displays a pulse on the light curves in microwave (34 GHz) and hard X-ray (HXR, 25-50 keV) bands before the flare maximum. Imaging observation shows that this pulse mainly comes from one footpoint of a solar flare loop. The slit of Interface Region Imaging Spectrograph (IRIS) stays at this footpoint during this solar flare. The Doppler velocities of Fe XXI 1354.09 Å and Si IV 1402.77 Å are extracted from the Gaussian fitting method. We find that the hot line of Fe XXI 1354.09 Å (logT~7.05) in corona exhibits blue shift, while the cool line of Si IV 1402.77 Å (logT~4.8) in transition region exhibits red shift, indicating explosive chromospheric evaporation. The evaporative upflows along the flare loop are also observed in the AIA 131 Å image. To our knowledge, this is the first report of chromospheric evaporation evidence from both spectral and imaging observations in the same flare. Both microwave and HXR pulses are well correlated with the Doppler velocities, suggesting that the chromospheric evaporation is driven by nonthermal electrons around this footpoint of a solar flare loop.

Authors: Li, Dong; Ning, Zongjun; Huang, Yu; Zhang, Qingmin
Projects: IRIS

Publication Status: accept by the Astrophysical Journal Letters
Last Modified: 2017-05-10 09:17
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Abstracts by Author
Two Kinds of Dynamic Behavior in a Quiescent Prominence Observed by the NVST
Spectroscopic and imaging observations of small-scale reconnection events
Non-damping oscillations at flaring loops
Observations of Electron-driven Evaporation during a Flare Precursor
Doppler shift oscillations from a hot line observed by IRIS
Quasi-periodic pulsations with multiple periods in hard X-ray emission
Explosive chromospheric evaporation driven by nonthermal electrons around one footpoint of a solar flare loop

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