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Abnormal oscillation modes in a waning light bridge  

Ding Yuan   Submitted: 2016-09-05 03:11

A sunspot acts as a waveguide in response to the dynamics of the solar interior; the trapped waves and oscillations could reveal its thermal and magnetic structures. We study the oscillations in a sunspot intruded by a light bridge, the details of the oscillations could reveal the fine structure of the magnetic topology. We use the Solar Dynamics Observatory/Atmospheric Imaging Assembly data to analyse the oscillations in the emission intensity of light bridge plasma at different temperatures and investigate their spatial distributions. The extreme ultraviolet emission intensity exhibits two persistent oscillations at five-minute and sub-minute ranges. The spatial distribution of the five-minute oscillation follows the spine of the bridge; whereas the sub-minute oscillations overlap with two flanks of the bridge. Moreover, the sub-minute oscillations are highly correlated in spatial domain, however, the oscillations at the eastern and western flanks are asymmetric with regard to the lag time. In the meanwhile, jet-like activities are only found at the eastern flank. Asymmetries in forms of oscillatory pattern and jet-like activities are found between two flanks of a granular light bridge. Based on our study and recent findings, we propose a new model of twisted magnetic field for a light bridge and its dynamic interactions with the magnetic field of a sunspot.

Authors: Ding Yuan, Robert W. Walsh
Projects: None

Publication Status: A&A accepted
Last Modified: 2016-09-07 12:12
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Secondary fast magnetoacoustic waves trapped in randomly structured plasmas  

Ding Yuan   Submitted: 2016-06-21 03:59

Fast magnetoacoustic wave is an important tool for inferring solar atmospheric parameters. We numerically simulate the propagation of fast wave pulses in randomly structured plasmas mimicking the highly inhomogeneous solar corona. A network of secondary waves is formed by a series of partial reflections and transmissions. These secondary waves exhibit quasi-periodicities in both time and space. Since the temporal and spatial periods are related simply through the fast wave speed, we quantify the properties of secondary waves by examining the dependence of the average temporal period (p) on the initial pulse width (w0) as well as the density contrast (δρ) and correlation length (Lc) that characterize the randomness of the equilibrium density profiles. For small-amplitude pulses, δρ does not alter p significantly. Large-amplitude pulses, on the other hand, enhance the density contrast when δρ is small but have a smoothing effect when δρ is sufficiently large. We found that p scales linearly with Lc and that the scaling factor is larger for a narrower pulse. However, in terms of the absolute values of p, broader pulses generate secondary waves with longer periods, and this effect is stronger in random plasmas with shorter correlation lengths. Secondary waves carry the signatures of both the leading wave pulse and background plasma, our study may find applications in MHD seismology by exploiting the secondary waves detected in the dimming regions after CMEs or EUV waves.

Authors: Ding Yuan, Bo Li, Robert W. Walsh
Projects: None

Publication Status: Accepted by ApJ
Last Modified: 2016-06-22 09:12
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Stochastic transients as a source of quasi-periodic processes in the solar atmosphere  

Ding Yuan   Submitted: 2016-03-31 14:33

Solar dynamics and turbulence occur at all heights of the solar atmosphere and could be described as stochastic processes. We propose that finite lifetime transients recurring at a certain place could trigger quasi-periodic processes in the associated structures. In this study, we developed a mathematical model for finite lifetime and randomly occurring transients, and found that quasi-periodic processes, with period longer than the time scale of the transients, are detectable intrinsically in form of trains. We simulate their propagation in an empirical solar atmospheric model with chromosphere, transition region and corona. We found that, due to the filtering effect of the chromospheric cavity, only the resonance period of the acoustic resonator is able to propagate to the upper atmosphere, such a scenario is applicable to slow magnetoacoustic waves in sunspots and active regions. If the thermal structure of the atmosphere is less wild and acoustic resonance does not take effect, the long period oscillations could propagate to the upper atmosphere. Such case would be more likely to occur at polar plumes.

Authors: Ding Yuan, Jiangtao Su, Fangran Jiao, Robert W. Walsh
Projects: None

Publication Status: ApJS in press
Last Modified: 2016-03-31 15:17
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Forward Modelling of Standing Kink Modes in Coronal Loops II. Applications  

Ding Yuan   Submitted: 2016-03-08 03:34

Magnetohydrodynamic waves are believed to play a significant role in coronal heating, and could be used for remote diagnostics of solar plasma. Both the heating and diagnostic applications rely on a correct inversion (or backward modelling) of the observables into the thermal and magnetic structures of the plasma. However, owing to the limited availability of observables, this is an ill-posed issue. Forward Modelling is to establish a plausible mapping of plasma structuring into observables. In this study, we set up forward models of standing kink modes in coronal loops and simulate optically thin emissions in the extreme ultraviolet bandpasses, and then adjust plasma parameters and viewing angles to match three events of transverse loop oscillations observed by the Solar Dynamics Observatory/Atmospheric Imaging Assembly. We demonstrate that forward models could be effectively used to identify the oscillation overtone and polarization, to reproduce the general profile of oscillation amplitude and phase, and to predict multiple harmonic periodicities in the associated emission intensity and loop width variation.

Authors: Ding Yuan, Tom Van Doorsselaere
Projects: SDO-AIA

Publication Status: ApJS in press
Last Modified: 2016-03-08 12:17
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Forward Modelling of Standing Kink Modes in Coronal Loops I. Synthetic Views  

Ding Yuan   Submitted: 2016-03-08 03:32

Kink magnetohydrodynamic (MHD) waves are frequently observed in various magnetic structures of the solar atmosphere. They may contribute significantly to coronal heating and could be used as a tool to diagnose the solar plasma. In this study, we synthesise the Fe IX λ171.073 emission of a coronal loop supporting a standing kink MHD mode. The kink MHD wave solution of a plasma cylinder is mapped into a semi-torus structure to simulate a curved coronal loop. We decompose the solution into a quasi-rigid kink motion and a quadrupole term, which dominate the plasma inside and outside the flux tube, respectively. At the loop edges, the line-of-sight integrates relatively more ambient plasma, and the background emission becomes significant. The plasma motion associated with the quadrupole term causes spectral line broadening and emission suppression. The periodic intensity suppression will modulate the integrated intensity and the effective loop width, which both exhibit oscillatory variations at half of the kink period. The quadrupole term can be directly observed as a pendular motion at front view.

Authors: Ding Yuan, Tom Van Doorsselaere
Projects: Hinode/EIS

Publication Status: ApJS in press
Last Modified: 2016-03-08 12:17
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Forward Modelling of standing slow modes of flaring coronal loops  

Ding Yuan   Submitted: 2015-05-17 11:43

Standing slow mode waves in hot flaring loops are exclusively observed in spectrometers and are used to diagnose the magnetic field strength and temperature of the loop structure. Due to the lack of spatial information, the longitudinal mode cannot be effectively identified. In this study, we simulate standing slow mode waves in flaring loops and compare the synthesized line emission properties with SUMER spectrographic and SDO/AIA imaging observations. We find that the emission intensity and line width oscillations are a quarter period out of phase with Doppler shift velocity both in time and spatial domain, which can be used to identify a standing slow mode wave from spectroscopic observations. However, the longitudinal overtones could be only measured with the assistance of imagers. We find emission intensity asymmetry in the positive and negative modulations, this is because the contribution function pertaining to the atomic emission process responds differently to positive and negative temperature variations. One may detect half periodicity close to the loop apex, where emission intensity modulation is relatively small. The line-of-sight projection affects the observation of Doppler shift significantly. A more accurate estimate of the amplitude of velocity perturbation is obtained by de-projecting the Doppler shift by a factor of 1-2\theta/π rather than the traditionally used \cos\theta. If a loop is heated to the hotter wing, the intensity modulation could be overwhelmed by background emission, while the Doppler shift velocity could still be detected to a certain extent.

Authors: Yuan, D., Van Doorsselaere, T., Banerjee, D., Antolin, P.
Projects: None

Publication Status: ApJ accepted
Last Modified: 2015-05-19 08:51
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Evolution of fast magnetoacoustic pulses in randomly structured coronal plasmas  

Ding Yuan   Submitted: 2014-12-08 11:15

Magnetohydrodynamic waves interact with structured plasmas and reveal the internal magnetic and thermal structures therein, thereby having seismological applications in the solar atmosphere. We investigate the evolution of fast magnetoacoustic pulses in randomly structured plasmas, in the context of large-scale propagating waves in the solar atmosphere. We perform one dimensional numerical simulations of fast wave pulses propagating perpendicular to a constant magnetic field in a low-beta plasma with a random density profile across the field. Both linear and nonlinear regimes are considered. We study how the evolution of the pulse amplitude and width depends on their initial values and the parameters of the random structuring. A randomly structured plasma acts as a dispersive medium for a fast magnetoacoustic pulse, causing amplitude attenuation and broadening of the pulse width. After the passage of the main pulse, secondary propagating and standing fast waves appear in the plasma. Width evolution of both linear and nonlinear pulses can be well approximated by linear functions; however, narrow pulses may have zero or negative broadening. This arises because a narrow pulse is prone to splitting, while a broad pulse usually deviates less from their initial Gaussian shape and form ripple structures on top of the main pulse. A linear pulse decays at almost a constant rate, while a nonlinear pulse decays exponentially. A pulse interacts most efficiently with a random medium which has a correlation length of about half of its initial pulse width. The development of a detailed model of a fast MHD pulse propagating in highly structured medium substantiates the interpretation of EIT waves as fast magnetoacoustic waves. Evolution of a fast pulse provides us with a novel method to diagnose the sub-resolution filamentation of the solar atmosphere.

Authors: D. Yuan, D.J. Pascoe, V.M. Nakariakov, B. Li, R. Keppens
Projects: None

Publication Status: ApJ accepted
Last Modified: 2014-12-10 18:59
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Oscillations in a sunspot with light bridges  

Ding Yuan   Submitted: 2014-07-10 12:33

Solar Optical Telescope onboard Hinode observed a sunspot (AR 11836) with two light bridges (LBs) on 31 Aug 2013. We analysed a 2-hour \ion{Ca}{2} H emission intensity data set and detected strong 5-min oscillation power on both LBs and in the inner penumbra. The time-distance plot reveals that 5-min oscillation phase does not vary significantly along the thin bridge, indicating that the oscillations are likely to originate from the underneath. The slit taken along the central axis of the wide light bridge exhibits a standing wave feature. However, at the centre of the wide bridge, the 5-min oscillation power is found to be stronger than at its sides. Moreover, the time-distance plot across the wide bridge exhibits a herringbone pattern that indicates a counter-stream of two running waves originated at the bridge sides. Thus, the 5-min oscillations on the wide bridge also resemble the properties of running penumbral waves. The 5-min oscillations are suppressed in the umbra, while the 3-min oscillations occupy all three cores of the sunspot's umbra, separated by the LBs. The 3-min oscillations were found to be in phase at both sides of the LBs. It may indicate that either LBs do not affect umbral oscillations, or umbral oscillations at different umbral cores share the same source. Also, it indicates that LBs are rather shallow objects situated in the upper part of the umbra. We found that umbral flashes follow the life cycles of umbral oscillations with much larger amplitudes. They cannot propagate across LBs. Umbral flashes dominate the 3-min oscillation power within each core, however, they do not disrupt the phase of umbral oscillation.

Authors: Ding Yuan, Valery M. Nakariakov, Zhenghua Huang, Bo Li, Jiangtao Su, Yihua Yan, Baolin Tan
Projects: None

Publication Status: ApJ accepted
Last Modified: 2014-07-11 14:34
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Distinct propagating fast wave trains associated with flaring energy releases  

Ding Yuan   Submitted: 2013-03-08 05:26

Large-scale fast waves with perturbation of the EUV emission intensity are well resolved both in temporal and spatial scale by SDO/AIA. These waves are prone to propagate along the magnetic field line. We aim to probe the link between propagating fast wave trains and flaring energy releases. By measuring the wave parameters, we reveal their nature and investigate the potential to diagnose the energy source and wave guide. The spatial and temporal evolution of the wave amplitude and propagating speed is studied. The correlation of individual wave trains with flare-generated radio bursts is tested. Propagating wave pattern comprises distinct wave trains with varying periods. This characteristic signature is consistent with the patterns formed by waveguide dispersion, when different spectral components propagate at different phase and group speeds. The wave train releases are found to be highly correlated in start time with the radio bursts emitted by the non-thermal electrons that were accelerated in bursty energy releases. The wave amplitude is seen to reach the maximum in the midway of its course. This can be caused by a combined effect of the waveguide spread in the transverse direction and density stratification. The cross-sectional amplitude distribution perpendicular to the wave vector is found to follow well a Gaussian profile. The spatial structure is consistent with the kink mode polarised along the line-of-sight. The propagating speed is subject to deceleration, from ~735-845 km s-1 to ~600 km s-1. This can be caused by the decrease in the local Alfvén speed and/or the projection effect.

Authors: Ding Yuan, Yuandeng Shen, Yu Liu, V.M. Nakariakov, Baolin Tan, Jing Huang
Projects: SDO-AIA

Publication Status: A&A letter (submitted)
Last Modified: 2013-03-08 08:49
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Field line reconstruction with magneto-acoustic cut-off frequency above sunspots  

Ding Yuan   Submitted: 2012-11-26 05:43

We used the Pixelised Wavelet Filtering (PWF) method to compute narrow-band power maps of SDO/AIA imaging datasets in the 1700 AA{}, 1600 AA{} and 304 AA{} bandpasses that correspond to different heights. The cut-off frequency was defined as contours where the spectral power droped to the median level. It was measured as a function of the spatial location. We inferred the magnetic field inclination according to the MAG wave theory in the low-eta limit and compared it with the potential field extrapolation.{We analysed intensity oscillations in a symmetric sunspot AR11131(08 Dec 2010) and an asymmetric sunspot AR11330 (27 Oct 2011). We reconstructed the magnetic field inclination in the radial direction for the symmetric sunspot and in both radial and azimuthal directions for the asymmetric sunspot. We observed 3D variation of the main oscillation periods in sunspots. We found that shorter-period oscillations were mostly constrained in sunspot umbrae, while longer-period oscillations formed an annular shape enclosing the umbra. Longer periods are found to be distributed further away from the sunspot centre. Our results indicate that 3-min oscillation are generated in the chromosphere, possibly by the acoustic resonator model, while 5-min and longer-period oscillations seemed to originate in a level under the photosphere. The reconstructed field inclinations gives the values of the field inclination that are systematically larger than the values obtained by the potential field extrapolation. The inclined magnetic field line can account for 60-80% of cut-offfrequency lowering only.

Authors: Ding Yuan, R. Sych, V. E. Reznikova, V. M. Nakariakov
Projects: SDO-AIA

Publication Status: submitted
Last Modified: 2012-11-27 15:06
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The measurement of the apparent phase speed of the propagating EUV disturbances  

Ding Yuan   Submitted: 2012-04-27 05:30

Propagating disturbances of the EUV emission intensity are commonly observed over a variety of coronal structures. Parameters of these disturbances, particularly the observed apparent (image-plane projected) propagation speed, are important tools for MHD coronal seismology. We design and test tools for reliable measurement of the apparent phase speed of propagating disturbances in imaging data sets. We design Cross-Fitting Technique (CFT), 2D Coupled Fitting (DCF) and Best Similarity Match (BSM) for the measurements of the apparent phase speed of propagating EUV disturbances in the running differences of time-distance plots (R) and background-removed and normalised time-distance plots (D). The methods were applied to the analysis of quasi-periodic EUV disturbances propagating at a coronal fan-structure of active region NOAA11330 on 27-Oct-2012, observed with the Atmospheric Imaging Assembly on SDO in the 171 Å bandpass. The noise propagation in the AIA image processing was estimated, resulting in the preliminary estimation of the uncertainties in the AIA image flux. This information was utilized in the measurements of the apparent phase speed of the propagating disturbances with the CFT, DCF and BSM methods, which gave consistent results. The average projected speed is measured at 47.6+-0.6 km s-1 and 49.0+-0.7 km s-1 for R and D, with the corresponding periods at 179.7+-0.2 s and 179.7+-0.3 s respectively. We analyzed the effects of the lag time and the detrending time in the running difference processing and the background-removed plot respectively, on the measurement of the speed, and found that they are fairly weak. The CFT, DCF and BSM methods are found to be reliable techniques for measuring the apparent (projected) phase speed. The samples of larger effective spatial length are more suitable for these methods. Time-distance plots with background removal and normalization allow for more robust measurements, with little effect of the choice of the detrending time. CFT provides reliable measurement on good samples (eg samples with large effective detection length and recurring features). DCF is found to be sensitive to the initial guess for parameters of the 2D fitting function. Thus DCF is only optimised in measuring one of the parameters (the phase speed in our application), while the period is poorly measured. BSM is robust for all kinds of samples and very tolerant to image pre-processing and regularisation (smoothing).

Authors: Yuan,D., Nakariakov, V.M
Projects: None,SDO-AIA

Publication Status: A&A (accepted)
Last Modified: 2012-04-27 09:07
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Abstracts by Author
Abnormal oscillation modes in a waning light bridge
Secondary fast magnetoacoustic waves trapped in randomly structured plasmas
Stochastic transients as a source of quasi-periodic processes in the solar atmosphere
Forward Modelling of Standing Kink Modes in Coronal Loops II. Applications
Forward Modelling of Standing Kink Modes in Coronal Loops I. Synthetic Views
Forward Modelling of standing slow modes of flaring coronal loops
Evolution of fast magnetoacoustic pulses in randomly structured coronal plasmas
Oscillations in a sunspot with light bridges
Distinct propagating fast wave trains associated with flaring energy releases
Field line reconstruction with magneto-acoustic cut-off frequency above sunspots
The measurement of the apparent phase speed of the propagating EUV disturbances

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