MagnetoFrictional Modeling of Coronal Nonlinear ForceFree Fields: II. Application to Observations 

Yang Guo Submitted: 20160907 20:45
A magnetofrictional module has been implemented and tested in the Message Passing Interface Adaptive Mesh Refinement Versatile Advection Code (MPIAMRVAC) in the first paper of this series. Here, we apply the magnetofrictional method to observations to demonstrate its applicability in both Cartesian and spherical coordinates, and in uniform and blockadaptive octree grids. We first reconstruct a nonlinear forcefree field (NLFFF) on a uniform grid of 1803 cells in Cartesian coordinates, with boundary conditions provided by the vector magnetic field observed by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) at 06:00 UT on 2010 November 11 in active region NOAA 11123. The reconstructed NLFFF successfully reproduces the sheared and twisted field lines and magnetic null points. Next, we adopt a threelevel
blockadaptive grid to model the same active region with a higher spatial resolution on the bottom boundary and a
coarser treatment of regions higher up. The forcefree and divergencefree metrics obtained are comparable to the
run with a uniform grid, and the reconstructed field topology is also very similar. Finally, a group of active regions, including NOAA 11401, 11402, 11405, and 11407, observed at 03:00 UT on 2012 January 23 by SDO/HMI is
modeled with a fivelevel blockadaptive grid in spherical coordinates, where we reach a local resolution of
0.06" pixel1 in an area of 790Mm?604Mm. Local high spatial resolution and a large field of view in NLFFF
modeling can be achieved simultaneously in parallel and blockadaptive magnetofrictional relaxations.
Authors: Y. Guo, C. Xia, R. Keppens
Projects: SDOAIA,SDOHMI

Publication Status: Published in ApJ
Last Modified: 20160908 06:55



MagnetoFrictional Modeling of Coronal Nonlinear ForceFree Fields: I. Testing with Analytic Solutions 

Yang Guo Submitted: 20160907 20:42
We report our implementation of the magnetofrictional method in the Message Passing Interface Adaptive Mesh
Refinement Versatile Advection Code (MPIAMRVAC). The method aims at applications where local adaptive
mesh refinement (AMR) is essential to make followup dynamical modeling affordable. We quantify its
performance in both domaindecomposed uniform grids and blockadaptive AMR computations, using all
frequently employed forcefree, divergencefree, and other vector comparison metrics. As test cases, we revisit the
semianalytic solution of Low and Lou in both Cartesian and spherical geometries, along with the topologically
challenging Titov"D"moulin model. We compare different combinations of spatial and temporal discretizations,
and find that the fourthorder central difference with a local Lax?Friedrichs dissipation term in a singlestep
marching scheme is an optimal combination. The initial condition is provided by the potential field, which is the
potential field source surface model in spherical geometry. Various boundary conditions are adopted, ranging from
fully prescribed cases where all boundaries are assigned with the semianalytic models, to solarlike cases where
only the magnetic field at the bottom is known. Our results demonstrate that all the metrics compare favorably to
previous works in both Cartesian and spherical coordinates. Cases with several AMR levels perform in accordance
with their effective resolutions. The magnetofrictional method in MPIAMRVAC allows us to model a region of
interest with high spatial resolution and large field of view simultaneously, as required by observationconstrained
extrapolations using vector data provided with modern instruments. The applications of the magnetofrictional
method to observations are shown in an accompanying paper.
Authors: Y. Guo, C. Xia, R. Keppens, G. Valori
Projects: None

Publication Status: Published in ApJ
Last Modified: 20160908 06:55



Slow Patchy Extremeultraviolet Propagating Fronts Associated With Fast Coronal Magnetoacoustic Waves In Solar Eruptions 

Yang Guo Submitted: 20150824 12:20
Using the high spatiotemporal resolution extreme ultraviolet (EUV) observations of the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory, we conduct a statistical study of the observational properties of the coronal EUV propagating fronts. We find that it might be a universal phenomenon for two types of fronts to coexist in a large solar eruptive event. It is consistent with the hybrid model of EUV propagating fronts, which predicts that coronal EUV propagating fronts consist of both a fast magnetoacoustic wave and a nonwave component. We find that the morphologies, propagation behaviors, and kinematic features of the two EUV propagating fronts are completely different from each other. The fast magnetoacoustic wave fronts are almost isotropic. They travel continuously from the flaring region across multiple magnetic polarities to global distances. On the other hand, the slow nonwave fronts appear as anisotropic and sequential patches of EUV brightening. Each patch propagates locally in the magnetic domains where the magnetic field lines connect to the bottom boundary and stops at the magnetic domain boundaries. Within each magnetic domain, the velocities of the slow patchy nonwave component are an order of magnitude lower than that of the fastwave component. However, the patches of the slow EUV propagating front can jump from one magnetic domain to a remote one. The velocities of such a transit between different magnetic domains are about onethird to onehalf of those of the fastwave component. The results show that the velocities of the nonwave component, both within one magnetic domain and between different magnetic domains, are highly nonuniform due to the inhomogeneity of the magnetic field in the lower atmosphere.
Authors: Y. Guo, M. D. Ding, P. F. Chen
Projects: SDOAIA

Publication Status: published in the The Astrophysical Journal Supplement Series
Last Modified: 20150825 16:49



MHD Seismology of a Coronal Loop System by the First Two Modes of Standing Kink Waves 

Yang Guo Submitted: 20141221 19:17
We report the observation of the first two harmonics of the horizontally polarized kink waves excited in a coronal loop system lying at southeast of AR 11719 on 2013 April 11. The detected periods of the fundamental mode (P_{1}), its first overtone (P_{2}) in the northern half, and that in the southern one are 530.2 pm 13.3, 300.4 pm 27.7, and 334.7 pm 22.1 s, respectively. The periods of the first overtone in the two halves are the same considering uncertainties in the measurement. We estimate the average electron density, temperature, and length of the loop system as (5.1 pm 0.8) imes 10^{8} cm^{3}, 0.65 pm 0.06 MK, and 203.8 pm 13.8 Mm, respectively. As a zeroth order estimation, the magnetic field strength, B = 8.2 pm 1.0 G, derived by the coronal seismology using the fundamental kink mode matches with that derived by a potential field model. The extrapolation model also shows the asymmetric and nonuniform distribution of the magnetic field along the coronal loop. Using the amplitude profile distributions of both the fundamental mode and its first overtone, we observe that the antinode positions of both the fundamental mode and its first overtone shift towards the weak field region along the coronal loop. The results indicate that the density stratification and the temperature difference effects are larger than the magnetic field variation effect on the period ratio. On the other hand, the magnetic field variation has a greater effect on the eigenfunction of the first overtone than the density stratification does for this case.
Authors: Y. Guo, R. Erdelyi, A. K. Srivastava, Q. Hao, X. Cheng, P. F. Chen, M. D. Ding, B. N. Dwivedi
Projects: SDOAIA,SDOHMI

Publication Status: accepted for publication in ApJ
Last Modified: 20141222 08:06



Twist Accumulation and Topology Structure of a Solar Magnetic Flux Rope 

Yang Guo Submitted: 20131112 02:11
To study the build up of a magnetic flux rope before a major flare and coronal mass ejection (CME), we compute the magnetic helicity injection, twist accumulation, and the topology structure of the three dimensional magnetic field, which is derived by the nonlinear forcefree field model. The Extremeultraviolet Imaging Telescope on board the Solar and Heliospheric Observatory observed a series of confined flares without any CME before a major flare with a CME at 23:02 UT on 2005 January 15 in active region NOAA 10720. We derive the vector velocity at eight time points from 18:27 UT to 22:20 UT with the differential affine velocity estimator for vector magnetic fields, which were observed by the Digital Vector Magnetograph at Big Bear Solar Observatory. The injected magnetic helicity is computed with the vector magnetic and velocity fields. The helicity injection rate was (16.47 pm 3.52) imes 10^{40} Mx^2/hr. We find that only about 1.8% of the injected magnetic helicity became finally the internal helicity of the magnetic flux rope, whose twist increasing rate was 0.18 pm 0.08 Turns/hr. The quasiseparatrix layers (QSLs) of the three dimensional magnetic field are computed by evaluating the squashing degree, Q. We find that the flux rope was wrapped by QSLs with large Q values, where the magnetic reconnection induced by the continuously injected magnetic helicity further produced the confined flares. We suggest that the flux rope was built up and heated by the magnetic reconnection in the QSLs.
Authors: Y. Guo, M. D. Ding, X. Cheng, J. Zhao, E. Pariat
Projects: SoHOEIT

Publication Status: accepted for publication in ApJ
Last Modified: 20131112 14:16



Modeling Magnetic Field Structure of a Solar Active Region Corona using Nonlinear ForceFree Fields in Spherical Geometry 

Yang Guo Submitted: 20121021 08:16
We test a nonlinear forcefree field (NLFFF) optimization code in spherical geometry using an analytical solution from Low and Lou. Several tests are run, ranging from idealized cases where exact vector field data are provided on all boundaries, to cases where noisy vector data are provided on only the lower boundary (approximating the solar problem). Analytical tests also show that the NLFFF code in the spherical geometry performs better than that in the Cartesian one when the field of view of the bottom boundary is large, say, 20^circ imes 20^circ. Additionally, We apply the NLFFF model to an active region observed by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) both before and after an M8.7 flare. For each observation time, we initialize the models using potential field source surface (PFSS) extrapolations based on either a synoptic chart or a fluxdispersal model, and compare the resulting NLFFF models. The results show that NLFFF extrapolations using the fluxdispersal model as the boundary condition have slightly lower, therefore better, forcefree and divergencefree metrics, and contain larger free magnetic energy. By comparing the extrapolated magnetic field lines with the extreme ultraviolet (EUV) observations by the Atmospheric Imaging Assembly (AIA) on board SDO, we find that the NLFFF performs better than the PFSS not only for the core field of the flare productive region, but also for large EUV loops higher than 50 Mm.
Authors: Y. Guo, M. D. Ding, Y. Liu, X. D. Sun, M. L. DeRosa, T. Wiegelmann
Projects: SDOAIA,SDOHMI

Publication Status: accepted for publication in ApJ
Last Modified: 20121022 10:26



Driving Mechanism and Onset Condition of a Confined Eruption 

Yang Guo Submitted: 20101103 21:18
We study a confined eruption accompanied by an M1.1 flare in solar active region (AR) NOAA 10767 on 2005 May 27, where a preeruptive magnetic flux rope was reported in a nonlinear forcefree field (NLFFF) extrapolation. The observations show a strong writhing motion of the erupting structure, suggesting that a flux rope was indeed present and converted some of its twist into writhe in the course of the eruption. Using the NLFFF extrapolation, we calculate the twist of the preeruptive flux rope and find that it is in very good agreement with thresholds of the helical kink instability found in numerical simulations.
We conclude that the activation and rise of the flux rope were triggered and driven by the instability. Using a potential field extrapolation, we also estimate the height distribution of the decay index of the external magnetic field in the AR one hour prior to the eruption. We find that the decay index stays below the threshold for the torus instability for a significant height range above the
erupting flux rope. This provides a possible explanation for the confinement of the eruption to the low corona.
Authors: Y. Guo, M.D. Ding, B. Schmieder, H. Li, T. Toeroek, T. Wiegelmann
Projects:

Publication Status: ApJ Letters, accepted
Last Modified: 20101105 07:08



Formation Heights of Extreme Ultraviolet Lines in an Active Region Derived by Correlation of Doppler Velocity and Magnetic Field 

Yang Guo Submitted: 20100311 03:52
We study the correlation heights, which indicate the formation heights of Extreme Ultraviolet (EUV) lines in an active region using observations from the EUV Imaging Spectrometer (EIS) and Solar Optical Telescope (SOT) on board Hinode. The nonlinear forcefree field (NLFFF) optimization method is adopted to extrapolate the 3D magnetic fields to higher layers. Three subregions with different characteristics are selected in the active region for this study. The results show that the formation heights in different subregions vary with their different magnetic fields or velocity patterns. After solving the line blending problem between the He 2 256.32 Å and Si 10 256.37 Å lines by the double Gaussian curve fitting, we find that the transition region lies higher in a strong magnetic area. In a preflare heating area there possibly exist multithermal loops as implied by comparing the Doppler velocity and the magnetic field on the solar disk.
Authors: Guo, Y., Ding, M. D., Jin, M., Wiegelmann, T.
Projects: Hinode/EIS,Hinode/SOT

Publication Status: ApJ (2009), 696, 1526
Last Modified: 20100312 09:44



Magnetic Field Structures in a Facular Region Observed by THEMIS and Hinode 

Yang Guo Submitted: 20100311 03:44
The main objective of this paper is to build and compare vector magnetic maps obtained by two spectral polarimeters, i.e. THEMIS/MTR and Hinode SOT/SP, using two inversion codes (UNNOFIT and MELANIE) based on the MilneEddington solar atmosphere model. To this end, we used observations of a facular region within active region NOAA 10996 on 23 May 2008, and found consistent results concerning the field strength, azimuth and inclination distributions. Because SOT/SP is free from the seeing effect and has better spatial resolution, we were able to resolve small magnetic polarities with sizes of 1'' to 2'', and we could detect strong horizontal magnetic fields, which converge or diverge in negative or positive facular polarities. These findings support models which suggest the existence of small vertical flux tube bundles in faculae. A new method is proposed to get the relative formation heights of the multilines observed by MTR assuming the validity of a flux tube model for the faculae. We found that the Fe 1 6302.5 Å line forms at a greater atmospheric height than the Fe 1 5250.2 Å line.
Authors: Guo, Y., Schmieder, B., Bommier, V., Gosain, S.
Projects: Hinode/SOT,THEMIS/MTR

Publication Status: Solar Physics (2010), 262, 35
Last Modified: 20100312 09:44



Coexisting Flux Rope and Dipped Arcade Sections along One Solar Filament 

Yang Guo Submitted: 20100311 03:09
We compute the threedimensional magnetic field of an active region in order
to study the magnetic configuration of active region filaments. The nonlinear
forcefree field model is adopted to compute the magnetic field above the
photosphere, where the vector magnetic field was observed by the Telescope
Heliographique pour l?Etude du Magnetisme et des Instabilites Solaires/Multi
Raies (THEMIS/MTR) on 2005 May 27. We propose a new method to remove
the 180◦ ambiguity of the transverse field. Next, we analyze the implications of
the preprocessing of the data by minimizing the total force and torque in the observed
vector fields. This step provides a consistent bottom boundary condition
for the nonlinear forcefree field model. Then, using the optimization method
to compute the coronal field, we find a magnetic flux rope along the polarity
inversion line. The magnetic flux rope aligns well with part of an Hα filament,
while the total distribution of the magnetic dips coincides with the whole Hα
filament. This implies that the magnetic field structure in one section of the
filament is a flux rope, while the other is a sheared arcade. The arcade induced a
leftbearing filament in the magnetic field of negative helicity, which is opposite
to the chirality of barbs that a flux rope would induce in a magnetic field of the
same helicity sign. The field strength in the center of the flux rope is about 700
gauss, and the twist of the field lines is ∼ 1.4 turns.
Authors: Guo, Y., Schmieder, B., Demoulin, P., Wiegelmann, T., Aulanier, G., Torok, T., Bommier, V.
Projects: THEMIS/MTR

Publication Status: ApJ, in press
Last Modified: 20100312 09:44




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