Comparison of STEREO/EUVI Loops with Potential Magnetic Field Models

Sandman, A.; Aschwanden, Markus J.; DeRosa, Marc L.; Wülser, J. P.; Alexander, David

doi:10.1007/s11207-009-9383-0

Cited by 47 publications

(51 citation statements)

References 16 publications

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“…We note that the validity of the potential approximation is questionable. For instance, Sandman et al (2009) and Aschwanden & Sandman (2010) found considerable misalignment between the magnetic field obtained from the potential approximation in comparison to the shapes of the coronal loops derived from the STEREO/EUVI observations (Wülser et al 2004). However, López Fuentes et al (2006) showed that linear force-free fields can also be quite poor approximations of the real magnetic field in active regions.…”

Section: Magnetic Field Extrapolationmentioning

confidence: 97%

Is it possible to model observed active region coronal emission simultaneously in EUV and X-ray filters?

Dudík

Dzifčáková

Karlický

et al. 2011

A&A

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Aims. We investigate the possibility of modeling the active region coronal emission in the EUV and X-ray filters using one, universal, steady heating function, tied to the properties of the magnetic field. Methods. We employ a simple, static model to compute the temperature and density distributions in the active region corona. The model allows us to explore a wide range of parameters of the heating function. The predicted EUV and X-ray emission in the filters of EIT/SOHO and XRT/Hinode are calculated and compared with observations. Using the combined improved filter-ratio (CIFR) method, a temperature diagnostic is employed to compare the modeled temperature structure of the active region with the temperature structure derived from the observations. Results. The global properties of the observations are most closely matched for heating functions scaling as B 0.7−0.8 0 /L 0.5 0 that depend on the spatially variable heating scale-length. The modeled X-ray emission originates from locations where large heating scale-lengths are found. However, the majority of the loops observed in the 171 and 195 filters can be modeled only by loops with very short heating scale-lengths. These loops are known to be thermally unstable. We are unable to find a model that both matches the observations in all EUV and X-ray filters, and contains only stable loops. As a result, although our model with a steady heating function can explain some of the emission properties of the 171 and 195 loops, it cannot explain their observed lifetimes. Thus, the model does not lead to a self-consistent solution. The performance of the CIFR method is evaluated and we find that the diagnosed temperature can be approximated with a geometric mean of the emission-measure weighted and maximum temperature along the line of sight. Conclusions. We conclude that if one universal heating function exists, it should be at least partially time-dependent.

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Section: Magnetic Field Extrapolationmentioning

confidence: 97%

Is it possible to model observed active region coronal emission simultaneously in EUV and X-ray filters?

Dudík

Dzifčáková

Karlický

et al. 2011

A&A

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“…The magnetic field in the environment of the oscillating loop can be modeled with potential-field or non-potential field models, but both are known to show misalignments with the 3D geometry of stereoscopically triangulated loops of the order of α mis ≈ 20 • − 40 • Sandman et al 2009), while simple potential field models calculated from a small set of unipolar magnetic charges (Aschwanden and Sandman 2010) or magnetic dipoles (Sandman and Aschwanden 2010) achieved a reduced misalignment of α mis = 13 • − 20 • . For a simple plausibility test of the magnetic field strength inferred from coronal seismology, we model the 3D field at the location of the oscillating loop with an analytical model of two unipolar charges with opposite magnetic polarities that are buried in depths z 1 and z 2 and have maximum longitudinal magnetic field strengths of B 1 = +187 G and B 2 = −63 G at the observed positions (x 1 , y 1 ) and (x 2 , y 2 ) of the nearest magnetic pores in the HMI magnetogram (marked with circles in Fig.…”

Section: Magnetic Field Modelingmentioning

confidence: 99%

Coronal Loop Oscillations Observed With Atmospheric Imaging Assembly—kink Mode With Cross-Sectional and Density Oscillations

Aschwanden

Schrijver

2011

ApJ

171

175

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A detailed analysis of a coronal loop oscillation event is presented, using data from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) for the first time. The loop oscillation event occurred on 2010 Oct 16, 19:05-19:35 UT, was triggered by an M2.9 GOES-class flare, located inside a highly inclined cone of a narrow-angle CME. This oscillation event had a number of unusual features: (i) Excitation of kink-mode oscillations in vertical polarization (in the loop plane); (ii) Coupled cross-sectional and density oscillations with identical periods; (iii) no detectable kink amplitude damping over the observed duration of four kink-mode periods (P = 6.3 min); (iv) multi-loop oscillations with slightly (≈ 10%) different periods; and (v) a relatively cool loop temperature of T ≈ 0.5 MK. We employ a novel method of deriving the electron density ratio external and internal to the oscillating loop from the ratio of Alfvénic speeds deduced from the flare trigger delay and the kink-mode period, i.e., n e /n i = (v A /v Ae ) 2 = 0.08 ± 0.01. The coupling of the kink mode and cross-sectional oscillations can be explained as a consequence of the loop length variation in the vertical polarization mode. We determine the exact footpoint locations and loop length with stereoscopic triangulation using STEREO/EUVI-A data. We model the magnetic field in the oscillating loop using HMI/SDO magnetogram data and a potential field model and find agreement with the seismological value of the magnetic field, B kink = 4.0 ± 0.7 G, within a factor of two.Subject headings: Sun: Flares -Sun : Corona -Sun: Extreme Ultra-Violet (EUV) -Sun : oscillations -Sun : waves -2 - INTRODUCTIONPropagating waves and standing waves (eigen-modes) in coronal plasma structures became an important tool to probe the physical parameters, the dynamics, and the magnetic field in the corona, in flare sites, and in coronal mass ejections (CMEs). Recent reviews on the theory and observations of coronal seismology can be found in Roberts and Nakariakov (2003), Erdelyi et al. (2003), Roberts (2004), Aschwanden (2004Aschwanden ( , 2006, Wang (2004), Nakariakov and Verwichte (2005), Banerjee et al. (2007), Andries et al. (2009), Ruderman and Erdelyi (2009), and Taroyan and Erdelyi (2009. Substantial progress was accomplished in applying MHD wave theory to coronal observations with previous instruments, such as the discovery of global waves with EIT/SOHO (Thompson et al. 1998(Thompson et al. , 1999, fast kink-mode loop oscillations with TRACE (Aschwanden et al. 1999;Nakariakov et al. 1999), fast sausage mode oscillations in radio wavelengths (Roberts et al. 1984;Asai et al. 2001;Melnikov et al. 2002;Aschwanden et al. 2004), slow (acoustic) mode oscillations with SUMER/SOHO (Wang et al. 2002;Kliem et al. 2002), slow (acoustic) propagating waves with UVCS/SOHO (Ofman et al. 1997) and Hinode (Erdelyi and Taroyan 2008), EIT/SOHO (DeForest and Gurman 1998), and TRACE (De Moortel et al. 2002a,b), fast Alfvénic waves with SECIS (Williams et a...

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“…Based on a study of three active regions modeled with potential field source surface (PFSS) magnetic fields, Sandman et al (2009) found misalignments of approximately 20 -40 degrees between the observed and modeled field structure. DeRosa et al (2009) modeled one of the three active regions with several different non-linear force-free field (NLFFF) models, using observations from Hinode's Solar Optical Telescope (SOT) in a restricted field of view.…”

Section: Introductionmentioning

confidence: 99%

A New Method for Modeling the Coronal Magnetic Field with STEREO and Submerged Dipoles

Sandman

Aschwanden

2011

Sol Phys

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Recent magnetic modeling efforts have shown substantial misalignment between theoretical models and observed coronal loop morphology as observed by STEREO/EUVI, regardless of the type of model used. Both potential field and non-linear force-free field (NLFFF) models yielded overall misalignment angles of 20 -40 degrees, depending on the complexity of the active region (Sandman et al., Solar Phys. 259, 1, 2009; DeRosa et al., Astrophys. J. 696, 1780 We demonstrate that with new, alternative forward-fitting techniques, we can achieve a significant reduction in the misalignment angles compared with potential field source surface (PFSS) models and NLFFF models. Fitting a series of submerged dipoles to the field directions of stereoscopically triangulated loops in four active regions (), we find that 3 -5 dipoles per active region yield misalignment angles of ∼ 11°-18°, a factor of two smaller than those given by previously established extrapolation methods. We investigate the spatial and temporal variation of misalignment angles with subsets of loops for each active region, as well as loops observed prior to and following a flare and filament eruption, and find that the spatial variation of median misalignment angles within an active region (up to 75%) exceeds the temporal variation associated with the flare (up to 40%). We also examine estimates of the stereoscopic error of our analysis. The corrected values yield a residual misalignment of 7°-13°, which is attributed to the non-potentiality due to currents in the active regions.

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Comparison of STEREO/EUVI Loops with Potential Magnetic Field Models

Cited by 47 publications

References 16 publications

Is it possible to model observed active region coronal emission simultaneously in EUV and X-ray filters?

Is it possible to model observed active region coronal emission simultaneously in EUV and X-ray filters?

Coronal Loop Oscillations Observed With Atmospheric Imaging Assembly—kink Mode With Cross-Sectional and Density Oscillations

A New Method for Modeling the Coronal Magnetic Field with STEREO and Submerged Dipoles

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