Publications (25)76.99 Total impact
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ABSTRACT: Magnetic reconnection is one of the primary mechanisms for triggering solar eruptive events, but direct observation of this rapid process has been a challenge. In this Letter, using a nonlinear forcefree field (NLFFF) extrapolation technique, we present a visualization of field line connectivity changes resulting from tethercutting reconnection over about 30 minutes during the 2011 February 13 M6.6 flare in NOAA AR 11158. Evidence for the tethercutting reconnection was first collected through multiwavelength observations and then by analysis of the field lines traced from positions of four conspicuous flare 1700 Å footpoints observed at the event onset. Right before the flare, the four footpoints are located very close to the regions of local maxima of the magnetic twist index. In particular, the field lines from the inner two footpoints form two strongly twisted flux bundles (up to ~1.2 turns), which shear past each other and reach out close to the outer two footpoints, respectively. Immediately after the flare, the twist index of regions around the footpoints diminishes greatly and the above field lines become lowlying and less twisted (<=0.6 turns), overarched by loops linking the two flare ribbons formed later. About 10% of the flux (~3 × 10^19 Mx) from the inner footpoints undergoes a footpoint exchange. This portion of flux originates from the edge regions of the inner footpoints that are brightened first. These rapid changes of magnetic field connectivity inferred from the NLFFF extrapolation are consistent with the tethercutting magnetic reconnection model.The Astrophysical Journal Letters 11/2013; 778(2):L36. · 6.35 Impact Factor  [show abstract] [hide abstract]
ABSTRACT: In this paper, we study the magnetic energy (ME) structure contained in the solar corona over the active region NOAA 11158. The time period is chosen as from 0:00 to 06:00 UT on 2011 February 15, during which an Xclass flare occurred. The nonlinear forcefree field (NLFFF) and the potential field extrapolation are carried out to model the coronal magnetic field over this active region, using highquality photospheric vector magnetograms observed by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory as boundary conditions. We find that the volume distribution for the density of the ME (B 2/8π) and the ohmic dissipation power (ODP, j 2/σ), in which j is the electric current density (c/4π∇ × B) and σ is the conductivity in the corona, can be readily fitted by a brokendown doublepower law. The turnover density for the spectrum of the ME and ODP is found to be fixed at ~1.0 × 104 erg cm–3 and ~2.0 × 10–15 W cm–3 (assuming σ = 105 Ω–1 m–1), respectively. Compared with their first powerlaw spectra (fitted below the corresponding turnover value) which remain unchanged, the second powerlaw spectra (fitted above the corresponding turnover value) for the NLFFF's ME and ODP show flareassociated changes. The potential field remains steady. These results indicate that a magnetic field with energy density larger than the turnover energy density plays a dominant role in powering the flare.The Astrophysical Journal 01/2013; 764(1):86. · 6.73 Impact Factor  [show abstract] [hide abstract]
ABSTRACT: The moss is the area at the footpoint of the hot (3 to 5 MK) loops forming the core of the active region where emission is believed to result from the heat flux conducted down to the transition region from the hot loops. Studying the variation of Doppler shift as a function of line formation temperatures over the moss area can give clues on the heating mechanism in the hot loops in the core of the active regions. We investigate the absolute Doppler shift of lines formed at temperatures between 1 MK and 2 MK in a moss area within active region NOAA 11243 using a novel technique that allows determining the absolute Doppler shift of EUV lines by combining observations from the SUMER and EIS spectrometers. The inner (brighter and denser) part of the moss area shows roughly constant blue shift (upward motions) of 5 km/s in the temperature range of 1 MK to 1.6 MK. For hotter lines the blue shift decreases and reaches 1 km/s for Fe xv 284 {\AA} (~2 MK). The measurements are discussed in relation to models of the heating of hot loops. The results for the hot coronal lines seem to support the quasisteady heating models for nonsymmetric hot loops in the core of active regions.Astronomy and Astrophysics 11/2012; · 5.08 Impact Factor 
Article: Solar Forcefree Magnetic Fields
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ABSTRACT: The structure and dynamics of the solar corona is dominated by the magnetic field. In most areas in the corona magnetic forces are so dominant that all nonmagnetic forces like plasma pressure gradient and gravity can be neglected in the lowest order. This model assumption is called the forcefree field assumption, as the Lorentz force vanishes. This can be obtained by either vanishing electric currents (leading to potential fields) or the currents are coaligned with the magnetic field lines. First we discuss a mathematically simpler approach that the magnetic field and currents are proportional with one global constant, the socalled linear forcefree field approximation. In the generic case, however, the relation between magnetic fields and electric currents is nonlinear and analytic solutions have been only found for special cases, like 1D or 2D configurations. For constructing realistic nonlinear forcefree coronal magnetic field models in 3D, sophisticated numerical computations are required and boundary conditions must be obtained from measurements of the magnetic field vector in the solar photosphere. This approach is currently of large interests, as accurate measurements of the photospheric field become available from groundbased (for example SOLIS) and spaceborn (for example Hinode and SDO) instruments. If we can obtain accurate forcefree coronal magnetic field models we can calculate the free magnetic energy in the corona, a quantity which is important for the prediction of flares and coronal mass ejections. Knowledge of the 3D structure of magnetic field lines also help us to interpret other coronal observations, e.g., EUVimages of the radiating coronal plasma.Living Reviews in Solar Physics. 08/2012;  [show abstract] [hide abstract]
ABSTRACT: Both magnetic and current helicities are crucial ingredients for describing the complexity of activeregion magnetic structure. In this Letter, we present the temporal evolution of these helicities contained in NOAA active region 11158 during five days from 2011 February 12 to 16. The photospheric vector magnetograms of the Helioseismic and Magnetic Imager on board the Solar Dynamic Observatory were used as the boundary conditions for the coronal field extrapolation under the assumption of nonlinear forcefree field, from which we calculated both relative magnetic helicity and current helicity. We construct a timealtitude diagram in which altitude distribution of the magnitude of current helicity density is displayed as a function of time. This diagram clearly shows a pattern of upwardly propagating current helicity density over two days prior to the X2.2 flare on February 15 with an average propagation speed of ~36 m s–1. The propagation is synchronous with the emergence of magnetic flux into the photosphere, and indicative of a gradual energy buildup for the X2.2 flare. The time profile of the relative magnetic helicity shows a monotonically increasing trend most of the time, but a pattern of increasing and decreasing magnetic helicity above the monotonic variation appears prior to each of two major flares, M6.6 and X2.2, respectively. The physics underlying this bump pattern is not fully understood. However, the fact that this pattern is apparent in the magnetic helicity evolution but not in the magnetic flux evolution makes it a useful indicator in forecasting major flares.The Astrophysical Journal Letters 05/2012; 752(1):L9. · 6.35 Impact Factor 
Article: 2D and 3D Polar Plume Analysis from the Three Vantage Positions of STEREO/EUVI A, B, and SOHO/EIT
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ABSTRACT: Polar plumes are seen as elongated objects starting at the solar polar regions. Here, we analyze these objects from a sequence of images taken simultaneously by the three spacecraft telescopes STEREO/EUVI A and B, and SOHO/EIT. We establish a method capable of automatically identifying plumes in solar EUV images close to the limb at 1.01  1.39 R in order to study their temporal evolution. This plumeidentification method is based on a multiscale Houghwavelet analysis. Then two methods to determined their 3D localization and structure are discussed: First, tomography using the filtered backprojection and including the differential rotation of the Sun and, secondly, conventional stereoscopic triangulation. We show that tomography and stereoscopy are complementary to study polar plumes. We also show that this systematic 2D identification and the proposed methods of 3D reconstruction are well suited, on one hand, to identify plumes individually and on the other hand, to analyze the distribution of plumes and interplume regions. Finally, the results are discussed focusing on the plume position with their crosssection area.Solar Physics 11/2011; 283(1). · 3.26 Impact Factor  [show abstract] [hide abstract]
ABSTRACT: Understanding the solar outer atmosphere requires concerted, simultaneous solar observations from the visible to the vacuum ultraviolet (VUV) and soft Xrays, at high spatial resolution (between 0.1" and 0.3"), at high temporal resolution (on the order of 10 s, i.e., the time scale of chromospheric dynamics), with a wide temperature coverage (0.01 MK to 20 MK, from the chromosphere to the flaring corona), and the capability of measuring magnetic fields through spectropolarimetry at visible and nearinfrared wavelengths. Simultaneous spectroscopic measurements sampling the entire temperature range are particularly important. These requirements are fulfilled by the Japanese SolarC mission (Plan B), composed of a spacecraft in a geosynchronous orbit with a payload providing a significant improvement of imaging and spectropolarimetric capabilities in the UV, visible, and nearinfrared with respect to what is available today and foreseen in the near future. The Large European Module for solar Ultraviolet Research (LEMUR), described in this paper, is a large VUV telescope feeding a scientific payload of highresolution imaging spectrographs and cameras. LEMUR consists of two major components: a VUV solar telescope with a 30 cm diameter mirror and a focal length of 3.6 m, and a focalplane package composed of VUV spectrometers covering six carefully chosen wavelength ranges between 17 and 127 nm. The LEMUR slit covers 280" on the Sun with 0.14" per pixel sampling. In addition, LEMUR is capable of measuring mass flows velocities (line shifts) down to 2 km/s or better. LEMUR has been proposed to ESA as the European contribution to the Solar C mission.09/2011;  [show abstract] [hide abstract]
ABSTRACT: We have developed a method to automatically segment chromospheric fibrils from Halpha observations and further identify their orientation. We assume that chromospheric fibrils are magnetic fieldaligned. By comparing the orientation of the fibrils with the azimuth of the embedding chromospheric magnetic field extrapolated from the photosphere or chromosphere with the help of a potential field model, the shear angle, a measure of nonpotentiality, along the fibrils is readily deduced. Following this approach, we make a quantitative assessment of the nonpotentiality of fibrils in the active region NOAA 9661 and NOAA 11092. The spatial distribution and the histogram of the shear angle along fibrils are presented.05/2011; 
Article: Magnetic loops in the quiet Sun
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ABSTRACT: We investigate the fine structure of magnetic fields in the atmosphere of the quiet Sun. We use photospheric magnetic field measurements from {\sc Sunrise}/IMaX with unprecedented spatial resolution to extrapolate the photospheric magnetic field into higher layers of the solar atmosphere with the help of potential and forcefree extrapolation techniques. We find that most magnetic loops which reach into the chromosphere or higher have one foot point in relatively strong magnetic field regions in the photosphere. $91%$ of the magnetic energy in the mid chromosphere (at a height of 1 Mm) is in field lines, whose stronger foot point has a strength of more than 300 G, i.e. above the equipartition field strength with convection. The loops reaching into the chromosphere and corona are also found to be asymmetric in the sense that the weaker foot point has a strength $B < 300$ G and is located in the internetwork. Such loops are expected to be strongly dynamic and have short lifetimes, as dictated by the properties of the internetwork fields. Comment: accepted for ApJL Sunrise special issue, 8 Pages, 4 FiguresThe Astrophysical Journal Letters 09/2010; · 6.35 Impact Factor  [show abstract] [hide abstract]
ABSTRACT: We present the results from a method to determine the 3D position and orientation of polar plumes from three corresponding images observed simultaneously by three spacecrafts, STEREO/SECCHI A, B, and SOHO/EIT. We have applied both conventional stereoscopic triangulation and a new detection tool based on a combination of Hough and wavelet transform. We show that the obtained plume orientation can help to verify magnetic field models in the pole region where surface observations are difficult and their extrapolation may be problematic. This automatic and systematic D reconstruction is well suited to identify plumes individually in time and to follow their intensity variation. Typical lifetimes observed were found between 12 days. The plumes we have reconstructed were not always rooted at a simultaneous EUV bright points and were sometime associated with a jet.01/2010;  [show abstract] [hide abstract]
ABSTRACT: We present the propagation of the polar jets observed from the field of view of EUVI, COR1 to COR2 on board STEREO. We provide a method to test the free fall model both in 2D and 3D dimensions by comparing the heighttime images extracted from observations with the free fall model. By assuming all the particles in polar jets are ejected at the same time when it is initiated, this method could produce the initial velocity distribution of the particles and tell us during the propagation whether the particles are ionized/recombined or experience some other processes. The derived 3D orientations of the polar jets are used to test different magnetic field models around polar regions where the observation and extrapolation are not reliable. The estimated 3D leading edge velocities by different telescopes are also investigated.01/2010;  [show abstract] [hide abstract]
ABSTRACT: In solar eruptions, like flares and coronal mass ejections, free magnetic energy stored in the solar corona is converted into kinetic energy. Unfortunately the coronal magnetic field cannot be measured directly. We can, however, reconstruct the coronal magnetic field from measurements of the photospheric magnetic field vector under the reasonable assumption of a forcefree coronal plasma. With a procedure dubbed preprocessing we derive forcefree consistent boundary conditions, which are extrapolated into the solar corona with a nonlinear forcefree extrapolation code. The resulting 3D coronal magnetic field allows us to derive the magnetic topology and to computed the magnetic energy as well as an upper limited of the free energy available for driving eruptive phenomena. We apply our code to measurements from several ground based vector magnetographs, e.g. the Solar Flare Telescope, SOLIS and the Big Bear Solar Observatory. Within our studies we find a clear relationship between the stored magnetic energy and the strength of eruptions. In most cases not the entire free energy is converted to kinetic energy, but only a fraction. Consequently, the postflare magnetic field configuration is usually not entirely current free, but significantly closer to a potential field as before the flare.01/2010;  [show abstract] [hide abstract]
ABSTRACT: SDO/HMI provides us high resolution full disk measurements of the photospheric magnetic field vector.We compute the field in the higher layers of the solar atmosphere from the measured photospheric field under the assumption that the corona is forcefree. However, those measured data are inconsistent with the above forcefree assumption. Therefore, one has to apply some transformations dubbed preprocessing to these data before nonlinear forcefree extrapolation codes can be applied. Our forcefree code is based on an optimization principle and takes the spherical geometry of the sun in to account. Untill now, these extrapolations were applied only to a small surface area of the Sun so that cartesian geometry could be applied. We carry out both full disk computations as well as computations of active regions. The code has been well tested with model equilibria and used with the ground based observations from SOLIS. We plan to show first Nonlinear forcefree coronal magnetic fields extrapolated from SDO/HMI in comparison with the coronal plasma in SDO/HMI.01/2010;  [show abstract] [hide abstract]
ABSTRACT: Nonlinear forcefree field (NLFFF) models are thought to be viable tools for investigating the structure, dynamics and evolution of the coronae of solar active regions. In a series of NLFFF modeling studies, we have found that NLFFF models are successful in application to analytic test cases, and relatively successful when applied to numerically constructed Sunlike test cases, but they are less successful in application to real solar data. Different NLFFF models have been found to have markedly different field line configurations and to provide widely varying estimates of the magnetic free energy in the coronal volume, when applied to solar data. NLFFF models require consistent, forcefree vector magnetic boundary data. However, vector magnetogram observations sampling the photosphere, which is dynamic and contains significant Lorentz and buoyancy forces, do not satisfy this requirement, thus creating several major problems for forcefree coronal modeling efforts. In this article, we discuss NLFFF modeling of NOAA Active Region 10953 using Hinode/SOTSP, Hinode/XRT, STEREO/SECCHIEUVI, and SOHO/MDI observations, and in the process illustrate the three such issues we judge to be critical to the success of NLFFF modeling: (1) vector magnetic field data covering larger areas are needed so that more electric currents associated with the full active regions of interest are measured, (2) the modeling algorithms need a way to accommodate the various uncertainties in the boundary data, and (3) a more realistic physical model is needed to approximate the photospheretocorona interface in order to better transform the forced photospheric magnetograms into adequate approximations of nearly forcefree fields at the base of the corona. We make recommendations for future modeling efforts to overcome these as yet unsolved problems.The Astrophysical Journal 02/2009; 696(2). · 6.73 Impact Factor  [show abstract] [hide abstract]
ABSTRACT: We compare a variety of nonlinear forcefree field (NLFFF) extrapolation algorithms, including optimization, magnetofrictional, and Grad – Rubinlike codes, applied to a solarlike reference model. The model used to test the algorithms includes realistic photospheric Lorentz forces and a complex field including a weakly twisted, right helical flux bundle. The codes were applied to both forced “photospheric” and more forcefree “chromospheric” vector magnetic field boundary data derived from the model. When applied to the chromospheric boundary data, the codes are able to recover the presence of the flux bundle and the field’s free energy, though some details of the field connectivity are lost. When the codes are applied to the forced photospheric boundary data, the reference model field is not well recovered, indicating that the combination of Lorentz forces and small spatial scale structure at the photosphere severely impact the extrapolation of the field. Preprocessing of the forced photospheric boundary does improve the extrapolations considerably for the layers above the chromosphere, but the extrapolations are sensitive to the details of the numerical codes and neither the field connectivity nor the free magnetic energy in the full volume are well recovered. The magnetic virial theorem gives a rapid measure of the total magnetic energy without extrapolation though, like the NLFFF codes, it is sensitive to the Lorentz forces in the coronal volume. Both the magnetic virial theorem and the Wiegelmann extrapolation, when applied to the preprocessed photospheric boundary, give a magnetic energy which is nearly equivalent to the value derived from the chromospheric boundary, but both underestimate the free energy above the photosphere by at least a factor of two. We discuss the interpretation of the preprocessed field in this context. When applying the NLFFF codes to solar data, the problems associated with Lorentz forces present in the low solar atmosphere must be recognized: the various codes will not necessarily converge to the correct, or even the same, solution.Solar Physics 01/2008; 247(2):269299. · 3.26 Impact Factor  [show abstract] [hide abstract]
ABSTRACT: In this study new results are presented regarding the relationships between the coronal magnetic field and the intensities and Doppler shifts of ultraviolet emission lines. This combination of magnetic field and spectroscopic data is used here to study material flows in association with the coronal field. We introduce the term ``coronal circulation'' to describe this flow, and to indicate that the plasma is not static but flows everywhere in the extended solar atmosphere. The blueshifts and redshifts often seen in transition region and coronal ultraviolet emission lines are interpreted as corresponding to upflows and downflows of the plasma on open (funnels) and closed (loops) coronal magnetic field lines, which tightly confine and strongly lead the flows in the lowbeta plasma. Evidence for these processes exists in the ubiquitous redshifts mostly seen at both legs of loops on all scales, and the sporadic blueshifts occurring in strong funnels. Therefore, there is no static magnetically stratified plasma in the corona, since panta rhei, but rather a continuous global plasma circulation, being the natural perpetuation of photospheric convection which ultimately is the driver.The Astrophysical Journal 01/2008; · 6.73 Impact Factor 
Article: Nonlinear forcefree field models
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ABSTRACT: The photospheric magnetic field vector is routinely measured with high accuracy from ground based and space born instruments. We use these measurements to prescribe suitable boundary conditions for modelling the coronal magnetic field. Because of the lowbeta plasma the magnetic field is in lowest order assumed to be forcefree in the corona and upper chromosphere, but not in the highbeta photosphere. We developed a program package which contains a preprocessing program and a nonlinear forcefree coronal magnetic extrapolation code. Both programs are based on optimization principles. The preprocessing routine uses the measured photospheric vector magnetogram as input and approximates the magnetic field vector in the forcefree upper chromosphere. These data are used as boundary condition for a nonlinear forcefree extrapolation of the coronal magnetic field. We applied our method to study the temporal evolution of a flaring active region as a sequence of nonlinear forcefree equilibria. We found that magnetic energy was build up before the occurance of a flare and released after it. Furthermore, the 3Dmagnetic field model allows us to trace the temporal evolution of the energy flows in the flaring region.01/2008;  [show abstract] [hide abstract]
ABSTRACT: CONTEXT: As the coronal magnetic field can usually not be measured directly, it has to be extrapolated from photospheric measurements into the corona. AIMS: We test the quality of a nonlinear forcefree coronal magnetic field extrapolation code with the help of a known analytical solution. METHODS: The nonlinear forcefree equations are numerically solved with the help of an optimization principle. The method minimizes an integral over the forcefree and solenoidal condition. As boundary condition we use either the magnetic field components on all six sides of the computational box in Case I or only on the bottom boundary in Case II. We check the quality of the reconstruction by computing how well forcefreeness and divergencefreeness are fulfilled and by comparing the numerical solution with the analytical solution. The comparison is done with magnetic field line plots and several quantitative measures, like the vector correlation, Cauchy Schwarz, normalized vector error, mean vector error and magnetic energy. RESULTS: For Case I the reconstructed magnetic field shows good agreement with the original magnetic field topology, whereas in Case II there are considerable deviations from the exact solution. This is corroborated by the quantitative measures, which are significantly better for Case I. CONCLUSIONS: Despite the strong nonlinearity of the considered forcefree equilibrium, the optimization method of extrapolation is able to reconstruct it; however, the quality of reconstruction depends significantly on the consistency of the input data, which is given only if the known solution is provided also at the lateral and top boundaries, and on the presence or absence of flux concentrations near the boundaries of the magnetogram.Astronomy and Astrophysics 01/2007; · 5.08 Impact Factor  [show abstract] [hide abstract]
ABSTRACT: AIMS: We develop an optimization principle for computing stationary MHD equilibria. METHODS: Our code for the selfconsistent computation of the coronal magnetic fields and the coronal plasma uses nonforcefree MHD equilibria. Previous versions of the code have been used to compute nonlinear forcefree coronal magnetic fields from photospheric measurements. The program uses photospheric vector magnetograms and coronal EUV images as input. We tested our reconstruction code with the help of a semianalytic MHDequilibrium. The quality of the reconstruction was judged by comparing the exact and reconstructed solution qualitatively by magnetic fieldline plots and EUVimages and quantitatively by several different numerical criteria. RESULTS: Our code is able to reconstruct the semianalytic test equilibrium with high accuracy. The stationary MHD optimization code developed here has about the same accuracy as its predecessor, a nonlinear forcefree optimization code. The computing time for MHDequilibria is, however, longer than for forcefree magnetic fields. We also extended a wellknown class of nonlinear forcefree equilibria to the nonforcefree regime for purposes of testing the code. CONCLUSIONS: We demonstrate that the code works in principle using tests with analytical equilibria, but it still needs to be applied to real data.Astronomy and Astrophysics 01/2007; · 5.08 Impact Factor  [show abstract] [hide abstract]
ABSTRACT: We compare six algorithms for the computation of nonlinear forcefree (NLFF) magnetic fields (including optimization, magnetofrictional, Grad–Rubin based, and Green's functionbased methods) by evaluating their performance in blind tests on analytical forcefreefield models for which boundary conditions are specified either for the entire surface area of a cubic volume or for an extended lower boundary only. Figures of merit are used to compare the input vector field to the resulting model fields. Based on these merit functions, we argue that all algorithms yield NLFF fields that agree best with the input field in the lower central region of the volume, where the field and electrical currents are strongest and the effects of boundary conditions weakest. The NLFF vector fields in the outer domains of the volume depend sensitively on the details of the specified boundary conditions; best agreement is found if the field outside of the model volume is incorporated as part of the model boundary, either as potential field boundaries on the side and top surfaces, or as a potential field in a skirt around the main volume of interest. For input field (B) and modeled field (b), the best method included in our study yields an average relative vector error En = 〈 B−b〉/〈 B〉 of only 0.02 when all sides are specified and 0.14 for the case where only the lower boundary is specified, while the total energy in the magnetic field is approximated to within 2%. The models converge towards the central, strong input field at speeds that differ by a factor of one million per iteration step. The fastestconverging, bestperforming model for these analytical test cases is the Wheatland, Sturrock, and Roumeliotis (2000) optimization algorithm as implemented by Wiegelmann (2004).Solar Physics 04/2006; 235(1):161190. · 3.26 Impact Factor
Publication Stats
207  Citations  
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76.99  Total Impact Points  
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Institutions

2013

Chinese Academy of Sciences
 Graduate School
Peping, Beijing, China


2012

Kyung Hee University
 School of Space Research
Sŏul, Seoul, South Korea


2006–2012

Max Planck Institute for Solar System Research
Göttingen, Lower Saxony, Germany


1998–2000

RuhrUniversität Bochum
 Institut für Theoretische Physik IV
Bochum, North RhineWestphalia, Germany
