TY - JOUR A1 - Kliem, Bernhard A1 - Seehafer, Norbert T1 - Helicity shedding by flux rope ejection JF - Astronomy and astrophysics : an international weekly journal N2 - We quantitatively address the conjecture that magnetic helicity must be shed from the Sun by eruptions launching coronal mass ejections in order to limit its accumulation in each hemisphere. By varying the ratio of guide and strapping field and the flux rope twist in a parametric simulation study of flux rope ejection from approximately marginally stable force-free equilibria, different ratios of self- and mutual helicity are set and the onset of the torus or helical kink instability is obtained. The helicity shed is found to vary over a broad range from a minor to a major part of the initial helicity, with self helicity being largely or completely shed and mutual helicity, which makes up the larger part of the initial helicity, being shed only partly. Torus-unstable configurations with subcritical twist and without a guide field shed up to about two-thirds of the initial helicity, while a highly twisted, kink-unstable configuration sheds only about one-quarter. The parametric study also yields stable force-free flux rope equilibria up to a total flux-normalized helicity of 0.25, with a ratio of self- to total helicity of 0.32 and a ratio of flux rope to external poloidal flux of 0.94. These results numerically demonstrate the conjecture of helicity shedding by coronal mass ejections and provide a first account of its parametric dependence. Both self- and mutual helicity are shed significantly; this reduces the total initial helicity by a fraction of ∼0.4--0.65 for typical source region parameters. KW - instabilities KW - magnetic fields KW - magnetohydrodynamics (MHD) KW - Sun KW - corona KW - coronal mass ejections (CMEs) KW - flares Y1 - 2022 U6 - https://doi.org/10.1051/0004-6361/202142422 SN - 0004-6361 SN - 1432-0746 VL - 659 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Lopez-Barquero, Vanessa A1 - Xu, S. A1 - Desiati, Paolo A1 - Lazarian, Alex A1 - Pogorelov, Nikolai V. A1 - Yan, Huirong T1 - TeV Cosmic-Ray Anisotropy from the Magnetic Field at the Heliospheric Boundary JF - The astrophysical journal : an international review of spectroscopy and astronomical physics KW - cosmic rays KW - magnetic fields KW - magnetohydrodynamics (MHD) KW - solar wind KW - Sun: heliosphere Y1 - 2017 U6 - https://doi.org/10.3847/1538-4357/aa74d1 SN - 0004-637X SN - 1538-4357 VL - 842 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Xu, Siyao A1 - Yan, Huirong A1 - Lazarian, A. T1 - DAMPING OF MAGNETOHYDRODYNAMIC TURBULENCE IN PARTIALLY IONIZED PLASMA: IMPLICATIONS FOR COSMIC RAY PROPAGATION JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - We study the damping processes of both incompressible and compressible magnetohydrodynamic (MHD) turbulence in a partially ionized medium. We start from the linear analysis of MHD waves, applying both single-fluid and two-fluid treatments. The damping rates derived from the linear analysis are then used in determining the damping scales of MHD turbulence. The physical connection between the damping scale of MHD turbulence and the cutoff boundary of linear MHD waves is investigated. We find two branches of slow modes propagating in ions and neutrals, respectively, below the damping scale of slow MHD turbulence, and offer a thorough discussion of their propagation and dissipation behavior. Our analytical results are shown to be applicable in a variety of partially ionized interstellar medium (ISM) phases and the solar chromosphere. The importance of neutral viscosity in damping the Alfvenic turbulence in the interstellar warm neutral medium and the solar chromosphere is demonstrated. As a significant astrophysical utility, we introduce damping effects to the propagation of cosmic rays in partially ionized ISM. The important role of turbulence damping in both transit-time damping and gyroresonance is identified. KW - cosmic rays KW - magnetohydrodynamics (MHD) KW - turbulence Y1 - 2016 U6 - https://doi.org/10.3847/0004-637X/826/2/166 SN - 0004-637X SN - 1538-4357 VL - 826 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Hassanin, Alshaimaa A1 - Kliem, Bernhard A1 - Seehafer, Norbert T1 - Helical kink instability in the confined solar eruption on 2002 May 27 JF - Astronomische Nachrichten = Astronomical notes KW - instabilities KW - magnetohydrodynamics (MHD) KW - Sun: corona KW - Sun: coronal mass ejections (CMEs) KW - Sun: flares Y1 - 2016 U6 - https://doi.org/10.1002/asna.201612446 SN - 0004-6337 SN - 1521-3994 VL - 337 SP - 1082 EP - 1089 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Ni, Lei A1 - Kliem, Bernhard A1 - Lin, Jun A1 - Wu, Ning T1 - Fast magnetic reconnection in the solar chromosphere mediated by theplasmoid instability JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - Magnetic reconnection in the partially ionized solar chromosphere is studied in 2.5 dimensional magnetohydrodynamic simulations including radiative cooling and ambipolar diffusion. A Harris current sheet with and without a guide field is considered. Characteristic values of the parameters in the middle chromosphere imply a high magnetic Reynolds number of similar to 10(6)-10(7) in the present simulations. Fast magnetic reconnection then develops as a consequence of the plasmoid instability without the need to invoke anomalous resistivity enhancements. Multiple levels of the instability are followed as it cascades to smaller scales, which approach the ion inertial length. The reconnection rate, normalized to the asymptotic values of magnetic field and Alfven velocity in the inflow region, reaches values in the range similar to 0.01-0.03 throughout the cascading plasmoid formation and for zero as well as for strong guide field. The outflow velocity reaches approximate to 40 km s(-1). Slow-mode shocks extend from the X-points, heating the plasmoids up to similar to 8 x 10(4) K. In the case of zero guide field, the inclusion of both ambipolar diffusion and radiative cooling causes a rapid thinning of the current sheet (down to similar to 30 m) and early formation of secondary islands. Both of these processes have very little effect on the plasmoid instability for a strong guide field. The reconnection rates, temperature enhancements, and upward outflow velocities from the vertical current sheet correspond well to their characteristic values in chromospheric jets. KW - magnetic reconnection KW - magnetohydrodynamics (MHD) KW - radiation: dynamics KW - Sun: activity KW - Sun: chromosphere Y1 - 2015 U6 - https://doi.org/10.1088/0004-637X/799/1/79 SN - 0004-637X SN - 1538-4357 VL - 799 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Dalmasse, Kevin A1 - Aulanier, Guillaume A1 - Demoulin, P. A1 - Kliem, Bernhard A1 - Török, Tibor A1 - Pariat, E. T1 - The origin of net electric currents in solar active regions JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - There is a recurring question in solar physics regarding whether or not electric currents are neutralized in active regions (ARs). This question was recently revisited using three-dimensional (3D) magnetohydrodynamic (MHD) numerical simulations of magnetic flux emergence into the solar atmosphere. Such simulations showed that flux emergence can generate a substantial net current in ARs. Other sources of AR currents are photospheric horizontal flows. Our aim is to determine the conditions for the occurrence of net versus neutralized currents with this second mechanism. Using 3D MHD simulations, we systematically impose line-tied, quasi-static, photospheric twisting and shearing motions to a bipolar potential magnetic field. We find that such flows: (1) produce both direct and return currents, (2) induce very weak compression currents-not observed in 2.5D-in the ambient field present in the close vicinity of the current-carrying field, and (3) can generate force-free magnetic fields with a net current. We demonstrate that neutralized currents are in general produced only in the absence of magnetic shear at the photospheric polarity inversion line-a special condition that is rarely observed. We conclude that. photospheric flows,. as magnetic flux emergence, can build up net currents in the solar atmosphere, in agreement with recent observations. These results thus provide support for eruption models based on pre-eruption magnetic fields that possess a net coronal current. KW - magnetohydrodynamics (MHD) KW - Sun: corona KW - Sun: coronal mass ejections (CMEs) KW - Sun: flares Y1 - 2015 U6 - https://doi.org/10.1088/0004-637X/810/1/17 SN - 0004-637X SN - 1538-4357 VL - 810 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Toeroek, T. A1 - Leake, J. E. A1 - Titov, Viacheslav S. A1 - Archontis, V. A1 - Mikic, Z. A1 - Linton, M. G. A1 - Dalmasse, K. A1 - Aulanier, Guillaume A1 - Kliem, Bernhard T1 - Distribution of electric currents in solar active regions JF - The astrophysical journal : an international review of spectroscopy and astronomical physics ; Part 2, Letters KW - magnetohydrodynamics (MHD) KW - Sun: corona KW - Sun: coronal mass ejections (CMEs) Y1 - 2014 U6 - https://doi.org/10.1088/2041-8205/782/1/L10 SN - 2041-8205 SN - 2041-8213 VL - 782 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Toeroek, T. A1 - Kliem, Bernhard A1 - Berger, M. A. A1 - Linton, M. G. A1 - Demoulin, P. A1 - van Driel-Gesztelyi, L. T1 - The evolution of writhe in kink-unstable flux ropes and erupting filaments JF - Plasma physics and controlled fusion N2 - The helical kink instability of a twisted magnetic flux tube has been suggested as a trigger mechanism for solar filament eruptions and coronal mass ejections (CMEs). In order to investigate if estimations of the pre-emptive twist can be obtained from observations of writhe in such events, we quantitatively analyze the conversion of twist into writhe in the course of the instability, using numerical simulations. We consider the line tied, cylindrically symmetric Gold-Hoyle flux rope model and measure the writhe using the formulae by Berger and Prior which express the quantity as a single integral in space. We find that the amount of twist converted into writhe does not simply scale with the initial flux rope twist, but depends mainly on the growth rates of the instability eigenmodes of higher longitudinal order than the basic mode. The saturation levels of the writhe, as well as the shapes of the kinked flux ropes, are very similar for considerable ranges of initial flux rope twists, which essentially precludes estimations of pre-eruptive twist from measurements of writhe. However, our simulations suggest an upper twist limit of similar to 6 pi for the majority of filaments prior to their eruption. KW - magnetohydrodynamics (MHD) KW - Sun: corona KW - Sun: filaments Y1 - 2014 U6 - https://doi.org/10.1088/0741-3335/56/6/064012 SN - 0741-3335 SN - 1361-6587 VL - 56 IS - 6 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - van Driel-Gesztelyi, L. A1 - Baker, Daniel N. A1 - Toeroek, T. A1 - Pariat, E. A1 - Green, L. M. A1 - Williams, D. R. A1 - Carlyle, J. A1 - Valori, G. A1 - Demoulin, P. A1 - Kliem, Bernhard A1 - Long, D. M. A1 - Matthews, S. A. A1 - Malherbe, J. -M. T1 - Coronal magnetic reconnection driven by CME expansion-the 2011 June 7 event JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - Coronal mass ejections (CMEs) erupt and expand in a magnetically structured solar corona. Various indirect observational pieces of evidence have shown that the magnetic field of CMEs reconnects with surrounding magnetic fields, forming, e.g., dimming regions distant from the CME source regions. Analyzing Solar Dynamics Observatory (SDO) observations of the eruption from AR 11226 on 2011 June 7, we present the first direct evidence of coronal magnetic reconnection between the fields of two adjacent active regions during a CME. The observations are presented jointly with a data-constrained numerical simulation, demonstrating the formation/intensification of current sheets along a hyperbolic flux tube at the interface between the CME and the neighboring AR 11227. Reconnection resulted in the formation of new magnetic connections between the erupting magnetic structure from AR 11226 and the neighboring active region AR 11227 about 200 Mm from the eruption site. The onset of reconnection first becomes apparent in the SDO/AIA images when filament plasma, originally contained within the erupting flux rope, is redirected toward remote areas in AR 11227, tracing the change of large-scale magnetic connectivity. The location of the coronal reconnection region becomes bright and directly observable at SDO/AIA wavelengths, owing to the presence of down-flowing cool, dense (1010 cm(-3)) filament plasma in its vicinity. The high-density plasma around the reconnection region is heated to coronal temperatures, presumably by slow-mode shocks and Coulomb collisions. These results provide the first direct observational evidence that CMEs reconnect with surrounding magnetic structures, leading to a large-scale reconfiguration of the coronal magnetic field. KW - magnetic reconnection KW - magnetohydrodynamics (MHD) KW - Sun: corona KW - Sun: coronal mass ejections (CMEs) KW - Sun: magnetic fields KW - Sun: UV radiation Y1 - 2014 U6 - https://doi.org/10.1088/0004-637X/788/1/85 SN - 0004-637X SN - 1538-4357 VL - 788 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Kliem, Bernhard A1 - Lin, J. A1 - Forbes, T. G. A1 - Priest, E. R. A1 - Toeroek, T. T1 - Catastrophe versus instability for the eruption of a toroadal solar magnetic flux JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - The onset of a solar eruption is formulated here as either a magnetic catastrophe or as an instability. Both start with the same equation of force balance governing the underlying equilibria. Using a toroidal flux rope in an external bipolar or quadrupolar field as a model for the current-carrying flux, we demonstrate the occurrence of a fold catastrophe by loss of equilibrium for several representative evolutionary sequences in the stable domain of parameter space. We verify that this catastrophe and the torus instability occur at the same point; they are thus equivalent descriptions for the onset condition of solar eruptions. KW - magnetohydrodynamics (MHD) KW - Sun: corona KW - Sun: coronal mass ejections (CMEs) KW - Sun: filaments, prominences KW - Sun: flares KW - Sun: magnetic fields Y1 - 2014 U6 - https://doi.org/10.1088/0004-637X/789/1/46 SN - 0004-637X SN - 1538-4357 VL - 789 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Kliem, Bernhard A1 - Toeroek, Tibor A1 - Titov, Viacheslav S. A1 - Lionello, Roberto A1 - Linker, Jon A. A1 - Liu, Rui A1 - Liu, Chang A1 - Wang, Haimin T1 - Slow rise and partial eruption of a double-decker filament. II. A double flux rope model JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - Force-free equilibria containing two vertically arranged magnetic flux ropes of like chirality and current direction are considered as a model for split filaments/prominences and filament-sigmoid systems. Such equilibria are constructed analytically through an extension of the methods developed in Titov & Demoulin and numerically through an evolutionary sequence including shear flows, flux emergence, and flux cancellation in the photospheric boundary. It is demonstrated that the analytical equilibria are stable if an external toroidal (shear) field component exceeding a threshold value is included. If this component decreases sufficiently, then both flux ropes turn unstable for conditions typical of solar active regions, with the lower rope typically becoming unstable first. Either both flux ropes erupt upward, or only the upper rope erupts while the lower rope reconnects with the ambient flux low in the corona and is destroyed. However, for shear field strengths staying somewhat above the threshold value, the configuration also admits evolutions which lead to partial eruptions with only the upper flux rope becoming unstable and the lower one remaining in place. This can be triggered by a transfer of flux and current from the lower to the upper rope, as suggested by the observations of a split filament in Paper I. It can also result from tether-cutting reconnection with the ambient flux at the X-type structure between the flux ropes, which similarly influences their stability properties in opposite ways. This is demonstrated for the numerically constructed equilibrium. KW - instabilities KW - magnetohydrodynamics (MHD) KW - Sun: coronal mass ejections (CMEs) KW - Sun: filaments, prominences KW - Sun: flares Y1 - 2014 U6 - https://doi.org/10.1088/0004-637X/792/2/107 SN - 0004-637X SN - 1538-4357 VL - 792 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Mizuno, Yosuke A1 - Pohl, Martin A1 - Niemiec, Jacek A1 - Zhang, Bing A1 - Nishikawa, Ken-Ichi A1 - Hardee, Philip E. T1 - Magnetic-field amplification by turbulence in a relativistic shockpropagating through an inhomogeneous medium JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - We perform two-dimensional relativistic magnetohydrodynamic simulations of a mildly relativistic shock propagating through an inhomogeneous medium. We show that the postshock region becomes turbulent owing to preshock density inhomogeneity, and the magnetic field is strongly amplified due to the stretching and folding of field lines in the turbulent velocity field. The amplified magnetic field evolves into a filamentary structure in two-dimensional simulations. The magnetic energy spectrum is flatter than the Kolmogorov spectrum and indicates that a so-called small-scale dynamo is occurring in the postshock region. We also find that the amount of magnetic-field amplification depends on the direction of the mean preshock magnetic field, and the timescale of magnetic-field growth depends on the shock strength. KW - gamma-ray burst: general KW - magnetohydrodynamics (MHD) KW - methods: numerical KW - relativistic processes KW - shock waves KW - turbulence Y1 - 2011 U6 - https://doi.org/10.1088/0004-637X/726/2/62 SN - 0004-637X VL - 726 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Fuhrmann, Marcel A1 - Seehafer, Norbert A1 - Valori, Gherardo A1 - Wiegelmann, T. T1 - A comparison of preprocessing methods for solar force-free magnetic field extrapolation JF - Astronomy and astrophysics : an international weekly journal N2 - Context. Extrapolations of solar photospheric vector magnetograms into three-dimensional magnetic fields in the chromosphere and corona are usually done under the assumption that the fields are force-free. This condition is violated in the photosphere itself and a thin layer in the lower atmosphere above. The field calculations can be improved by preprocessing the photospheric magnetograms. The intention here is to remove a non-force-free component from the data. Aims. We compare two preprocessing methods presently in use, namely the methods of Wiegelmann et al. (2006, Sol. Phys., 233, 215) and Fuhrmann et al. (2007, A&A, 476, 349). Methods. The two preprocessing methods were applied to a vector magnetogram of the recently observed active region NOAA AR 10 953. We examine the changes in the magnetogram effected by the two preprocessing algorithms. Furthermore, the original magnetogram and the two preprocessed magnetograms were each used as input data for nonlinear force-free field extrapolations by means of two different methods, and we analyze the resulting fields. Results. Both preprocessing methods managed to significantly decrease the magnetic forces and magnetic torques that act through the magnetogram area and that can cause incompatibilities with the assumption of force-freeness in the solution domain. The force and torque decrease is stronger for the Fuhrmann et al. method. Both methods also reduced the amount of small-scale irregularities in the observed photospheric field, which can sharply worsen the quality of the solutions. For the chosen parameter set, the Wiegelmann et al. method led to greater changes in strong-field areas, leaving weak-field areas mostly unchanged, and thus providing an approximation of the magnetic field vector in the chromosphere, while the Fuhrmann et al. method weakly changed the whole magnetogram, thereby better preserving patterns present in the original magnetogram. Both preprocessing methods raised the magnetic energy content of the extrapolated fields to values above the minimum energy, corresponding to the potential field. Also, the fields calculated from the preprocessed magnetograms fulfill the solenoidal condition better than those calculated without preprocessing. KW - Sun: magnetic topology KW - Sun: atmosphere KW - magnetohydrodynamics (MHD) Y1 - 2011 U6 - https://doi.org/10.1051/0004-6361/201015453 SN - 0004-6361 VL - 526 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Kliem, Bernhard A1 - Su, Y. N. A1 - van Ballegooijen, A. A. A1 - DeLuca, E. E. T1 - Magnetohydrodynamic modeling of the solar eruption on 2010 APRIL 8 JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - The structure of the coronal magnetic field prior to eruptive processes and the conditions for the onset of eruption are important issues that can be addressed through studying the magnetohydrodynamic (MHD) stability and evolution of nonlinear force-free field (NLFFF) models. This paper uses data-constrained NLFFF models of a solar active region (AR) that erupted on 2010 April 8 as initial conditions in MHD simulations. These models, constructed with the techniques of flux rope insertion and magnetofrictional relaxation (MFR), include a stable, an approximately marginally stable, and an unstable configuration. The simulations confirm previous related results of MFR runs, particularly that stable flux rope equilibria represent key features of the observed pre-eruption coronal structure very well, and that there is a limiting value of the axial flux in the rope for the existence of stable NLFFF equilibria. The specific limiting value is located within a tighter range, due to the sharper discrimination between stability and instability by the MHD description. The MHD treatment of the eruptive configuration yields a very good agreement with a number of observed features, like the strongly inclined initial rise path and the close temporal association between the coronal mass ejection and the onset of flare reconnection. Minor differences occur in the velocity of flare ribbon expansion and in the further evolution of the inclination; these can be eliminated through refined simulations. We suggest that the slingshot effect of horizontally bent flux in the source region of eruptions can contribute significantly to the inclination of the rise direction. Finally, we demonstrate that the onset criterion, formulated in terms of a threshold value for the axial flux in the rope, corresponds very well to the threshold of the torus instability in the considered AR. KW - magnetohydrodynamics (MHD) KW - Sun: corona KW - Sun: coronal mass ejections (CMEs) KW - Sun: filaments, prominences KW - Sun: flares KW - Sun: magnetic fields Y1 - 2013 U6 - https://doi.org/10.1088/0004-637X/779/2/129 SN - 0004-637X SN - 1538-4357 VL - 779 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER -