TY - JOUR A1 - Cheng, Xin A1 - Kliem, Bernhard A1 - Ding, Mingde T1 - Unambiguous evidence of filament splitting-induced partial eruptions JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - Coronal mass ejections are often considered to result from the full eruption of a magnetic flux rope (MFR). However, it is recognized that, in some events, the MFR may release only part of its flux, with the details of the implied splitting not completely established due to limitations in observations. Here, we investigate two partial eruption events including a confined and a successful one. Both partial eruptions are a consequence of the vertical splitting of a filament-hosting MFR involving internal reconnection. A loss of equilibrium in the rising part of the magnetic flux is suggested by the impulsive onset of both events and by the delayed onset of reconnection in the confined event. The remaining part of the flux might be line-tied to the photosphere in a bald patch (BP) separatrix surface, and we confirm the existence of extended BP sections for the successful eruption. The internal reconnection is signified by brightenings in the body of one filament and between the rising and remaining parts of both filaments. It evolves quickly into the standard current sheet reconnection in the wake of the eruption. As a result, regardless of being confined or successful, both eruptions produce hard X-ray sources and flare loops below the erupting but above the surviving flux, as well as a pair of flare ribbons enclosing the latter. KW - Sun: magnetic fields KW - Sun: corona KW - Sun: coronal mass ejections (CMEs) KW - Sun: flares Y1 - 2018 U6 - https://doi.org/10.3847/1538-4357/aab08d SN - 0004-637X SN - 1538-4357 VL - 856 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Diercke, Andrea A1 - Kuckein, Christoph A1 - Verma, Meetu A1 - Denker, Carsten T1 - Counter-streaming flows in a giant quiet-Sun filament observed in the extreme ultraviolet JF - Astronomy and astrophysics : an international weekly journal N2 - Aims. The giant solar filament was visible on the solar surface from 2011 November 8-23. Multiwavelength data from the Solar Dynamics Observatory (SDO) were used to examine counter-streaming flows within the spine of the filament. Methods. We use data from two SDO instruments, the Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic Imager (HMI), covering the whole filament, which stretched over more than half a solar diameter. H alpha images from the Kanzelhohe Solar Observatory (KSO) provide context information of where the spine of the filament is defined and the barbs are located. We apply local correlation tracking (LCT) to a two-hour time series on 2011 November 16 of the AIA images to derive horizontal flow velocities of the filament. To enhance the contrast of the AIA images, noise adaptive fuzzy equalization (NAFE) is employed, which allows us to identify and quantify counter-streaming flows in the filament. We observe the same cool filament plasma in absorption in both H alpha and EUV images. Hence, the counter-streaming flows are directly related to this filament material in the spine. In addition, we use directional flow maps to highlight the counter-streaming flows. Results. We detect counter-streaming flows in the filament, which are visible in the time-lapse movies in all four examined AIA wavelength bands (lambda 171 angstrom, lambda 193 angstrom, lambda 304 angstrom, and lambda 211 angstrom). In the time-lapse movies we see that these persistent flows lasted for at least two hours, although they became less prominent towards the end of the time series. Furthermore, by applying LCT to the images we clearly determine counter-streaming flows in time series of lambda 171 angstrom and lambda 193 angstrom images. In the lambda 304 angstrom wavelength band, we only see minor indications for counter-streaming flows with LCT, while in the lambda 211 angstrom wavelength band the counter-streaming flows are not detectable with this method. The diverse morphology of the filament in H alpha and EUV images is caused by different absorption processes, i.e., spectral line absorption and absorption by hydrogen and helium continua, respectively. The horizontal flows reach mean flow speeds of about 0.5 km s(-1) for all wavelength bands. The highest horizontal flow speeds are identified in the lambda 171 angstrom band with flow speeds of up to 2.5 km s(-1). The results are averaged over a time series of 90 minutes. Because the LCT sampling window has finite width, a spatial degradation cannot be avoided leading to lower estimates of the flow velocities as compared to feature tracking or Doppler measurements. The counter-streaming flows cover about 15-20% of the whole area of the EUV filament channel and are located in the central part of the spine. Conclusions. Compared to the ground-based observations, the absence of seeing effects in AIA observations reveal counter-streaming flows in the filament even with a moderate image scale of 0 '.6 pixel(-1). Using a contrast enhancement technique, these flows can be detected and quantified with LCT in different wavelengths. We confirm the omnipresence of counter-streaming flows also in giant quiet-Sun filaments. KW - methods: observational KW - Sun: filaments, prominences KW - Sun: activity KW - Sun: chromosphere KW - Sun: corona KW - techniques: image processing Y1 - 2017 U6 - https://doi.org/10.1051/0004-6361/201730536 SN - 1432-0746 VL - 611 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Veronig, Astrid M. A1 - Podladchikova, Tatiana A1 - Dissauer, Karin A1 - Temmer, Manuela A1 - Seaton, Daniel B. A1 - Long, David A1 - Guo, Jingnan A1 - Vrsnak, Bojan A1 - Harra, Louise A1 - Kliem, Bernhard T1 - Genesis and Impulsive Evolution of the 2017 September 10 Coronal Mass Ejection JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - The X8.2 event of 2017 September 10 provides unique observations to study the genesis, magnetic morphology, and impulsive dynamics of a very fast coronal mass ejection (CME). Combining GOES-16/SUVI and SDO/AIA EUV imagery, we identify a hot (T approximate to 10-15 MK) bright rim around a quickly expanding cavity, embedded inside a much larger CME shell (T approximate to 1-2 MK). The CME shell develops from a dense set of large AR loops ( greater than or similar to 0.5R(s)) and seamlessly evolves into the CME front observed in LASCO C2. The strong lateral overexpansion of the CME shell acts as a piston initiating the fast EUV wave. The hot cavity rim is demonstrated to be a manifestation of the dominantly poloidal flux and frozen-in plasma added to the rising flux rope by magnetic reconnection in the current sheet beneath. The same structure is later observed as the core of the white-light CME, challenging the traditional interpretation of the CME three-part morphology. The large amount of added magnetic flux suggested by these observations explains the extreme accelerations of the radial and lateral expansion of the CME shell and cavity, all reaching values of 5-10 km s(-2). The acceleration peaks occur simultaneously with the first RHESSI 100-300 keV hard X-ray burst of the associated flare, further underlining the importance of the reconnection process for the impulsive CME evolution. Finally, the much higher radial propagation speed of the flux rope in relation to the CME shell causes a distinct deformation of the white-light CME front and shock. KW - Sun: activity KW - Sun: corona KW - Sun: coronal mass ejections (CMEs) KW - Sun: flares Y1 - 2018 U6 - https://doi.org/10.3847/1538-4357/aaeac5 SN - 0004-637X SN - 1538-4357 VL - 868 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Kuckein, Christoph A1 - Diercke, Andrea A1 - González Manrique, Sergio Javier A1 - Verma, Meetu A1 - Loehner-Boettcher, Johannes A1 - Socas-Navarro, H. A1 - Balthasar, Horst A1 - Sobotka, M. A1 - Denker, Carsten T1 - Ca II 8542 angstrom brightenings induced by a solar microflare JF - Astronomy and astrophysics : an international weekly journal N2 - Aims. We study small-scale brightenings in Ca II 8542 angstrom line-core images to determine their nature and effect on localized heating and mass transfer in active regions. Methods. High-resolution two-dimensional spectroscopic observations of a solar active region in the near-infrared Ca II 8542 angstrom line were acquired with the GREGOR Fabry-Perot Interferometer attached to the 1.5-m GREGOR telescope. Inversions of the spectra were carried out using the NICOLE code to infer temperatures and line-of-sight (LOS) velocities. Response functions of the Ca II line were computed for temperature and LOS velocity variations. Filtergrams of the Atmospheric Imaging Assembly (AIA) and magnetograms of the Helioseismic and Magnetic Imager (HMI) were coaligned to match the ground-based observations and to follow the Ca II brightenings along all available layers of the atmosphere. Results. We identified three brightenings of sizes up to 2 ' x 2 ' that appeared in the Ca II 8542 angstrom line-core images. Their lifetimes were at least 1.5 min. We found evidence that the brightenings belonged to the footpoints of a microflare (MF). The properties of the observed brightenings disqualified the scenarios of Ellerman bombs or Interface Region Imaging Spectrograph (IRIS) bombs. However, this MF shared some common properties with flaring active-region fibrils or flaring arch filaments (FAFs): (1) FAFs and MFs are both apparent in chromospheric and coronal layers according to the AIA channels; and (2) both show flaring arches with lifetimes of about 3.0-3.5 min and lengths of similar to 20 ' next to the brightenings. The inversions revealed heating by 600 K at the footpoint location in the ambient chromosphere during the impulsive phase. Connecting the footpoints, a dark filamentary structure appeared in the Ca II line-core images. Before the start of the MF, the spectra of this structure already indicated average blueshifts, meaning upward motions of the plasma along the LOS. During the impulsive phase, these velocities increased up to -2.2 km s(-1). The structure did not disappear during the observations. Downflows dominated at the footpoints. However, in the upper photosphere, slight upflows occurred during the impulsive phase. Hence, bidirectional flows are present in the footpoints of the MF. KW - Sun: photosphere KW - Sun: chromosphere KW - Sun: corona KW - Sun: activity KW - techniques: imaging spectroscopy Y1 - 2017 U6 - https://doi.org/10.1051/0004-6361/201731319 SN - 1432-0746 VL - 608 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Liu, Rui A1 - Kliem, Bernhard A1 - Titov, Viacheslav S. A1 - Chen, Jun A1 - Wang, Yuming A1 - Wang, Haimin A1 - Liu, Chang A1 - Xu, Yan A1 - Wiegelmann, Thomas T1 - STRUCTURE, STABILITY, AND EVOLUTION OF MAGNETIC FLUX ROPES FROM THE PERSPECTIVE OF MAGNETIC TWIST JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - We investigate the evolution of NOAA Active Region (AR) 11817 during 2013 August 10–12, when it developed a complex field configuration and produced four confined, followed by two eruptive, flares. These C-and-above flares are all associated with a magnetic flux rope (MFR) located along the major polarity inversion line, where shearing and converging photospheric flows are present. Aided by the nonlinear force-free field modeling, we identify the MFR through mapping magnetic connectivities and computing the twist number ${{ \mathcal T }}_{w}$ for each individual field line. The MFR is moderately twisted ($| {{ \mathcal T }}_{w}| \lt 2$) and has a well-defined boundary of high squashing factor Q. We found that the field line with the extremum $| {{ \mathcal T }}_{w}| $ is a reliable proxy of the rope axis, and that the MFR's peak $| {{ \mathcal T }}_{w}| $ temporarily increases within half an hour before each flare while it decreases after the flare peak for both confined and eruptive flares. This pre-flare increase in $| {{ \mathcal T }}_{w}| $ has little effect on the AR's free magnetic energy or any other parameters derived for the whole region, due to its moderate amount and the MFR's relatively small volume, while its decrease after flares is clearly associated with the stepwise decrease in the whole region's free magnetic energy due to the flare. We suggest that ${{ \mathcal T }}_{w}$ may serve as a useful parameter in forewarning the onset of eruption, and therefore, the consequent space weather effects. The helical kink instability is identified as the prime candidate onset mechanism for the considered flares. KW - coronal mass ejections (CMEs) KW - Sun: corona KW - Sun: filaments, pominences KW - Sun: flares KW - Sun: magnetic fields Y1 - 2016 U6 - https://doi.org/10.3847/0004-637X/818/2/148 SN - 0004-637X SN - 1538-4357 VL - 818 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 - 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 -