Author name code: kliem ADS astronomy entries on 2022-09-14 author:"Kliem, Bernhard" ------------------------------------------------------------------------ Title: Torus instability in magnetic flux ropes Authors: Kliem, Bernhard Bibcode: 2022cosp...44.2410K Altcode: The torus instability is a lateral kink of an arched (toroidal) current channel carrying a net current, which is in force-free equilibrium provided by an external poloidal ("strapping") field. The instability leads to expansion of the current channel and associated magnetic flux rope in major radius (rise) as well as in minor radius (cavity formation). Thus, it is a prime candidate for a standard model of the coronal mass ejection (CME) component of solar eruptions. In this presentation, I will introduce the instability, discussing the differences between the MHD case with ambient magnetized plasma and the vacuum case relevant in the tokamak device. Magnetic reconnection is a key ingredient of the instability in the former case. Next, I will review recent investigations of the parametric dependencies of the threshold, the decay index of the strapping field. These aim to substantiate the support for the occurrence of the instability from solar observations, which typically indicate a threshold in the range between 1 and 2. Finally, the height profile of the decay index holds one of the keys for understanding fast vs. slow CMEs and, eventually, the confinement of eruptions. Title: A Model of Homologous Confined and Ejective Eruptions Involving Kink Instability and Flux Cancellation Authors: Hassanin, Alshaimaa; Kliem, Bernhard; Seehafer, Norbert; Török, Tibor Bibcode: 2022ApJ...929L..23H Altcode: 2022arXiv220411767H In this study, we model a sequence of a confined and a full eruption, employing the relaxed end state of the confined eruption of a kink-unstable flux rope as the initial condition for the ejective one. The full eruption, a model of a coronal mass ejection, develops as a result of converging motions imposed at the photospheric boundary, which drive flux cancellation. In this process, parts of the positive and negative external flux converge toward the polarity inversion line, reconnect, and cancel each other. Flux of the same amount as the canceled flux transfers to a flux rope, increasing the free magnetic energy of the coronal field. With sustained flux cancellation and the associated progressive weakening of the magnetic tension of the overlying flux, we find that a flux reduction of ≍11% initiates the torus instability of the flux rope, which leads to a full eruption. These results demonstrate that a homologous full eruption, following a confined one, can be driven by flux cancellation. Title: Helicity shedding by flux rope ejection Authors: Kliem, B.; Seehafer, N. Bibcode: 2022A&A...659A..49K Altcode: 2021arXiv211205833K Context. 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.
Aims: 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.
Methods: 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.
Results: 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. Title: Fast plasmoid-mediated reconnection in a solar flare Authors: Yan, Xiaoli; Xue, Zhike; Jiang, Chaowei; Priest, E. R.; Kliem, Bernhard; Yang, Liheng; Wang, Jincheng; Kong, Defang; Song, Yongliang; Feng, Xueshang; Liu, Zhong Bibcode: 2022NatCo..13..640Y Altcode: Magnetic reconnection is a multi-faceted process of energy conversion in astrophysical, space and laboratory plasmas that operates at microscopic scales but has macroscopic drivers and consequences. Solar flares present a key laboratory for its study, leaving imprints of the microscopic physics in radiation spectra and allowing the macroscopic evolution to be imaged, yet a full observational characterization remains elusive. Here we combine high resolution imaging and spectral observations of a confined solar flare at multiple wavelengths with data-constrained magnetohydrodynamic modeling to study the dynamics of the flare plasma from the current sheet to the plasmoid scale. The analysis suggests that the flare resulted from the interaction of a twisted magnetic flux rope surrounding a filament with nearby magnetic loops whose feet are anchored in chromospheric fibrils. Bright cusp-shaped structures represent the region around a reconnecting separator or quasi-separator (hyperbolic flux tube). The fast reconnection, which is relevant for other astrophysical environments, revealed plasmoids in the current sheet and separatrices and associated unresolved turbulent motions. Title: Flux rope reformation as a model for homologous solar flares and coronal mass ejections Authors: Saad Hassanin, Alshaimaa; Kliem, Bernhard; Torok, Tibor; Seehafer, Norbert Bibcode: 2021AGUFMSH32A..09S Altcode: In this study we model for the first time a sequence of a confined and a full eruption, employing the flux rope reformed in the confined eruption as the initial condition for the ejective one. The full eruption develops as a result of imposed converging motions in the photospheric boundary which drive flux cancellation. In this process, a part of the positive and negative sunspot flux converge toward the polarity inversion line, reconnect, and cancel each other. Flux of the same amount as the canceled flux transfers to the flux rope, building up free magnetic energy. With sustained flux cancellation and the associated progressive weakening of the magnetic tension of the overlying flux, we find that a flux reduction of 8.9% leads to the ejective eruption. These results demonstrate that homologous eruptions, eventually leading to a coronal mass ejection (CME), can be driven by flux cancellation. Title: Partial Eruption, Confinement, and Twist Buildup and Release of a Double-decker Filament Authors: Chen, Jialin; Su, Yingna; Liu, Rui; Kliem, Bernhard; Zhang, Qingmin; Ji, Haisheng; Liu, Tie Bibcode: 2021ApJ...923..142C Altcode: 2021arXiv211113174C We investigate the failed partial eruption of a filament system in NOAA AR 12104 on 2014 July 5, using multiwavelength EUV, magnetogram, and Hα observations, as well as magnetic field modeling. The filament system consists of two almost co-spatial segments with different end points, both resembling a C shape. Following an ejection and a precursor flare related to flux cancellation, only the upper segment rises and then displays a prominent twisted structure, while rolling over toward its footpoints. The lower segment remains undisturbed, indicating that the system possesses a double-decker structure. The erupted segment ends up with a reverse-C shape, with material draining toward its footpoints, while losing its twist. Using the flux rope insertion method, we construct a model of the source region that qualitatively reproduces key elements of the observed evolution. At the eruption onset, the model consists of a flux rope atop a flux bundle with negligible twist, which is consistent with the observational interpretation that the filament possesses a double-decker structure. The flux rope reaches the critical height of the torus instability during its initial relaxation, while the lower flux bundle remains in stable equilibrium. The eruption terminates when the flux rope reaches a dome-shaped quasi-separatrix layer that is reminiscent of a magnetic fan surface, although no magnetic null is found. The flux rope is destroyed by reconnection with the confining overlying flux above the dome, transferring its twist in the process. Title: Pre-eruption Splitting of the Double-decker Structure in a Solar Filament Authors: Pan, Hanya; Liu, Rui; Gou, Tingyu; Kliem, Bernhard; Su, Yingna; Chen, Jun; Wang, Yuming Bibcode: 2021ApJ...909...32P Altcode: 2021arXiv210103296P Solar filaments often erupt partially. Although how they split remains elusive, the splitting process has the potential of revealing the filament structure and eruption mechanism. Here we investigate the pre-eruption splitting of an apparently single filament and its subsequent partial eruption on 2012 September 27. The evolution is characterized by three stages with distinct dynamics. During the quasi-static stage, the splitting proceeds gradually for about 1.5 hr, with the upper branch rising at a few kilometers per second and displaying swirling motions about its axis. During the precursor stage that lasts for about 10 minutes, the upper branch rises at tens of kilometers per second, with a pair of conjugated dimming regions starting to develop at its footpoints; with the swirling motions turning chaotic, the axis of the upper branch whips southward, which drives an arc-shaped extreme-ultraviolet front propagating in a similar direction. During the eruption stage, the upper branch erupts with the onset of a C3.7-class two-ribbon flare, while the lower branch remains stable. Judging from the well-separated footpoints of the upper branch from those of the lower one, we suggest that the pre-eruption filament processes a double-decker structure composed of two distinct flux bundles, whose formation is associated with gradual magnetic flux cancellations and converging photospheric flows around the polarity inversion line. Title: Nonequilibrium Flux Rope Formation by Confined Flares Preceding a Solar Coronal Mass Ejection Authors: Kliem, Bernhard; Lee, Jeongwoo; Liu, Rui; White, Stephen M.; Liu, Chang; Masuda, Satoshi Bibcode: 2021ApJ...909...91K Altcode: 2021arXiv210102181K We present evidence that a magnetic flux rope was formed before a coronal mass ejection (CME) and its associated long-duration flare during a pair of preceding confined eruptions and associated impulsive flares in a compound event in NOAA Active Region 12371. Extreme-ultraviolet images and the extrapolated nonlinear force-free field show that the first two (impulsive) flares, SOL2015-06-21T01:42, result from the confined eruption of highly sheared low-lying flux, presumably a seed flux rope. The eruption spawns a vertical current sheet, where magnetic reconnection creates flare ribbons and loops, a nonthermal microwave source, and a sigmoidal hot channel that can only be interpreted as a magnetic flux rope. Until the subsequent long-duration flare, SOL2015-06-21T02:36, the sigmoid's elbows expand, while its center remains stationary, suggesting nonequilibrium but not yet instability. The "flare reconnection" during the confined eruptions acts like "tether-cutting reconnection" whose flux feeding of the rope leads to instability. The subsequent full eruption is seen as an accelerated rise of the entire hot channel, seamlessly evolving into the fast halo CME. Both the confined and ejective eruptions are consistent with the onset of the torus instability in the dipped decay index profile that results from the region's two-scale magnetic structure. We suggest that the formation or enhancement of a nonequilibrium but stable flux rope by confined eruptions is a generic process occurring prior to many CMEs. Title: Laboratory Study of the Torus Instability Threshold in Solar-relevant, Line-tied Magnetic Flux Ropes Authors: Alt, Andrew; Myers, Clayton E.; Ji, Hantao; Jara-Almonte, Jonathan; Yoo, Jongsoo; Bose, Sayak; Goodman, Aaron; Yamada, Masaaki; Kliem, Bernhard; Savcheva, Antonia Bibcode: 2021ApJ...908...41A Altcode: 2020arXiv201010607A Coronal mass ejections (CMEs) occur when long-lived magnetic flux ropes (MFRs) anchored to the solar surface destabilize and erupt away from the Sun. This destabilization is often described in terms of an ideal magnetohydrodynamic instability called the torus instability. It occurs when the external magnetic field decreases sufficiently fast such that its decay index, ${n}_{}=-z\,\partial (\mathrm{ln}{B}_{})/\partial z$ , is larger than a critical value, $n\gt {n}_{\mathrm{cr}}^{}$ , where ${n}_{\mathrm{cr}}^{}=1.5$ for a full, large aspect ratio torus. However, when this is applied to solar MFRs, a range of conflicting values for ${n}_{\mathrm{cr}}^{}$ is found in the literature. To investigate this discrepancy, we have conducted laboratory experiments on arched, line-tied flux ropes and applied a theoretical model of the torus instability. Our model describes an MFR as a partial torus with foot points anchored in a conducting surface and numerically calculates various magnetic forces on it. This calculation yields better predictions of ${n}_{\mathrm{cr}}^{}$ that take into account the specific parameters of the MFR. We describe a systematic methodology to properly translate laboratory results to their solar counterparts, provided that the MFRs have a sufficiently small edge safety factor or, equivalently, a large enough twist. After this translation, our model predicts that ${n}_{\mathrm{cr}}^{}$ in solar conditions falls near ${n}_{\mathrm{cr}}^{\mathrm{solar}}\sim 0.9$ and within a larger range of ${n}_{\mathrm{cr}}^{\mathrm{solar}}\sim (0.7,1.2)$ , depending on the parameters. The methodology of translating laboratory MFRs to their solar counterparts enables quantitative investigations of CME initiation through laboratory experiments. These experiments allow for new physics insights that are required for better predictions of space weather events but are difficult to obtain otherwise. Title: A Statistical Study of Flare Ribbons in Confined Precursor Eruptions Authors: Kliem, Bernhard Bibcode: 2021cosp...43E1774K Altcode: We present a statistical study of flare ribbons in confined eruptions which precede a major ejective eruption (CME), using the complete sample of CMEs associated with > M5.0 flares in 2011--2015 and source distance from Sun center of < 50 deg (32 events, from Baumgartner et al. 2018). Ribbons of precursor events within 12 hr from the onset of the CME-associated (eruptive) flare are compared with the ribbons of the main event to assess a potential contribution of the precursor events to the buildup of a flux rope prior to CME onset. It is found that 26 CMEs (81%) have one or several precursors with bright ribbons that cover a part of the ribbons in the main event, hence, clearly contribute to the buildup of the flux rope erupting in the CME. Two further events develop such ribbons during an enhancement that is part of the slow-rise phase of the GOES light curve, i.e., prior to the onset of the main exponential rise of the soft X-ray flux. Two CMEs (6%) only have one or several precursors with weak, very short, or very transient ribbons that cover a part of the ribbons in the main event. These precursors provide at least a minor contribution to the buildup of the flux rope erupting in the CME. Precursor events were absent, or did not produce ribbons overlapping those of the main event, for the remaining two CMEs. Of the four events lacking separate precursor events with relevant ribbons, three are characterized by a large ribbon separation from their onset, suggesting a high-lying flux rope, and one is triggered by the emergence of parasitic flux. In conclusion, a significant role of flare reconnection for the buildup of a flux rope in precursor events or during the slow-rise phase is indicated for 88% of the considered major ejective eruptions. This implies that the formation of a flux rope progresses significantly already prior to the onset of the main acceleration phase in these events. Since all studied precursor events produce shorter ribbons than the main event, the forming flux rope is likely not yet fully coherent along the whole length of the eventually erupting flux. Title: Decay Index Profile and Coronal Mass Ejection Speed Authors: Kliem, Bernhard; Zhang, Jie; Torok, Tibor; Chintzoglou, Georgios Bibcode: 2021cosp...43E.997K Altcode: The velocity of coronal mass ejections (CMEs) is one of the primary parameters determining their potential geoeffectiveness. A great majority of very fast CMEs receive their main acceleration already in the corona. We study the magnetic source region structure for a complete sample of 15 very fast CMEs (v > 1500 km/s) during 2000--2006, originating within 30 deg from central meridian. We find a correlation between CME speed and the decay index profile of the coronal field estimated by a PFSS extrapolation. The correlation is considerably weaker for an extended sample that includes slower CMEs. We also study how the decay index profile is related to the structure of the photospheric field distribution. This is complemented by a parametric simulation study of flux-rope eruptions using the analytic Titov-D\'emoulin active-region model for simple bipolar and quadrupolar source regions. The simulations provide simple relationships between the photospheric field distribution and the coronal decay index profile. They also help identifying source regions which are likely to produce slow CMEs only, thus improving the correlation for the extended CME sample. Very fast, moderate-velocity, and even confined eruptions are found, and the conditions for their occurrence are quantified. Title: Decoding the Pre-Eruptive Magnetic Field Configurations of Coronal Mass Ejections Authors: Patsourakos, S.; Vourlidas, A.; Török, T.; Kliem, B.; Antiochos, S. K.; Archontis, V.; Aulanier, G.; Cheng, X.; Chintzoglou, G.; Georgoulis, M. K.; Green, L. M.; Leake, J. E.; Moore, R.; Nindos, A.; Syntelis, P.; Yardley, S. L.; Yurchyshyn, V.; Zhang, J. Bibcode: 2020SSRv..216..131P Altcode: 2020arXiv201010186P A clear understanding of the nature of the pre-eruptive magnetic field configurations of Coronal Mass Ejections (CMEs) is required for understanding and eventually predicting solar eruptions. Only two, but seemingly disparate, magnetic configurations are considered viable; namely, sheared magnetic arcades (SMA) and magnetic flux ropes (MFR). They can form via three physical mechanisms (flux emergence, flux cancellation, helicity condensation). Whether the CME culprit is an SMA or an MFR, however, has been strongly debated for thirty years. We formed an International Space Science Institute (ISSI) team to address and resolve this issue and report the outcome here. We review the status of the field across modeling and observations, identify the open and closed issues, compile lists of SMA and MFR observables to be tested against observations and outline research activities to close the gaps in our current understanding. We propose that the combination of multi-viewpoint multi-thermal coronal observations and multi-height vector magnetic field measurements is the optimal approach for resolving the issue conclusively. We demonstrate the approach using MHD simulations and synthetic coronal images. Title: Coronal Mini-jets in an Activated Solar Tornado-like Prominence Authors: Chen, Huadong; Zhang, Jun; De Pontieu, Bart; Ma, Suli; Kliem, Bernhard; Priest, Eric Bibcode: 2020ApJ...899...19C Altcode: 2020arXiv200608252C High-resolution observations from the Interface Region Imaging Spectrometer reveal the existence of a particular type of small solar jet, which arose singly or in clusters from a tornado-like prominence suspended in the corona. In this study, we perform a detailed statistical analysis of 43 selected mini-jets in the tornado event. Our results show that the mini-jets typically have (1) a projected length of 1.0-6.0 Mm, (2) a width of 0.2-1.0 Mm, (3) a lifetime of 10-50 s, (4) a velocity of 100-350 km s-1, and (5) an acceleration of 3-20 km s-2. Based on spectral diagnostics and EM-Loci analysis, these jets seem to be multithermal small-scale plasma ejections with an estimated average electron density of ∼2.4 × 1010 cm-3 and an approximate mean temperature of ∼2.6 × 105 K. Their mean kinetic energy density, thermal energy density, and dissipated magnetic field strength are roughly estimated to be ∼9 erg cm-3, 3 erg cm-3, and 16 G, respectively. The accelerations of the mini-jets, the UV and EUV brightenings at the footpoints of some mini-jets, and the activation of the host prominence suggest that the tornado mini-jets are probably created by fine-scale external or internal magnetic reconnections (a) between the prominence field and the enveloping or background field or (b) between twisted or braided flux tubes within the prominence. The observations provide insight into the geometry of such reconnection events in the corona and have implications for the structure of the prominence magnetic field and the instability that is responsible for the eruption of prominences and coronal mass ejections. Title: sagemath/sage: 9.1 Authors: Stein, William; Chapoton, Frédéric; Demeyer, Jeroen; Köppe, Matthias; Krenn, Daniel; Rüth, Julian; Nathanncohen; Braun, Volker; Delecroix, Vincent; Dcoudert; Palmieri, John H.; Pasechnik, Dima; Bray, E. M.; Cheuberg; Stephan, Ralf; Bradshaw, Robert; Thiéry, Nicolas M.; Grinberg, Darij; Jm58660; Hansen, Mike; Gourgoulhon, Eric; King, Simon; Bruin, Peter; Hackl, Benjamin; Roed314; Kliem; Cremona, John; Rubey, Martin; Mezzarobba, Marc; Yi Bibcode: 2020zndo...4066866S Altcode: Mirror of the Sage source tree -- please do not submit PRs here -- everything must be submitted via https://trac.sagemath.org/ Title: Initiation and Early Kinematic Evolution of Solar Eruptions Authors: Cheng, X.; Zhang, J.; Kliem, B.; Török, T.; Xing, C.; Zhou, Z. J.; Inhester, B.; Ding, M. D. Bibcode: 2020ApJ...894...85C Altcode: 2020arXiv200403790C We investigate the initiation and early evolution of 12 solar eruptions, including six active-region hot channel and six quiescent filament eruptions, which were well observed by the Solar Dynamics Observatory, as well as by the Solar Terrestrial Relations Observatory for the latter. The sample includes one failed eruption and 11 coronal mass ejections, with velocities ranging from 493 to 2140 km s-1. A detailed analysis of the eruption kinematics yields the following main results. (1) The early evolution of all events consists of a slow-rise phase followed by a main-acceleration phase, the height-time profiles of which differ markedly and can be best fit, respectively, by a linear and an exponential function. This indicates that different physical processes dominate in these phases, which is at variance with models that involve a single process. (2) The kinematic evolution of the eruptions tends to be synchronized with the flare light curve in both phases. The synchronization is often but not always close. A delayed onset of the impulsive flare phase is found in the majority of the filament eruptions (five out of six). This delay and its trend to be larger for slower eruptions favor ideal MHD instability models. (3) The average decay index at the onset heights of the main acceleration is close to the threshold of the torus instability for both groups of events (although, it is based on a tentative coronal field model for the hot channels), suggesting that this instability initiates and possibly drives the main acceleration. Title: Extreme-ultraviolet Late Phase of Solar Flares Authors: Chen, Jun; Liu, Rui; Liu, Kai; Awasthi, Arun Kumar; Zhang, Peijin; Wang, Yuming; Kliem, Bernhard Bibcode: 2020ApJ...890..158C Altcode: 2020arXiv200106929C A second peak in the extreme ultraviolet sometimes appears during the gradual phase of solar flares, which is known as the EUV late phase (ELP). Stereotypically ELP is associated with two separated sets of flaring loops with distinct sizes, and it has been debated whether ELP is caused by additional heating or extended plasma cooling in the longer loop system. Here we carry out a survey of 55 M-and-above GOES-class flares with ELP during 2010-2014. Based on the flare-ribbon morphology, these flares are categorized as circular-ribbon (19 events), two-ribbon (23 events), and complex-ribbon (13 events) flares. Among them, 22 events (40%) are associated with coronal mass ejections, while the rest are confined. An extreme ELP, with the late-phase peak exceeding the main-phase peak, is found in 48% of two-ribbon flares, 37% of circular-ribbon flares, and 31% of complex-ribbon flares, suggesting that additional heating is more likely present during ELP in two-ribbon than in circular-ribbon flares. Overall, cooling may be the dominant factor causing the delay of the ELP peak relative to the main-phase peak, because the loop system responsible for the ELP emission is generally larger than, and well separated from, that responsible for the main-phase emission. All but one of the circular-ribbon flares can be well explained by a composite "dome-plate" quasi-separatrix layer (QSL). Only half of these show a magnetic null point, with its fan and spine embedded in the dome and plate, respectively. The dome-plate QSL, therefore, is a general and robust structure characterizing circular-ribbon flares. Title: Genesis and impulsive evolution of the fast CME associated with the X8.2 flare on 2017 September 10 Authors: Veronig, A.; Podladchikova, T.; Dissauer, K.; Temmer, M.; Seaton, D. B.; Long, D.; Guo, J.; Vrsnak, B.; Harra, L. K.; Kliem, B. Bibcode: 2019AGUFMSH13A..02V Altcode: The X8.2 event of 2017 September 10 provides unique observations to study the genesis, magnetic morphology, impulsive dynamics and shock formation in a very fast coronal mass ejection (CME). As will be discussed in this presentation, fundamental insight in the processes of magnetic reconnection, CME acceleration and shock formation are provided through EUV observations of the middle corona.

Combining the large field-of-view and high-cadence imagery from GOES-16/SUVI and SDO/AIA EUV, respectively, we identify a hot (T ≈ 10-15 MK) bright rim around a quickly expanding cavity, embedded inside a much larger CME shell (T ≈ 1-2 MK). The CME shell develops from a dense set of large AR loops (>0.5Rs) 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 and globally propagating EUV shock 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 (Veronig et al. 2018).

The large amount of added magnetic flux suggested by these observations can explain the extreme accelerations of the radial and lateral expansion of the CME shell and cavity, all reaching values up to 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 in the low and middle corona. Title: Sheared Magnetic Arcades and the Pre-eruptive Magnetic Configuration of Coronal Mass Ejections: Diagnostics, Challenges and Future Observables Authors: Patsourakos, Spiros; Vourlidas, A.; Anthiochos, S. K.; Archontis, V.; Aulanier, G.; Cheng, X.; Chintzoglou, G.; Georgoulis, M. K.; Green, L. M.; Kliem, B.; Leake, J.; Moore, R. L.; Nindos, A.; Syntelis, P.; Torok, T.; Yardley, S. L.; Yurchyshyn, V.; Zhang, J. Bibcode: 2019shin.confE.194P Altcode: Our thinking about the pre-eruptive magnetic configuration of Coronal Mass Ejections has been effectively dichotomized into two opposing and often fiercely contested views: namely, sheared magnetic arcades and magnetic flux ropes. Finding a solution to this issue will have important implications for our understanding of CME initiation. We first discuss the very value of embarking into the arcade vs. flux rope dilemma and illustrate the corresponding challenges and difficulties to address it. Next, we are compiling several observational diagnostics of pre-eruptive sheared magnetic arcades stemming from theory/modeling, discuss their merits, and highlight potential ambiguities that could arise in their interpretation. We finally conclude with a discussion of possible new observables, in the frame of upcoming or proposed instrumentation, that could help to circumvent the issues we are currently facing. Title: Genesis, magnetic morphology and impulsive evolution of the coronal mass ejection associated with the X8.2 flare on 2017 September 10 Authors: Veronig, Astrid; Podladchikova, Tatiana; Dissauer, Karin; Temmer, Manuela; Seaton, Daniel; Long, David; Guo, Jingnan; Vrsnak, Bojan; Harra, Louise; Kliem, Bernhard Bibcode: 2019EGUGA..21.9243V Altcode: The extreme X8.2 event of 2017 September 10 provides unique observations to study the genesis, magnetic morphology, impulsive dynamics and shock formation in a very fast coronal mass ejection (CME). Combining GOES-16/SUVI and SDO/AIA EUV imagery, we identify a hot (T ≈ 10-15 MK) bright rim around a quickly expanding cavity, embedded inside a much larger CME shell (T ≈ 1-2 MK). The CME shell develops from a dense set of large AR loops (>0.5Rs) 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 shock 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 up to 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. Title: The Birth of a Coronal Mass Ejection Authors: Liu, Rui; Gou, Tingyu; Kliem, Bernhard; Wang, Yuming; Veronig, Astrid Bibcode: 2019EGUGA..21.5194L Altcode: The Sun's atmosphere is frequently disrupted by coronal mass ejections (CMEs), coupled with flares and energetic particles. In the standard picture, the coupling is explained by magnetic reconnection at a vertical current sheet connecting the flare loops and the CME, with the latter embedding a helical magnetic structure known as flux rope. As it jumps upward due to instabilities or loss of equilibrium, the flux rope stretches the overlying coronal loops so that oppositely directed field is brought together underneath, creating the current sheet. However, both the origin of flux ropes and their nascent paths toward eruption remain elusive. Here we present an observation of how a stellar-sized CME bubble evolves continuously from plasmoids, mini flux ropes that are barely resolved, within half an hour. The eruption initiates when plasmoids springing from a vertical current sheet merge into a leading plasmoid occupying the upper tip of the current sheet. Rising at increasing speed to stretch the overlying loops, this leading plasmoid then expands impulsively into the CME bubble, in tandem with hard X-ray bursts. This observation illuminates for the first time a complete CME evolutionary path that has the capacity to accommodate a wide variety of plasma phenomena by bridging the gap between micro-scale dynamics and macro-scale activities. Title: The Birth of A Coronal Mass Ejection Authors: Gou, Tingyu; Liu, Rui; Kliem, Bernhard; Wang, Yuming; Veronig, Astrid M. Bibcode: 2019SciA....5.7004G Altcode: 2018arXiv181104707G The Sun's atmosphere is frequently disrupted by coronal mass ejections (CMEs), coupled with flares and energetic particles. The coupling is usually attributed to magnetic reconnection at a vertical current sheet connecting the flare and CME, with the latter embedding a helical magnetic structure known as flux rope. However, both the origin of flux ropes and their nascent paths toward eruption remain elusive. Here, we present an observation of how a stellar-sized CME bubble evolves continuously from plasmoids, mini flux ropes that are barely resolved, within half an hour. The eruption initiates when plasmoids springing from a vertical current sheet merge into a leading plasmoid, which rises at increasing speeds and expands impulsively into the CME bubble, producing hard x-ray bursts simultaneously. This observation illuminates a complete CME evolutionary path capable of accommodating a wide variety of plasma phenomena by bridging the gap between microscale and macroscale dynamics. Title: Genesis and Impulsive Evolution of the 2017 September 10 Coronal Mass Ejection Authors: Veronig, Astrid M.; Podladchikova, Tatiana; Dissauer, Karin; Temmer, Manuela; Seaton, Daniel B.; Long, David; Guo, Jingnan; Vršnak, Bojan; Harra, Louise; Kliem, Bernhard Bibcode: 2018ApJ...868..107V Altcode: 2018arXiv181009320V 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 ≈ 10-15 MK) bright rim around a quickly expanding cavity, embedded inside a much larger CME shell (T ≈ 1-2 MK). The CME shell develops from a dense set of large AR loops (≳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. Title: Evolution of flux rope, CME and associated EUV wave in the 10-Sep-2018 X8.2 event Authors: Podladchikova, Tatiana; Veronig, Astrid M.; Dissauer, Karin; Temmer, Manuela; Seaton, Daniel B.; Long, David; Guo, Jingnan; Vršnak, Bojan; Harra, Louise; Kliem, Bernhard Bibcode: 2018csc..confE..38P Altcode: We combine the high-cadence and large field-of-view EUV imagery of the Atmospheric Imaging Assembly (AIA) onboard SDO and the Solar Ultraviolet Imager (SUVI) onboard GOES-16 to study the origin and impulsive evolution of the fast CME that originated in the September 10th 2017 X8.2 event as well as the initiation of the associated EUV wave. In the LASCO field-of-view, the CME reveals speeds >3000 km/s. In the low-to-mid corona, it shows a distinct bubble in the EUV imagery that reveals a significant lateral overexpansion. In addition, is also shows a distinct expanding cavity that is interpreted as manifestation of the flux rope driving the eruption. We present a method to automatically identify and segment the CME bubble in SUVI images and to derive its radial and lateral evolution up to about 2 solar radii, in terms of velocity and acceleration. These measurements are set into context with the evolution of the embedded flux rope/cavity observed by AIA. The observations show clear signatures of new poloidal flux added to the flux rope by magnetic reconnection in the current sheet beneath the eruptive structure, which is important for the high accelerations observed in this event. The radial propagation of the CME shell revealed a peak value of the acceleration of about 5.3 km/s2, whereas the lateral expansion reached a peak value of 10.1 km/s2, which is the largest value reported so far. The flux rope/cavity reveals a radial acceleration of 6.7 km/s2 and lateral acceleration of 5.3 km/s2. We note that at this early evolution phase, the speed of the cavity/flux rope is higher than that of the CME bubble (front). The EUV wave associated with this eruption was observed by AIA, SUVI and STEREO-A EUVI, which had a separation angle with Earth of 128°, and the common field of view of the spacecraft was 52°. AIA and SUVI images above the solar limb reveal the initiation of the EUV wave by the accelerating flanks of the CME bubble, followed by detachment and propagation of the wave with a speed of 1100 km/s. The EUV wave shows a global propagation over the full hemisphere visible to Earth view as well as into the STEREO-A field-of-view. We study the propagation and kinematics of the direct as well as the various reflected and refracted EUV wave components on the solar sphere, finding speeds in the range from 370 to 1010 km/s. Finally, we note that this EUV wave is also distinct as it reveals propagation and transmission through the polar coronal holes. Title: Data-constrained simulation of a Double-decker Eruption Authors: Savcheva, Antonia; Kliem, B.; Downs, Copper; Torok, Tibor Bibcode: 2018shin.confE..91S Altcode: We present the challenges we encountered in producing the initial condition for the hypnotized double-decker flux rope eruption on 12/07/12. We then use this I.C. in Kliem-Torok zero-beta MHD simulation to produce an eruption and reproduce overall magnetic field structure of the eruption. We also produce a full thermodynamic simulation with MAS of a simpler IC which also produces similarities to the observed ribbons and dimmings. Both approaches have their advantages. Title: Unambiguous Evidence of Filament Splitting-Induced Partial Eruptions Authors: Cheng, Xin; Kliem, Bernhard Bibcode: 2018cosp...42E.619C Altcode: Coronal mass ejections are usually believed to be produced by the full eruption of a magnetic flux rope (MFR). However, it is recently recognised that the MFR may only release part of flux during once eruption, leading to a partial eruption. Here, we investigate two partial eruption events, the first one appears as a failed filament eruption, while the second manifests as a successful hot channel eruption. Thanks to the high resolution, high cadence, and multi-temperature capability of the Atmospheric Imaging Assembly, it is found that both partial eruptions are a consequence of the splitting of an intact MFR-filament system caused by the internal reconnection. For the failed one, the internal reconnection, most likely induced by the rising and writhing motions of the filament, initially appears as EUV brightenings in the middle of the filament. Quickly, it evolves to the current sheet reconnection in the wake of the eruption of the upper filament. For the successful one, the internal reconnection initiates prior to the eruption, which is also evidenced by obvious heating in the middle of the filament. As the internal reconnection proceeds to the current sheet reconnection, the filament is split into two; the upper flux escapes as an erupting hot channel. As a result, regardless of being failed or successful, both of them produce hard X-ray sources and cusp-shaped loops below the erupting flux but above the surviving flux, as well as two ribbons at two sides of the latter. Title: Magnetic Structure of a Composite Solar Microwave Burst Authors: Lee, Jeongwoo; White, Stephen M.; Liu, Chang; Kliem, Bernhard; Masuda, Satoshi Bibcode: 2018ApJ...856...70L Altcode: A composite flare consisting of an impulsive flare SOL2015-06-21T01:42 (GOES class M2.0) and a more gradual, long-duration flare SOL2015-06-21T02:36 (M2.6) from NOAA Active Region 12371, is studied using observations with the Nobeyama Radioheliograph (NoRH) and the Solar Dynamics Observatory (SDO). While composite flares are defined by their characteristic time profiles, in this paper we present imaging observations that demonstrate the spatial relationship of the two flares and allow us to address the nature of the evolution of a composite event. The NoRH maps show that the first flare is confined not only in time, but also in space, as evidenced by the stagnation of ribbon separation and the stationarity of the microwave source. The NoRH also detected another microwave source during the second flare, emerging from a different location where thermal plasma is so depleted that accelerated electrons could survive longer against Coulomb collisional loss. The AIA 131 Å images show that a sigmoidal EUV hot channel developed after the first flare and erupted before the second flare. We suggest that this eruption removed the high-lying flux to let the separatrix dome underneath reconnect with neighboring flux and the second microwave burst follow. This scenario explains how the first microwave burst is related to the much-delayed second microwave burst in this composite event. Title: Unambiguous Evidence of Filament Splitting-induced Partial Eruptions Authors: Cheng, X.; Kliem, B.; Ding, M. D. Bibcode: 2018ApJ...856...48C Altcode: 2018arXiv180204932C 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. Title: Magnetic Source Region Characteristics Influencing the Velocity of Solar Eruptions in the Corona Authors: Kliem, B.; Chintzoglou, G.; Torok, T.; Zhang, J.; Downs, C. Bibcode: 2016AGUFMSH13B2292K Altcode: The velocity of coronal mass ejections (CMEs) is one of the primary parameters determining their potential geoeffectiveness. The great majority of very fast CMEs receive their main acceleration already in the corona. We study the magnetic source region structure for a complete sample of 15 very fast CMEs (v > 1500 km/s) during 2000-2006, originating within 30 deg from central meridian and find a correlation between CME speed and the decay index profile of the coronal field estimated by a PFSS extrapolation. Such a correlation is not found for a comparison sample of slower CMEs. We also study how the decay index profile is related to the structure of the photospheric field distribution. This is complemented by a parametric simulation study of flux rope eruptions using the analytic Titov-Demoulin active-region model for simple bipolar and quadrupolar source regions, which provide simple relationships between the photospheric field distribution and the coronal decay index profile. Very fast, moderate-velocity, and even confined eruptions are found. Detailed, data-constrained MHD modeling of a very fast and a relatively slow CME, including a comparison of their source region characteristics, will also be presented. Support by NSF and NASA's LWS program is acknowledged. Title: Helical Kink Instability in a Confined Solar Eruption Authors: Hassanin, Alshaimaa; Kliem, Bernhard Bibcode: 2016ApJ...832..106H Altcode: 2016arXiv160900673H A model for strongly writhing confined solar eruptions suggests an origin in the helical kink instability of a coronal flux rope that remains stable against the torus instability. This model is tested against the well observed filament eruption on 2002 May 27 in a parametric MHD simulation study that comprises all phases of the event. Good agreement with the essential observed properties is obtained. These include the confinement, terminal height, writhing, distortion, and dissolution of the filament, and the flare loops. The agreement is robust against variations in a representative range of parameter space. Careful comparisons with the observation data constrain the ratio of the external toroidal and poloidal field components to {B}{et}/{B}{ep}≈ 1 and the initial flux rope twist to {{Φ }}≈ 4π . Different from ejective eruptions, two distinct phases of strong magnetic reconnection can occur. First, the erupting flux is cut by reconnection with overlying flux in the helical current sheet formed by the instability. If the resulting flux bundles are linked as a consequence of the erupting rope’s strong writhing, they subsequently reconnect in the vertical current sheet between them. This reforms the overlying flux and a far less twisted flux rope, offering a pathway to homologous eruptions. Title: Magnetic Structure of Solar Eruptions: Flux Rope Formation, Instability, Confinement, and Ejection Authors: Kliem, B. Bibcode: 2016AGUFMSH12B..01K Altcode: Recent observations of sigmoids and filaments and the associated modeling of a nonlinear force-free coronal field increasingly suggest the formation of a magnetic flux rope prior to the onset of an eruption, albeit the degree of coherence, amount of twist, and exact timing of the rope are not yet well known. Torus instability of a flux rope, equivalent to the well known catastrophe and flux imbalance descriptions, appears to be the mechanism of destabilization in the majority of eruptions. Although the three-halves onset condition for the field's decay index is a good quantitative guideline, its parametric dependence requires further study. The helical kink explains the onset of some strongly writhing eruptions, and, in particular, provides a pathway to confined eruptions, which often writhe strongly. However, many confined eruptions do not indicate the occurrence of a helical kink, and magnetic environments that allow us to explain them solely in terms of the torus instability are not yet well established. This overview will also stress that some of the parameters determining the geoeffectiveness of eruptions - their speed and magnetic orientation - may be largely set already in the low corona. Finally, the expansion of unstable flux ropes can be impulsive, explaining the triggering of EUV waves without the recently suggested formation of a blast wave, which we do not expect in low-beta plasma. Title: Helical kink instability in the confined solar eruption on 2002 May 27 Authors: Hassanin, A.; Kliem, B.; Seehafer, N. Bibcode: 2016AN....337.1082H Altcode: 2016arXiv161101008H This paper presents an improved MHD modeling of the confined filament eruption in solar active region NOAA 9957 on 2002 May 27 by extending the parametric studies of the event in \citet{Torok&Kliem2005} and \citet{Hassanin&Kliem2016}. Here the initial flux rope equilibrium is chosen to possess a small apex height identical to the observed initial filament height, which implies a more realistic inclusion of the photospheric line tying. The model matches the observations as closely as in the preceding studies, with the closest agreement again being obtained for an initial average flux rope twist of about 4π. Thus, the model for strongly writhing confined solar eruptions, which assumes that a kink-unstable flux rope in the stability domain of the torus instability exists at the onset of the eruption's main acceleration phase, is further substantiated. Title: Observing the release of twist by magnetic reconnection in a solar filament eruption Authors: Xue, Zhike; Yan, Xiaoli; Cheng, Xin; Yang, Liheng; Su, Yingna; Kliem, Bernhard; Zhang, Jun; Liu, Zhong; Bi, Yi; Xiang, Yongyuan; Yang, Kai; Zhao, Li Bibcode: 2016NatCo...711837X Altcode: Magnetic reconnection is a fundamental process of topology change and energy release, taking place in plasmas on the Sun, in space, in astrophysical objects and in the laboratory. However, observational evidence has been relatively rare and typically only partial. Here we present evidence of fast reconnection in a solar filament eruption using high-resolution H-alpha images from the New Vacuum Solar Telescope, supplemented by extreme ultraviolet observations. The reconnection is seen to occur between a set of ambient chromospheric fibrils and the filament itself. This allows for the relaxation of magnetic tension in the filament by an untwisting motion, demonstrating a flux rope structure. The topology change and untwisting are also found through nonlinear force-free field modelling of the active region in combination with magnetohydrodynamic simulation. These results demonstrate a new role for reconnection in solar eruptions: the release of magnetic twist. Title: Structure, Stability, and Evolution of Magnetic Flux Ropes from the Perspective of Magnetic Twist Authors: Liu, Rui; Kliem, Bernhard; Titov, Viacheslav S.; Chen, Jun; Wang, Yuming; Wang, Haimin; Liu, Chang; Xu, Yan; Wiegelmann, Thomas Bibcode: 2016ApJ...818..148L Altcode: 2015arXiv151202338L 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 {{ T }}w for each individual field line. The MFR is moderately twisted (| {{ T }}w| \lt 2) and has a well-defined boundary of high squashing factor Q. We found that the field line with the extremum | {{ T }}w| is a reliable proxy of the rope axis, and that the MFR's peak | {{ 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 | {{ 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 {{ 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. Title: Erratum: ``Slow Rise and Partial Eruption of a Double-decker Filament. I Observations and Interpretation'(2012, ApJ, 756, 59) Authors: Liu, Rui; Kliem, Bernhard; Török, Tibor; Liu, Chang; Titov, Viacheslav S.; Lionello, Roberto; Linker, Jon A.; Wang, Haimin Bibcode: 2015ApJ...814..164L Altcode: No abstract at ADS Title: Triggering an Eruptive Flare by Emerging Flux in a Solar Active-Region Complex Authors: Louis, Rohan E.; Kliem, Bernhard; Ravindra, B.; Chintzoglou, Georgios Bibcode: 2015SoPh..290.3641L Altcode: 2015arXiv150608035L; 2015SoPh..tmp...81L A flare and fast coronal mass ejection originated between solar active regions NOAA 11514 and 11515 on 2012 July 1 (SOL2012-07-01) in response to flux emergence in front of the leading sunspot of the trailing region 11515. Analyzing the evolution of the photospheric magnetic flux and the coronal structure, we find that the flux emergence triggered the eruption by interaction with overlying flux in a non-standard way. The new flux neither had the opposite orientation nor a location near the polarity inversion line, which are favorable for strong reconnection with the arcade flux under which it emerged. Moreover, its flux content remained significantly smaller than that of the arcade (≈40 % ). However, a loop system rooted in the trailing active region ran in part under the arcade between the active regions, passing over the site of flux emergence. The reconnection with the emerging flux, leading to a series of jet emissions into the loop system, caused a strong but confined rise of the loop system. This lifted the arcade between the two active regions, weakening its downward tension force and thus destabilizing the considerably sheared flux under the arcade. The complex event was also associated with supporting precursor activity in an enhanced network near the active regions, acting on the large-scale overlying flux, and with two simultaneous confined flares within the active regions. Title: Structure, Stability and Evolution of a Magnetic Flux Rope Authors: Liu, R.; Kliem, B.; Titov, V. S.; Chen, J.; Wang, Y.; Wang, H.; Liu, C.; Xu, Y.; Wiegelmann, T. Bibcode: 2015AGUFMSH13D2457L Altcode: We investigate the evolution of NOAA Active Region 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. With the aid of nonlinear force-free field modeling, we identify the MFR through mapping magnetic connectivities and computing the twist number Tw for each individual field line. The MFR is moderately twisted (Tw < 2) and has a well-defined boundary of high squashing factor Q. Its axis coincides with the field line with the peak Tw in the rope. We find that the MFR's peak Tw 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 Tw has little effect on the active region'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 free magnetic energy due to the flare. We suggest that Tw may serve as a useful parameter in prewarning 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. Title: The Origin of Net Electric Currents in Solar Active Regions Authors: Dalmasse, K.; Aulanier, G.; Démoulin, P.; Kliem, B.; Török, T.; Pariat, E. Bibcode: 2015ApJ...810...17D Altcode: 2015arXiv150705060D 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. Title: Electric current neutralization in solar active regions Authors: Dalmasse, Kévin; Aulanier, Guillaume; Török, Tibor; Démoulin, Pascal; Pariat, Etienne; Kliem, Bernhard Bibcode: 2015TESS....111303D Altcode: There is a recurring question in solar physics of whether or not photospheric vertical electric currents are neutralized in solar active regions, i.e., whether or not the total electric current integrated over a single magnetic polarity of an active region vanishes. While different arguments have been proposed in favor of, or against, the neutralization of electric currents, both theory and observations are still not fully conclusive. Providing the answer to this question is crucial for theoretical models of solar eruptions. Indeed, if currents are neutralized in active regions, then any eruption model based on net - i.e., non-zero - electric currents, such as the torus instability, requires further consideration. We address the question of electric current neutralization in active regions using 3D zero-beta MHD simulations of line-tied, slow photospheric driving motions imposed on an initially potential magnetic field. We compare our results to a recent study of the build-up of coronal electric currents in an MHD simulation of the emergence of a current-neutralized twisted flux tube into the solar atmosphere. Our parametric study shows that, in accordance with the flux emergence simulation, photospheric motions are associated with the formation of both direct and return currents. It further shows that both processes (flux emergence and photospheric flows) can lead to the formation of strong net currents in the solar corona, and that the non-neutralization of electric currents is related to the presence of magnetic shear at the polarity inversion line. We discuss the implications of our results for the observations and for theoretical models of solar eruptions. Title: Fast Magnetic Reconnection in the Solar Chromosphere Mediated by the Plasmoid Instability Authors: Ni, Lei; Kliem, Bernhard; Lin, Jun; Wu, Ning Bibcode: 2015ApJ...799...79N Altcode: 2015arXiv150906895N 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 ~106-107 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 Alfvén velocity in the inflow region, reaches values in the range ~0.01-0.03 throughout the cascading plasmoid formation and for zero as well as for strong guide field. The outflow velocity reaches ≈40 km s-1. Slow-mode shocks extend from the X-points, heating the plasmoids up to ~8 × 104 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 ~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. Title: Slow Rise and Partial Eruption of a Double-decker Filament. II. A Double Flux Rope Model Authors: Kliem, Bernhard; Török, Tibor; Titov, Viacheslav S.; Lionello, Roberto; Linker, Jon A.; Liu, Rui; Liu, Chang; Wang, Haimin Bibcode: 2014ApJ...792..107K Altcode: 2014arXiv1407.2272K 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 & Démoulin 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. Title: Formation of a Double-decker Magnetic Flux Rope in the Sigmoidal Solar Active Region 11520 Authors: Cheng, X.; Ding, M. D.; Zhang, J.; Sun, X. D.; Guo, Y.; Wang, Y. M.; Kliem, B.; Deng, Y. Y. Bibcode: 2014ApJ...789...93C Altcode: 2014arXiv1405.4923C In this paper, we address the formation of a magnetic flux rope (MFR) that erupted on 2012 July 12 and caused a strong geomagnetic storm event on July 15. Through analyzing the long-term evolution of the associated active region observed by the Atmospheric Imaging Assembly and the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory, it is found that the twisted field of an MFR, indicated by a continuous S-shaped sigmoid, is built up from two groups of sheared arcades near the main polarity inversion line a half day before the eruption. The temperature within the twisted field and sheared arcades is higher than that of the ambient volume, suggesting that magnetic reconnection most likely works there. The driver behind the reconnection is attributed to shearing and converging motions at magnetic footpoints with velocities in the range of 0.1-0.6 km s-1. The rotation of the preceding sunspot also contributes to the MFR buildup. Extrapolated three-dimensional non-linear force-free field structures further reveal the locations of the reconnection to be in a bald-patch region and in a hyperbolic flux tube. About 2 hr before the eruption, indications of a second MFR in the form of an S-shaped hot channel are seen. It lies above the original MFR that continuously exists and includes a filament. The whole structure thus makes up a stable double-decker MFR system for hours prior to the eruption. Eventually, after entering the domain of instability, the high-lying MFR impulsively erupts to generate a fast coronal mass ejection and X-class flare; while the low-lying MFR remains behind and continuously maintains the sigmoidicity of the active region. Title: Catastrophe versus Instability for the Eruption of a Toroidal Solar Magnetic Flux Rope Authors: Kliem, B.; Lin, J.; Forbes, T. G.; Priest, E. R.; Török, T. Bibcode: 2014ApJ...789...46K Altcode: 2014arXiv1404.5922K 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. Title: Rapid CME Cavity Formation and Expansion Authors: Kliem, Bernhard; Forbes, Terry G.; Patsourakos, Spiros; Vourlidas, Angelos Bibcode: 2014AAS...22421206K Altcode: A cavity is supposed to be a general feature of well-developed CMEs at the stage they can be imaged by white-light coronagraphs (in the outer corona and solar wind). The cavity is interpreted as the cross section of the CME flux rope in the plane of sky. Preexisting cavities are observed around some quiescent erupting prominences, but usually not in active regions. Observations of CME cavities in the inner corona, where most of them appear to form, have become possible only with the STEREO and SDO missions. These reveal a very rapid formation and expansion of "EUV cavities" in fast and impulsively commencing eruptions early in the phase of main CME acceleration and impulsive flare rise. Different from the white-light observations, the EUV cavity initially appears to be larger than the CME flux rope. However, it evolves into the white-light cavity subsequently. MHD simulations of flux rope eruptions conform to this picture of initially larger cavity but subsequently approaching cavity and flux rope size. The initial expansion of ambient flux can be understood as a "reverse pinch effect", driven by decreasing flux rope current as the rope rises. Title: The evolution of writhe in kink-unstable flux ropes and erupting filaments Authors: Török, T.; Kliem, B.; Berger, M. A.; Linton, M. G.; Démoulin, P.; van Driel-Gesztelyi, L. Bibcode: 2014PPCF...56f4012T Altcode: 2014arXiv1403.1565T 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 ∼6π for the majority of filaments prior to their eruption. Title: Distribution of electric currents in source regions of solar eruptions Authors: Torok, Tibor; Leake, James E.; Titov, Viacheslav; Archontis, Vasilis; Mikic, Zoran; Linton, Mark; Dalmasse, Kevin; Aulanier, Guillaume; Kliem, Bernhard Bibcode: 2014AAS...22431202T Altcode: There has been a long-lasting debate on the question of whether or not electric currents in the source regions of solar eruptions are neutralized. That is, whether or not the direct coronal currents connecting the photospheric polarities in such regions are surrounded by return currents of equal amount and opposite direction. In order to address this question, we consider several mechanisms of source region formation (flux emergence, photospheric shearing/twisting flows, and flux cancellation) and quantify the evolution of the electric currents, using 3D MHD simulations. For the experiments conducted so far, we find a clear dominance of the direct currents over the return currents in all cases in which the models produce significant magnetic shear along the source region's polarity inversion line. This suggests that pre-eruptive magnetic configurations in strongly sheared active regions and filament channels carry substantial net currents. We discuss the implications of this result for the modeling of solar eruptions. Title: Coronal Magnetic Reconnection Driven by CME Expansion—the 2011 June 7 Event Authors: van Driel-Gesztelyi, L.; Baker, D.; Török, T.; Pariat, E.; Green, L. M.; Williams, D. R.; Carlyle, J.; Valori, G.; Démoulin, P.; Kliem, B.; Long, D. M.; Matthews, S. A.; Malherbe, J. -M. Bibcode: 2014ApJ...788...85V Altcode: 2014arXiv1406.3153V 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. Title: Distribution of Electric Currents in Solar Active Regions Authors: Török, T.; Leake, J. E.; Titov, V. S.; Archontis, V.; Mikić, Z.; Linton, M. G.; Dalmasse, K.; Aulanier, G.; Kliem, B. Bibcode: 2014ApJ...782L..10T Altcode: 2014arXiv1401.2931T There has been a long-standing debate on the question of whether or not electric currents in solar active regions are neutralized. That is, whether or not the main (or direct) coronal currents connecting the active region polarities are surrounded by shielding (or return) currents of equal total value and opposite direction. Both theory and observations are not yet fully conclusive regarding this question, and numerical simulations have, surprisingly, barely been used to address it. Here we quantify the evolution of electric currents during the formation of a bipolar active region by considering a three-dimensional magnetohydrodynamic simulation of the emergence of a sub-photospheric, current-neutralized magnetic flux rope into the solar atmosphere. We find that a strong deviation from current neutralization develops simultaneously with the onset of significant flux emergence into the corona, accompanied by the development of substantial magnetic shear along the active region's polarity inversion line. After the region has formed and flux emergence has ceased, the strong magnetic fields in the region's center are connected solely by direct currents, and the total direct current is several times larger than the total return current. These results suggest that active regions, the main sources of coronal mass ejections and flares, are born with substantial net currents, in agreement with recent observations. Furthermore, they support eruption models that employ pre-eruption magnetic fields containing such currents. Title: Sunspot splitting triggering an eruptive flare Authors: Louis, Rohan E.; Puschmann, Klaus G.; Kliem, Bernhard; Balthasar, Horst; Denker, Carsten Bibcode: 2014A&A...562A.110L Altcode: 2013arXiv1311.5054L
Aims: We investigate how the splitting of the leading sunspot and associated flux emergence and cancellation in active region NOAA 11515 caused an eruptive M5.6 flare on 2012 July 2.
Methods: Continuum intensity, line-of-sight magnetogram, and dopplergram data of the Helioseismic and Magnetic Imager were employed to analyse the photospheric evolution. Filtergrams in Hα and He I 10830 Å of the Chromospheric Telescope at the Observatorio del Teide, Tenerife, track the evolution of the flare. The corresponding coronal conditions were derived from 171 Å and 304 Å images of the Atmospheric Imaging Assembly. Local correlation tracking was utilized to determine shear flows.
Results: Emerging flux formed a neutral line ahead of the leading sunspot and new satellite spots. The sunspot splitting caused a long-lasting flow towards this neutral line, where a filament formed. Further flux emergence, partly of mixed polarity, as well as episodes of flux cancellation occurred repeatedly at the neutral line. Following a nearby C-class precursor flare with signs of interaction with the filament, the filament erupted nearly simultaneously with the onset of the M5.6 flare and evolved into a coronal mass ejection. The sunspot stretched without forming a light bridge, splitting unusually fast (within about a day, complete ≈6 h after the eruption) in two nearly equal parts. The front part separated strongly from the active region to approach the neighbouring active region where all its coronal magnetic connections were rooted. It also rotated rapidly (by 4.9° h-1) and caused significant shear flows at its edge.
Conclusions: The eruption resulted from a complex sequence of processes in the (sub-)photosphere and corona. The persistent flows towards the neutral line likely caused the formation of a flux rope that held the filament. These flows, their associated flux cancellation, the emerging flux, and the precursor flare all contributed to the destabilization of the flux rope. We interpret the sunspot splitting as the separation of two flux bundles differently rooted in the convection zone and only temporarily joined in the spot. This explains the rotation as the continued rise of the separating flux, and it implies that at least this part of the sunspot was still connected to its roots deep in the convection zone.

Movie available in electronic form at http://www.aanda.org Title: Fast magnetic reconnection in solar chromosphere with radiation cooling and ambipolar diffusion Authors: Ni, Lei; Lin, Jun; Kliem, Bernhard Bibcode: 2014cosp...40E2270N Altcode: Magnetic reconnection in solar chromosphere with radiation cooling and ambipolar diffusion is studied through 2.5-dimensional numerical MHD simulations employing a force-free Harris current sheet. Although} the magnetic Reynolds number is high ( 10 (5) -10 (6) ), fast magnetic reconnection still can happen because of multiple levels of the plasmoid instability occurring in the current sheet. As secondary instabilities start to appear, slow-mode shocks are detected between secondary plasmoids and secondary fragments of the current sheet. The temperature apparently increases around the shock front. Our numerical simulations also indicate that the radiation cooling can strongly accelerate magnetic reconnection for the case with weak guide field. The radiation cooling can make the secondary plasmoids appear earlier and the maximum current density at the main X-point reach a much higher value than in the case without radiation cooling. For the case with strong guide field, the radiation cooling becomes less important during reconnection process. By analyze our numerical results, we find that the ambipolar diffusion around the middle of the chromosphere can also increases the recoonection rate. However, the higher orders of plasmoid instabilities dominate eventually. The upward out-flow velocity in our simulations approaches the velocity of chromospheric jets, with the maximum velocity reaching 40 km s(-1) . Title: Electric currents in solar active regions Authors: Dalmasse, Kévin; Pariat, Etienne; Kliem, Bernhard; Aulanier, Guillaume; Demoulin, Pascal; Torok, Tibor Bibcode: 2014cosp...40E.613D Altcode: There is a recurring question in solar physics about whether or not photospheric vertical electric currents are neutralized in solar active regions, i.e. if the total electric current integrated over a single photospheric magnetic polarity of an active region vanishes. Different arguments have been proposed in favor of, or against, the neutralization of electric currents, but both theory and observations are still not fully conclusive. The answer to this question has implications for eruption models. Indeed, if currents are neutralized in active regions, then any eruption model based on non-neutralized electric currents, such as the torus instability, would need to be further analyzed. We addressed the question of electric currents neutralization in active regions using 3D zero-beta, line-tied, slow driving motions of an initially potential magnetic field. We compared our results to a recent study of electric currents build-up in a MHD numerical simulation of the emergence of a current-neutralized twisted flux tube. Our parametric analyses show that, as for the emergence, photospheric motions are associated with the formation of both direct and return currents. It further shows that both processes can lead to the formation of strong net currents in the solar corona, and that the non-neutralization of electric currents is related to the presence of magnetic shear at the polarity inversion line. We will discuss the implications of our results for the observations and for the different solar eruption models. Title: Observations of flux rope formation prior to coronal mass ejections Authors: Green, Lucie M.; Kliem, Bernhard Bibcode: 2014IAUS..300..209G Altcode: 2013arXiv1312.4388G Understanding the magnetic configuration of the source regions of coronal mass ejections (CMEs) is vital in order to determine the trigger and driver of these events. Observations of four CME productive active regions are presented here, which indicate that the pre-eruption magnetic configuration is that of a magnetic flux rope. The flux ropes are formed in the solar atmosphere by the process known as flux cancellation and are stable for several hours before the eruption. The observations also indicate that the magnetic structure that erupts is not the entire flux rope as initially formed, raising the question of whether the flux rope is able to undergo a partial eruption or whether it undergoes a transition in specific flux rope configuration shortly before the CME. Title: Magnetic reconnection driven by filament eruption in the 7 June 2011 event Authors: van Driel-Gesztelyi, L.; Baker, D.; Török, T.; Pariat, E.; Green, L. M.; Williams, D. R.; Carlyle, J.; Valori, G.; Démoulin, P.; Matthews, S. A.; Kliem, B.; Malherbe, J. -M. Bibcode: 2014IAUS..300..502V Altcode: During an unusually massive filament eruption on 7 June 2011, SDO/AIA imaged for the first time significant EUV emission around a magnetic reconnection region in the solar corona. The reconnection occurred between magnetic fields of the laterally expanding CME and a neighbouring active region. A pre-existing quasi-separatrix layer was activated in the process. This scenario is supported by data-constrained numerical simulations of the eruption. Observations show that dense cool filament plasma was re-directed and heated in situ, producing coronal-temperature emission around the reconnection region. These results provide the first direct observational evidence, supported by MHD simulations and magnetic modelling, that a large-scale re-configuration of the coronal magnetic field takes place during solar eruptions via the process of magnetic reconnection. Title: Magnetohydrodynamic Modeling of the Solar Eruption on 2010 April 8 Authors: Kliem, B.; Su, Y. N.; van Ballegooijen, A. A.; DeLuca, E. E. Bibcode: 2013ApJ...779..129K Altcode: 2013arXiv1304.6981K 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. Title: Observation of a Moreton Wave and Wave-Filament Interactions Associated with the Renowned X9 Flare on 1990 May 24 Authors: Liu, Rui; Liu, Chang; Xu, Yan; Liu, Wei; Kliem, Bernhard; Wang, Haimin Bibcode: 2013ApJ...773..166L Altcode: Using Big Bear Solar Observatory film data recently digitized at NJIT, we investigate a Moreton wave associated with an X9 flare on 1990 May 24, as well as its interactions with four filaments F1-F4 located close to the flaring region. The interaction yields interesting insight into physical properties of both the wave and the filaments. The first clear Moreton wavefront appears at the flaring-region periphery at approximately the same time as the peak of a microwave burst and the first of two γ-ray peaks. The wavefront propagates at different speeds ranging from 1500-2600 km s-1 in different directions, reaching as far as 600 Mm away from the flaring site. Sequential chromospheric brightenings are observed ahead of the Moreton wavefront. A slower diffuse front at 300-600 km s-1 is observed to trail the fast Moreton wavefront about one minute after the onset. The Moreton wave decelerates to ~550 km s-1 as it sweeps through F1. The wave passage results in F1's oscillation which is featured by ~1 mHz signals with coherent Fourier phases over the filament, the activation of F3 and F4 followed by gradual recovery, but no disturbance in F2. Different height and magnetic environment together may account for the distinct responses of the filaments to the wave passage. The wavefront bulges at F4, whose spine is oriented perpendicular to the upcoming wavefront. The deformation of the wavefront is suggested to be due to both the forward inclination of the wavefront and the enhancement of the local Alfvén speed within the filament channel. Title: Numerical simulations of the CME on 2010 April 8 Authors: Su, Yingna; Kliem, Bernhard; van Ballegooijen, Adriaan; Deluca, Edward Bibcode: 2013IAUS..294..575S Altcode: We present 3D zero-beta ideal MHD simulations of the solar flare/CME event that occurred in Active Region 11060 on 2010 April 8. The initial magnetic configurations of the two simulations are stable nonlinear force-free field and unstable magnetic field models constructed by Su et al. (2011) using the flux rope insertion method. The MHD simulations confirm that the stable model relaxes to a stable equilibrium, while the unstable model erupts as a CME. Comparisons between observations and MHD simulations of the CME are also presented. Title: An MHD Model of a Solar Eruption Starting from NLFFF Initial Conditions Authors: DeLuca, Edward E.; Su, Y.; Kliem, B.; Van Ballegooijen, A. A. Bibcode: 2013SPD....4410301D Altcode: 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 stability and evolution of nonlinear force-free field (NLFFF) models. This talk uses data-constrained NLFFF models of a solar active region that erupted on 2010 Apri 8 as initial conditions in MHD simulations. These models, constructed with the techniques of flux rope insertion and magnetofrictional relaxation, include a stable, an approximately marginally stable, and an unstable configuration. The simulations confirm previous related results of magnetofrictional relaxation runs, in particular 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 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 active region. Title: MHD Modeling of the Solar Eruption on 2010 April 8 Authors: Kliem, B.; Su, Y.; Van Ballegooijen, A. A.; DeLuca, E. Bibcode: 2012AGUFMSH51A2194K Altcode: We present a numerical MHD study of the solar eruption on 2010 April 8, extending the previous modeling of the source region in Su et al. (2011) which had employed the flux rope insertion method and magnetofrictional relaxation. The threshold of the rope's axial flux for the loss of equilibrium obtained in Su et al. is confirmed. We find that the inserted flux rope partly splits for slightly subcritical axial flux. Starting with slightly supercritical axial flux in the rope, the MHD simulation yields a fast and strongly inclined eruption as observed by the STEREO and SDO instruments. The causes of the inclination will be explored. We also model photospheric changes that may have driven the flux rope from a stable to the unstable configuration. Title: CME Initiation and Evolution in the Corona Authors: Kliem, B. Bibcode: 2012AGUFMSH14A..01K Altcode: Evidence for the existence of a flux rope in the source volume of coronal mass ejections (CMEs) has been accumulating in recent years, supporting models of CME initiation that involve the loss of a flux rope equilibrium. Such models allow detailed, comparative studies of the various interactions of the CME flux rope with the coronal field, which will be reviewed. It has been demonstrated that the height profile of the overlying field strongly influences the CME speed. Important processes governing the magnetic orientation are the rotation of the CME flux rope by the coronal shear field component, the writhing of the rope by the helical kink mode, reconnection with complex ambient field, and eventually the alignment with the heliospheric current sheet. Interactions with coronal holes or nearby flux concentrations, the guidance by a helmet streamer, reconnection with the ambient field, and the curvature of the filament channel can cause considerable deviations of the CME path from the radial direction. Title: A Parametric Study of Erupting Flux Rope Rotation. Modeling the "Cartwheel CME" on 9 April 2008 Authors: Kliem, B.; Török, T.; Thompson, W. T. Bibcode: 2012SoPh..281..137K Altcode: 2011arXiv1112.3389K; 2012SoPh..tmp...91K The rotation of erupting filaments in the solar corona is addressed through a parametric simulation study of unstable, rotating flux ropes in bipolar force-free initial equilibrium. The Lorentz force due to the external shear-field component and the relaxation of tension in the twisted field are the major contributors to the rotation in this model, while reconnection with the ambient field is of minor importance, due to the field's simple structure. In the low-beta corona, the rotation is not guided by the changing orientation of the vertical field component's polarity inversion line with height. The model yields strong initial rotations which saturate in the corona and differ qualitatively from the profile of rotation vs. height obtained in a recent simulation of an eruption without preexisting flux rope. Both major mechanisms writhe the flux rope axis, converting part of the initial twist helicity, and produce rotation profiles which, to a large part, are very similar within a range of shear-twist combinations. A difference lies in the tendency of twist-driven rotation to saturate at lower heights than shear-driven rotation. For parameters characteristic of the source regions of erupting filaments and coronal mass ejections, the shear field is found to be the dominant origin of rotations in the corona and to be required if the rotation reaches angles of order 90 degrees and higher; it dominates even if the twist exceeds the threshold of the helical kink instability. The contributions by shear and twist to the total rotation can be disentangled in the analysis of observations if the rotation and rise profiles are simultaneously compared with model calculations. The resulting twist estimate allows one to judge whether the helical kink instability occurred. This is demonstrated for the erupting prominence in the "Cartwheel CME" on 9 April 2008, which has shown a rotation of ≈ 115∘ up to a height of 1.5 R⊙ above the photosphere. Out of a range of initial equilibria which include strongly kink-unstable (twist Φ=5π), weakly kink-unstable (Φ=3.5π), and kink-stable (Φ=2.5π) configurations, only the evolution of the weakly kink-unstable flux rope matches the observations in their entirety. Title: LEMUR: Large European module for solar Ultraviolet Research. European contribution to JAXA's Solar-C mission Authors: Teriaca, Luca; Andretta, Vincenzo; Auchère, Frédéric; Brown, Charles M.; Buchlin, Eric; Cauzzi, Gianna; Culhane, J. Len; Curdt, Werner; Davila, Joseph M.; Del Zanna, Giulio; Doschek, George A.; Fineschi, Silvano; Fludra, Andrzej; Gallagher, Peter T.; Green, Lucie; Harra, Louise K.; Imada, Shinsuke; Innes, Davina; Kliem, Bernhard; Korendyke, Clarence; Mariska, John T.; Martínez-Pillet, Valentin; Parenti, Susanna; Patsourakos, Spiros; Peter, Hardi; Poletto, Luca; Rutten, Robert J.; Schühle, Udo; Siemer, Martin; Shimizu, Toshifumi; Socas-Navarro, Hector; Solanki, Sami K.; Spadaro, Daniele; Trujillo-Bueno, Javier; Tsuneta, Saku; Dominguez, Santiago Vargas; Vial, Jean-Claude; Walsh, Robert; Warren, Harry P.; Wiegelmann, Thomas; Winter, Berend; Young, Peter Bibcode: 2012ExA....34..273T Altcode: 2011ExA...tmp..135T; 2011arXiv1109.4301T The solar outer atmosphere is an extremely dynamic environment characterized by the continuous interplay between the plasma and the magnetic field that generates and permeates it. Such interactions play a fundamental role in hugely diverse astrophysical systems, but occur at scales that cannot be studied outside the solar system. Understanding this complex system requires concerted, simultaneous solar observations from the visible to the vacuum ultraviolet (VUV) and soft X-rays, 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 near-infrared wavelengths. Simultaneous spectroscopic measurements sampling the entire temperature range are particularly important. These requirements are fulfilled by the Japanese Solar-C 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 near-infrared 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 high-resolution 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 focal-plane package composed of VUV spectrometers covering six carefully chosen wavelength ranges between 170 Å and 1270 Å. 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 - 1 or better. LEMUR has been proposed to ESA as the European contribution to the Solar C mission. Title: Slow Rise and Partial Eruption of a Double-decker Filament. I. Observations and Interpretation Authors: Liu, Rui; Kliem, Bernhard; Török, Tibor; Liu, Chang; Titov, Viacheslav S.; Lionello, Roberto; Linker, Jon A.; Wang, Haimin Bibcode: 2012ApJ...756...59L Altcode: 2012arXiv1207.1757L We study an active-region dextral filament that was composed of two branches separated in height by about 13 Mm, as inferred from three-dimensional reconstruction by combining SDO and STEREO-B observations. This "double-decker" configuration sustained for days before the upper branch erupted with a GOES-class M1.0 flare on 2010 August 7. Analyzing this evolution, we obtain the following main results. (1) During the hours before the eruption, filament threads within the lower branch were observed to intermittently brighten up, lift upward, and then merge with the upper branch. The merging process contributed magnetic flux and current to the upper branch, resulting in its quasi-static ascent. (2) This transfer might serve as the key mechanism for the upper branch to lose equilibrium by reaching the limiting flux that can be stably held down by the overlying field or by reaching the threshold of the torus instability. (3) The erupting branch first straightened from a reverse S shape that followed the polarity inversion line and then writhed into a forward S shape. This shows a transfer of left-handed helicity in a sequence of writhe-twist-writhe. The fact that the initial writhe is converted into the twist of the flux rope excludes the helical kink instability as the trigger process of the eruption, but supports the occurrence of the instability in the main phase, which is indeed indicated by the very strong writhing motion. (4) A hard X-ray sigmoid, likely of coronal origin, formed in the gap between the two original filament branches in the impulsive phase of the associated flare. This supports a model of transient sigmoids forming in the vertical flare current sheet. (5) Left-handed magnetic helicity is inferred for both branches of the dextral filament. (6) Two types of force-free magnetic configurations are compatible with the data, a double flux rope equilibrium and a single flux rope situated above a loop arcade. Title: Hinode Observations of an Eruption from a Sigmoidal Active Region Authors: Green, L. M.; Wallace, A. J.; Kliem, B. Bibcode: 2012ASPC..454..391G Altcode: We analyse the evolution of a bipolar active region which produces an eruption during its decay phase. The soft X-ray arcade develops high shear over a time span of two days and transitions to sigmoidal shortly before the eruption. We propose that the continuous sigmoidal soft X-ray threads indicate that a flux rope has formed which is lying low in the solar atmosphere with a bald patch separatrix surface topology. The formation of the flux rope is driven by the photospheric evolution which is dominated by fragmentation of the main polarities, motion due to supergranular flows and cancellation at the polarity inversion line. Title: Energy Storage and Release in the Solar Atmosphere - Key SDO Results Authors: Kliem, Bernhard Bibcode: 2012cosp...39..937K Altcode: 2012cosp.meet..937K The Solar Dynamics Observatory (SDO) has opened several new windows in solar observing. Energy release processes can now be studied in a very wide temperature range at high resolution, high cadence, and full-Sun field of view. In particular, the Atmospheric Imaging Assembly (AIA) simultaneously images plasmas from transition region up to flare temperatures (11 MK). This has stimulated a series of discoveries as well as verifications of key elements in energy release models. Only some of these can be reviewed. On the largest scales, the corona of a whole hemisphere was found to be magnetically coupled and involved in a sequence of eruption events. At the heart of such events lies a magnetic flux rope, and, for the first time, this structure has been directly imaged in the impulsive phase, and found to be very hot. The association of the rope with the flare current sheet was also verified. SDO allows us to observe a key process in reconnection physics, the breakup of current sheets into filamentary structures. Rapidly forming and expanding cavities provide insight into the genesis of coronal mass ejections and begin to reveal how coronal EUV waves and chromospheric Moreton waves are triggered. Indications for the triggering of two EUV wave fronts were found, possibly resolving the long-standing puzzle of different kinematic properties seen in such waves. At the smallest scales, the multi-temperature data support the hypothesis of nanoflare heating in coronal loops. The EUV Variability Experiment (EVE) data complete our view of the radiative flare energy, allow very detailed diagnostics of flare plasmas, and have enabled the discovery of a second, delayed peak in the EUV flare emission. A very intriguing sun quake was recently discovered in Helioseismic and Magnetic Imager (HMI) data of the first X-class flare of the current solar cycle. This quake challenges current trigger models based on particle precipitation, as it occurred remote from the strong hard X-ray sources. The first series of vector magnetograms from HMI, which is now released, is beginning to boost the studies of energy storage to a new level. These high-cadence, uniform-quality data show how the magnetic field and the current system evolve in space and time from flux emergence, via flux shearing, twisting and cancellation, to an X-class flare. Using nonlinear force-free field extrapolation, the evolution of the free magnetic energy and relative helicity up to the event can be quantified, and strong indications for the formation of a flux rope low in the corona prior to the eruption are found. Title: CME development in the corona and interplanetary medium : A multi-wavelength approach Authors: Pick, M.; Kliem, B. Bibcode: 2012EAS....55..299P Altcode: 2014arXiv1407.2271P This review focuses on the so called three-part CMEs which essentially represent the standard picture of a CME eruption. It is shown how the multi-wavelength observations obtained in the last decade, especially those with high cadence, have validated the early models and contributed to their evolution. These observations cover a broad spectral range including the EUV, white-light, and radio domains. Title: Observation & Modeling of An Erupting Double-Decker Filament Authors: Liu, Rui; Kliem, B.; Toeroek, T.; Liu, C.; Titov, V. S.; Lionello, R.; Linker, J. A.; Wang, H. Bibcode: 2012AAS...22032203L Altcode: We study an active-region dextral filament which was composed of two branches separated in height by about 13 Mm. This ``double-decker'' configuration sustained for days before the upper branch erupted on 2010 August 7. Main results are as follows. 1) During hours before the eruption, filament threads within the lower branch were observed to intermittently brighten up, lift upward, and then merge with the upper branch. The merging process contributed magnetic flux to the upper branch, resulting in its quasi-static ascent. 2) This flux transfer might serve as the key mechanism for the upper branch to lose equilibrium by reaching the limiting flux that can be stably held down by the overlying field or by reaching the threshold of the torus instability. 3) The erupting branch first straightened from a reverse S shape that followed the polarity

inversion line and then writhed into a forward S shape. This shows a transfer of left-handed helicity in a sequence of writhe-twist-writhe. The fact that the initial writhe is converted into the twist of the flux rope excludes the helical kink instability as the trigger process of the eruption, but allows for a role of the instability in the main phase. 4) A hard X-ray sigmoid, likely of coronal origin, formed in the gap between the two original filament branches in the impulsive phase of the associated flare. This supports a model of transient sigmoids forming in the vertical flare current sheet. 5) Using MHD modeling, we demonstrate that a configuration with two force-free flux ropes of like handedness can form in the slow-rise phase before an eruption and that it admits stable equilibria as well as the instability of only the upper rope. Title: 3D Reconstruction of a Rotating Erupting Prominence Authors: Thompson, W. T.; Kliem, B.; Török, T. Bibcode: 2012SoPh..276..241T Altcode: 2011arXiv1112.3388T A bright prominence associated with a coronal mass ejection (CME) was seen erupting from the Sun on 9 April 2008. This prominence was tracked by both the Solar Terrestrial Relations Observatory (STEREO) EUVI and COR1 telescopes, and was seen to rotate about the line of sight as it erupted; therefore, the event has been nicknamed the "Cartwheel CME." The threads of the prominence in the core of the CME quite clearly indicate the structure of a weakly to moderately twisted flux rope throughout the field of view, up to heliocentric heights of 4 solar radii. Although the STEREO separation was 48°, it was possible to match some sharp features in the later part of the eruption as seen in the 304 Å line in EUVI and in the Hα-sensitive bandpass of COR1 by both STEREO Ahead and Behind. These features could then be traced out in three-dimensional space, and reprojected into a view in which the eruption is directed toward the observer. The reconstructed view shows that the alignment of the prominence to the vertical axis rotates as it rises up to a leading-edge height of ≈ 2.5 solar radii, and then remains approximately constant. The alignment at 2.5 solar radii differs by about 115° from the original filament orientation inferred from Hα and EUV data, and the height profile of the rotation, obtained here for the first time, shows that two thirds of the total rotation are reached within ≈ 0.5 solar radii above the photosphere. These features are well reproduced by numerical simulations of an unstable moderately twisted flux rope embedded in external flux with a relatively strong shear field component. Title: Helicity transport in a simulated coronal mass ejection Authors: Kliem, B.; Rust, S.; Seehafer, N. Bibcode: 2011IAUS..274..125K Altcode: 2010arXiv1012.2297K It has been suggested that coronal mass ejections (CMEs) remove the magnetic helicity of their coronal source region from the Sun. Such removal is often regarded to be necessary due to the hemispheric sign preference of the helicity, which inhibits a simple annihilation by reconnection between volumes of opposite chirality. Here we monitor the relative magnetic helicity contained in the coronal volume of a simulated flux rope CME, as well as the upward flux of relative helicity through horizontal planes in the simulation box. The unstable and erupting flux rope carries away only a minor part of the initial relative helicity; the major part remains in the volume. This is a consequence of the requirement that the current through an expanding loop must decrease if the magnetic energy of the configuration is to decrease as the loop rises, to provide the kinetic energy of the CME. Title: Photospheric flux cancellation and associated flux rope formation and eruption Authors: Green, L. M.; Kliem, B.; Wallace, A. J. Bibcode: 2011A&A...526A...2G Altcode: 2010arXiv1011.1227G
Aims: We study an evolving bipolar active region that exhibits flux cancellation at the internal polarity inversion line, the formation of a soft X-ray sigmoid along the inversion line and a coronal mass ejection. The aim is to investigate the quantity of flux cancellation that is involved in flux rope formation in the time period leading up to the eruption.
Methods: The active region is studied using its extreme ultraviolet and soft X-ray emissions as it evolves from a sheared arcade to flux rope configuration. The evolution of the photospheric magnetic field is described and used to estimate how much flux is reconnected into the flux rope.
Results: About one third of the active region flux cancels at the internal polarity inversion line in the 2.5 days leading up to the eruption. In this period, the coronal structure evolves from a weakly to a highly sheared arcade and then to a sigmoid that crosses the inversion line in the inverse direction. These properties suggest that a flux rope has formed prior to the eruption. The amount of cancellation implies that up to 60% of the active region flux could be in the body of the flux rope. We point out that only part of the cancellation contributes to the flux in the rope if the arcade is only weakly sheared, as in the first part of the evolution. This reduces the estimated flux in the rope to ~30% or less of the active region flux. We suggest that the remaining discrepancy between our estimate and the limiting value of ~10% of the active region flux, obtained previously by the flux rope insertion method, results from the incomplete coherence of the flux rope, due to nonuniform cancellation along the polarity inversion line. A hot linear feature is observed in the active region which rises as part of the eruption and then likely traces out the field lines close to the axis of the flux rope. The flux cancellation and changing magnetic connections at one end of this feature suggest that the flux rope reaches coherence by reconnection immediately before and early in the impulsive phase of the associated flare. The sigmoid is destroyed in the eruption but reforms quickly, with the amount of cancellation involved being much smaller than in the course of its original formation. Title: Writhing and rotation of erupting prominences and CMEs Authors: Torok, T.; Kliem, B.; Thompson, W. T.; Berger, M. A. Bibcode: 2010AGUFMSH43C..01T Altcode: Erupting prominences often exhibit a writhing motion as they rise in the corona to become the core of a coronal mass ejection (CME). The writhing points towards the presence of a magnetic flux rope whose top part rotates about the direction of ascent. Understanding what causes the writhing, and which parameters determine its amount, will help us to 1) constrain CME initiation models and 2) predict the magnetic orientation of CMEs when they hit the Earth. Two mechanisms have been suggested to cause significant writhing/rotation in the low corona, namely the helical kink instability (KI) and the interaction of the shear field component of the ambient coronal field with the flux rope current. Here we present the first height profile of the rotation of an erupting prominence, obtained from STEREO observations. The prominence rotated by about 120 degree from its pre-eruptive orientation, until it reached a heliocentric height of about 2.5 solar radii. The data are compared to a series of numerical simulations that study the corresponding rotation of an erupting magnetic flux rope, by varying the initial flux rope twist and the external shear field component. The parameter range compatible with the data is constrained by the observed rotation-height and height-time profiles. It is found that, for the set of geometrical model parameters considered here, the observed strong rotation cannot be caused by the KI alone, but requires the presence of a shear field component. Moreover, the simulations suggest that the contribution of the shear field component was dominant in the observed event, indicating that the flux rope was only moderately twisted. We also briefly present the first measurements of the evolution of twist and writhe in numerical simulations of confined and ejective flux rope eruptions, and we discuss the implications of the results for filament eruptions and CMEs. Title: Simulations of Overexpanding CME Cavities Authors: Kliem, B.; Forbes, T.; Vourlidas, A.; Patsourakos, S. Bibcode: 2010AGUFMSH51A1661K Altcode: Coronal mass ejection (CME) cavities seen in white-light coronagraphs expand nearly self similarly in the outer corona and inner solar wind. Little is known about their initial expansion in the inner corona. A two-phase evolution, consisting of an initial overexpansion up to a heliocentric front height of about 1.5 solar radii, followed by nearly self-similar expansion, was recently discovered in STEREO/SECCHI observations of a fast CME (Patsourakos et al. 2010). The overexpansion is expressed as a decrease of the cavity aspect ratio (center height by radius) by at least a factor of 2 during the rise phase of the main CME acceleration. We present MHD simulations of erupting flux ropes that show the initial overexpansion of a cavity in line with the observed evolution. The contributions of ideal-MHD expansion and of magnetic reconnection to the growth of the flux rope and cavity in the simulations will be quantified to identify the primary cause of the overexpansion. This assesses the diagnostic potential of the overexpansion for the change of flux rope current and the role of magnetic reconnection in the early evolution of CMEs. Title: Helicity Shedding in a Simulated CME Authors: Seehafer, N.; Kliem, B. Bibcode: 2010AGUFMSH23B1855S Altcode: It has been suggested that coronal mass ejections remove the magnetic helicity of active regions from the Sun. Such removal is often regarded to be necessary due to the hemispheric sign preference of the helicity, which inhibits a simple annihilation by reconnection between volumes of opposite chirality. We have monitored the relative magnetic helicity contained in the coronal volume of a simulated flux rope CME, as well as the upward flux of helicity through a horizontal plane in the simulation box. The unstable and erupting flux rope carries away only part of the initial helicity through the open upper boundary of the box; the larger part remains in the volume. We offer a simple physical explanation for this result. Since most active regions erupt only once in their lifetime, our finding suggests that the major part of the helicity which is transported into the corona by emerging active regions is redistributed into the coronal field upon the dispersal of the active regions and is eventually transported back into the solar interior as the field submerges. Title: Toward understanding the early stages of an impulsively accelerated coronal mass ejection. SECCHI observations Authors: Patsourakos, S.; Vourlidas, A.; Kliem, B. Bibcode: 2010A&A...522A.100P Altcode: 2010arXiv1008.1171P Context. The expanding magnetic flux in coronal mass ejections (CMEs) often forms a cavity. Studies of CME cavities have so far been limited to the pre-event configuration to evolved CMEs at great heights, and to two-dimensional imaging data.
Aims: Quantitative analysis of three-dimensional cavity evolution at CME onset can reveal information that is relevant to the genesis of the eruption.
Methods: A spherical model was simultaneously fit to Solar Terrestrial Relations Observatory (STEREO) Extreme Ultraviolet Imager (EUVI) and Inner Coronagraph (COR1) data of an impulsively accelerated CME on 25 March 2008, which displays a well-defined extreme ultraviolet (EUV) and white-light cavity of nearly circular shape already at low heights h ≈ 0.2 R_⊙. The center height h(t) and radial expansion r(t) of the cavity were obtained in the whole height range of the main acceleration. We interpret them as the axis height and as a quantity proportional to the minor radius of a flux rope.
Results: The three-dimensional expansion of the CME exhibits two phases in the course of its main upward acceleration. From the first h and r data points, taken shortly after the onset of the main acceleration, the erupting flux shows an overexpansion compared to its rise, as expressed by the decrease in the aspect ratio from κ = h/r ≈ 3 to κ ≈ (1.5-2). This phase is approximately coincident with the impulsive rise in the acceleration and is followed by a phase of very gradual change in the aspect ratio (a nearly self-similar expansion) toward κ ~ 2.5 at h ~ 10 R_⊙. The initial overexpansion of the CME cavity can be caused by flux conservation around a rising flux rope of decreasing axial current and by the addition of flux to a growing, or by even newly formed, flux rope by magnetic reconnection. Further analysis will be required to decide which of these contributions is dominant. The data also suggest that the horizontal component of the impulsive cavity expansion (parallel to the solar surface) triggers the associated EUV wave, which subsequently detaches from the CME volume. Title: Reconnection of a Kinking Flux Rope Triggering the Ejection of a Microwave and Hard X-Ray Source II. Numerical Modeling Authors: Kliem, B.; Linton, M. G.; Török, T.; Karlický, M. Bibcode: 2010SoPh..266...91K Altcode: 2010SoPh..tmp..149K; 2010arXiv1007.2147K Numerical simulations of the helical (m=1) kink instability of an arched, line-tied flux rope demonstrate that the helical deformation enforces reconnection between the legs of the rope if modes with two helical turns are dominant as a result of high initial twist in the range Φ≳6π. Such a reconnection is complex, involving also the ambient field. In addition to breaking up the original rope, it can form a new, low-lying, less twisted flux rope. The new flux rope is pushed downward by the reconnection outflow, which typically forces it to break as well by reconnecting with the ambient field. The top part of the original rope, largely rooted in the sources of the ambient flux after the break-up, can fully erupt or be halted at low heights, producing a "failed eruption." The helical current sheet associated with the instability is squeezed between the approaching legs, temporarily forming a double current sheet. The leg - leg reconnection proceeds at a high rate, producing sufficiently strong electric fields that it would be able to accelerate particles. It may also form plasmoids, or plasmoid-like structures, which trap energetic particles and propagate out of the reconnection region up to the top of the erupting flux rope along the helical current sheet. The kinking of a highly twisted flux rope involving leg - leg reconnection can explain key features of an eruptive but partially occulted solar flare on 18 April 2001, which ejected a relatively compact hard X-ray and microwave source and was associated with a fast coronal mass ejection. Title: Reconnection of a Kinking Flux Rope Triggering the Ejection of a Microwave and Hard X-ray Source I. Observations and Interpretation Authors: Karlický, M.; Kliem, B. Bibcode: 2010SoPh..266...71K Altcode: 2010arXiv1007.2146K; 2010SoPh..tmp..139K Imaging microwave observations of an eruptive, partially occulted solar flare on 18 April 2001 suggest that the global structure of the event can be described by the helical kink instability of a twisted magnetic flux rope. This model is suggested by the inverse gamma shape of the source exhibiting crossing legs of a rising flux loop and by evidence that the legs interact at or near the crossing point. The interaction is reflected by the location of peak brightness near the crossing point and by the formation of superimposed compact nonthermal sources most likely at or near the crossing point. These sources propagate upward along both legs, merge into a single, bright source at the top of the structure, and continue to rise at a velocity >1000 km s−1. The compact sources trap accelerated electrons which radiate in the radio and hard X-ray ranges. This suggests that they are plasmoids, although their internal structure is not revealed by the data. They exhibit variations of the radio brightness temperature at a characteristic time scale of ∼ 40 s, anti-correlated to their area, which also support their interpretation as plasmoids. Their propagation path differs from the standard scenario of plasmoid formation and propagation in the flare current sheet, suggesting the helical current sheet formed by the instability instead. Title: Testing magnetofrictional extrapolation with the Titov-Démoulin model of solar active regions Authors: Valori, G.; Kliem, B.; Török, T.; Titov, V. S. Bibcode: 2010A&A...519A..44V Altcode: 2010arXiv1005.0254V We examine the nonlinear magnetofrictional extrapolation scheme using the solar active region model by Titov and Démoulin as test field. This model consists of an arched, line-tied current channel held in force-free equilibrium by the potential field of a bipolar flux distribution in the bottom boundary. A modified version with a parabolic current density profile is employed here. We find that the equilibrium is reconstructed with very high accuracy in a representative range of parameter space, using only the vector field in the bottom boundary as input. Structural features formed in the interface between the flux rope and the surrounding arcade - “hyperbolic flux tube” and “bald patch separatrix surface” - are reliably reproduced, as are the flux rope twist and the energy and helicity of the configuration. This demonstrates that force-free fields containing these basic structural elements of solar active regions can be obtained by extrapolation. The influence of the chosen initial condition on the accuracy of reconstruction is also addressed, confirming that the initial field that best matches the external potential field of the model quite naturally leads to the best reconstruction. Extrapolating the magnetogram of a Titov-Démoulin equilibrium in the unstable range of parameter space yields a sequence of two opposing evolutionary phases, which clearly indicate the unstable nature of the configuration: a partial buildup of the flux rope with rising free energy is followed by destruction of the rope, losing most of the free energy. Title: The writhe of helical structures in the solar corona Authors: Török, T.; Berger, M. A.; Kliem, B. Bibcode: 2010A&A...516A..49T Altcode: 2010arXiv1004.3918T Context. Helicity is a fundamental property of magnetic fields, conserved in ideal MHD. In flux rope geometry, it consists of twist and writhe helicity. Despite the common occurrence of helical structures in the solar atmosphere, little is known about how their shape relates to the writhe, which fraction of helicity is contained in writhe, and how much helicity is exchanged between twist and writhe when they erupt.
Aims: Here we perform a quantitative investigation of these questions relevant for coronal flux ropes.
Methods: The decomposition of the writhe of a curve into local and nonlocal components greatly facilitates its computation. We use it to study the relation between writhe and projected S shape of helical curves and to measure writhe and twist in numerical simulations of flux rope instabilities. The results are discussed with regard to filament eruptions and coronal mass ejections (CMEs).
Results: (1) We demonstrate that the relation between writhe and projected S shape is not unique in principle, but that the ambiguity does not affect low-lying structures, thus supporting the established empirical rule which associates stable forward (reverse) S shaped structures low in the corona with positive (negative) helicity. (2) Kink-unstable erupting flux ropes are found to transform a far smaller fraction of their twist helicity into writhe helicity than often assumed. (3) Confined flux rope eruptions tend to show stronger writhe at low heights than ejective eruptions (CMEs). This argues against suggestions that the writhing facilitates the rise of the rope through the overlying field. (4) Erupting filaments which are S shaped already before the eruption and keep the sign of their axis writhe (which is expected if field of one chirality dominates the source volume of the eruption), must reverse their S shape in the course of the rise. Implications for the occurrence of the helical kink instability in such events are discussed. (5) The writhe of rising loops can easily be estimated from the angle of rotation about the direction of ascent, once the apex height exceeds the footpoint separation significantly.
Conclusions: Writhe can straightforwardly be computed for numerical data and can often be estimated from observations. It is useful in interpreting S shaped coronal structures and in constraining models of eruptions. Title: Flux Rope Formation Preceding Coronal Mass Ejection Onset Authors: Kliem, Bernhard; Green, L. M. Bibcode: 2009SPD....41.2120K Altcode: We analyse the evolution of a sigmoidal (S shaped) active region toward eruption, which includes a coronal mass ejection (CME) but leaves part of the filament in place. The X-ray sigmoid is found to trace out three different magnetic topologies in succession: a highly sheared arcade of coronal loops in its long-lived phase, a bald-patch separatrix surface (BPSS) in the hours before the CME, and the first flare loops in its major transient intensity enhancement. The coronal evolution is driven by photospheric changes which involve the convergence and cancellation of flux elements under the sigmoid and filament. The data yield unambiguous evidence for the existence of a BPSS, and hence a flux rope, in the corona prior to the onset of the CME. Title: Evidence for Mixed Helicity in Erupting Filaments Authors: Muglach, K.; Wang, Y. -M.; Kliem, B. Bibcode: 2009ApJ...703..976M Altcode: 2009arXiv0907.4446M Erupting filaments are sometimes observed to undergo a rotation about the vertical direction as they rise. This rotation of the filament axis is generally interpreted as a conversion of twist into writhe in a kink-unstable magnetic flux rope. Consistent with this interpretation, the rotation is usually found to be clockwise (as viewed from above) if the post-eruption arcade has right-handed helicity, but counterclockwise if it has left-handed helicity. Here, we describe two non-active-region filament events recorded with the Extreme-Ultraviolet Imaging Telescope on the Solar and Heliospheric Observatory in which the sense of rotation appears to be opposite to that expected from the helicity of the post-event arcade. Based on these observations, we suggest that the rotation of the filament axis is, in general, determined by the net helicity of the erupting system, and that the axially aligned core of the filament can have the opposite helicity sign to the surrounding field. In most cases, the surrounding field provides the main contribution to the net helicity. In the events reported here, however, the helicity associated with the filament "barbs" is opposite in sign to and dominates that of the overlying arcade. Title: Flux Rope Formation Preceding Coronal Mass Ejection Onset Authors: Green, L. M.; Kliem, B. Bibcode: 2009ApJ...700L..83G Altcode: 2009arXiv0906.4794G We analyze the evolution of a sigmoidal (S-shaped) active region toward eruption, which includes a coronal mass ejection (CME) but leaves part of the filament in place. The X-ray sigmoid is found to trace out three different magnetic topologies in succession: a highly sheared arcade of coronal loops in its long-lived phase, a bald-patch separatrix surface (BPSS) in the hours before the CME, and the first flare loops in its major transient intensity enhancement. The coronal evolution is driven by photospheric changes which involve the convergence and cancellation of flux elements under the sigmoid and filament. The data yield unambiguous evidence for the existence of a BPSS, and hence a flux rope, in the corona prior to the onset of the CME. Title: Endpoint Brightenings in Erupting Filaments Authors: Wang, Y. -M.; Muglach, K.; Kliem, B. Bibcode: 2009ApJ...699..133W Altcode: Two well known phenomena associated with erupting filaments are the transient coronal holes that form on each side of the filament channel and the bright post-event arcade with its expanding double row of footpoints. Here we focus on a frequently overlooked signature of filament eruptions: the spike- or fan-shaped brightenings that appear to mark the far endpoints of the filament. From a sample of non-active-region filament events observed with the Extreme-Ultraviolet Imaging Telescope on the Solar and Heliospheric Observatory, we find that these brightenings usually occur near the outer edges of the transient holes, in contrast to the post-event arcades, which define their inner edges. The endpoints are often multiple and are rooted in and around strong network flux well outside the filament channel, a result that is consistent with the axial field of the filament being much stronger than the photospheric field inside the channel. The extreme ultraviolet brightenings, which are most intense at the time of maximum outward acceleration of the filament, can be used to determine unambiguously the direction of the axial field component from longitudinal magnetograms. Their location near the outer boundary of the transient holes suggests that we are observing the footprints of the current sheet formed at the leading edge of the erupting filament, as distinct from the vertical current sheet behind the filament which is the source of the post-event arcade. Title: Temperature Tomography of a Coronal Sigmoid Supporting the Gradual Formation of a Flux Rope Authors: Tripathi, Durgesh; Kliem, Bernhard; Mason, Helen E.; Young, Peter R.; Green, Lucie M. Bibcode: 2009ApJ...698L..27T Altcode: 2009arXiv0904.4782T Multiwavelength observations of a sigmoidal (S-shaped) solar coronal source by the EUV Imaging Spectrometer and the X-Ray Telescope aboard the Hinode spacecraft and by the EUV Imager aboard STEREO are reported. The data reveal the coexistence of a pair of J-shaped hot arcs at temperatures T>2 MK with an S-shaped structure at somewhat lower temperatures (T ≈ 1-1.3 MK). The middle section of the S-shaped structure runs along the polarity inversion line of the photospheric field, bridging the gap between the arcs. Flux cancellation occurs at the same location in the photosphere. The sigmoid forms in the gradual decay phase of the active region, which does not experience an eruption. These findings correspond to the expected signatures of a flux rope forming, or being augmented, gradually by a topology transformation inside a magnetic arcade. In such a transformation, the plasma on newly formed helical field lines in the outer flux shell of the rope (S-shaped in projection) is expected to enter a cooling phase once the reconnection of their parent field line pairs (double-J shaped in projection) is complete. Thus, the data support the conjecture that flux ropes can exist in the corona prior to eruptive activity. Title: 3D Reconstruction of an Erupting Prominence Authors: Thompson, William T.; Kliem, B.; Toeroek, T. Bibcode: 2009SPD....40.2111T Altcode: A bright prominence associated with a coronal mass ejection was seen erupting from the Sun on April 9, 2008. This prominence was tracked in both the STEREO EUVI and COR1 telescopes, and was seen to rotate or ``swirl'' as it erupted. Although the STEREO separation was 48 degrees, it was possible to match some sharp features in the later part of the eruption as seen in the 304 A line in EUVI by both STEREO Ahead and Behind. These features could then be traced out in three-dimensional space, and reprojected into a view in which the eruption is directed towards the observer. The reconstructed view shows that the alignment of the prominence rotates as it rises through the EUVI field-of-view out to 1.4 solar radii, and then remains constant as seen by COR1. The alignment at 1.4 solar radii differed by about 120 degrees from the original filament orientation. We will match the filament observations against a model of the event as kink instability in a flux rope. Title: Quadrature STEREO Observations Determine the Nature of EUV Waves Authors: Kliem, Bernhard; Patsourakos, S.; Vourlidas, A.; Ontiveros, V. Bibcode: 2009SPD....40.2603K Altcode: One of the major discoveries of EIT on SOHO was the observation of large-scale EUV intensity disturbances which travel over significant fractions of the solar disk. These `EUV waves' are associated with CME onsets and can be either an MHD wave triggered by the eruption or the footprints of the associated CME, which currently is a subject of intense debate. EUV waves are better observed when their source region is close to disk center, whereas CME onsets and CMEs in general are better observed off-limb. Therefore, simultaneous multi-viewpoint observations of EUV waves are best suited to clarify the nature of these transients and to determine their true relationship with CMEs.

We present here the first quadrature STEREO observations of an EUV wave. The wave was observed on 2009 February 13 by both satellites, which were at a separation of 90 degrees. The wave originated from an erupting active region near disk center as seen from SC B and propagated over almost the entire visible solar disk. For SC A the active region was at the east limb and showed a small erupting bubble, expanding impulsively in both radial and lateral directions and inducing deflections of nearby and remote coronal structures. We present high cadence EUVI and COR1 measurements of both the wave (SC B), and the expanding EUV bubble (SC A), and of the resulting white-light CME (SC A; COR1). These would allow to quantify for the first time the true sizes and expansion characteristics of both the EUV wave and the associated CME.

Finally, we search for wave-associated features in 3D MHD simulations of CME onsets based on ideal MHD instabilities. These are compared with the STEREO observations. Title: Intermittent Reconnection Downflow Enhancements In A Simulated Flux Rope Eruption Authors: Kliem, Bernhard; Linton, M. G. Bibcode: 2009SPD....40.2006K Altcode: Supra-arcade downflows in X-ray and EUV flare emissions and post-eruption inflows in coronagraph data have been interpreted to be signatures of the downward reconnection outflow from a vertical (flare) current sheet. These downflows show an intermittent occurrence pattern, indicating that the reconnection is bursty in time or patchy in space, or both. We present MHD simulations of such reconnection in the realistic configuration of a vertical current sheet formed beneath and driven by an erupting flux rope. The reconnection is found to develop bursty outflows, both upward and downward, with the upward outflows generally showing the stronger variablity. While the reconnection starts early in the rise of the flux rope and its peak upward outflow velocity is closely correlated with the rope's rise velocity, the burstiness develops in a clear fashion only as the rope's height has increased from the initial position by about an order of magnitude, so that the current sheet has reached a sufficient vertical extent. The reconnection downflow shows a series of enhancements, each of them starting at a successively greater height from a newly developed magnetic X line. The plasma temporarily accelerated downward in such an enhancement soon turns into a gradual deceleration and then eventually comes to rest on top of previously accelerated plasma. These findings are consistent with the observations of intermittent downflows. Title: Endpoint Brightenings in Erupting Filaments Authors: Muglach, Karin; Wang, Y.; Kliem, B. Bibcode: 2009SPD....40.2109M Altcode: Two well-known phenomena associated with erupting filaments are the transient coronal holes that form on each side of the filament channel and the bright post-event arcade with its expanding double row of footpoints. Here we focus on a frequently overlooked signature of filament eruptions: the spike- or fan-shaped brightenings that appear to mark the far endpoints of the filament. From a sample of non-active-region filament events observed with the Extreme-Ultraviolet Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO), we find that these brightenings usually occur near the outer edges of the transient holes, in contrast to the post-event arcades, which define their inner edges. The endpoints are often multiple and are rooted in and around strong network flux well outside the filament channel, a result that is consistent with the axial field of the filament being much stronger than the photospheric field inside the channel. The EUV brightenings, which are most intense at the time of maximum outward acceleration of the filament, can be used to determine unambiguously the direction of the axial field component from longitudinal magnetograms. Their location near the outer boundary of the transient holes suggests that we are observing the footprints of the current sheet formed at the leading edge of the erupting filament, as distinct from the vertical current sheet behind the filament which is the source of the post-event arcade. Title: Simulations of the CME-Flare Relationship Authors: Kliem, B.; Török, T.; Forbes, T. G. Bibcode: 2008AGUFMSH23B1648K Altcode: Observations of coronal mass ejections (CMEs) and solar flares have revealed a high correlation between the acceleration of the ejecta and the plasma heating and particle acceleration signified by the soft and hard X-ray emissions of the associated flare. The latter are generally thought to result from magnetic reconnection. This finding has stimulated the discussion of the CME-flare relationship, but at the same time it has made it difficult to find a conclusive answer as to whether magnetic reconnection or an ideal MHD instability is the prime cause of the eruptions. Numerical simulations of unstable flux ropes will be presented that are in very satisfactory quantitative agreement with erupting filaments, both, confined to the corona and ejective (i.e., developing into a CME). Some of these simulations indeed show a high degree of synchronization between the initial exponential acceleration of the flux rope, due to the ideal MHD instability, and the development of reconnection flows. However, others show a very delayed onset of reconnection, even after the flux rope's acceleration peak. In addition, the reconnection flows generally lag behind the motions driven by the ideal instability as the flux rope rise velocity nears the saturation phase. Comparison of the simulation results with observations suggests that the ideal MHD process is the primary driver of the coupled CME-flare phenomenon. The strong differences in the degree of synchronization, which the simulated systems show in the main rise phase of the eruption, are related to the magnetic topology prior to the eruption. Given the observational result of a high correlation between CME and flare development (Zhang & Dere 2006), these simulations yield constraints on the topology and lead us to conclude that a seed for a reconnecting current sheet must typically be present already at the onset of the eruption. Title: Coronal Magnetic Field Extrapolation from Photospheric Measurements Applied to an Active Region Authors: Fuhrmann, M.; Kliem, B.; Valori, G.; Seehafer, N. Bibcode: 2008ESPM...12.3.37F Altcode: We outline an MHD relaxation method that permits to extrapolate photospheric vector magnetograms into coronal nonlinear force-free fields. The method is applied to a magnetogram taken before an eruptive event in NOAA AR 7792 on 25 October 1994. The event produced a coronal mass ejection (CME) and eruptive flare with a prominent sigmoidal soft X-ray source. Multiwavelength observations as well as theoretical modeling indicate the importance of twisted magnetic configurations in solar active regions(ARs) in the initiation of such events. Manoharan et al. (1996) proposed a model for this event that included the merging of two slightly twisted flux bundles near the polarity inversion line into a flux rope of larger total twist and an overlying flux system much closer to a potential-field state. For the extrapolation we use a vector magnetogram taken at the Mees Solar Observatory 16 hours before the actual event. The magnetic field extrapolation is able to recover main parts of the structures suggested in the model by Manoharan et al. We find the overlying nearly potential flux and part of the sigmoidal field, i.e., one of the suggested weakly twisted flux bundles, in the location observed. This supports the notion that sigmoids are coronal manifestations of twisted magnetic flux tubes which start expanding in eruptive events and may exist even before the onset of such events. We tentatively attribute the incomplete reconstruction of the sigmoidal field structure to the strong evolution of the photospheric field at the suggested location of flux tube merging between the time of the magnetogram and the eruption, as indicated by a magnetogram on the following day. Title: Magnetofrictional Extrapolations of Current-Carrying Flux Ropes Authors: Valori, G.; Kliem, B.; Toeroek, T. Bibcode: 2008ESPM...12.2.90V Altcode: The quiescent solar corona is regularly modified by very fast ejections of coronal material and magnetic field (CME) that occur preferably above active regions. Most CME models require the formation of twisted magnetic field structures (flux ropes) before or during such events.

Unfortunately, the coronal magnetic field is not directly measurable at present, and therefore it is difficult to verify the validity of different CME models.

In order to remove this obstacle, the extrapolation of photospheric magnetic field measurements can be used to reconstruct the missing coronal information. As an application of our magneto-frictional code, we present an extrapolation of a measured magnetogram where a flux rope is found.

In such applications it is necessary to estimate how well our extrapolation code can reproduce all aspects of highly nonlinear structures such as flux ropes. This is of course possible only using test fields.

The Titov and Demoulin force-free equilibrium (Titov and Demoulin, Astr. and Astrophys. 351, 707, (1999), hereafter TD) models a semi-circular, 3D current-carrying flux rope by means of a current ring embedded in a potential field. The parameters of the TD model can be adjusted to create both stable and kink- and torus-unstable configurations.

Its solar relevance was confirmed by the quantitative reproduction of some specific CME features (see e.g., Toeroek and Kliem, Astroph. J. Lett. 630 L97 (2005)).

Therefore, the TD solution is by far the most realistic analytical equilibrium available to date for the modeling of solar active regions.

Employing the TD equilibrium as a test-field, we show that the magnetofrictional extrapolation code can reproduce the energy and the twist of the magnetic field within a percent accuracy.

This information is essential for the reconstruction of coronal fields involved in eruptions because the twist is, together with the height profile of the overlying potential field, the most important stability parameter -- at least as long as the TD equilibrium is a good model of the considered active region.

Perfectly reproduced are also X-type magnetic topology features, sometimes referred to as Hyperbolic Flux Tubes, which are regarded to be essential to the physics of CMEs and flares because they are preferred locations for the formation of current sheets.

On the other hand, we also show how the scale-height of the potential field that is used as initial condition in the extrapolation influences the quality of the reconstructed field: different initial conditions reproduce correctly the twist and the topology, but less accurately the height and the shape of the flux rope.

Consequently, care must be taken when comparing the shapes of soft X-ray and EUV loops, especially those in the nearly potential field overlying filaments, with the field lines obtained from the extrapolation of the corresponding magnetogram. Title: Simulations of the CME-Flare Relationship Authors: Kliem, B.; Török, T. Bibcode: 2008ESPM...12.3.67K Altcode: Observations of coronal mass ejections (CMEs) and solar flares have revealed a high correlation between the acceleration of the ejecta and the plasma heating and particle acceleration signified by the soft and hard X-ray emissions of the associated flare. The latter are generally thought to result from magnetic reconnection. This finding has stimulated the discussion of the CME-flare relationship, but at the same time it has made it difficult to find a conclusive answer as to whether magnetic reconnection or an ideal MHD instability is the prime cause of the eruptions.

Numerical simulations of unstable flux ropes will be presented that are in very satisfactory quantitative agreement with erupting filaments, both, confined to the corona and ejective (i.e., developing into a CME). Some of these simulations indeed show a high degree of synchronization between the initial exponential acceleration of the flux rope, due to the ideal MHD instability, and the development of reconnection flows. However, others show a very delayed onset of reconnection, even after the flux rope's acceleration peak. In addition, the reconnection flows generally lag behind the motions driven by the ideal instability as the flux rope rise velocity nears the saturation phase. Both findings indicate that the ideal MHD process is the primary driver of the coupled CME-flare phenomenon.

The strong differences in the degree of synchronization, which the simulated systems show in the main rise phase of the eruption, are related to the magnetic topology prior to the eruption. Given the observational result of a high correlation between CME and flare development (Zhang & Dere 2006), these simulations yield constraints on the topology and lead us to conclude that a seed for a reconnecting current sheet must typically be present already at the onset of the eruption. Title: Twist, Writhe and Rotation of Magnetic Flux Ropes in Filament Eruptions and Coronal Mass Ejections Authors: Török, T.; Berger, M. A.; Kliem, B.; Démoulin, P.; Linton, M.; van Driel-Gesztelyi, L. Bibcode: 2008ESPM...12.3.54T Altcode: We present the first quantitative analysis of the conversion of twist into writhe in the course of ideal MHD instabilities in erupting coronal magnetic flux ropes. For our analysis, we consider numerical simulations of two instabilities which have been suggested as trigger and initial driving mechanisms in filament eruptions and coronal mass ejections, namely the helical kink instability and the torus instability. We use two different coronal flux rope models as initial conditions in the simulations, namely the cylindrical Gold-Hoyle equilibrium and the toroidal Titov-Demoulin equilibrium.

For each model, we perform a series of simulations with different amounts of initial flux rope twist. In order to study both confined and ejective eruptions, we additionally use different initial potential fields overlying the flux rope in the simulations of the Titov-Demoulin model.

In all simulations, we measure the writhe of the flux rope and the corresponding rotation of its axis in vertical projection by making use of recently developed expressions which permit us to calculate writhe as a single integral in space. We discuss the implications of our results for filament eruptions, coronal mass ejections and magnetic clouds. Title: Observations and Modeling of the Early Acceleration Phase of Erupting Filaments Involved in Coronal Mass Ejections Authors: Schrijver, Carolus J.; Elmore, Christopher; Kliem, Bernhard; Török, Tibor; Title, Alan M. Bibcode: 2008ApJ...674..586S Altcode: 2007arXiv0710.1609S We examine the early phases of two near-limb filament destabilizations involved in coronal mass ejections (CMEs) on 2005 June 16 and July 27, using high-resolution, high-cadence observations made with the Transition Region and Coronal Explorer (TRACE), complemented by coronagraphic observations by the Mauna Loa Solar Observatory (MLSO) and the Solar and Heliospheric Observatory (SOHO). The filaments' heights above the solar limb in their rapid-acceleration phases are best characterized by a height dependence h(t) propto tm with m near, or slightly above, 3 for both events. Such profiles are incompatible with published results for breakout, MHD-instability, and catastrophe models. We show numerical simulations of the torus instability that approximate this height evolution in case a substantial initial velocity perturbation is applied to the developing instability. We argue that the sensitivity of magnetic instabilities to initial and boundary conditions requires higher fidelity modeling of all proposed mechanisms if observations of rise profiles are to be used to differentiate between them. The observations show no significant delays between the motions of the filament and of overlying loops: the filaments seem to move as part of the overall coronal field until several minutes after the onset of the rapid-acceleration phase. Title: What kinking filament eruptions tell us about the physical nature of transient coronal sigmoids ? Authors: van Driel-Gesztelyi, Lidia; Green, Lucie M.; Kliem, Bernhard; Toeroek, Tibor; Attrill, Gemma Bibcode: 2008cosp...37.3289V Altcode: 2008cosp.meet.3289V Soft X-ray images of the Sun have shown that some active regions contain loops, or collections of loops, which appear forward or reverse 'S' in shape. These features have been termed sigmoids. These structures are of interest because their presence in an active region has been linked to eruptive activity and the sense of sigmoid orientation is taken to indicate the sense of shear and twist (or helicity) in the magnetic field. Differing models have been put forward in order to explain the physical nature of sigmoids and the role they play in an eruption. We use multiwavelength observations (Yohkoh/SXT, TRACE, SOHO/EIT and MDI, H-alpha) to investigate how transient sigmoids are formed. We also investigate filament eruptions from these active regions, which show a clear sign of rotation of their apex. We find that for positive (negative) helicity the filament apex rotates clockwise (counterclockwise), consistent with the flux rope taking on a reverse (forward) S shape, which is opposite to that observed for the sigmoid. These observations put constraints on sigmoid models, excluding some of them. We conclude that transient sigmoids are associated with the formation of current sheets and heating along field lines under a dynamic flux rope. Title: Transient Coronal Sigmoids and Rotating Erupting Flux Ropes Authors: Green, L. M.; Kliem, B.; Török, T.; van Driel-Gesztelyi, L.; Attrill, G. D. R. Bibcode: 2007SoPh..246..365G Altcode: To determine the relationship between transient coronal (soft X-ray or EUV) sigmoids and erupting flux ropes, we analyse four events in which a transient sigmoid could be associated with a filament whose apex rotates upon eruption and two further events in which the two phenomena were spatially but not temporally coincident. We find the helicity sign of the erupting field and the direction of filament rotation to be consistent with the conversion of twist into writhe under the ideal MHD constraint of helicity conservation, thus supporting our assumption of flux rope topology for the rising filament. For positive (negative) helicity the filament apex rotates clockwise (counterclockwise), consistent with the flux rope taking on a reverse (forward) S shape, which is opposite to that observed for the sigmoid. This result is incompatible with two models for sigmoid formation: one identifying sigmoids with upward arching kink-unstable flux ropes and one identifying sigmoids with a current layer between two oppositely sheared arcades. We find instead that the observations agree well with the model by Titov and Démoulin (Astron. Astrophys.351, 707, 1999), which identifies transient sigmoids with steepened current layers below rising flux ropes. Title: Force-free magnetic fields in the solar atmosphere Authors: Seehafer, N.; Fuhrmann, M.; Valori, G.; Kliem, B. Bibcode: 2007AN....328.1166S Altcode: Reliable measurements of the solar magnetic field are restricted to the level of the photosphere. For about half a century attempts have been made to calculate the field in the layers above the photosphere, i.e. in the chromosphere and in the corona, from the measured photospheric field. The procedure is known as magnetic field extrapolation. In the superphotospheric parts of active regions the magnetic field is approximately force-free, i.e. electric currents are aligned with the magnetic field. The practical application to solar active regions has been largely confined to constant-α or linear force-free fields, with a spatially constant ratio, α, between the electric current and the magnetic field. We review results obtained from extrapolations with constant-α force-free fields, in particular on magnetic topologies favourable for flares and on magnetic and current helicities. Presently, different methods are being developed to calculate non-constant-α or nonlinear force-free fields from photospheric vector magnetograms. We also briefly discuss these methods and present a comparison of a linear and a nonlinear force-free magnetic field extrapolation applied to the same photospheric boundary data. Title: Numerical simulations of fast and slow coronal mass ejections Authors: Török, T.; Kliem, B. Bibcode: 2007AN....328..743T Altcode: 2007arXiv0705.2100T Solar coronal mass ejections (CMEs) show a large variety in their kinematic properties. CMEs originating in active regions and accompanied by strong flares are usually faster and accelerated more impulsively than CMEs associated with filament eruptions outside active regions and weak flares. It has been proposed more than two decades ago that there are two separate types of CMEs, fast (impulsive) CMEs and slow (gradual) CMEs. However, this concept may not be valid, since the large data sets acquired in recent years do not show two distinct peaks in the CME velocity distribution and reveal that both fast and slow CMEs can be accompanied by both weak and strong flares. We present numerical simulations which confirm our earlier analytical result that a flux-rope CME model permits describing fast and slow CMEs in a unified manner. We consider a force-free coronal magnetic flux rope embedded in the potential field of model bipolar and quadrupolar active regions. The eruption is driven by the torus instability which occurs if the field overlying the flux rope decreases sufficiently rapidly with height. The acceleration profile depends on the steepness of this field decrease, corresponding to fast CMEs for rapid decrease, as is typical of active regions, and to slow CMEs for gentle decrease, as is typical of the quiet Sun. Complex (quadrupolar) active regions lead to the fastest CMEs. Title: Magnetofrictional Extrapolations of Low and Lou's Force-Free Equilibria Authors: Valori, G.; Kliem, B.; Fuhrmann, M. Bibcode: 2007SoPh..245..263V Altcode: We present a careful investigation of the magnetofrictional relaxation and extrapolation technique applied to the reconstruction of two test fields. These fields are taken from the family of nonlinear force-free magnetic equilibria constructed by Low and Lou (Astrophys. J.352, 343, 1990), which have emerged as standard tests for extrapolation techniques in recent years. For the practically relevant case that only the field values in the bottom plane of the considered volume (vector magnetogram) are used as input information (i.e., not including the knowledge about the test field at the side and top boundaries), the test field is reconstructed to a higher accuracy than obtained previously. Detailed diagnostics of the reconstruction accuracy show that the implementation of fourth-order spatial discretization was essential to reach this accuracy for the given test fields and to achieve near machine precision in satisfying the solenoidal condition. Different variants of boundary conditions are tested, which all yield comparable accuracy. In its present implementation, the technique yields a scaling of computing time with total number of grid points only slightly below N5/3, which is too steep for applications to large (≥10242) magnetograms, except on supercomputers. Directions for improvement are outlined. Title: The Evolving Sigmoid: Evidence for Magnetic Flux Ropes in the Corona Before, During, and after CMES Authors: Gibson, S. E.; Fan, Y.; Török, T.; Kliem, B. Bibcode: 2007sdeh.book..131G Altcode: No abstract at ADS Title: Testing non-linear force-free coronal magnetic field extrapolations with the Titov-Démoulin equilibrium Authors: Wiegelmann, T.; Inhester, B.; Kliem, B.; Valori, G.; Neukirch, T. Bibcode: 2006A&A...453..737W Altcode: 2006astro.ph.12650W 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 non-linear force-free coronal magnetic field extrapolation code with the help of a known analytical solution.
Methods. The non-linear force-free equations are numerically solved with the help of an optimization principle. The method minimizes an integral over the force-free 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 force-freeness and divergence-freeness 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 force-free 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. Title: Relativistic electron scattering by electrostatic upper hybrid waves in the radiation belt Authors: Shklyar, D. R.; Kliem, B. Bibcode: 2006JGRA..111.6204S Altcode: 2006JGRA..11106204S Observations of particle precipitation into the Earth's atmosphere performed on low-altitude satellites have excited substantial interest in relativistic electron precipitation bursts from the outer zone of the radiation belt. As underlying mechanism of such bursts, we suggest particle scattering into the loss cone due to higher-order cyclotron resonance interaction between relativistic electrons and intense narrow-band upper hybrid waves, which are frequently observed outside the plasmapause. The case of a single wave and the case of a wide wave number spectrum are considered, and approximate expressions for the relativistic particle diffusion coefficients in phase space are calculated for conditions of an inhomogeneous plasma. It is found that relativistic electrons have a preference over lower-energy electrons from the viewpoint of the number of cyclotron resonances that a particle crosses during each bounce period. Owing to the predominantly longitudinal direction of the upper hybrid wave group velocity, the resonant wave-particle interaction can take place over many electron bounce periods, which facilitates the particle scattering into the loss cone. The theory developed in the present paper accounts for two main features of the relativistic electron precipitation bursts, namely, a strong energy dependence in the electron precipitation process and the small-scale burst structure, which is primarily attributed to the localization and strong inhomogeneity of the growth region of the upper hybrid modes responsible for the scattering. The existence of intrinsic temporal structure of the precipitation on timescales comparable to the bounce period is also consistent with the theory due to the high efficiency of the scattering process. Title: The Evolving Sigmoid: Evidence for Magnetic Flux Ropes in the Corona Before, During, and After CMES Authors: Gibson, S. E.; Fan, Y.; Török, T.; Kliem, B. Bibcode: 2006SSRv..124..131G Altcode: 2007SSRv..tmp...52G It is generally accepted that the energy that drives coronal mass ejections (CMEs) is magnetic in origin. Sheared and twisted coronal fields can store free magnetic energy which ultimately is released in the CME. We explore the possibility of the specific magnetic configuration of a magnetic flux rope of field lines that twist about an axial field line. The flux rope model predicts coronal observables, including heating along forward or inverse S-shaped, or sigmoid, topological surfaces. Therefore, studying the observed evolution of such sigmoids prior to, during, and after the CME gives us crucial insight into the physics of coronal storage and release of magnetic energy. In particular, we consider (1) soft-X-ray sigmoids, both transient and persistent; (2) The formation of a current sheet and cusp-shaped post-flare loops below the CME; (3) Reappearance of sigmoids after CMEs; (4) Partially erupting filaments; (5) Magnetic cloud observations of filament material. Title: Flux Ropes and CMEs: The Kink and Torus Instabilities, Catastrophe, and Magnetic Reconnection Authors: Kliem, Bernhard; Toeroek, T. Bibcode: 2006SPD....37.0820K Altcode: 2006BAAS...38..234K Prior to eruption, the coronal magnetic field evolves along an equilibrium sequence due to slow photospheric changes. Configurations containing a flux rope can erupt when the rope passes the threshold of an instability in the sequence or when the rope is driven beyond an end point of the sequence in parameter space (catastrophe). The kink instability of a flux rope yields quantitative agreement with characteristic properties of many CMEs during their onset and early evolution: development of helical shape and exponential-to-linear rise profiles. The large-scale evolution of kinking flux ropes is governed by the torus (expansion) instability (TI). This instability yields a unified description of fast and slow CMEs, the preferred occurrence of very fast CMEs in quadrupolar active regions, and an indication why the minor flux rope radius expands overproportionally in the course of the eruption, creating or deepening the cavity seen in three-part CMEs. If an eruption is triggered by a flux rope catastrophe, we expect its evolution to possess characteristics similar to the TI-driven case. The magnetic reconnection that commences in the wake of a rising unstable flux rope is an integral part of the eruption and proceeds in a highly dynamic and complex manner, forming many intermittent X- and O-type structures along a vertical current sheet. Title: Torus Instability Authors: Kliem, B.; Török, T. Bibcode: 2006PhRvL..96y5002K Altcode: 2006physics...5217K The expansion instability of a toroidal current ring in low-beta magnetized plasma is investigated. Qualitative agreement is obtained with experiments on spheromak expansion and with essential properties of solar coronal mass ejections, unifying the two apparently disparate classes of fast and slow coronal mass ejections. Title: Confined and Ejective Eruptions of Kink-unstable Flux Ropes Authors: Török, T.; Kliem, B. Bibcode: 2005ApJ...630L..97T Altcode: 2005astro.ph..7662T The ideal helical kink instability of a force-free coronal magnetic flux rope, anchored in the photosphere, is studied as a model for solar eruptions. Using the flux rope model of Titov and Démoulin as the initial condition in MHD simulations, both the development of helical shape and the rise profile of a confined (or failed) filament eruption (on 2002 May 27) are reproduced in very good agreement with the observations. By modifying the model such that the magnetic field decreases more rapidly with height above the flux rope, a full (or ejective) eruption of the rope is obtained in very good agreement with the developing helical shape and the exponential-to-linear rise profile of a fast coronal mass ejection (CME) on 2001 May 15. This confirms that the helical kink instability of a twisted magnetic flux rope can be the mechanism of the initiation and the initial driver of solar eruptions. The agreement of the simulations with properties that are characteristic of many eruptions suggests that they are often triggered by the kink instability. The decrease of the overlying field with height is a main factor in deciding whether the instability leads to a confined event or to a CME. Title: Eruption of a Kink-unstable Filament in NOAA Active Region 10696 Authors: Williams, David R.; Török, Tibor; Démoulin, Pascal; van Driel-Gesztelyi, Lidia; Kliem, Bernhard Bibcode: 2005ApJ...628L.163W Altcode: 2005astro.ph..7661W We present rapid-cadence Transition Region and Coronal Explorer (TRACE) observations that show evidence of a filament eruption from NOAA active region 10696, accompanied by an X2.5 flare, on 2004 November 10. The eruptive filament, which manifests as a fast coronal mass ejection some minutes later, rises as a kinking structure with an apparently exponential growth of height within TRACE's field of view. We compare the characteristics of this filament eruption with MHD numerical simulations of a kink-unstable magnetic flux rope, finding excellent qualitative agreement. We suggest that while tether weakening by breakout-like quadrupolar reconnection may be the release mechanism for the previously confined flux rope, the driver of the expansion is most likely the MHD helical kink instability. Title: Extrapolation of a nonlinear force-free field containing a highly twisted magnetic loop Authors: Valori, G.; Kliem, B.; Keppens, R. Bibcode: 2005A&A...433..335V Altcode: The stress-and-relax method for the extrapolation of nonlinear force-free coronal magnetic fields from photospheric vector magnetograms is formulated and implemented in a manner analogous to the evolutionary extrapolation method. The technique is applied to a numerically constructed force-free equilibrium that has a simple bipolar structure of the normal field component in the bottom (magnetogram) plane but contains a highly twisted loop and a shear (current) layer, with a smooth but strong variation of the force-free parameter α in the magnetogram. A standard linear force-free extrapolation of this magnetogram, using the so-called α_best value, is found to fail in reproducing the twisted loop (or flux rope) and the shear layer; it yields a loop pair instead and the shear is not concentrated in a layer. With the nonlinear extrapolation technique, the given equilibrium is readily reconstructed to a high degree of accuracy if the magnetogram is sufficiently resolved. A parametric study quantifies the requirements on the resolution for a successful nonlinear extrapolation. Permitting magnetic reconnection by a controlled use of resistivity improved the extrapolation at a resolution comparable to the smallest structures in the magnetogram. Title: The Kink Instability in Solar Eruptions Authors: Török, T.; Kliem, B. Bibcode: 2004ESASP.575...56T Altcode: 2004soho...15...56T No abstract at ADS Title: The kink instability of a coronal magnetic loop as a trigger mechanism for solar eruptions Authors: Török, T.; Kliem, B. Bibcode: 2004PADEU..14..165T Altcode: The kink instability of twisted magnetic flux tubes in the solar corona is regarded as a possible initiation process of solar eruptions. We study the stability properties and the dynamic evolution of such coronal magnetic loops using 3D numerical simulations within the framework of ideal MHD. The analytical force-free coronal loop model by Titov and Demoulin (1999) is used as initial condition in the simulations. The loop model is found to be kink-unstable if a critical twist is exceeded. The growing kink perturbation leads to the formation of current sheets, which steepen exponentially and define the locations of plasma heating. Due to the twist in the magnetic field, the heated structures are S shaped - in very good agreement with soft X-ray observations of solar eruptions. The model, however, does not yet show a successful eruption, rather the kink instability starts to saturate. We present an improvement of the model which is promising with regard to eruption: a modification of the equilibrium so that the magnetic field surrounding the loop decreases more rapidly with height above the photosphere. Furthermore, we briefly discuss how the simulation results can be related to observations of solar eruptive phenomena. Title: Pulsating Microwave Emission from the Star AD Leo Authors: Zaitsev, V. V.; Kislyakov, A. G.; Stepanov, A. V.; Kliem, B.; Furst, E. Bibcode: 2004AstL...30..319Z Altcode: We have performed a spectral analysis of the quasi-periodic low-frequency modulation of microwave emission from a flare on the star AD Leo. We used the observations of the May 19, 1997 flare in the frequency range 4.5-5.1 GHz with a total duration of the burst phase of about 50 s obtained in Effelsberg with a time resolution of 1 ms. The time profile of the radio emission was analyzed by using the Wigner-Ville transformation, which yielded the dynamic spectrum of low-frequency pulsations with a satisfactory frequency-time resolution. In addition to the noise component, two regular components were found to be present in the low-frequency modulation spectrum of the stellar radio emission: a quasi-periodic component whose frequency smoothly decreased during the flare from ~2 to ~0.2 Hz and a periodic sequence of pulses with a repetition rate of about 2 Hz, which was approximately constant during the flare. We consider the possibility of the combined effect of MHD and LCR oscillations of the radio source on the particle acceleration in the stellar atmosphere and give estimates of the source's parameters that follow from an analysis of the low-frequency modulation spectra. Title: SUMER Observations of Heating and Cooling of Coronal Loops Authors: Curdt, W.; Wang, T. J.; Dwivedi, B. N.; Kliem, B.; Dammasch, I. E. Bibcode: 2004ESASP.547..333C Altcode: 2004soho...13..333C Hot-loop transient events observed by SUMER in hot EUV lines are known to trigger loop oscillations, as reported by Kliem et al. [1] and Wang et al. [2], [3]. Apart from the inference of physical parameters in the solar corona, these observations also carry the signatures of heating and cooling of coronal loops. We present the light curves for various highly-ionized ions which were simultaneously observed during and after the trigger. Even though the majority of SUMER events occur on sub-flare level, it is clear that the heating is impulsive and drives the plasma to a very high temperature of up to 10 MK within minutes. During the cooling phase, however, we find the plasma in gradually decreasing ionization stages which implies that the entire loop system involved in such events is basically in the isothermal state. Such events may also help in our understanding of mass supply and energy transport in the corona. Title: Ideal kink instability of a magnetic loop equilibrium Authors: Török, T.; Kliem, B.; Titov, V. S. Bibcode: 2004A&A...413L..27T Altcode: 2003astro.ph.11198T The force-free coronal loop model by \cite{Tit:Dem-99} is found to be unstable with respect to the ideal kink mode, which suggests this instability as a mechanism for the initiation of flares. The long-wavelength (m = 1) mode grows for average twists Φ⪆3.5π (at a loop aspect ratio of ≈5). The threshold of instability increases with increasing major loop radius, primarily because the aspect ratio then also increases. Numerically obtained equilibria at subcritical twist are very close to the approximate analytical equilibrium; they do not show indications of sigmoidal shape. The growth of kink perturbations is eventually slowed down by the surrounding potential field, which varies only slowly with radius in the model. With this field a global eruption is not obtained in the ideal MHD limit. Kink perturbations with a rising loop apex lead to the formation of a vertical current sheet below the apex, which does not occur in the cylindrical approximation. Title: Formation of current sheets and sigmoidal structure by the kink instability of a magnetic loop Authors: Kliem, B.; Titov, V. S.; Török, T. Bibcode: 2004A&A...413L..23K Altcode: 2003astro.ph.11199K We study dynamical consequences of the kink instability of a twisted coronal flux rope, using the force-free coronal loop model by \cite{Tit:Dem-99} as the initial condition in ideal-MHD simulations. When a critical value of the twist is exceeded, the long-wavelength (m = 1) kink mode develops. Analogous to the well-known cylindrical approximation, a helical current sheet is then formed at the interface with the surrounding medium. In contrast to the cylindrical case, upward-kinking loops form a second, vertical current sheet below the loop apex at the position of the hyperbolic flux tube (generalized X line) in the model. The current density is steepened in both sheets and eventually exceeds the current density in the loop (although the kink perturbation starts to saturate in our simulations without leading to a global eruption). The projection of the field lines that pass through the vertical current sheet shows an S shape whose sense agrees with the typical sense of transient sigmoidal (forward or reverse S-shaped) structures that brighten in soft X rays prior to coronal eruptions. The upward-kinked loop has the opposite S shape, leading to the conclusion that such sigmoids do not generally show the erupting loops themselves but indicate the formation of the vertical current sheet below them that is the central element of the standard flare model. Title: The kink instability of a coronal magnetic loop as a trigger mechanism for solar eruptions Authors: Török, T.; Kliem, B.; Titov, V. S. Bibcode: 2004cosp...35.3327T Altcode: 2004cosp.meet.3327T MHD instabilities of twisted magnetic flux tubes in the solar corona are regarded as a possible initiation process of solar eruptions. We study the stability properties and the dynamic evolution of coronal magnetic loops using 3D numerical simulations within the framework of ideal MHD. The analytical force-free coronal loop model by Titov and Démoulin (1999) is used as initial condition in the simulations. The loop model is found to be kink-unstable if a critical twist is exceeded. The growing kink perturbation leads to the formation of current sheets, which steepen exponentially and define the locations of plasma heating. Due to the twist in the magnetic field, the heated structures are S shaped -- in very good agreement with sigmoidal soft X-ray structures that brighten in solar eruptions. The model, however, does not yet show a successful eruption, rather the kink instability starts to saturate. We present an improvement of the model which is promising with regard to eruption: a modification of the equilibrium so that the magnetic field surrounding the loop decreases more rapidly with height above the photosphere. Furthermore, we discuss how the simulation results can be related to observations of solar eruptive phenomena. Title: Flare/CME Relationship Authors: Kliem, B. Bibcode: 2004cosp...35.4097K Altcode: 2004cosp.meet.4097K In recent years evidence has accumulated showing that flares and CMEs are different observational manifestations of a single process -- the destabilization and reorganization of magnetic fields at active region spatial scales. Neupert et al. (2001) and Zhang et al. (2001) have clearly shown the connection between the two in a couple of events. I will present a further well-observed example showing the same connection, the 2002 April 21 solar X flare. Combined data from the TRACE, SUMER, RHESSI, NoRH, UVCS, and LASCO instruments show erupting core flux, associated with nonthermal and thermal flare emissions and evolving into one of the fastest CMEs ever observed. Although the observations are very detailed, they still do not seem to permit a firm conclusion regarding the destabilization mechanism, but they point to an instability of a complex flux rope structure, with some elements of the tether cutting and magnetic breakout models possibly being included. The evolution of unstable magnetic flux from the impulsive rise phase of flare emissions to a fully developed CME typically happens in the inner and middle corona, a region too sparsely sampled by current instrumentation. It is therefore still largely ambiguous which height-time characteristic should be fitted to the data and whether a distinct acceleration phase of the ejecta occurs during the impulsive flare phase. Guidance by theoretical models is needed. I will briefly discuss a few height-time characteristics suggested in the literature, including the one implied by a recently proposed destabilization mechanism which is based on the kink instability of a flux rope. Title: The evolution of twisting coronal magnetic flux tubes Authors: Török, T.; Kliem, B. Bibcode: 2003A&A...406.1043T Altcode: We simulate the twisting of an initially potential coronal flux tube by photospheric vortex motions, centred at two photospheric flux concentrations, using the compressible zero-beta ideal MHD equations. A twisted flux tube is formed, surrounded by much less twisted and sheared outer flux. Under the action of continuous slow driving, the flux tube starts to evolve quasi-statically along a sequence of force-free equilibria, which rise slowly with increasing twist and possess helical shape. The flux bundle that extends from the location of peak photospheric current density (slightly displaced from the vortex centre) shows a sigmoidal shape in agreement with observations of sigmoidal soft X-ray loops. There exists a critical twist, above which no equilibrium can be found in the simulation and the flux tube ascends rapidly. Then either stable equilibrium ceases to exist or the character of the sequence changes such that neighbouring stable equilibria rise by enormous amounts for only modest additions of twist. A comparison with the scalings of the rise of flux in axisymmetric geometry by \cite{Stur:al-95} suggests the former. Both cases would be observed as an eruption. The critical end-to-end twist, for a particular set of parameters describing the initial potential field, is found to lie in the range 2.5pi <Phic<2.75pi . There are some indications for the growth of helical perturbations at supercritical twist. Depending on the radial profiles of the photospheric flux concentration and vortex velocity, the outer part or all of the twisted flux expands from the central field line of the flux tube. This effect is particularly efficient in the dynamic phase, provided the density is modeled realistically, falling off sufficiently rapidly with height. It is expected to lead to the formation of a cavity in which the twisted flux tube is embedded, analogous to the typical structure of coronal mass ejections. Title: Hot coronal loop oscillations observed with SUMER: Examples and statistics Authors: Wang, T. J.; Solanki, S. K.; Curdt, W.; Innes, D. E.; Dammasch, I. E.; Kliem, B. Bibcode: 2003A&A...406.1105W Altcode: We give an extensive overview of Doppler shift oscillations in hot active region loops obtained with SUMER. The oscillations have been detected in loops sampled 50-100 arcsec off the limb of the Sun in ultraviolet lines, mainly Fe Xix and Fe Xxi, with formation temperature greater than 6 MK. The spectra were recorded along a 300 arcsec slit placed at a fixed position in the corona above the active regions. Oscillations are usually seen along an extended section of the slit and often appear to be from several different portions of the loops (or from different loops). Different portions are sometimes in phase, sometimes out of phase and sometimes show phase shifts along the slit. We measure physical parameters of 54 Doppler shift oscillations in 27 flare-like events and give geometric parameters of the associated hot loops when soft X-ray (SXR) images are available. The oscillations have periods in the range 7-31 min, with decay times 5.7-36.8 min, and show an initial large Doppler shift pulse with peak velocities up to 200 km s-1. The oscillation periods are on average a factor of three longer than the TRACE transverse loop oscillations. The damping times and velocity amplitude are roughly the same, but the derived displacement amplitude is four or five times larger than the transverse oscillation amplitude measured in TRACE images. Unlike TRACE oscillations, only a small fraction of them are triggered by large flares, and they often recur 2-3 times within a couple of hours. All recurring events show initial shifts of the same sign. These data provide the following evidence to support the conclusion that these oscillations are slow magnetoacoustic standing waves in hot loops: (1) the phase speeds derived from observed periods and loop lengths roughly agree with the sound speed; (2) the intensity fluctuation lags the Doppler shifts by 1/4 period; (3) The scaling of the dissipation time of slow waves with period agrees with the observed scaling for 49 cases. They seem to be triggered by micro- or subflares near a footpoint, as revealed in one example with SXR image observations. However other mechanisms cannot as yet be ruled out. Some oscillations showed phase propagation along the slit in one or both directions with apparent speeds in the range of 8-102 km s-1, together with distinctly different intensity and line width distributions along the slit. These features can be explained by the excitation of the oscillation at a footpoint of an inhomogeneous coronal loop, e.g. a loop with fine structure.

Table \ref{osctab} and Appendices A and B are only available in electronic form at http://www.edpsciences.org Title: The Initial Phase of the 2002 April 21 Solar X Flare: Aspects of Agreement and of Disagreement with Solar Flare Models Authors: Kliem, Bernhard Bibcode: 2003ANS...324R..11K Altcode: 2003ANS...324..B06K No abstract at ADS Title: Formation of Current Sheets and Sigmoidal Structure by the Ideal Kink Instability of a Magnetic Loop Authors: Kliem, Bernhard; Török, Tibor; Titov, Viatcheslav S. Bibcode: 2003ANS...324...73K Altcode: 2003ANS...324..I16K No abstract at ADS Title: Recent Progress in Understanding Energy Conversion and Particle Acceleration in the Solar Corona Authors: Kliem, B.; MacKinnon, A.; Trottet, G.; et al. Bibcode: 2003LNP...612..263K Altcode: 2003ecpa.conf..263K We report on results of the working group sessions at the CESRA 2001 workshop on "Energy Conversion and Particle Acceleration in the Solar Corona" which was focused on radio observations and related modeling. Progress reached in the following areas is summarized: (1) diagnostics of coronal magnetic fields and the morphology of the field in flares and filament eruptions; (2) evidence of magnetic reconnection and MHD turbulence in radio emissions; (3) acceleration site, propagation, and trapping of radio-emitting energetic particles in flares; (4) the sites of particle acceleration in long duration events, as evidenced by the 2000 July 14 ("Bastille Day") flare; (5) radio imaging of CMEs and filament eruptions; (6) the relationship of coronal and interplanetary shock waves to flares, CMEs, and other coronal waves; and (7) the origin of solar energetic particles. Title: The evolution of coronal magnetic flux tubes twisted by photospheric vortex motions Authors: Török, T.; Kliem, B. Bibcode: 2002ESASP.506..781T Altcode: 2002svco.conf..781T; 2002ESPM...10..781T We simulate the twisting of initially potential coronal magnetic flux by slow photospheric vortex motions using the compressional, zero-beta ideal MHD equations. The twisted flux tube starts to evolve quasi-statically along a sequence of force-free equilibria, rising slowly and possessing helical shape. As a critical amount of twist is reached, the evolution becomes dynamic and the tube rises and expands rapidly. No neighbouring equilibrium can be found in the simulation domain at this stage of the evolution, as confirmed by relaxation runs. Hence, above the critical twist either the flux tube becomes unstable or neighbouring equilibria rise by enormous amounts for only small additional twist. Both cases would be observed as an eruption. The critical end-to-end twist of the flux tube is found to lie in the range 2.5π < Φc < 3.0π. Title: Doppler oscillations in hot coronal loops Authors: Curdt, W.; Wang, T. J.; Innes, D. E.; Solanki, S. K.; Dammasch, I. E.; Kliem, B.; Ofman, L. Bibcode: 2002ESASP.506..581C Altcode: 2002ESPM...10..581C; 2002svco.conf..581C Recently, a new kind of damped oscillations of hot coronal loops was revealed by the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer on SOHO. Such events seem to be a common feature observed in active region loops, seen very often when these lines brighten. The oscillations always have an impulsive trigger and are strongly damped while they cool down. However, in lines formed at coronal temperatures of ≍2 MK never any signature of these oscillations has been observed. In this study, we present the main properties of Doppler oscillations derived from a statistical study of 17 flare-like events, and a comparison with TRACE transverse loop oscillations. We also discuss the oscillation modes and their damping mechanism. Title: Drifting decimetric pulsation structures in the initial phase of solar flares Authors: Karlický, M.; Kliem, B.; Mészárosová, H.; Jiřička, K. Bibcode: 2002ESASP.506..653K Altcode: 2002svco.conf..653K; 2002ESPM...10..653K Two new cases of slowly negatively drifting pulsation structures in the decimetric wavelength range are reported. It is shown that the August 18, 1998 flare started with a slowly drifting pulsation structure associated with a plasmoid ejection observed by Yohkoh/SXT. This drifting pulsation structure is an example of a narrow-band one, which even shows intensity enhancements at its high- and low-frequency edges. Using the cross-correlation method no significant relation between the radio emission of this drifting structure and the hard X-rays was found. On the other hand, the second drifting structure observed during the hard X-ray decay phase of the March 24, 2000 flare shows a clear separation from the large-scale coronal shock wave released by the flare (a partly simultaneous type II radio burst occurred at much lower frequencies). The slowly drifting structures are analyzed and possible underlying physical processes are discussed assuming the plasmoid ejection model of eruptive solar flares. Title: Correlated Dynamics of Hot and Cool Plasmas in the Main Phase of a Solar Flare Authors: Kliem, B.; Dammasch, I. E.; Curdt, W.; Wilhelm, K. Bibcode: 2002ApJ...568L..61K Altcode: We report far-ultraviolet observations of a solar limb flare obtained by the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer. At a fixed pointing of the slit above the limb, spectra were simultaneously obtained in several emission lines that covered a wide temperature range from ~104 to ~107 K. The temporal evolution of the spectra revealed, for the first time, a high degree of correlation between the dynamical behavior of hot (T~107 K) and cool (T~104 K) coronal material during the main phase of a flare. We note that the data did not show any indication of the presence of a prominence. Hot and cool plasmas brightened at nearly the same location. Their Doppler shifts, which were opposite to each other, reached peak values simultaneously. Thereafter, the two components showed anticorrelated, rapidly damped, and oscillatory Doppler shifts and a very similar decay of the line widths, but with the cool plasma reaching maximum brightness before the hot plasma. This behavior points to an active role for cool plasma in the dynamics of this flare, different from the usual picture of passive cooling after the impulsive phase. We suggest a model in which the localized cooling of coronal plasma by the thermal instability triggers magnetic reconnection through the resulting enhanced resistivity, the combined processes leading to the correlated dynamics of hot and cool plasmas in a loop-loop interaction geometry. Title: The evolution of coronal magnetic flux tubes subject to footpoint twisting motions Authors: Kliem, B.; Török, T. Bibcode: 2002ocnd.confE..25K Altcode: No abstract at ADS Title: Correlated Dynamics of Hot and Cool Plasmas in Two Solar Flares Authors: Kliem, B.; Dammasch, I. E.; Curdt, W.; Wilhelm, K. Bibcode: 2002mwoc.conf..271K Altcode: We report far-ultraviolet observations of a solar limb flare by the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer. At a fixed pointing of the slit above the limb, spectra were simultaneously obtained in several emission lines that covered a wide temperature range of ≅ 104 -- 107 K. The temporal evolution of the spectra revealed a high degree of correlation between the dynamical behavior of hot (T ~107 K) and cool (T ~104 K) material during the main flare phase. Hot and cool plasma brightened at nearly the same place, with cool plasma reaching maximum brightness before the hot plasma. The opposite line-of-sight velocities reached their peak values simultaneously. A correlated, rapidly damped oscillatory motion followed, while the excess (turbulent) line widths decayed in a similar manner. This behavior points to an active role of cool plasma in the dynamics of this flare, different from the usual picture of passive cooling after the impulsive phase. An interpretation is suggested in terms of magnetic reconnection which is triggered by rapid localized cooling due to the thermal instability in a loop-loop interaction geometry. Title: Microwave plasma emission of a flare on AD Leo Authors: Stepanov, A. V.; Kliem, B.; Zaitsev, V. V.; Fürst, E.; Jessner, A.; Krüger, A.; Hildebrandt, J.; Schmitt, J. H. M. M. Bibcode: 2001A&A...374.1072S Altcode: 2001astro.ph..6369S An intense radio flare on the dMe star AD Leo, observed with the Effelsberg radio telescope and spectrally resolved in a band of 480 MHz centred at 4.85 GHz is analysed. A lower limit of the brightness temperature of the totally right handed polarized emission is estimated as T_b ~ 5*E10 K (with values T_bga3 *E13 K considered to be more probable), which requires a coherent radio emission process. In the interpretation we favour fundamental plasma radiation by mildly relativistic electrons trapped in a hot and dense coronal loop above electron cyclotron maser emission. This leads to densities and magnetic field strengths in the radio source of n ~ 2*E11 cm-3 and B ~ 800 G. Quasi-periodic pulsations during the decay phase of the event suggest a loop radius of r ~ 7*E8 cm. A filamentary corona is implied in which the dense radio source is embedded in hot thin plasma with temperature T>=2*E7 K and density n_ext<=10-2n. Runaway acceleration by sub-Dreicer electric fields in a magnetic loop is found to supply a sufficient number of energetic electrons. Title: The evolution of coronal magnetic flux tubes under the influence of footpoint twisting motions Authors: Török, T.; Kliem, B. Bibcode: 2001sps..proc..364T Altcode: We present first results of an MHD simulation study of the quasi-static evolution of coronal magnetic flux tubes under the influence of vortex motions at their footpoints. An initial vacuum field of two sub-photospheric dipoles is subjected to slow vortex motions at the photospheric level, centred at the projected positions of the dipoles. The flux tube connecting the vortices becomes strongly twisted with time. This leads to an ascending and also laterally expanding motion of the upper parts of the tube, which takes an S-type, or sigmoidal, shape. Most of the field lines emanating at the sides of the vortices lean sidewards (to let the central flux tube ascend freely). If the amount of twist increases, the flux tube tends to take uniform width, i.e., field strength, along its upper parts. Decreasing the separation of the flux tube footpoints leads to the formation of a central current sheet between them. Title: Spectroscopic Signatures of a Flare Observed by SUMER Onboard SOHO Authors: Dammasch, I. E.; Curdt, W.; Kliem, B.; Dwivedi, B. N.; Wilhelm, K. Bibcode: 2001IAUS..203..264D Altcode: On 06 November 1999, SUMER (the Solar Ultraviolet Measurements of Emitted Radiation spectrometer on SOHO) observed a post-flare site above active region NOAA 8758 which was approaching the north-east limb. SUMER recorded a time series taken with a constant slit position and several spectral windows covering a wide temperature range (10 000 - 10 000 000 K), preceded and followed by contextual raster scans. During this operation, a flare of size C4.6 occurred in the observed region, also observed by the GOES 8 X-ray flux monitor, the SOHO/EIT imager and the YOHKOH/SXT instrument. All data sets have been coaligned. The temporal evolution seen in SUMER spectra is presented and compared with the other observations. Title: Dynamic three-dimensional spontaneous reconnection in a sheared current sheet Authors: Kliem, B.; Schumacher, J. Bibcode: 2001sps..proc..264K Altcode: We present one-fluid MHD simulations of threedimensional spontaneous magnetic reconnection, triggered by the localized occurrence of anomalous resistivity in a current sheet. The resistivity is coupled to the dynamics of the fluid through a threshold criterion based on the electron-ion drift velocity. A sheared current sheet with a uniform guide field component is considered, which is relevant, e.g., to the reconnection between newly emerging and preexisting flux on the Sun. A guide field component (Bz) of intermediate strength (comparable to the antiparallel component Bx) leads to efficient coupling and expansion of the reconnection process along the z direction and enables an evolution towards a macroscopic reconnection regime also in the present case, where the initial perturbation is of microscopic scale in all three dimensions. However, the reconnection rate decreases rapidly for increasing Bz/Bx >∼ 2. The splitting of the central X line (so-called secondary tearing) results in the spontaneous formation of interlinked flux tubes, which occur in this system in place of plasmoids. The interaction between the interlinked flux tubes leads to high and variable electric fields but also hinders the complete reconnection of the flux systems, at least temporarily. Title: Solar flare radio pulsations as a signature of dynamic magnetic reconnection Authors: Kliem, B.; Karlický, M.; Benz, A. O. Bibcode: 2000A&A...360..715K Altcode: 2000astro.ph..6324K Decimetric radio observations of the impulsive solar flare on October 5, 1992, 09:25 UT show a long series of quasi-periodic pulsations deeply modulating a continuum in the 0.6-2 GHz range that is slowly drifting toward lower frequencies. We propose a model in which the pulsations of the radio flux are caused by quasi-periodic particle acceleration episodes that result from a dynamic phase of magnetic reconnection in a large-scale current sheet. The reconnection is dominated by repeated formation and subsequent coalescence of magnetic islands (known as "secondary tearing" or "impulsive bursty" regime of reconnection), while a continuously growing plasmoid is fed by newly coalescing islands. Such a model, involving a current sheet and a growing plasmoid, is consistent with the Yohkoh observations of the same flare (Ohyama & Shibata ?). We present two-dimensional MHD simulations of dynamic magnetic reconnection that support the model. Within the framework of the proposed interpretation, the radio observations reveal details of plasmoid formation in flares. Title: Three-dimensional spontaneous magnetic reconnection in neutral current sheets Authors: Schumacher, Jörg; Kliem, Bernhard; Seehafer, Norbert Bibcode: 2000PhPl....7..108S Altcode: 1999astro.ph.12443S Magnetic reconnection in an antiparallel uniform Harris current sheet equilibrium, which is initially perturbed by a region of enhanced resistivity limited in all three dimensions, is investigated through compressible magnetohydrodynamic simulations. Variable resistivity, coupled to the dynamics of the plasma by an electron-ion drift velocity criterion, is used during the evolution. A phase of magnetic reconnection amplifying with time and leading to eruptive energy release is triggered only if the initial perturbation is strongly elongated in the direction of current flow or if the threshold for the onset of anomalous resistivity is significantly lower than in the corresponding two-dimensional case. A Petschek-like configuration is then built up for ~102 Alfvén times, but remains localized in the third dimension. Subsequently, a change of topology to an O-line at the center of the system (``secondary tearing'') occurs. This leads to enhanced and time-variable reconnection, to a second pair of outflow jets directed along the O-line, and to expansion of the reconnection process into the third dimension. High parallel current density components are created mainly near the region of enhanced resistivity. Title: Second-Harmonic Plasma Radiation of Magnetically Trapped Electrons in Stellar Coronae Authors: Stepanov, A. V.; Kliem, B.; Krüger, A.; Hildebrandt, J.; Garaimov, V. I. Bibcode: 1999ApJ...524..961S Altcode: Plasma radiation near the second harmonic of the plasma frequency driven by the loss-cone instability of magnetically trapped energetic electrons in stellar coronae is considered. Growth rates of longitudinal waves near the upper hybrid frequency are determined for warm background plasma and sufficiently high plasma densities, ωp>ωc, where the electrostatic instability prevails over the electromagnetic cyclotron maser instability, with particular attention given to the intermediate magnetic field condition, 1<ω2p/ω2c<~5. The plasma turbulence level and the brightness temperature of the second-harmonic plasma radiation arising from the coalescence of upper hybrid waves are estimated. The brightness temperature can reach ~1014 K for spontaneous conversion of the waves and ~1016 K for induced conversion. The radiation pattern of the second-harmonic plasma emission is also calculated; it shows a prevalence of the extraordinary mode. Analyzing the problem of the escape of radiation from stellar coronae, it is found that the escape window is wider for the o-mode because the x-mode radiation is strongly absorbed by the warm background plasma at the low harmonic gyrolevels, and thus the observed radiation can be polarized in the ordinary sense in the intermediate magnetic field case. Because of the high temperature of the plasma in the coronae of X-ray-emitting stars, the characteristic length scale of the wave conversion and the efficiency of the plasma radiation mechanism can be much higher than on the Sun. The results are discussed in the context of nonthermal quiescent and flare radio emission from active stars. Title: The evolution of solar bipolar jets observed in the UV by SUMER on SOHO. Authors: Mendoza-Torres, J. E.; Wilhelm, K.; Innes, D. E.; Curd, W.; Kliem, B.; Brekke, P. Bibcode: 1998larm.confE..17M Altcode: Observations of bipolar jets made in the UV by SUMER onboard SOHO are presented. The observations were carried out in the interval 74.9-79.0 nm which contains some chromospheric and transition region lines. The theory predicts that under the conditions of the solar chromosphere the reconnection of magnetic lines accelerates plasma into two oppositely directed jets. In the chromosphere, as well as in other solar layers, different phenomena, where plasma acceleration take place, are observed. Of them the UV jets, observed by SUMER, represent one of the most clear evidences of the link between magnetic reconnection and plasma acceleration. The jets are clearly revealed by a considerably intensity increase and large doppler shifts of the line wings. It should be noted that the enhancement of only one wing also occurs and that the central component of the line also undergoes velocity shifts and amplitude enhancement. The behavior of the wings found in this work is consistent with results obtained for other events (Innes et al. 1997) namely: 1)The wings are observed during few minutes 2) The doppler velocities of the line wings (red and blue) are similar to each other and typical values are of the order of 100 km/sec, 3) In general, the blue component is more long-lived than the red one, 4)the red component is, in general, more intense than the blue one. In this work, an analysis of the velocities of the three spectral components (the central and the wings) as a function of the location on the slit, used for the observations, is also done. The ratios of two OV (density sensitive) and two OIV (temperature sensitive) lines, whose wavelengths are in the observed range, are also analyzed. An important result is that these parameters show gradients along the slit. An analysis of the spatial distributions and the correlations between them is done. A discussion of these results, in relation with the appearance and the evolution of the jets, will be done. Title: Wavelet Analysis of Solar Flare Hard X-Rays Authors: Aschwanden, Markus J.; Kliem, Bernhard; Schwarz, Udo; Kurths, Jürgen; Dennis, Brian R.; Schwartz, Richard A. Bibcode: 1998ApJ...505..941A Altcode: We apply a multiresolution analysis to hard X-ray (HXR) time profiles f(t) of solar flares. This method is based on a wavelet transform (with triangle-shaped wavelets), which yields a dynamic decomposition of the power at different timescales T, the scalogram P(T, t). For stationary processes, time-averaged power coefficients, the scalegram S(T), can be calculated. We develop an algorithm to transform these (multiresolution) scalegrams S(T) into a standard distribution function of physical timescales, N(T). We analyze 647 solar flares observed with the Compton Gamma Ray Observatory (CGRO), recorded at energies >=25 keV with a time resolution of 64 ms over 4 minutes in each flare. The main findings of our wavelet analysis are:

1. In strong flares, the shortest detected timescales are found in the range Tmin ~ 0.1-0.7 s. These minimum timescales are found to correlate with the flare loop size r (measured from Yohkoh images in 46 flares), according to the relation Tmin(r) ~ 0.5(r/109 cm) s. Moreover, these minimum timescales are subject to a cutoff, Tmin(ne) >~ TDefl(ne), which corresponds to the electron collisional deflection time at the loss-cone site of the flare loops (inferred from energy-dependent time delays in CGRO data).

2. In smoothly varying flares, the shortest detected timescales are found in the range Tmin ~ 0.5-5 s. Because these smoothly varying flares exhibit also large trap delays, the lack of detected fine structure is likely to be caused by the convolution with trapping times.

3. In weak flares, the shortest detected timescales cover a large range, Tmin ~ 0.5-50 s, mostly affected by Poisson noise.

4. The scalegrams S(T) show a power-law behavior with slopes of βmax ~ 1.5-3.2 (for strong flares) over the timescale range of [Tmin, Tpeak]. Dominant peaks in the timescale distribution N(T) are found in the range Tpeak ~ 0.5-102 s, often coinciding with the upper cutoff of N(T). These observational results indicate that the fastest significant HXR time structures detected with wavelets (in strong flares) are related to physical parameters of propagation and collision processes. If the minimum timescale Tmin is associated with an Alfvénic crossing time through elementary acceleration cells, we obtain sizes of racc ~ 75-750 km, which have a scale-invariant ratio racc/r ~ 0.03 to flare loops and are consistent with cell sizes inferred from the frequency bandwidth of decimetric millisecond spikes. Title: Plasma jets in the solar atmosphere observed in EUV emission lines by SUMER on SOHO Authors: Wilhelm, K.; Innes, E. E.; Curdt, W.; Kliem, B.; Brekke, P. Bibcode: 1998ESASP.421..103W Altcode: 1998sjcp.conf..103W No abstract at ADS Title: Coronal magnetic-field changes of a strong, very fast developing compact solar active region and related processes Authors: Shibasaki, K.; Kosugi, T.; Bogod, V.; Garaimov, V.; Gelfreikh, G.; Stepanov, A. V.; Kliem, B.; Hildebrandt, J.; Kruger, A.; Hofmann, A.; Urpo, S. Bibcode: 1998cee..workE..49S Altcode: We have studied the development of the large active region NOAA AR 7321, which suddenly appeared near the central meridian of the Sun, during the first days of its remarkably fast evolution. To analyse the structural and evolutional features of the magnetosphere of the active region spatially resolved spectral-polarization observations of the large Radioheliograph at Nobeyama (Japan), the RATAN-600 telescope at Zelenchuk (Russia), and mm-wave data of the Metsahovi Radio Research Station (Finland) have been used and compared with model calculations of the radio emission. Our main goals concern the evolution of the magnetic scale height above the sunspots centre deduced from the gyromagnetic emission and its relation to the flare activity, as well as the nature of the energy release responsible for the source of long-duration coronal mm-wave radiation. Title: On the SORS project of CORONAS I and F Authors: Fomichev, V. V.; Oraevsky, V. N.; Pulinets, S. A.; Prutensky, I. S.; Gorgutsa, R. V.; Klos, Z.; Kiraga, A.; Rothkaehl, H.; Kruger, A.; Hildebrandt, J.; Aurass, H.; Klassen, A.; Mann, G.; Kliem, B.; Estel, C. Bibcode: 1998cee..workE..48F Altcode: The scientific experiment SOLAR RADIO SPECTROMETER (abbreviated: SORS) of the CORONAS-I satellite project is described and prospects to the planned mission of CORONAS-F are given. The astrophysical goal of these experiments is the investigation of solar radio emission in a wide frequency band detecting low-frequency solar flare emission outside the terrestrial radio window and its physical interpretation. The principal capacity of SORS could be demonstrated by observation of dynamic spectra of type III bursts during a relatively short operational period aboard CORONAS-I restricted by low solar activity. Using the experience of CORONAS-I it can be anticipated that a new SORS instrument on CORONAS-F will deliver a broader material for the exploration of coronal and heliospheric plasma processes. Title: Test particle acceleration in 2D current sheets undergoing dynamical phases of reconnection Authors: Kliem, B.; Schumacher, J.; Shklyar, D. R. Bibcode: 1998AdSpR..21..563K Altcode: Calculations of collision-free test particle orbits in a dynamic, filamentary current sheet are presented. The electro-magnetic fields are obtained from a two-dimensional MHD simulation; thus this study is restricted to energization in perpendicular electric fields. Starting from a multiple X-line initial perturbation, the current sheet undergoes complex dynamics, comprising a tearing-like phase, multiple island coalescence and plasmoid formation, and finally the acceleration and ejection of the plasmoid. The dynamic, multiple X-line configuration possessses two important properties supporting strong particle acceleration: efficient particle confinement which permits multiple or long-lasting acceleration phases, and appropriate asymmetries of the average electric field about the O-lines, which permits acceleration of the majority of test particles. Dynamic current sheets may thus be responsible for the efficient bulk acceleration of particles observed in the impulsive phase of solar flares. For parameters of the lower solar corona, sub-second acceleration times and maximum energies in the 10^7-10^8 eV range are obtained. Title: On the source height of decimetre-wave burst emission (abstract) Authors: Krüger, A.; Kliem, B.; Hildebrandt, J.; Karlický, M.; Zlobec, P. Bibcode: 1998PAICz..88...93K Altcode: No abstract at ADS Title: Test Particle Acceleration in 2d Current Sheets Undergoing Dynamical Phases of Reconnection Authors: Kliem, B.; Schumacher, J.; Shklyar, D. R. Bibcode: 1998paac.conf..563K Altcode: No abstract at ADS Title: An Attempt to Classify Solar Microwave-Bursts by Source Localization Characteristics and Dynamics of Flare-Energy Release Authors: Kruger, A.; Kliem, B.; Hildebrandt, J.; Nefedev, V. P.; Agalakov, B. V.; Smolkov, G. Ya. Bibcode: 1998IAUS..188..205K Altcode: An overview of spatially resolved observations of solar radio bursts obtained by the Siberian Solar Radio Telescope at 5.8 GHz during the last ten years reveals the occurrence of different classes of burst emission defined by their source characteristics. Four major classes of bursts according to the source position relative to sunspots, the source size and structure, and the source height, could be tentatively distinguished and compared with burst spectral characteristics as well as with soft X-ray emission oberved by YOHKOH. Possible consequences of these findings on the understanding of the underlying flare process are briefly discussed. Title: On the double plasma resonance effect in solar and stellar coronae Authors: Kliem, B.; Stepanov, A.; Kruger, A. Bibcode: 1998cee..workE..50K Altcode: The double plasma resonance (DPR) implies the equality of a plasma eigen mode near the upper hybrid frequency and a harmonic of the electron gyrofrequency. Under the DPR-condition, the growth rate of a plasma instability can exceed the growth rate of the nonresonant case by more than an order of magnitude. This effect has been used to interpret some fine structures of solar radio bursts, in particular the Zebra pattern (Rosenberg, 1972; Zheleznyakov & Zlotnik, 1975). We present calculations on the DPR of the upper hybrid mode instability in a two-component plasma (warm background and energetic loss cone component) that are based on the rigorous dispersion equation derived by Karpman et al. (1975) and take the background plasma temperature into account. We find that the DPR under the intermediate magnetic field condition, where the upper hybrid frequency coincides with one of the low harmonics of the electron gyrofrequency, requires high temperature ratios, T_h / T_0 ga 100. Such ratios are not expected to occur in the coronae of red dwarfs and close binaries like RS CVn and Algol where the background component is already rather hot, T_0 = 10^7-10^8K. Hence, the generation of Zebra patterns in the radio emission of these objects appears improbable. The DPR effect is more important for the solar corona under the weak magnetic field condition, omega_p ga 10 omega_c and less hot background plasma in a flare loop. Title: Logistic Avalanche Processes, Elementary Time Structures, Frequency Distributions, and Wavelet Analysis of Solar Flares Authors: Aschwanden, M. J.; Dennis, B. R.; Schwartz, R. A.; Benz, A. O.; Kliem, B.; Schwarz, U.; Kurths, J. Bibcode: 1998cee..workE..10A Altcode: Do elementary time scales exist in solar flares that could provide us a clue on the spatial fragmentation of the primary energy release process? Or is there a continuous distribution, composed of a hierarchy of unresolved time structures? We present results of two recent studies on these fundamental questions. We applied a multi-resolution analysis (using triangle-shaped wavelet transforms) to 647 solar flares observed with the COMPTON Gamma Ray Observatory (CGRO) at hard X-ray energies of >= 25 keV with a time resolution of 64 ms. From the wavelet scalegrams we infer a distribution of time scales N(T) for each flare and find a cutoff for the shortest detected time scales Tmin that is independent of the Poisson noise for strong flares. These shortest time scales Tmin are found to correlate with the flare loop radius r (Fig.1), i.e. Tmin = 0.5 (r/10^9 cm)s (measured with Yohkoh) and the collisional deflection time, Tmin > ~tDefl(n_e), (determined from the electron density n_e of trapped electrons by measuring energy-dependent time delays detected with CGRO). From these observations we infer spatial sizes of racc = 75-750 km for elementary acceleration cells. In a second study we determined the frequency distributions of elementary time structures in over 600 flares, based on some 10^4 hard X-ray pulses at 25 and 50 keV, 4000 radio type III bursts, 4000 decimetric quasi-periodic broadband pulsation events, and 10^4 decimetric millisecond spike events. All elementary time structures have a quasi-Gaussian shape and can be modeled with the logistic equation, which describes the exponential growth phase and nonlinear saturation (caused by the limited amount of available free energy) of a general instability. We derive a theoretical description of frequency distributions in terms of this logistic avalanche model and find that the power-law slope of observed frequency distributions provides a powerful diagnostic on coherent versus incoherent instabilities. Title: Multiresolution analysis of solar mm-wave bursts Authors: Schwarz, U.; Kurths, J.; Kliem, B.; Kruger, A.; Urpo, S. Bibcode: 1998A&AS..127..309S Altcode: Two methods of multi-scale time series analysis are applied to solar mm-wavelength flux time profiles in order to assess the diagnostic power of these tools for the exploration of nonlinear energy release processes. Both the multiresolution analysis (MRA), a method based on the wavelet transform, and the structure function analysis (SFA) permit the treatment of non-stationary time series. In addition, the MRA offers a local decomposition of the scaling behavior of the flux variations. Our main emphasis is directed at a decomposition of the contributions of the different time scales to the overall flux profile. The methods yield consistent values of the ``spectral index" which describes the scalings contained in the time series. We find that time profiles of bursts are qualitatively analogous to fractional Brownian motion (fBm), possessing long-range temporal correlations. Such correlations are not found in quiet Sun observations. The MRA of six solar mm-wave bursts shows that the radio flux is always composed of contributions from a broad range of time scales. Also during the main phase of bursts, which appears to be structurally analogous to the pre- and post-burst phases at a resolution limit of 1 s, flux fluctuations are enhanced in a broad range of time scales. This suggests that the mm-wave bursts are composed of unresolved elements, just as the pre- and post-burst time profiles. The underlying energy release thus appears to be fragmentary. These results are discussed in terms of the avalanche model and plasma physical models for solar energy release events. Title: Plasma radiation of power-law electrons in magnetic loops: application to solar decimeter-wave continua Authors: Zaitsev, V. V.; Kruger, A.; Hildebrandt, J.; Kliem, B. Bibcode: 1997A&A...328..390Z Altcode: It is shown that flare-produced fast electrons with a power-law energy distribution trapped in magnetic loops are capable to produce plasma waves at the upper hybrid frequency due to a loss-cone instability. Conditions for instability are obtained in dependence of the parameters of the power-law distribution and the magnetic-loop parameters. The growth rate of the plasma waves has been calculated and their energy density has been estimated in the frame of quasi-linear saturation. This instability has been considered as the cause of the solar decimetric continuum which exhibits a strong temporal and spatial correlation with regions of flare-energy release and sources of microwave bursts. The strong absorption of the first harmonic in the decimeter range and also the peculiarities of the conversion of the plasma waves into electromagnetic waves yield a preference of the generation of the decimetric continuum at the second harmonic of the plasma frequency. In this case the polarization of the radiation corresponds to the ordinary wave mode in a wide cone of propagation angles around the direction perpendicular to the magnetic field. Title: Electrostatic Wave Instability and Plasma Radiation from Coronal Loops Authors: Stepanov, A. V.; Kliem, B.; Krüger, A.; Hildebrandt, J. Bibcode: 1997SoPh..176..147S Altcode: Polarization properties of solar and stellar radio emission require, in some cases, emission below the third or fourth coronal electron gyro level, ω < 3,ω_c; 4, ω_c. In the context of plasma radiation, the source parameters should be such that the `intermediate magnetic field condition' 1 < ωp2/ωc2 < 3 is satisfied. Supposing this condition, we investigate the generation of electrostatic waves in a `warm' background plasma with a high-energy component of magnetically trapped electrons. We invoke the conversion of upper-hybrid waves and Bernstein waves into electromagnetic radiation as being responsible for intense radio emission from a coronal magnetic loop. Moreover, `odd-half' harmonic emissions in the solar radio spectrum as well as the o-mode polarization at the second harmonic of the plasma frequency are natural consequence of this proposed model. Title: Particle Acceleration by Magnetic Reconnection in Filamentary Current Sheets Authors: Kliem, B. Bibcode: 1997ICRC....1..149K Altcode: 1997ICRC...25a.149K No abstract at ADS Title: Coronal Energy Release and Magnetic Fields at Low Solar Activity Authors: Krüger, A.; Hildebrandt, J.; Kliem, B.; Hofmann, A.; Nefedev, V. P.; Agalakov, A. B.; Smolkov, G. Ya. Bibcode: 1997ESASP.404..469K Altcode: 1997cswn.conf..469K No abstract at ADS Title: Transient fast reconnection in dynamic current sheets with anomalous resistivity Authors: Schumacher, J.; Kliem, B. Bibcode: 1997AdSpR..19.1797S Altcode: Two-dimensional compressible magnetohydrodynamic simulations of current sheet dynamics under the influence of multiple anomalous resistivity areas and slight asymmetries are presented. Following induced tearing and multiple coalescence, a plasmoid is formed and accelerated. Dominant X-points drive the dynamical evolution and lead to transient occurrence of a Petschek-like reconnection geometry. The dependence of current density extrema, plasmoid bulk velocity and maximum reconnection rate on the Lundquist number is examined. Title: On the Origin of Decimetric-Wave Continuum of Solar Flares Authors: Zaitsev, V. V.; Krüger, A.; Hildebrandt, J.; Kliem, B. Bibcode: 1997pre4.conf..453Z Altcode: It is shown that flare-produced fast electrons with a power-law energy distribution trapped in magnetic loops are capable to produce plasma waves at the upper hybrid frequency due to a loss-cone instability. This instability has been considered as the cause of the solar decimetric continuum which exhibits a strong temporal and spatial correlation with regions of flare-energy release and sources of microwave bursts. The strong absorption of the first harmonic in the decimeter range and also the peculiarities of the conversion of the plasma waves into electromagnetic waves yield a preference of the generation of the decimetric continuum at the second harmonic of the plasma frequency. In this case the polarization of the radiation corresponds to the ordinary wave mode in a wide cone of propagation angles around the direction perpendicular to the magnetic field. Title: Plasma Radiation and Polarization of Stellar Radio Emission Authors: Stepanov, A. V.; Hildebrandt, J.; Kliem, B.; Kruger, A. Bibcode: 1996ASPC...93..333S Altcode: 1996ress.conf..333S No abstract at ADS Title: VLA Observations of Decimetric Microbuzsts in the Solar Corona Authors: Kliem, B.; Bastian, T. S. Bibcode: 1996ASPC...93..372K Altcode: 1996ress.conf..372K No abstract at ADS Title: MHD Simulations of Coronal Current Sheet Dynamics Authors: Kliem, B.; Schumacher, J. Bibcode: 1996mpsa.conf..305K Altcode: 1996IAUCo.153..305K No abstract at ADS Title: Book-Review - Computational Plasma Physics - with Applications to Fusion and Astrophysics Authors: Tajima, T.; Kliem, B. Bibcode: 1995AN....316..195T Altcode: No abstract at ADS Title: VLA observations of decimetric microbursts in the solar corona. Authors: Kliem, B. Bibcode: 1995AGAb...11..118K Altcode: No abstract at ADS Title: Coupled Magnetohydrodynamic and Kinetic Development of Current Sheets in the Solar Corona Authors: Kliem, B. Bibcode: 1995LNP...444...93K Altcode: 1995cmer.conf...93K This review considers (1) the stabilization of current sheets by shear flows, (2) the dynamical behaviour of coronal current sheets due to the interaction of magnetohydrodynamic (i.e., tearing and coalescence) and kinetic currentdriven instabilities, (3) the resulting spatial and temporal scales of energy release, (4) the acceleration of particles in such an environment. The dynamical current sheet model naturally explains fragmentary energy release, and it may lead to a better understanding of particle acceleration and of the cross-field propagation of the flare disturbance. Title: MHD simulations of coronal current sheet dynamics. Authors: Schumacher, J.; Kliem, B. Bibcode: 1995AGAb...11..322S Altcode: No abstract at ADS Title: Microwave Burst Timescales and Solar Flare Acceleration Processes Authors: Krueger, A.; Kliem, B.; Hildebrandt, J.; Zaitsev, V. V. Bibcode: 1994ApJS...90..683K Altcode: 1994IAUCo.142..683K The occurrence of quite different timescales in solar microwave bursts is considered and possible ways of their physical interpretation are discussed. An interesting feature is the existence of hierarchic time structures, an example of which is provided by the solar event of 1991 March 23. Plasma parameter sets to be invoked for the interpretation of timescales are estimated on the base of different models such as the collisionless conduction front model. The twisted magnetic loop model, the coalescence model, and the electric circuit model. With emphasis on the interpretation of burst fine structures with timescales of a few seconds the coalescence model has been favored. On the other hand, the simultaneous occurrence of a large range of different timescales appears well suited to be described by the circuit model. Title: Particle Orbits, Trapping, and Acceleration in a Filamentary Current Sheet Model Authors: Kliem, Bernhard Bibcode: 1994ApJS...90..719K Altcode: 1994IAUCo.142..719K Test particle orbits in the two-dimensional Fadeev equilibrium with a perpendicular electric field added are analyzed to show that impulsive bursty reconnection, which has been proposed as a model for fragmentary energy release in solar flares, may account also for particle acceleration to (near) relativistic energies within a fraction of a second. The convective electric field connected with magnetic island dynamics can play an important role in the acceleration process. Title: Particle acceleration in a filamentary current sheet model. Authors: Kliem, Bernhard Bibcode: 1993ppcn.conf..223K Altcode: Considering the observational indications that the energy release and particle acceleration in solar flares occur in a fragmentary manner as an important clue to the underlying plasma processes, the behaviour of test particles in a filamentary current sheet, which had earlier been proposed as a model for fragmentary energy release, is investigated. For simplicity, an analytic current sheet model with antiparallel external fields is used and a perpendicular electric field is added. By considering test particle orbits in detail it is demonstrated that the perpendicular electric field can accelerate particles efficiently and that the multiple X-line configuration permits higher particle energies than a single X-line configuration for similar plasma parameters. Taking typical plasma parameters of impulsive flare regions in the lower solar corona, many of the observed particle charcteristics can be reproduced; on the other hand, the relevance of the considered process to particle acceleration in galactic and extragalactic plasmas requires further study. Title: U-shaped multiband type II radio burst from a behind-limb solar flare Authors: Kliem, B.; Kruger, A.; Urbars, H. W.; Fomichev, R. V.; Gorgutsa, V. V.; Chertok, I. M. Bibcode: 1993AZh....70..188K Altcode: Spectral characteristics of an unusual type II radio burst from a flare site far behind (130 deg W) the solar limb are analyzed. The type II burst consists of three or four simultaneous analogous U-shaped bands with a slow variation of the frequency drift sign. It is proposed that the form of the dynamic spectrum corresponds to the generation of type II burst emission from the interaction of a blast shock wave with a dense large-scale arch structure of a loop-shaped coronal transient. Analysis of different variants of an interpretation of the spectral band structure points to the possibility of the generation of type II burst emission at higher harmonics (n is geater than 2) of the local plasma frequency. Title: Solar Coronal Magnetic Fields Authors: Hildebrandt, J.; Kliem, B.; Kruger, A. Bibcode: 1993IAUS..157...59H Altcode: No abstract at ADS Title: A U-shaped, multiband, type II radio burst from a solar flare beyond the limb Authors: Kliem, B.; Kruger, A.; Urbarz, H. W.; Gorgutsa, R. V.; Fomichev, V. V.; Chertok, I. M. Bibcode: 1993ARep...37...99K Altcode: 1993SvA....37...99K No abstract at ADS Title: Fiber Bursts in Type-Iv DM Radio Continua as a Signature of Coronal Current Sheet Dynamics Authors: Aurass, H.; Kliem, B. Bibcode: 1992SoPh..141..371A Altcode: In the late phase of some complex flare events which produce type IV radio emission, a narrow-band decimetric component with generally a high abundance of spectral fine structures is observed. We identify this late dm continuum as a hypothetical `switch-off' signature of the equivalent two-ribbon flare current system. The quasi-periodic and highly dynamic fiber-burst pattern studied in the February 5, 1986 example is understood as radio evidence for a final reactivation of a cyclic sequence of instabilities in the driven reconnecting current sheet below the rising prominence. Title: Third Plasma Harmonic Radiation in Type-II Bursts Authors: Kliem, B.; Krueger, A.; Treumann, R. A. Bibcode: 1992SoPh..140..149K Altcode: We present observational evidence for simultaneous fundamental, second and third harmonic radio emission during an excessively strong type II burst on February 16, 1984. This burst was emitted from an active region behind the limb allowing for fair resolution of the wave bands. If interpreted as a triple harmonic system, three different, nearly equally probable mechanisms for higher harmonic emission are qualitatively discussed. These are a four-wave process which involves very strong Langmuir waves, a decay process first proposed by Cairns (1987) for higher harmonic emission near the Earth's bow shock, and time evolution of the emitted frequency during Langmuir wave collapse. In sufficiently strong coronal shock waves, both of the former mechanisms may be more efficient than under solar wind conditions. In the third mechanism, Langmuir wave collapse may be driven by strong electron beams as are expected to exist in quasiparallel shocks where electron reflection may be strongest. We discuss the differences between the signatures of these mechanisms. Title: Particle orbits and acceleration in a filamentary current sheet model Authors: Kliem, B. Bibcode: 1992sws..coll..469K Altcode: Test particle orbits in the 2D Fadeev equilibrium with a perpendicular electric field added are analyzed to show that impulsive bursty reconnection, which has been proposed as a model for fragmentary energy release in solar flares, may account also for particle acceleration to (near) relativistic energies within a fraction of a second. The convective electric field connected with magnetic island dynamics can play an important role in the acceleration process. Title: Erratum: Microwave Characteristics of a behind-the-Limb Proton Flare Authors: Krüger, A.; Hildebrandt, J.; Kliem, B.; Aurass, H.; Kurths, J. Bibcode: 1991SoPh..135..423K Altcode: No abstract at ADS Title: Microwave characteristics of a behind-the-limb proton flare Authors: Krüger, A.; Hildebrandt, J.; Kliem, B.; Aurass, H.; Kurths, J.; Fomichev, V. V.; Chertok, I. M.; Křivský, L. Bibcode: 1991SoPh..134..171K Altcode: Behind-the-limb flares provide a unique opportunity for the study of vertical source structures of microwave bursts and dynamic flare processes. Based on complex observational data related to the outstanding solar proton event on 16 February, 1984, the development of burst emission at a height z ≥ 200000 km above the photosphere has been investigated. A comparison with the associated X-ray emission measured aboard various spacecraft yields a time lag of about ≤ 1 min between the onset of the unocculted impulsive HXR-emission and the onsets of the X-ray and microwave emissions occulted by the solar limb. The lag corresponds to a range of speeds of the propagation of the flare volume of about 3000-5000 km s−1. Considering competing transport agents that could account for such expansion of the source volume, a qualitative model of shock-wave activation of loops successively reaching into larger coronal heights is proposed. Title: Third plasma harmonic radiation in type 2 bursts Authors: Kliem, B.; Krueger, A.; Treumann, R. A. Bibcode: 1991STIN...9212982K Altcode: Some observational evidence for simultaneous fundamental, second, and third harmonic radio emission during an excessively strong type II burst on February 16, 1984 is presented. This burst was emitted from an active region behind the limb allowing for good resolution of the wave bands. Three different nearly equally probable mechanisms for higher harmonic emission are qualitatively discussed, however, preferred choice is given either to a four wave process which involves very strong Langmuir waves and a decay process for higher harmonic emission near the Earth's bow shock or Langmuir wave collapse. In sufficiently strong coronal shock waves both mechanisms may even be more efficient than under solar wind conditions. Langmuir wave collapse may be driven by strong electron beams. Observations like the present one should open additional diagnostic opportunities and should stimulate further search for higher than second harmonic emission in strong solar radio bursts. Title: Plasma Heating by Current Sheets in Solar Active Regions Authors: Kliem, B.; Seehafer, N. Bibcode: 1991mcch.conf..564K Altcode: No abstract at ADS Title: Fragmentary energy release due to tearing and coalescence in coronal current sheets Authors: Kliem, B. Bibcode: 1990AN....311..399K Altcode: No abstract at ADS Title: Book-Review - Laboratory and Space Plasmas Authors: Kikuchi, H.; Kliem, B. Bibcode: 1990AN....311..423K Altcode: No abstract at ADS Title: Plasma heating by current sheets in solar active regions. Authors: Kliem, B.; Seehafer, N. Bibcode: 1990AGAb....5...40K Altcode: No abstract at ADS Title: A survey of the peculiar radio emission of the solar behind-limb event on 16th February 1984 Authors: Chertok, I. M.; Fomichev, V. V.; Gorgutsa, R. V.; Markeev, A. K.; Podstrigach, T. S.; Aurass, H.; Hildebrandt, J.; Kliem, B.; Krüger, A.; Kurths, J.; Karlický, M.; Tlamicha, A.; Urbarz, H. W.; Zlobec, P. Bibcode: 1990AN....311...55C Altcode: This paper presents radio emission measurements, taken from different European stations, of the February 16th, 1984 solar behind-limb burst, which started in the active region AR 4408 at a longitude of about 130 deg W. The event was related to a very strong relativistic particle emission, the presence of cm-wave radiation from altitudes above 200,000 km above the photosphere, and an unusual U-shaped type II burst pattern consisting of four simultaneous emission bands. The spectrum in the cm-dm range exhibited different stages of evolution related to the burst onset, the burst maximum, and the postburst increase. It appears possible that, at dm-waves during the burst maximum, a partial overlapping of plasma radiation with gyrosynchroton radiation governing the cm-wave range took place. Title: Higher Harmonic Plasma Radiation in Solar Type-II Radio Bursts Authors: Fomichev, V. V.; Chertok, I. M.; Gorgutsa, R. V.; Markeev, A. K.; Kliem, B.; Aurass, H.; Kruger, A.; Kurts, J.; Urbarz, H. Bibcode: 1990IAUS..142..517F Altcode: No abstract at ADS Title: Source heights of radio emission derived from behind-limb flares. Authors: Krüger, A.; Hildebrandt, J.; Kliem, B. Bibcode: 1990AGAb....5...36K Altcode: No abstract at ADS Title: Book-Review - Instabilities in Space and Laboratory Plasmas Authors: Melrose, D. B.; Kliem, B. Bibcode: 1990AN....311...40M Altcode: No abstract at ADS Title: Book-Review - Instabilities in Space and Laboratory Plasmas Authors: Melrose, D. B.; Kliem, B. Bibcode: 1989AN....310..434M Altcode: No abstract at ADS Title: Time Scales of Solar Microwave Bursts and Scenarios of Flare Energy Release Authors: Krueger, A.; Kliem, B.; Hildebrandt, J. Bibcode: 1989ESASP.285..169K Altcode: 1989rsp..conf..169K Based on observational evidence that characteristic time scales of different solar microwave burst types are distributed over a wide range (0.001 to 10.000 sec), different mechanisms of energy release are considered to account for the impulsive flux increase (time scale less than 1000 sec). The coalescence instability is found to be a promising candidate to combine sufficiently short time scales with substantial energy release. Title: Dynamic Development of Coronal Current Sheets Authors: Kliem, B. Bibcode: 1989ESASP.285..117K Altcode: 1989rsp..conf..117K The possible interplay of tearing, coalescence, and current-driven kinetic instabilities in current sheets is considered on the basis of order of magnitude estimates of instability thresholds and scalings. Arguments are given that favor the development of internal dynamics for current sheets with stationary inflow and appropriate combination of parameters (the impulsive bursty regime of reconnection). The model relates newly emerging flux to flare energy release and provides the basis for an understanding of moving type 4 radio bursts associated with coronal mass ejections. Title: Examination of solar radio burst onsets in view of different flare scenarios and preflare structures. Authors: Krueger, A.; Aurass, H.; Kliem, B.; Urpo, S. Bibcode: 1987PAICz..66..245K Altcode: 1987eram....1..245K The time profiles of the solar microwave emission exhibit various phenomena reflecting the evolution of magnetic flux tubes before and during the onset of flare events. Different scenarios are possible to describe the processes of energy release in a flux tube and the interaction of a number of tubes during the preflare stage and the early flare development. Multi-peak structures at quite different time scales displayed by flux records at mm-, cm-, and dm-waves are examined. Title: Book-Review - Plasma Astrophysics Authors: Melrose, D. B.; Kliem, B. Bibcode: 1987AN....308...87M Altcode: No abstract at ADS Title: Radiostrahlung durch Temperaturanisotropieinstabilität elektronenzyklotronharmonischer Wellen in der Sonnenkorona Title: Radiostrahlung durch Temperaturanisotropieinstabilität elektronenzyklotronharmonischer Wellen in der Sonnenkorona Title: Radio emission by temperature anisotropy instability of electron cyclotron harmonic waves in the solar corona; Authors: Kliem, Bernhard Bibcode: 1987PhDT.......184K Altcode: No abstract at ADS Title: Book-Review - Radiotelescopes - ED.2 Authors: Christiansen, W. N.; Hogbom, J. A.; Kliem, B. Bibcode: 1986AN....307..384C Altcode: No abstract at ADS Title: A new plasma emission model for bright, slowly moving type IV solar radio bursts Authors: Kliem, B. Bibcode: 1986CoSka..15..709K Altcode: As a model of bright, slowly moving type IV bursts, coalescence of oblique Bernstein modes into predominantly o-mode waves is proposed. Bernstein mode instability is supposed in an adiabatically compressed plasma within the current sheet associated with the underlying traveling coronal disturbance. Title: Small time-scale features of impulsive solar nm-bursts Authors: Krüger, A.; Kliem, B.; Urpo, S. Bibcode: 1986CoSka..15..139K Altcode: High time-resolution observations of the Metsähhovi Radio Research Station of the University of Technology of Helsinki are investigated with respect to an analysis of the impulsive phase of solar microwave burst radiation. Results of an earlier statistical study (Urpo et al., 1986) are used to discuss the steepness of the time profiles of the radio flux (which was observed at 36.8 and 22.2 GHz with an angular resolution of 2.4 and 4.0 arc min, respectively) as well as their temporal fine structures, in the frames of current physical models. Title: Brightness temperature of radiation from the coupling of oblique Bernstein modes in coronal plasma Authors: Kliem, B. Bibcode: 1986CoSka..15..703K Altcode: The probability for coalescence of electrostatic Bernstein modes propagating obliquely to an ambient magnetic field and having random phases is evaluated in a kinetic weak turbulence description. Using knowledge of dispersion from linear theory and of the saturation spectrum of the waves in a temperature anisotropy instability from numerical simulation (Gitomer et al., 1972), the brightness temperature of the emitted high-frequency electro-magnetic waves is calculated.