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.
Coronal jets represent important manifestations of ubiquitous solar transients, which may be the source of significant mass and energy input to the upper solar atmosphere and the solar wind. While ...the energy involved in a jet-like event is smaller than that of "nominal" solar flares and coronal mass ejections (CMEs), jets share many common properties with these phenomena, in particular, the explosive magnetically driven dynamics. Studies of jets could, therefore, provide critical insight for understanding the larger, more complex drivers of the solar activity. On the other side of the size-spectrum, the study of jets could also supply important clues on the physics of transients close or at the limit of the current spatial resolution such as spicules. Furthermore, jet phenomena may hint to basic process for heating the corona and accelerating the solar wind; consequently their study gives us the opportunity to attack a broad range of solar-heliospheric problems.
Exploring the effects of bank vegetation on fluvial morphodynamics has long been an essential part of fluvial morphodynamic-related research. In a practical sense, a central question is: does ...increased vegetation density increase or decrease the channel width? Several aspects concerning the role of vegetation may result in examples of both width decrease and increase. In this study, we examined more than 170 alluvial river sections. Our goal was to detect the phenomena that ultimately determine riparian woody vegetation-induced width variation. We found that bed material is a governing factor. In the case of fine-grained material, i.e. median size D
< 2 mm, increasingly densely forested riparian vegetation reduces the bankfull Shields number, and destabilizes the banks toward a wider bankfull channel. In the case of coarse-grained material (i.e. median size D
≥ 16 mm), the effect is the opposite; increased density is correlated with a higher bankfull Shields number and a narrower bankfull channel. The extent of the role of vegetation varies depending on the ratio of characteristic root zone depth to channel depth and channel width. We present an improved estimator for bankfull Shields number, which considers riparian vegetation density. The bankfull Shields number can be estimated up to 19% more accurately with our corrected estimator.
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.
Our key conclusion is that the differentiation of pre-eruptive configurations in terms of SMAs and MFRs seems artificial. Both observations and modeling can be made consistent if the pre-eruptive configuration exists in a hybrid state that is continuously evolving from an SMA to an MFR. Thus, the ‘dominant’ nature of a given configuration will largely depend on its evolutionary stage (SMA-like early-on, MFR-like near the eruption).
Sympathetic eruptions on the Sun have been observed for several decades, but the mechanisms by which one eruption can trigger another remain poorly understood. We present a three-dimensional MHD ...simulation that suggests two possible magnetic trigger mechanisms for sympathetic eruptions. We consider a configuration that contains two coronal flux ropes located within a pseudo-streamer and one rope located next to it. A sequence of eruptions is initiated by triggering the eruption of the flux rope next to the streamer. The expansion of the rope leads to two consecutive reconnection events, each of which triggers the eruption of a flux rope by removing a sufficient amount of overlying flux. The simulation qualitatively reproduces important aspects of the global sympathetic event on 2010 August 1 and provides a scenario for the so-called twin filament eruptions. The suggested mechanisms are also applicable for sympathetic eruptions occurring in other magnetic configurations.
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.
Aims. We investigate the formation of flux ropes in a flux emergence region and their rise into the outer atmosphere of the Sun. Methods. We perform 3D numerical experiments by solving the ...time-dependent and resistive MHD equations. Results. A sub-photospheric twisted flux tube rises from the solar interior and expands into the corona. A flux rope is formed within the expanding field, due to shearing and reconnection of field lines at low atmospheric heights. If the tube emerges into a non-magnetized atmosphere, the flux rope rises, but remains confined inside the expanding magnetized volume. In contrast, if the expanding tube is allowed to reconnect with a pre-existing coronal field, the flux rope experiences a full eruption with a rise profile that is in qualitative agreement with erupting filaments and Coronal Mass Ejections.
Abstract
It has been suggested that the ratio of photospheric direct to return current, ∣DC/RC∣, may be a better proxy for assessing the ability of solar active regions to produce a coronal mass ...ejection (CME) than others such as the amount of shear along the polarity inversion line (PIL). To test this conjecture, we measure both quantities prior to eruptive and confined flares of varying magnitude. We find that eruptive-flare source regions have ∣DC/RC∣ > 1.63 and PIL shear above 45° (average values of 3.°2 and 68°, respectively), tending to be larger for stronger events, while both quantities are on average smaller for confined-flare source regions (2.°2 and 68°, respectively), albeit with substantial overlap. Many source regions, especially those of eruptive X-class flares, exhibit elongated direct currents (EDCs) bracketing the eruptive PIL segment, which typically coincide with areas of continuous PIL shear above 45°. However, a small subset of confined-flare source regions have ∣DC/RC∣ close to unity, very low PIL shear (<38°), and no clear EDC signatures, rendering such regions less likely to produce a CME. A simple quantitative analysis reveals that ∣DC/RC∣ and PIL shear are almost equally good proxies for assessing CME-productivity, comparable to other proxies suggested in the literature. We also show that an inadequate selection of the current-integration area typically yields a substantial underestimation of ∣DC/RC∣, discuss specific cases that require careful consideration for ∣DC/RC∣ calculation and interpretation of the results, and suggest improving photospheric CME-productivity proxies by incorporating coronal measures such as the decay index.
Abstract Observations have shown a clear association of filament/prominence eruptions with the emergence of magnetic flux in or near filament channels. Magnetohydrodynamic (MHD) simulations have been ...employed to systematically study the conditions under which such eruptions occur. These simulations to date have modeled filament channels as 2D flux ropes or 3D uniformly sheared arcades. Here we present MHD simulations of flux emergence into a more realistic configuration consisting of a bipolar active region containing a line-tied 3D flux rope. We use the coronal flux-rope model of Titov et al. as the initial condition and drive our simulations by imposing boundary conditions extracted from a flux emergence simulation by Leake et al. We identify three mechanisms that determine the evolution of the system: (i) reconnection displacing footpoints of field lines overlying the coronal flux rope, (ii) changes of the ambient field due to the intrusion of new flux at the boundary, and (iii) interaction of the (axial) electric currents in the preexisting and newly emerging flux systems. The relative contributions and effects of these mechanisms depend on the properties of the preexisting and emerging flux systems. Here we focus on the location and orientation of the emerging flux relative to the coronal flux rope. Varying these parameters, we investigate under which conditions an eruption of the latter is triggered.
Abstract
The so-called regularized Biot–Savart laws (
R
BS
L
s) provide an efficient and flexible method for modeling pre-eruptive magnetic configurations of coronal mass ejections (CMEs) whose ...characteristics are constrained by observational images and magnetic field data. This method allows one to calculate the field of magnetic flux ropes (MFRs) with small circular cross sections and an arbitrary axis shape. The field of the whole configuration is constructed as a superposition of (1) such a flux-rope field and (2) an ambient potential field derived, for example, from an observed magnetogram. The
R
BS
L
kernels are determined from the requirement that the MFR field for a straight cylinder must be exactly force free. For a curved MFR, however, the magnetic forces are generally unbalanced over the whole path of the MFR. To minimize these forces, we apply a modified Gauss–Newton method to find optimal MFR parameters. This is done by iteratively adjusting the MFR axis path and axial current. We then try to relax the resulting optimized configuration in a subsequent line-tied zero-beta magnetohydrodynamic simulation toward a force-free equilibrium. By considering two models of the sigmoidal pre-eruption configuration for the 2009 February 13 CME, we demonstrate how this approach works and what it is capable of. We show, in particular, that the building blocks of the core magnetic structure described by these models match morphological features typically observed in such types of configurations. Our method will be useful for both the modeling of particular eruptive events and theoretical studies of idealized pre-eruptive MFR configurations.
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