C. R. DeVore

A Model for Solar Polar Jets

We propose a model for the jetting activity that is commonly observed in the Suns corona, especially in the open-field regions of polar coronal holes. Magnetic reconnection is the process driving the jets and a relevant magnetic configuration is the well known null-point and fan-separatrix topology. The primary challenge in explaining the observations is that reconnection must occur in a short-duration energetic burst, rather than quasi-continuously as is implied by the observations of long-lived structures in coronal holes, such as polar plumes. The key idea underlying our model for jets is that reconnection is forbidden for an axisymmetrical null-point topology. Consequently, by imposing a twisting motion that maintains the axisymmetry, magnetic stress can be built up to high levels until an ideal instability breaks the symmetry and leads to an explosive release of energy via reconnection. Using three-dimensional magnetohydrodynamic simulations, we demonstrate that this mechanism does produce massive, high-speed jets driven by nonlinear Alfven waves. We discuss the implications of our results for observations of the solar corona.

Topological Evolution of a Fast Magnetic Breakout CME in Three Dimensions

We present the extension of the magnetic breakout model for CME initiation to a fully three-dimensional, spherical geometry. Given the increased complexity of the dynamic magnetic field interactions in three dimensions, we first present a summary of the well known axisymmetric breakout scenario in terms of the topological evolution associated with the various phases of the eruptive process. In this context, we discuss the analogous topological evolution during the magnetic breakout CME initiation process in the simplest three-dimensional multipolar system. We show that an extended bipolar active region embedded in an oppositely directed background dipole field has all the necessary topological features required for magnetic breakout, i.e., a fan separatrix surface between the two distinct flux systems, a pair of spine field lines, and a true three-dimensional coronal null point at their intersection. We then present the results of a numerical MHD simulation of this three-dimensional system where boundary shearing flows introduce free magnetic energy, eventually leading to a fast magnetic breakout CME. The eruptive flare reconnection facilitates the rapid conversion of this stored free magnetic energy into kinetic energy and the associated acceleration causes the erupting field and plasma structure to reach an asymptotic eruption velocity of 1100 km s−1 over an ~15 minute time period. The simulation results are discussed using the topological insight developed to interpret the various phases of the eruption and the complex, dynamic, and interacting magnetic field structures.

A Numerical Study of the Breakout Model for Coronal Mass Ejection Initiation

A leading theory for the initiation of coronal mass ejections (CMEs) is the breakout model, in which magnetic reconnection above a filament channel is responsible for disrupting the coronal magnetic field. We present the first simulations of the complete breakout process including the initiation, the plasmoid formation and ejection, and the eventual relaxation of the coronal field to a more potential state. These simulations were performed using a new numerical code that solves the numerical cavitation problems that prevented previous simulations from calculating a complete ejection. Furthermore, the position of the outer boundary in the new simulations is increased out to 30 R☉, which enables determination of the full structure and dynamics of the ejected plasmoid. Our results show that the ejection occurs at a speed on the order of the coronal Alfven speed and hence that the breakout model can produce fast CMEs. Another key result is that the ejection speed is not sensitive to the refinement level of the grid used in the calculations, which implies that, at least for the numerical resistivity of these simulations, the speed is not sensitive to the Lundquist number. We also calculate, in detail, the helicity of the system and show that the helicity is well conserved during the breakout process. Most of the helicity is ejected from the Sun with the escaping plasmoid, but a significant fraction (of order 10%) remains in the corona. The implications of these results for observation and prediction of CMEs and eruptive flares is discussed.

Three-Dimensional Modeling of Quasi-Homologous Solar Jets

Recent solar observations (e.g., obtained with Hinode and STEREO) have revealed that coronal jets are a more frequent phenomenon than previously believed. This higher frequency results, in part, from the fact that jets exhibit a homologous behavior: successive jets recur at the same location with similar morphological features. We present the results of three-dimensional (3D) numerical simulations of our model for coronal jets. This study demonstrates the ability of the model to generate recurrent 3D untwisting quasi-homologous jets when a stress is constantly applied at the photospheric boundary. The homology results from the property of the 3D null-point system to relax to a state topologically similar to its initial configuration. In addition, we find two distinct regimes of reconnection in the simulations: an impulsive 3D mode involving a helical rotating current sheet that generates the jet and a quasi-steady mode that occurs in a 2D-like current sheet located along the fan between the sheared spines. We argue that these different regimes can explain the observed link between jets and plumes.

Rotation of Coronal Mass Ejections during Eruption

Understanding the connection between coronal mass ejections (CMEs) and their interplanetary counterparts (ICMEs) is one of the most important problems in solar-terrestrial physics. We calculate the rotation of erupting field structures predicted by numerical simulations of CME initiation via the magnetic breakout model. In this model, the initial potential magnetic field has a multipolar topology and the system is driven by imposing a shear flow at the photospheric boundary. Our results yield insight on how to connect solar observations of the orientation of the filament or polarity inversion line (PIL) in the CME source region, the orientation of the CME axis as inferred from coronagraph images, and the ICME flux rope orientation obtained from in situ measurements. We present the results of two numerical simulations that differ only in the direction of the applied shearing motions (i.e., the handedness of the sheared-arcade systems and their resulting CME fields). In both simulations, eruptive flare reconnection occurs underneath the rapidly expanding sheared fields transforming the ejecta fields into three-dimensional flux rope structures. As the erupting flux ropes propagate through the low corona (from 2 to 4 R ☉) the right-handed breakout flux rope rotates clockwise and the left-handed breakout flux rope rotates counterclockwise, in agreement with recent observations of the rotation of erupting filaments. We find that by 3.5 R ☉ the average rotation angle between the flux rope axes and the active region PIL is approximately 50°. We discuss the implications of these results for predicting, from the observed chirality of the pre-eruption filament and/or other properties of the CME source region, the direction and amount of rotation that magnetic flux rope structures will experience during eruption. We also discuss the implications of our results for CME initiation models.

FORMATION AND EVOLUTION OF A MULTI-THREADED SOLAR PROMINENCE

We investigate the process of formation and subsequent evolution of prominence plasma in a filament channel and its overlying arcade. We construct a three-dimensional time-dependent model of an intermediate quiescent prominence suitable to be compared with observations. We combine the magnetic field structure of a three-dimensional sheared double arcade with one-dimensional independent simulations of many selected flux tubes, in which the thermal nonequilibrium process governs the plasma evolution. We have found that the condensations in the corona can be divided into two populations: threads and blobs. Threads are massive condensations that linger in the flux tube dips. Blobs are ubiquitous small condensations that are produced throughout the filament and overlying arcade magnetic structure, and rapidly fall to the chromosphere. The threads are the principal contributors to the total mass, whereas the blob contribution is small. The total prominence mass is in agreement with observations, assuming reasonable filling factors of order 0.001 and a fixed number of threads. The motion of the threads is basically horizontal, while blobs move in all directions along the field. We have generated synthetic images of the whole structure in an H? proxy and in two EUV channels of the Atmospheric Imaging Assembly instrument on board Solar Dynamics Observatory, thus showing the plasma at cool, warm, and hot temperatures. The predicted differential emission measure of our system agrees very well with observations in the temperature range log T = 4.6-5.7. We conclude that the sheared-arcade magnetic structure and plasma behavior driven by thermal nonequilibrium fit the abundant observational evidence well for typical intermediate prominences.

Structure and Dynamics of the Sun’s Open Magnetic Field

The solar magnetic field is the primary agent that drives solar activity and couples the Sun to the heliosphere. Although the details of this coupling depend on the quantitative properties of the field, many important aspects of the corona-solar wind connection can be understood by considering only the general topological properties of those regions on the Sun where the field extends from the photosphere out to interplanetary space, the so-called open field regions that are usually observed as coronal holes. From the simple assumptions that underlie the standard quasi-steady corona-wind theoretical models, and that are likely to hold for the Sun as well, we derive two conjectures as to the possible structure and dynamics of coronal holes: (1) coronal holes are unique in that every unipolar region on the photosphere can contain at most one coronal hole, and (2) coronal holes of nested polarity regions must themselves be nested. Magnetic reconnection plays the central role in enforcing these constraints on the field topology. From these conjectures we derive additional properties for the topology of open field regions, and propose several observational predictions for both the slowly varying and transient corona/solar wind.

Prominence Formation by Thermal Nonequilibrium in the Sheared-Arcade Model

The existence of solar prominences—cool, dense, filamented plasma suspended in the corona above magnetic neutral lines—has long been an outstanding problem in solar physics. In earlier numerical studies we identified a mechanism, thermal nonequilibrium, by which cool condensations can form in long coronal flux tubes heated locally above their footpoints. To understand the physics of this process, we began by modeling idealized symmetric flux tubes with uniform cross-sectional area and a simplified radiative-loss function. The present work demonstrates that condensations also form under more realistic conditions, in a typical flux tube taken from our three-dimensional MHD simulation of prominence magnetic structure produced by the sheared arcade mechanism. We compare these results with simulations of an otherwise identical flux tube with uniform cross-sectional area, to determine the influence of the overall three-dimensional magnetic configuration on the condensation process. We also show that updating the optically thin radiative loss function yields more rapidly varying, dynamic behavior in better agreement with the latest prominence observations than our earlier studies. These developments bring us substantially closer to a fully self-consistent, three-dimensional model of both magnetic field and plasma in prominences.

Prominence Magnetic Dips in Three-dimensional Sheared Arcades

We calculate the distribution of field-line dips in the three-dimensional sheared arcade model for prominence/filament magnetic fields. We consider both moderately and highly sheared configurations computed by fully time-dependent three-dimensional MHD simulations in which the field was relaxed to a static equilibrium end state. In agreement with previous low spatial resolution measurements of the magnetic field inside prominences, we find that for all configurations, the field in the great majority of the calculated dips exhibits inverse polarity. But for each configuration we also find well-defined narrow regions with stable dips of normal polarity. These tend to be located on the edges of the filament ends and at the top of the central part of the prominence. This distinctive mixture of normal/inverse polarity dips that we find in sheared arcades is not likely to be present in twisted flux rope prominence models. Therefore, our results provide a rigorous and unique observational test that can distinguish between the two classes of models, as well as new predictions for future high spatial resolution spectropolarimetric observations of filaments and prominences.

INTERCHANGE RECONNECTION AND CORONAL HOLE DYNAMICS

We investigate the effect of magnetic reconnection between open and closed fields, often referred to as interchange reconnection, on the dynamics and topology of coronal hole boundaries. The most important and most prevalent three-dimensional topology of the interchange process is that of a small-scale bipolar magnetic field interacting with a large-scale background field. We determine the evolution of such a magnetic topology by numerical solution of the fully three-dimensional MHD equations in spherical coordinates. First, we calculate the evolution of a small-scale bipole that initially is completely inside an open field region and then is driven across a coronal hole boundary by photospheric motions. Next the reverse situation is calculated in which the bipole is initially inside the closed region and driven toward the coronal hole boundary. In both cases, we find that the stress imparted by the photospheric motions results in deformation of the separatrix surface between the closed field of the bipole and the background field, leading to rapid current sheet formation and to efficient reconnection. When the bipole is inside the open field region, the reconnection is of the interchange type in that it exchanges open and closed fields. We examine, in detail, the topology of the field as the bipole moves across the coronal hole boundary and find that the field remains well connected throughout this process. Our results, therefore, provide essential support for the quasi-steady models of the open field, because in these models the open and closed flux are assumed to remain topologically distinct as the photosphere evolves. Our results also support the uniqueness hypothesis for open field regions as postulated by Antiochos et?al. On the other hand, the results argue against models in which open flux is assumed to diffusively penetrate deeply inside the closed field region under a helmet streamer. We discuss the implications of this work for coronal observations.