B. J. Lynch

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.

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.

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.

Sun to 1 AU propagation and evolution of a slow streamer-blowout coronal mass ejection

[1]xa0We present a comprehensive analysis of the evolution of the classic, slow streamer-blowout CME of 1 June 2008 observed by the STEREO twin spacecraft to infer relevant properties of the pre-eruption source region which includes a substantial portion of the coronal helmet streamer belt. The CME was directed ∼40° East of the Sun-Earth line and the Heliospheric Imager observations are consistent with the CME propagating essentially radially to 1 AU. The elongation-time J-map constructed from the STEREO-A HI images tracks the arrival of two density peaks that bound the magnetic flux rope ICME seen at STEREO-B on 6 June 2008. From the STEREO-A elongation-time plots we measure the ICME flux rope radial size Rc(t) and find it well approximated by the constant expansion value Vexp = 24.5 km/s obtained from the STEREO-B declining velocity profile within the magnetic cloud. The flux rope spatial orientation, determined by forward modeling fits to the STEREO COR2 and HI1 data, approaches the observed 1 AU flux rope orientation and suggests large-scale rotation during propagation, as predicted by recent numerical simulations. We compare the ICME flux content to the PFSS model coronal field for Carrington Rotation 2070 and find sufficient streamer belt flux to account for the observed ICME poloidal/twist flux if reconnection during CME initiation process is responsible for the conversion of overlying field into the flux rope twist component in the standard fashion. However, the PFSS model field cannot account for the ICME toroidal/axial flux component. We estimate the field strength of the pre-eruption sheared/axial component in the low corona and the timescales required to accumulate this energized pre-eruption configuration via differential rotation and flux cancelation by supergranular diffusion at the polarity inversion line. We show that both mechanisms are capable of generating the desired shear component over time periods of roughly 1–2 months. We discuss the implications for slow streamer-blowout CMEs arising as a natural consequence of the coronas re-adjustment to the long term evolutionary driving of the photospheric fields.

RECONNECTION-DRIVEN DYNAMICS OF CORONAL-HOLE BOUNDARIES

We investigate the effect of magnetic reconnection on the boundary between open and closed magnetic field in the solar corona. The magnetic topology for our numerical study consists of a global dipole that gives rise to polar coronal holes and an equatorial streamer belt, and a smaller active-region bipole embedded inside the closed-field streamer belt. The initially potential magnetic field is energized by a rotational motion at the photosphere that slowly twists the embedded-bipole flux. Due to the applied stress, the bipole field expands outward and reconnects with the surrounding closed flux, eventually tunneling through the streamer boundary and encountering the open flux of the coronal hole. The resulting interchange reconnection between closed and open field releases the magnetic twist and free energy trapped inside the bipole onto open field lines, where they freely escape into the heliosphere along with the entrained closed-field plasma. Thereafter, the bipole field relaxes and reconnects back down into the interior of the streamer belt. Our simulation shows that the detailed properties of magnetic reconnection can be crucial to the coronal magnetic topology, which implies that both potential-field source-surface and quasi-steady magnetohydrodynamic models may often be an inadequate description of the corona and solar wind. We discuss the implications of our results for understanding the dynamics of the boundary between open and closed field on the Sun and the origins of the slow wind.

SYMPATHETIC MAGNETIC BREAKOUT CORONAL MASS EJECTIONS FROM PSEUDOSTREAMERS

We present high-resolution 2.5D MHD simulation results of magnetic breakout-initiated coronal mass ejections (CMEs) originating from a coronal pseudostreamer configuration. The coronal null point in the magnetic topology of pseudostreamers means that the initiation of consecutive sympathetic eruptions is a natural consequence of the systems evolution. A generic source region energization process—ideal footpoint shearing parallel to the pseudostreamer arcade polarity inversion lines—is all that is necessary to store sufficient magnetic energy to power consecutive CME eruptions given that the pseudostreamer topology enables the breakout initiation mechanism. The second CME occurs because the eruptive flare reconnection of the first CME simultaneously acts as the overlying pre-eruption breakout reconnection for the sympathetic eruption. We examine the details of the magnetic and kinetic energy evolution and the signatures of the overlying null point distortion, current sheet formation, and magnetic breakout reconnection giving rise to the runaway expansion that drives the flare reconnection below the erupting sheared field core. The numerical simulations spatial resolution and output cadence are sufficient to resolve the formation of magnetic islands during the reconnection process in both the breakout and eruptive flare current sheets. We quantify the flux transfer between the pseudostreamer arcades and show that the eruptive flare reconnection processes flux ~10xa0times faster than the pre-eruption breakout reconnection. We show that the breakout reconnection jets cause bursty, intermittent upflows along the pseudostreamer stalk, as well as downflows in the adjacent pseudostreamer arcade, both of which may be observable as pre-eruption signatures. Finally, we examine the flux rope CME trajectories and show that the breakout current sheet provides a path of least resistance as an imbalance in the surrounding magnetic energy density and results in a non-radial CME deflection early in the eruption.

THE SOLAR MAGNETIC FIELD AND CORONAL DYNAMICS OF THE ERUPTION ON 2007 MAY 19

The solar eruption on 2007 May 19, from AR 10956 near solar disk center, consisted of a B9.5 flare (12:48 UT), a filament eruption, an EUV dimming, a coronal wave, and a multifront CME. The eruption was observed by the twin STEREO spacecraft at a separation angle of 8.5°. We report analysis of the source region photospheric magnetic field and its preeruption evolution using MDI magnetograms, the coronal magnetic field topology estimated via PFSS modeling, and the coronal dynamics of the eruption through STEREO EUVI wavelet-enhanced anaglyph movies. Despite its moderate magnitude and size, AR 10956 was a complex and highly nonpotential active region with a multipolar configuration, and hosted frequent flares, multiple filament eruptions, and CMEs. In the 2 days prior to the May 19 eruption, the total unsigned magnetic flux of the region decreased by ~17%. We interpret the photospheric magnetic field evolution, the coronal field topology, and the observed coronal dynamics in the context of current models of CME initiation and discuss the prospects for future MHD modeling inspired by these analyses.

Magnetic clouds and origins in STEREO era

When a coronal mass ejection (CME) encounters the Earth, the Earths electromagnetic environment is disturbed, especially when it is a magnetic cloud (MC) with enhanced, steady, and long-lasting southward field. The speed and the magnetic field of an MC are the two important properties for its geoeffectiveness. The correspondence between a CME and its resulting MC is not straightforward, partly due to the CME velocity and the complications during propagation through corona and the solar wind. From 2007 to 2012, we have three observing points at 1u2009AU near the ecliptic plane (ACE and STEREO A and B). We search for MC events encountered at one of the three observers and study the statistics independently and in comparison. We found that the annual number of MCs at each receiver varies significantly and the temporal variation at each receiver does not always follow the solar activity level. The speed and the magnetic field strength of the MCs do vary with the solar activity level. The polarity of MC magnetic field at ACE and STEREO also shows large fluctuations. We have also identified the CME and solar activity sources for the L1 MC events. STEREO SECCHI images served critical roles in the determination of the CMEs both in solar quiet times and active times. We found that halo CMEs are not necessarily good indicators for receiving MCs. Further studies of CME initial velocity and the propagation through the heliosphere are needed in order to improve our space weather forecasting capability.

IONIC COMPOSITION STRUCTURE OF CORONAL MASS EJECTIONS IN AXISYMMETRIC MAGNETOHYDRODYNAMIC MODELS

We present the ionic charge state composition structure derived from axisymmetric MHD simulations of coronal mass ejections (CMEs), initiated via the flux-cancellation and magnetic breakout mechanisms. The flux-cancellation CME simulation is run on the Magnetohydrodynamics-on-A-Sphere code developed at Predictive Sciences, Inc., and the magnetic breakout CME simulation is run on ARC7 developed at NASA GSFC. Both MHD codes include field-aligned thermal conduction, radiative losses, and coronal heating terms which make the energy equations sufficient to calculate reasonable temperatures associated with the steady-state solar wind and model the eruptive flare heating during CME formation and eruption. We systematically track a grid of Lagrangian plasma parcels through the simulation data and calculate the coronal density and temperature history of the plasma in and around the CME magnetic flux ropes. The simulation data are then used to integrate the continuity equations for the ionic charge states of several heavy ion species under the assumption that they act as passive tracers in the MHD flow. We construct two-dimensional spatial distributions of commonly measured ionic charge state ratios in carbon, oxygen, silicon, and iron that are typically elevated in interplanetary coronal mass ejection (ICME) plasma. We find that the slower CME eruption has relatively enhanced ionic charge states and the faster CME eruption shows basically no enhancement in charge states—which is the opposite trend to what is seen in the in situ ICME observations. The primary cause of the difference in the ionic charge states in the two simulations is not due to the different CME initiation mechanisms per se. Rather, the difference lies in their respective implementation of the coronal heating which governs the steady-state solar wind, density and temperature profiles, the duration of the connectivity of the CME to the eruptive flare current sheet, and the contribution of the flare-heated plasma associated with the reconnection jet outflow into the ejecta. Despite the limitations inherent in the first attempt at this novel procedure, the simulation results provide strong evidence in support of the conclusion that enhanced heavy ion charge states within CMEs are a direct consequence of flare heating in the low corona. We also discuss future improvements through combining numerical CME modeling with quantitative ionic charge state calculations.

Magnetic-Island Contraction and Particle Acceleration in Simulated Eruptive Solar Flares

The mechanism that accelerates particles to the energies required to produce the observed high-energy impulsive emission in solar flares is not well understood. Drake et al. (2006) proposed a mechanism for accelerating electrons in contracting magnetic islands formed by kinetic reconnection in multi-layered current sheets. We apply these ideas to sunward-moving flux ropes (2.5D magnetic islands) formed during fast reconnection in a simulated eruptive flare. A simple analytic model is used to calculate the energy gain of particles orbiting the field lines of the contracting magnetic islands in our ultrahigh-resolution 2.5D numerical simulation. We find that the estimated energy gains in a single island range up to a factor of five. This is higher than that found by Drake et al. for islands in the terrestrial magnetosphere and at the heliopause, due to strong plasma compression that occurs at the flare current sheet. In order to increase their energy by two orders of magnitude and plausibly account for the observed high-energy flare emission, the electrons must visit multiple contracting islands. This mechanism should produce sporadic emission because island formation is intermittent. Moreover, a large number of particles could be accelerated in each magnetohydrodynamic-scale island, which may explain the inferred rates of energetic-electron production in flares. We conclude that island contraction in the flare current sheet is a promising candidate for electron acceleration in solar eruptions.