BREAKOUT CORONAL MASS EJECTION OR STREAMER BLOWOUT: THE BUGLE EFFECT

The Astrophysical Journal, 693:1178 1187, 2009 March 10 C 2009. The American Astronomical Society. All rights reserved. Printed in the U.S.A. doi:10.1088/0004-637x/693/2/1178 BREAKOUT CORONAL MASS EJECTION OR STREAMER BLOWOUT: THE BUGLE EFFECT B. van der Holst, W. Manchester IV, I.V. Sokolov, G. Tóth, T.I. Gombosi, D. DeZeeuw, and O. Cohen Center for Space Environment Modeling, University of Michigan, 2455 Hayward Street, Ann Arbor, MI 48109, USA; bartvand@umich.edu Received 2008 September 5; accepted 2008 December 3; published 2009 March 5 ABSTRACT We present three-dimensional numerical magnetohydrodynamic (MHD) simulations of coronal mass ejections (CMEs) initiated by the breakout mechanism. The initial steady state consists of a bipolar active region embedded in the solar wind. The field orientation of the active region is opposite to that of the overarching helmet streamer, so that this pre-eruptive region consists of three arcades with a magnetic null line on the leading edge of the central arcade. By applying footpoint motion near the polarity inversion line of the central arcade, the breakout reconnection is turned on. During the eruption, the plasma in front of the breakout arcade gets swept up. The latter effect causes a pre-event swelling of the streamer. The width of the helmet streamer increases in time and follows a bugle pattern. In this paper, we will demonstrate that if this pre-event streamer swelling is insufficient, reconnection on the sides of the erupting breakout arcade/flux rope sets in. This will ultimately disconnect the helmet top, resulting in a streamer blowout CME. On the other hand, if this pre-event swelling is effective enough, the breakout reconnection will continue all the way to the top of the helmet streamer. The breakout mechanisms will then succeed in creating a breakout CME. Key words: Sun: coronal mass ejections (CMEs) solar wind MHD Online-only material: color figures 1. INTRODUCTION Coronal mass ejections (CMEs) are large-scale eruptive events that are among the most violent phenomena in the solar system. They typically eject large amounts of mass of the order of 10 15 10 16 g into the interplanetary space with a release of kinetic energies of 10 31 10 32 erg. The speed of the CMEs is typically in the range of 400 500 km s 1, but can reach more than 2500 km s 1. Most of the CMEs originate from a catastrophic disruption of the slowly evolving largescale helmet streamers (Hundhausen 1993). Some low speed CMEs are related to streamer blowouts (Howard et al. 1985). The less common fast CMEs originate from the so-called active regions that are small areas of concentrated magnetic flux. The pre-eruptive state of a CME is quite generally located above the polarity inversion line (PIL) at the photospheric level and show the strongly sheared magnetic field along this inversion line separating magnetic flux of opposite sign. Magnetic shear indicates that the field has a large component parallel to the PIL and has a significant amount of free magnetic energy above the minimum magnetic energy of a potential field. A theory for explaining sheared fields at PILs was put forth by Manchester &Low(2000). Many of the models for CME initiation can be considered as storage and release models (Klimchuk 2001). Additional free magnetic energy can be stored into the corona as a result of photospheric surface flows as well as emergence and cancellation of magnetic flux. The excess of energy can result in a loss of stable equilibrium, potentially giving rise to a CME eruption. For reviews on CMEs and CME models, see for instance Low (2001) and Forbes et al. (2006). One class of CME models assume a pre-existing fluxrope. The analytical force-free flux tube model of Titov & Démoulin (1999), which has been proposed to explain CMEs and solar flares in twisted configurations, was used as a starting point for the numerical experiments by Roussev et al. (2003). In their simulations, they demonstrated that to achieve a CME eruption the arcade field associated with the line current had to be removed. Three-dimensional analytical models have also been derived to capture the CME propagation. The self-similar CME model of Gibson & Low (1998) was instrumental in showing the observed three-part structure (Illing & Hundhausen 1985): a bright leading loop, a dark void surrounded by the loop, and a high-density core in the cavity. About 30% of the CMEs configurations show this three-part structure (Gopalswamy 2006). Another class of CME models are arcade type. Manchester (2003) showed fully self-consistent CME simulations with shear flows driven by the Lorentz force. This model produced spontaneous multiple eruptions. In the flux cancellation method (van Ballegooijen & Martens 1989; Amari et al. 2003; Linker et al. 2003) the magnetic flux at the photosphere is annihilated by converging footpoint motions of the pre-existing sheared arcade. The restraining overarching magnetic field is converted by the flux cancellation into a flux rope (removal of magnetic tethers) and from a certain moment a loss of equilibrium can occur when a certain threshold is reached. Roussev et al. (2004) demonstrated the flux cancellation in a more realistic threedimensional solar corona by combining the solar wind with synoptic map information. In the flux cancellation mechanism the reconnection takes place at the photospheric level. In the magnetic breakout mechanism (Antiochos 1998; Antiochos et al. 1999), the cutting of magnetic tethers happens on the leading edge of the pre-eruptive arcade. This is possible for configurations where the restraining overarching field is oppositely directed to the arcade field, so that a magnetic null appears in the corona. Due to photospheric shear flows, the arcade gets energized and as a result expands. This sheared arcade reconnects with the overlying unsheared magnetic field and thereby creates a passage during the eruption phase. The breakout model was further analyzed by MacNeice et al. (2004); Phillips et al. (2005); DeVore & Antiochos (2005); van der Holst et al. (2007). The first three-dimensional breakout scenario was presented by Lynch et al. (2005, 2008). DeVore & Antiochos (2008) demonstrated in a three-dimensional setting that the breakout 1178

No. 2, 2009 BREAKOUT CME OR STREAMER BLOWOUT: THE BUGLE EFFECT 1179 mechanism supports multiple eruptions without renewal of magnetic flux at the photosphere. They showed that this is possible due to the flare reconnection that follows each eruption reforms the magnetic null in the corona. Both the flux cancellation and magnetic breakout mechanism are possible in simple bipolar active regions. In the case of the breakout configuration, the direction of the overarching magnetic field must be opposite to that of the pre-eruptive arcade. The combination of both initiation mechanisms in a quadrupolar active region configuration was analyzed by Amari et al. (2007). They showed that the presence of the magnetic null above the arcade makes the eruption faster due to the weaker restraining of the overarching field. In this paper, we will analyze the breakout mechanism in the solar wind. The overarching magnetic field that is needed for the magnetic breakout is arranged by placing the bipolar active region at the base of the helmet streamer. In van der Holst et al. (2007) a similar approach was followed and shown that, if the breakout CME was launched in the midplane of a helmet streamer, the breakout reconnection failed to continue all the way to the top of the helmet streamer. Instead, the breakout plasmoid reconnected on the sides with the magnetic field of the helmet streamer. This resulted in a disconnection of the helmet top and retarded the breakout process. We will demonstrate in this paper that under certain circumstances the breakout reconnection can succeed. The important condition is that the streamer swells before the breakout CME reconnects with the overarching helmet field. The pre-event swelling of streamers was observed by Hundhausen (1993). The paper is organized as follows: in Section 2, we describe the initial steady state solar wind configuration containing a helmet streamer and breakout arcade. In Section 3, we describe two CME scenarios for the breakout eruption. The first one is a breakout CME, where the breakout reconnection succeeded to remove all the overarching magnetic field of the helmet streamer. In the second scenario, the breakout reconnection fails to remove all the overarching helmet field and instead of a breakout CME a streamer blowout will happen. Finally, in Section 4, we discuss the results of the simulations and their significance for future research on breakout CMEs. 2. THE MODEL For the simulation of the breakout CME propagation through the helmet streamer, a representative magnetohydrodynamic (MHD) model is required for the steady state that includes both the helmet streamer and the breakout arcade from the onset. The essential features of a helmet streamer were numerically analyzed by Pneuman & Kopp (1971). The magnetic configuration consists of closed field lines near the Sun outlining the helmet streamer. A thin current sheet is formed at the tip of the streamer that separates open magnetic field lines of opposite direction. In this paper, we will create the initial breakout arcade in the helmet streamer via adding a bipolar active region. The dipole moment of the active region will be opposite to that of the helmet field, so that a magnetic null line is initially formed inside the helmet field. While we include the steady state solar wind in this study, it is not the intention of this work to reproduce a particular solar wind. We leave the breakout CME simulation in a realistic three-dimensional solar wind based on magnetogram data for future research. In this paper, we will simplify the analysis to a non-tilted dipole configuration with one helmet streamer only. The solar wind will therefore be closer to a solar minimum configuration. The solar wind model is constructed in the framework of an ideal MHD on a spherical grid (r, θ, ϕ). The full system of equations are in conservative form given by (ρu) t ρ t [ + ρuu + B t [ E t + u + (ρu) = 0, (1) (p + B2 8π ) I BB ] = ρ GM e 4π r 2 r, (2) + (ub Bu) = 0, B = 0, (3) (E + p + B2 8π ) u BB ] = 0, (4) 4π where ρ, u, B, p are the mass density, the velocity, the magnetic field, and the plasma pressure, respectively. Other symbols indicate the gravitational constant G, the solar mass M, a unit vector in the radial direction e r, and the polytropic index γ. The latter is chosen to be spatially varying to account for the unknown coronal heating mechanisms as outlined by Cohen et al. (2007). The total energy expressed in terms of this polytropic index E = ρu2 2 + p γ 1 + B2 8π ρgm (5) r is conserved. This solar wind model uses the Wang Sheeley Arge (WSA) model (Arge & Pizzo 2000; Argeetal.2004) and the Bernoulli integral along the magnetic field lines to determine γ. These equations are solved in slightly reorganized form (gravitational energy is implemented as a source term), using the block-adaptive tree solar wind Roe-type upwind scheme (BATS- R-US) code (Powell et al. 1999). The numerical scheme employed in this work is the multi-dimensional implementation of Roe s MHD approximate Riemann solver (Sokolov et al. 2007). For the background magnetic field, we start from the simplest possible configuration that still yields a helmet streamer: a dipole field. In our application, the field strength is 3.6 G at the pole. To obtain the triple arcade system in the equatorial streamer belt, an extra magnetic field is added at low latitude λ = π/2 θ. Instead of using a scalar potential field, we opted for a nonpotential magnetic field expressed in terms of a vector potential, B = A, given by f (λ)g(ϕ) A = A 0 r 4 sin θ e ϕ, (6) where f (λ) = { cos 2 (0.5πλ/Δλ), if λ < Δλ, 0, otherwise, and 1, ϕ π <π/9 = 20, g(ϕ) = cos 2 (9 ϕ π π), ϕ π <π/6 = 30, 0, otherwise. The latitudinal extent of the active region is taken to be Δλ = 0.32 18. The amplitude A 0 is such that the maximum

1180 VAN DER HOLST ET AL. Vol. 693 unsigned radial magnetic field component B r in the active region is 14.3 G. The use of a magnetic field that is not scalar potential will add extra magnetic free energy to the system and therefore making the breakout configuration more energized. However, it is to be noted that the overall configuration has to be relaxed to a steady state solar wind that will have a different final magnetic field topology from that described above. 2.1. Computational Grid The computational grid for simulations is spherical extending over the range R r 24 R,0 θ π, and 0 ϕ 2π. The final mesh employs 134,752 self-similar 4 4 4 adaptive mesh refinement (AMR) blocks yielding a total amount of 8.6 million internal mesh cells. The construction of the grid is as follows. We start with a grid with 128 128 256 cells for (r, θ, ϕ). The mesh is uniform in the angular direction, but uses a natural logarithmic stretching for the radial coordinate. Near the poles of the spherical coordinate system the resolution is decreased by one AMR level to avoid too small cells at high latitude. The resolution near the current sheets in the solar wind is increased by one AMR level. To accurately capture the early evolution of the erupting breakout arcade inside the helmet streamer, the number of AMR levels is increased by two in the region (r, θ, ϕ) [R, 3.14 R ] [ 14, 14 ] [ 36, 36 ]. The smallest cell sizes are obtained at the bottom of the arcade, where (Δr, Δθ,Δϕ) (0.0062 R, 0.35, 0.35 ). 2.2. Boundary Conditions The boundary conditions at the inner boundary (r = 1 R ) and the outer boundary (r = 24 R ) of the coronal domain are specified as follows. At the inner boundary, we fix the mass density and temperature uniformly: ρ = 3.35 10 16 gcm 3, T = (T e + T i )/2 = 10 6 K. For the construction of a semiempirical solar wind model with a non-uniform distribution of the density and temperature that better fits the observations at 1 AU we refer the reader to Cohen et al. (2007). For the magnetic field, we fix the radial component B r, but extrapolate the tangential components B t. The velocity components at the coronal base cannot be taken at will, but have to satisfy the fixed B r boundary condition as shown by Amari et al. (2003). From the induction equation B/ t + E = 0, (7) where the electric field is E = u B, it is concluded that under the constrain of fixed B r, i.e. B r / t = 0, the tangential velocity components has to satisfy u t = u r B t 1 [Φ(θ,ϕ,t)e r ], (8) B r B r where Φ(θ,ϕ,t) is the electrostatic potential. The flow at the lower boundary should therefore be parallel to the magnetic field up to horizontal shear flows given by the electrostatic potential at the boundary. In what follows, the radial velocity component u r at the boundary is set to zero and consequently there will be no flow along the magnetic field lines at the base. The plasma is not allowed to leave or enter via the lower boundary. While the velocity in the coronal hole boundary is in reality not zero, the magnitude of the flows is so tiny that even a very small numerical diffusion can capture the flow near the boundary. The combination of zero flow through the boundary and fixing the density ignores the potential density enhancement due to flows directed toward the Sun, like the flows resulting from the flare reconnection. Nullifying u r and fixing B r allows us to analyze the claim of Antiochos et al. (1999) that the breakout mechanism can trigger CMEs by applying only the shear flows to the breakout arcade. At this point, arbitrary electrostatic potential will be constrained as to produce zero flow at the PIL in a similar fashion to Amari et al. (2003) and furthermore constrained to only produce shear flows near the active region of interest: Φ(θ,ϕ,t) = Φ 0 B 3 r e (B r /B rm ) 2 M(θ,ϕ)T (t), (9) where B rm indicates the maximum unsigned radial magnetic field component of the active region and Φ 0 is an input amplitude. To spatially confine the shear flows near the active region of interest, a mask M(θ,ϕ) = e 1 2 [ ( θ θ0 Δθ ) 2+ ( ) ] ϕ ϕ0 2 Δϕ (10) is introduced, where we will set θ 0 = π/2, ϕ 0 = 0, Δθ = π/12 = 15, and Δϕ = π/6 = 30 for our simulations. To smoothly ramp up or down the applied shear flows, a time profile T (t) = { min(t/1200, 1), 0 <t<t0, max(1 (t t 0 )/1200, 0), t t 0, (11) is used, where t 0 is the time that the ramp down sets in and time is measured in seconds. In the simulations presented in this paper, we will vary two parameters, namely the amplitude of the applied velocity Φ 0 and the time t 0 when the ramp down sets in. At the outer boundary of the corona, the flow is super fast. As a consequence, all waves that are exited at the outer boundary will leave the corona. Therefore, all MHD variables are extrapolated at the outer boundary. 2.3. Steady State Solution The steady state solution is obtained by time evolving the initial system subject to the boundary conditions. Local time stepping is used for fast convergence to the steady state. The MHD equations are solved in a frame co-rotating with the Sun. At this point, the footpoint motions are not yet applied to the breakout arcade. Figure 1 depicts the steady state solution close to the Sun. The distribution of B r on the coronal base as well as the three-dimensional field line topology of the triple arcades and helmet streamer are shown in the left. The maximum unsigned B r in the active region is 14.3 G, yielding a maximum Alfvén speed at the coronal base of 2.2 10 3 km s 1. The gray scale of the B r contour has endpoints 4 and 4 and therefore the elliptical shaped contour lines of the active region appear somewhat rectangular. The selected field lines colored in blue belong to the helmet streamer and the open field lines at higher latitude. The selected field lines of the central (breakout) arcade are colored in red and the orientation is opposite to the overarching streamer field. A consequence of this field reversal is the appearance of a magnetic null line at the leading edge of the breakout arcade. The outermost location of the magnetic null line is at r = 1.4 R. Two neighboring arcades are also formed at midlatitudes and the corresponding field lines are indicated in green. On the right of Figure 1, the radial velocity distribution and projected field lines in the y = 0 plane are shown. A current sheet is formed at the tip of the helmet streamer that separates open field lines of opposite direction.

No. 2, 2009 BREAKOUT CME OR STREAMER BLOWOUT: THE BUGLE EFFECT 1181 R Figure 1. (Left) Initial magnetic field topology. The field lines are colored to indicate the various flux systems: the overarching helmet streamer and the open field in blue, breakout arcade in red, and the neighboring high latitude arcades in green. The signed radial magnetic field strength is shown in saturated white to black gray contours of positive to negative values. (Right) The radial velocity distribution in the y = 0 plane. Streamlines represent field lines by ignoring the ϕ component to show the direction of the field confined to the x z plane. 3. RESULTS 3.1. The Breakout Case We first present the results for the three-dimensional numerical experiment that resulted in the breakout CME. For time t > 0, shearflow type footpoint motions are applied that are localized near the PIL of the bipolar active region and tangential to the solar surface. The shape of the velocity profile as described in Section 2.2 is used. Here, we first linearly ramp-up the velocities to a maximum of 40 km s 1 during a time frame from t = 0tot = 1200 s. Between t = 1200 s and t = 8800 s, the footpoint velocities are maintained at this level. These velocities are then linearly ramped down in time from t = 8800 s to t = 10, 000 s. These shear flows cause a maximum footpoint separation of 1.0 R, which is larger than expected for the Sun. However, it should be realized that the size of helmet streamer is larger, and the field strength of the bipolar active region is smaller than reality. The changes in the physical numbers are due to the constraints in computational time. A smaller helmet streamer and a stronger field strength is expected to help to reduce the footpoint separation, since less overlying field of the helmet streamer has to be removed by a stronger breakout arcade field. It is also to be noted that a shear velocity of 40 km s 1 near the PIL is higher than the observed photospheric velocities of at most a few km s 1. The applied shear velocity is only 2% of the Alfvén speed. Since the dynamical evolution of eruptions are mainly caused by magnetic reconfiguration in the corona, one might intuitively therefore make the assumption that the initial evolution is quasi-statical. However, we will demonstrate in this paper that the so-called bugle effect depends strongly on the shear flow amplitude. We will therefore contrast the results in Section 3.2 with a closer to realistic shear flow velocity of 10 km s 1 and show that the results will be drastically different. Which one of these two scenarios will happen for a realistic solar wind configuration has to be investigated for each given magnetogram. In this paper, we will restrict the analysis to demonstrative examples showing both cases. In Figure 2, the evolution of the breakout CME inside the helmet streamer is shown. Figure 2(a) depicts the initial breakout phase at time t = 1 hr 40 minutes. Due to the applied shear velocity near the PIL of the central arcade (colored in red in the left panel), the footpoints anchored at the opposite polarity start to move in opposite direction along the PIL of the bipolar active region. This generates extra longitudinal magnetic field, B ϕ,in the sheared core of the breakout arcade. The magnetic pressure rises, and as a response the unsheared part of the arcade expands. The combination of the tension of the overlying helmet field with the expanding arcade results in a flattening of the magnetic null line on the leading edge. Once the distance between the two flux systems is of the order of the grid spacing, reconnection sets in due to the numerical dissipation. Magnetic flux of the overlying helmet streamer and the unsheared breakout arcade is transferred to the neighboring midlattitude arcades (colored in green). This so-called breakout reconnection creates a passage for the breakout arcade on the way out of the helmet streamer. Figure 2(c) shows the erupting breakout CME at a later time t = 2 hr 20 minutes. The erupting breakout structure colored in red has now almost reached the top of the helmet streamer. The breakout reconnection between the streamer field and unsheared breakout field is still progressing on the leading edge. Near the Sun the flare reconnection has set in at time t 1 hr 50 minutes. The connection between the breakout configuration and flare reconnection is not new. Sweet (1958) showed that flare reconnection can set in by merging the footpoints of the breakout arcade. In Figure 2, the flare reconnection sets in due to the radial expansion of the magnetic field in the wake of the erupting breakout arcade. Due to the flare reconnection, the breakout structure has changed from arcade like to flux rope like. The flux rope consists of the strongly sheared core of the expelled sheared arcade, and therefore consists of mainly longitudinal magnetic field, B ϕ. The footpoint of this field is either located inside the active region or away from the active

1182 VAN DER HOLST ET AL. Vol. 693 (a) Time = 1 hr 40 minutes (b) Time = 1 hr 40 minutes (c) Time = 2 hr 20 minutes (d) Time = 2 hr 20 minutes R Figure 2. Evolution of the breakout CME inside the streamer, at times t = 1hr40minutes((a) and(b)) and t = 2 hr 20 minutes ((c) and(d)). Selected field lines are shown in the left panels ((a)and(c)). The snapshots in the right ((b)and(d)) show the radial velocity in color contours y = 0 plane. The minimum and maximum velocities of the legend do not represent the actual extremal velocities. region near the coronal hole. The origin of the latter can be understood from the left panel of Figure 1: due to the finite extent in the longitudinal direction, green colored field of the neighboring arcades do not only connect north-southward, but also east-westward at the outer flanks of the active region. Part of this field moves along the PIL during the shearing phase and eventually flare reconnection causes this field to become part of the erupted flux rope. Due to the flare reconnection, flare loops are also formed near the solar base (the red colored, small loops near the PIL of the active region in panel (c) of Figure 2). Panels (b), (d) of Figure 2 displays the radial velocity in color contour at time t = 1 hr 40 minutes (panel (b)) and t = 2hr 20 minutes (panel (d)). At time t = 1 hr 40 minutes, just before the onset of the flare reconnection, the radial velocity is still modest with a maximum velocity of 200 km s 1 at the unsheared top of the erupting arcade. However, at time t = 2 hr 20 minutes, the fast reconnective upflows and downflows are observed. The maximum value of the upflow is u R = 1215 km s 1 and maximum value of the downflows is 345 km s 1. To further demonstrate the importance of the flare reconnection, the kinetic and magnetic energy of the corona are shown in Figure 3 as a function of time. For convenience, the energy contributions of the t = 0 steady state is subtracted. Due to the footpoint motion in the initial phase, longitudinal magnetic flux is created and this increases the magnetic energy. The additional magnetic pressure also causes expansion of the unsheared part of the breakout arcade, so that the kinetic energy gradually increases. However, from time t 1 hr 50 minutes the flare reconnections sets in and the kinetic energy rises steeply as the magnetic energy gets converted, showing that flare reconnection is energetically far more significant than breakout reconnection.

No. 2, 2009 BREAKOUT CME OR STREAMER BLOWOUT: THE BUGLE EFFECT 1183 magnetic energy [erg] 6.0x10 +31 5.0x10 +31 4.0x10 +31 3.0x10 +31 2.0x10 +31 1.0x10 +31 0 1 2 3 Time [hr] 1.0x10 +31 8.0x10 +30 6.0x10 +30 4.0x10 +30 2.0x10 +30 Figure 3. Change in the magnetic (solid line) and kinetic (dashed line) energies from the initial steady state. The vertical dotted line indicates the onset of the flare reconnection. At time t = 3 hr 40 minutes, the breakout CME is well outside the helmet streamer as shown in Figure 4. A threedimensional view is shown in the left panel together with a radial velocity distribution in the translucent z = 0 plane. The radial velocity in the y = 0 plane is shown in the right panel. The CME front has reached 8R. The maximum radial velocity of the CME is approximately 620 km s 1. The front displays a deep indentation at low latitudes. In Manchester et al. (2005), it was shown that the interaction of the front with the bimodal structure of the solar wind can generate this dimple close to the Sun. In our simulation the dimple is mostly due to the presence of the helmet streamer. It is also to be noted that the field lines of the breakout CME are rotated out of the meridional plane. This is of course associated with the footpoint motion that was applied to the initial breakout configuration, since that process introduced 0 kinetic energy [erg] azimuthal field. Close to the Sun, the field restructures itself into the original triple arcade structure and the helmet streamer. In van der Holst et al. (2007), it was demonstrated that if the breakout arcade is initiated along the midplane of the helmet streamer, then reconnection sets in on the sides of the erupting central arcade. This type of reconnection between the helmet streamer and breakout CME would eventually disconnect the helmet streamer top. Clearly, this process did not happen in Figure 2. Here, the breakout reconnection did succeed all the way out to the top of the helmet streamer. The reason why the breakout reconnection can sustain itself can be seen in the right panels of Figure 2. Due to the rise of the central arcade, the material in front this arcade gets swept up. It is this snow plowing effect that swells up the helmet streamer before the breakout arcade arrives. This pre-event swelling of the helmet streamer was already observed by Hundhausen (1993). In their work, this effect was found by analyzing the SMM coronagraph/ polarimeter data. They found that the helmet streamer was swelling before the passage of the CME and coined the name as the bugle effect. To understand this name, the time evolution of the swelling of the streamer is analyzed in more detail. In Figure 5, the z-coordinates of the last closed field line of the helmet streamer is followed in time for fixed value of y = 0 and x = 3. The resulting pattern depicts the shape of a bugle. 3.2. The Blowout Case In Section 3.1, it was shown that by applying shearflow type footpoint motion near the PIL of the bipolar active region the breakout mechanism succeeded in generating a CME. First the breakout reconnection was switched on, followed by a fast rise of the CME once the flare reconnection sets in. The maximum velocity of the footpoint motion used in this simulation was 40 km s 1. However, the observed photospheric velocities are at most a few km s 1. In this section, we will therefore investigate if the breakout mechanism still succeeds if footpoint motions are applied with a velocity amplitude of at most 10 km s 1. We will first linearly ramp-up the Time = 3 hr 40 minutes Time = 3 hr 40 minutes R Figure 4. Breakout CME at t = 3 hr 40 minutes. On the left is the three-dimensional view. The solid lines are selected field lines; the saturated color contours show the current density, J. The radial velocity, u R, is shown on a translucent z = 0. The latitudinal dependence of the radial velocity can be depicted from the y = 0 plane on the right. Streamlines represent field lines by ignoring the ϕ component.

1184 VAN DER HOLST ET AL. Vol. 693 Figure 5. The z-coordinate of the last closed field line of the helmet streamer followed as a function of time, but at fixed y = 0andx = 3. The figure displays the shape of a bugle. velocity to a maximum of 10 km s 1 during the time period ranging from t = 0 to t = 1200 s. Then the obtained velocity profile is maintained till t = 26,800 s. These motions are then smoothly diminished till time t = 28,000 s. The maximum footpoint separation achieved by this shear flow is 0.77 R. The initial phase of the evolution is similar to the breakout case described in Section 3.1. Due to the applied footpoint motion, the longitudinal magnetic field is created. This increases the magnetic pressure in the sheared core of the arcade, which expands outward pushing the unsheared portion of the arcade ahead of it. The breakout reconnection on the leading edge sets in and removes flux of the overarching helmet field. Flare reconnection will eventually start in the wake of erupting arcade. However, since the applied shear flows at the coronal base is smaller than in the breakout case, the initial expansion of the arcade is slower. Therefore, the material in front of the central arcade is less effectively swept up and consequently the streamer swelling, as described in Section 3.1, is less pronounced. In the left panel of Figure 6, the result is demonstrated for time t = 6 hr 40 minutes. At the locations (x,z) = (1.77, 0.56)R and (1.76, 0.57)R, there is reconnection between the erupting breakout structure and the helmet streamer field. These side reconnection are similar to those found in van der Holst et al. (2007). Due to these reconnections, flux is added to the midlatitude arcades and a new flux rope is formed ahead of the central breakout arcade. The associated flows directed toward the neighboring arcades and the flux rope on the leading edge are observed. A three-dimensional view of this flux rope can be depicted in the right panel of Figure 6. The stream lines colored in purple are a few representative field lines of this flux rope. The footpoints of these field lines are anchored at the solar base. This flux rope on the leading edge of the breakout arcade eventually prevents the breakout reconnection from removing flux from the overarching helmet field and therefore retards the ejection of the breakout arcade. In Figure 7, the radial velocity is shown in the y = 0 plane for time t = 7 hr 20 minutes along with a few projected field lines. Three reconnection locations can clearly be distinguished. The flare reconnection at (x,z) (1.22 R, 0) has an associated upflow radial velocity that reaches 800 km s 1 into the sheared breakout fluxrope and a downflow of as much as 285 km s 1 toward the footpoints of the post-flare loops. The reconnection on the sides of the breakout fluxrope has progressively removed flux on the sides of the helmet streamer. The corresponding reconnective flows are directed toward the footpoints of the midlatitude arcades and into the helmet top. In the displayed meridional cut, the top of the helmet streamer is almost disconnected. This ejection can therefore be qualified Time = 6 hr 40 minutes Time = 6 hr 40 minutes Figure 6. (Left) Saturated color representation of the radial velocity on the y = 0 plane at time t = 6 hr 40 minutes. Streamlines represent field lines by ignoring the ϕ component. (Right) The three-dimensional view of the magnetic topology showing the reconnection on the sides of the rising arcade for the blowout case. The selected field lines of the flux rope on the leading edge of the rising arcade is colored in purple. The other flux systems are colored in the same way as in Figure 1.

No. 2, 2009 BREAKOUT CME OR STREAMER BLOWOUT: THE BUGLE EFFECT 1185 Time = 7 hr 20 minutes 4.0x10 +31 3.0x10 +30 magnetic energy [erg] 3.0x10 +31 2.0x10 +31 1.0x10 +31 2.0x10 +30 1.0x10 +30 kinetic energy [erg] Figure 7. Saturated color representation of the radial velocity on the y = 0 plane at time t = 7 hr 20 minutes. Streamlines represent field lines by ignoring the ϕ component. as a streamer blowout. It is also to be noted that the breakout reconnection is absent. At time t = 8 hr 40 minutes (see Figure 8), part of the helmet streamer top is completely removed. From the radial velocity in the z = 0 plane in the left panel and the y = 0 plane in the right panel, it follows that the velocity of the CME is at most 500 km s 1. This is not much faster than the slow solar wind. Similar to the breakout CME scenario, the field lines of the streamer blowout are rotated out of the meridional plane due to the azimuthal magnetic field introduced by the footpoint motion of the initial breakout arcade. In the wake of the CME, the magnetic field is restructuring to the original triple arcade structure and helmet streamer. In Figure 9, the kinetic and magnetic energies are shown as a function of time. The energy contributions of the t = 0 0 2 4 6 8 Time [hr] Figure 9. Change in the magnetic (solid line) and kinetic (dashed line) energies from the initial steady state. The vertical dotted line indicates the onset of the flare reconnection. steady state are subtracted. Again, like the breakout case, extra longitudinal magnetic field is created due to the footpoint motion. This in turn increases the magnetic energy in the corona. The additional magnetic pressure also causes the breakout arcade to move upward and hence slightly increases the kinetic energy. At time t 6 hr 30 minutes, the flare reconnection sets in. A steep incline in the kinetic energy due to the conversion of the magnetic energy by the flare reconnection is visible. In this section, we have demonstrated that the breakout CME model produces a streamer blowout for sufficiently slow footpoint shearing motion. The difference between the fast footpoint motion, which leads to a breakout CME, and the slow shearing motion is the formation of magnetic islands ahead of the erupting breakout arcade in the latter case. For small shear flow at the PIL, the central arcade also rises slowly. Therefore the sweeping up of the material has less impact, 0 Time = 8 hr 40 minutes Time = 8 hr 40 minutes Figure 8. Streamer blowout at t = 8 hr 40 minutes. On the left is the three-dimensional view. The solid lines are selected field lines; the saturated color contours shows the current density, J. The radial velocity, U R, is shown on a translucent z = 0. The latitudinal dependence of the radial velocity can be depicted from the y = 0 plane on the right. Streamlines represent field lines by ignoring the ϕ component.

1186 VAN DER HOLST ET AL. Vol. 693 resulting in an insufficient pre-event widening of the streamer and consequently reconnection on the side with the helmet streamer sets in. However, one might argue that this island formation is not due to the lack of swept up plasma, but instead due to a tearing instability of the breakout current sheet. If that would be the case, these magnetic islands can be suppressed by sufficiently reducing the magnetic resistivity. Since we relied on the numerical diffusion for the resistivity, this effectively means increasing the grid resolution. While a much higher resolution is needed to obtain realistic values for the resistivity in the corona, it is not yet feasible to perform such simulations. To demonstrate that the physical processes shown in this paper are grid converged, the calculations are also performed with a lower resolution. The domain size of the highest AMR level, as described in Section 2.1, is in the radial direction decreased from [R, 3.14 R ]to[r, 1.4 R ], so that at the location of the island formation the grid resolution is reduced. Figure 10 shows the island formation in the streamer blowout case in the left panel and the breakout CME in the right panel, both with this coarse grid. While there are some minor quantitative differences with the high resolution simulations, the basic features like the bugle effect and island formation are recaptured. 4. DISCUSSION AND CONCLUSIONS We have investigated the time evolution of a breakout eruption in the solar wind. The initial magnetic configuration consists of a simple large, but fully three-dimensional bipolar active region located at the base of the helmet streamer. The two polarities of this photospheric region are separated as well as enclosed by a polarity inversion line. Since the magnetic field of the overarching helmet streamer is oppositely directed to the central breakout arcade, a magnetic null line is found higher up in the streamer. This line separates the configuration into a breakout arcade, two neighboring midlatitude arcades, and the overarching streamer. By forcing a shear type displacement to the footpoints of the central arcade, the magnetic field gets more non-potential and therefore energized. The result of this motion is the buildup of a strong magnetic field component parallel to the central polarity inversion line. The added magnetic pressure causes the central arcade to slowly expand radially so that the breakout reconnection with the overlying helmet field sets in. A consequence of the rising arcade is that the low-lying sheared field opens up resulting from flare reconnection. Once this happens the breakout arcade transforms to a flux rope that gets expelled much faster and post-flare loops are formed close to the solar base along with a coronal magnetic null line on the leading edge of these loops. In this paper, two scenarios are shown for the further evolution of the breakout flux rope and the interaction with the solar wind. The first case is a simulation where the breakout reconnection succeeded to remove the restraining overarching helmet field all the way up to the top of the helmet streamer. During this eruptive phase inside the helmet streamer, the rising flux rope is snow plowing the plasma ahead of the flux rope. It is this swept up material that broadens the helmet streamer before the flux rope and in doing so it facilitates the magnetic breakout reconnection between the flux rope and the helmet field. This phenomenon of pre-event swelling of the helmet streamer was observed by Hundhausen (1993) in the statistical analysis of the SMM coronagraph/polarimeter data set. In our second scenario, we decreased the velocity of the footpoint motions from 40 km s 1 to a somewhat more realistic value of 10 km s 1. As a consequence the initial rise of the breakout arcade is also slower. Therefore, the swept up material ahead of the arcade gains less momentum and does not succeed to broaden the streamer enough. The breakout reconnection stops prematurely and instead reconnection sets in on the sides resulting in a disconnection of the overlying helmet top. The resulting CME is a streamer blowout. However, it is possible that for realistic values of the Lundquist number the magnetic island formation ahead of the breakout arcade does not occur. This would be the case if these islands are the result of a tearing instability of the breakout current sheet. For the grid resolutions that are currently possible, we cannot distinguish between the tearing instability and the lack of swept up plasma. A grid convergence study, however, indicates that the latter scenario is a more likely explanation. These simulation results show that for realistic shear flows velocities the breakout eruption results in a streamer blowout. Time = 6 hr 40 minutes Time = 2 hr 20 minutes Figure 10. (Left) Streamer blowout case at time t = 6 hr 40 minutes as in the left panel of Figure 6, but with lower resolution. (Right) The low resolution result of the breakout CME in Figure 4(d) at time t = 2hr20minutes.

No. 2, 2009 BREAKOUT CME OR STREAMER BLOWOUT: THE BUGLE EFFECT 1187 Still more realistic simulations are needed to test which scenario, breakout or blowout, will happen for a realistic solar wind. First of all, in our simulation the breakout arcade is initially placed in the midplane of the streamer. It is possible that for an asymmetrically placed arcade, not all of the side reconnection will happen. Whether the resulting CME will be a breakout CME or a streamer blowout is still an open question. Another issue that needs to be investigated further is whether or not the side reconnections, that result into magnetic island formation, do also set in for the slow footpoint driving scenario if the resistivity has a much higher, realistic Lundquist number. Thirdly, active region in realistic solar wind configurations are smaller with more concentrated magnetic flux. Rising breakout arcades with strong magnetic field are maybe better in snow plowing the material ahead and therefore are potentially better in swelling the streamer before the breakout flux rope and thus facilitating the breakout reconnection. Whether this will result in a fast breakout CME has to be investigated in a realistic solar wind based on magnetogram input. The results in this paper demonstrate that the magnetic breakout model in combination with flare reconnection can explain several phenomena like CMEs that do not disconnect the streamer top, streamer blowout CMEs, and the bugle effect. This work was supported by NSF ATM grants 0325332 and 0642309, NASA grant NNX07AC16G, and AFOSR grant FA9550-07-1-0434 at the University of Michigan. W. Manchester IV is supported by NASA grant LWS NNX06AC36G. The numerical simulations have been performed on the Columbia supercomputer at NASA Ames. REFERENCES Amari, T., Luciani, J. F., Aly, J. J., Mikic, Z., & Linker, J. 2003, ApJ, 595, 1231 Amari, T., Aly, J. J., Mikić, Z., & Linker, J. 2007, ApJ, 671, 189 Antiochos, S. K. 1998, ApJ, 502, 181 Antiochos, S. K., DeVore, C. R., & Klimchuck, J. A. 1999, ApJ, 510, 485 Arge, C. N., & Pizzo, V. J. 2000, J. Geophys. Res., 105, 10465 Arge, C. N., Luhmann, J. G., Odstrcil, D., Schrijver, C. J., & Li, Y. 2004, J. Atmos. Terr. Phys., 66, 1295 Cohen, O., et al. 2007, ApJ, 654, L163 DeVore, C. R., & Antiochos, S. K. 2005, ApJ, 628, 1031 DeVore, C. R., & Antiochos, S. K. 2008, ApJ, 680, 740 Forbes, T. G., et al. 2006, Space Sci. Rev., 123, 251 Gopalswamy, N., Mikić, Z., Maia, D., Alexander, D., Cremades, H., Kaufmann, P., Tripathi, D., & Wang, Y.-M. 2006, Space Sci. Rev., 123, 303 Gibson, S., & Low, B. C. 1998, ApJ, 493, 460 Howard, R. A., Sheeley, N. R. Jr., Koomen, M. J., & Michels, D. J. 1985, J. Geophys. Res., 90, 8173 Hundhausen, A. J. 1993, J. Geophys. Res., 98, 13177 Illing, R. M.E., & Hundhausen, A. J. 1985, J. Geophys. Res., 90, 275 Klimchuk, J. A. 2001, in Space Weather, ed. P. Song, H. J. Singer, & G. L. Siscoe (Washington: AGU) 143 Linker, J. A., Mikić, Z., Lionello, R., Riley, P., Amari, T., & Odstrcil, D. 2003, Phys. Plasmas, 10, 1971 Low, B. C. 2001, J. Geophys. Res., 106, 25141 Lynch, B. J., Antiochos, S. K., DeVore, C. R., & Zurbuchen, T. H. 2005, in Proc. Solar Wind 11 SOHO 16, Connecting Sun and Heliosphere (ESA SP-592; Noordwijk: ESA), 297 Lynch, B. J., Antiochos, S. K., DeVore, C. R., Luhmann, J. G., & Zurbuchen, T. H. 2008, ApJ, 683, 1192 MacNeice, P., Antiochos, S. K., Phillips, A., Spicer, D. S., DeVore, C. R., & Olson, K. 2004, ApJ, 614, 1028 Manchester, W. B. IV, & Low, B. C. 2000, Phys. Plasmas, 7, 1263 Manchester, W. B. IV. 2003, J. Geophys. Res., 108, 1162 Manchester, W. B., et al. 2005, ApJ, 622, 1225 Phillips, A. D., MacNeice, P. J., & Antiochos, S. K. 2005, ApJ, 624, 129 Pneuman, G. W., & Kopp, R. A. 1971, Sol. Phys., 18, 258 Powell, K. G., Roe, P. L., Linde, T. J., Gombosi, T. I., & DeZeeuw, D. L. 1999, J. Comput. Phys., 154, 284 Roussev, I. I., Forbes, T. G., Gombosi, T. I., Sokolov, I. V., DeZeeuw, D. L., & Birn, J. 2003, ApJ, 588, 45 Roussev, I. I., Sokolov, I. V., Forbes, T. G., Gombosi, T. I., Lee, M. A., & Sakai, J. I. 2004, ApJ, 605, 73 Sokolov, I. V., Powell, K. G., Cohen, O., & Gombosi, T. I. 2007, in ASP Conf. Ser. 385, Numerical Modeling of Space Plasma Flows, ed. N. V. Pogorelov, E. Audit, & G. P. Zank (San Francisco, CA: ASP) 291 Sweet, P. A. 1958, in IAU Symp. 6, Electromagnetic Phenomena in Cosmical Physics, ed. B. Lehnert (Dordrecht: Kluwer) 123 Titov, V. S., & Démoulin, P. 1999, A&A, 351, 707 van Ballegooijen, A. A., & Martens, P. C.H. 1989, ApJ, 343, 971 van der Holst, B., Jacobs, C., & Poedts, S. 2007, ApJ, 671, L77