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Breakout Coronal Mass Ejection Or Streamer Blowout: the Bugle Effect B The Astrophysical Journal, 693:1178–1187, 2009 March 10 doi:10.1088/0004-637X/693/2/1178 C 2009. The American Astronomical Society. All rights reserved. Printed in the U.S.A. BREAKOUT CORONAL MASS EJECTION OR STREAMER BLOWOUT: THE BUGLE EFFECT B. van der Holst, W. Manchester IV, I.V. Sokolov, G. Toth´ , T.I. Gombosi, D. DeZeeuw, and O. Cohen Center for Space Environment Modeling, University of Michigan, 2455 Hayward Street, Ann Arbor, MI 48109, USA; [email protected] 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 arcade field associated with the line current had to be removed. Three-dimensional analytical models have also been derived to Coronal mass ejections (CMEs) are large-scale eruptive capture the CME propagation. The self-similar CME model of events that are among the most violent phenomena in the Gibson & Low (1998) was instrumental in showing the observed solar system. They typically eject large amounts of mass of three-part structure (Illing & Hundhausen 1985): a bright lead- the order of 1015–1016 g into the interplanetary space with a ing loop, a dark void surrounded by the loop, and a high-density release of kinetic energies of 1031–1032 erg. The speed of the core in the cavity. About 30% of the CMEs configurations show CMEs is typically in the range of 400–500 km s−1, but can this three-part structure (Gopalswamy 2006). reach more than 2500 km s−1. Most of the CMEs originate Another class of CME models are arcade type. Manchester from a catastrophic disruption of the slowly evolving large- (2003) showed fully self-consistent CME simulations with shear scale helmet streamers (Hundhausen 1993). Some low speed flows driven by the Lorentz force. This model produced sponta- CMEs are related to streamer blowouts (Howard et al. 1985). neous multiple eruptions. In the flux cancellation method (van The less common fast CMEs originate from the so-called active Ballegooijen & Martens 1989; Amari et al. 2003; Linker et al. regions that are small areas of concentrated magnetic flux. The 2003) the magnetic flux at the photosphere is annihilated by pre-eruptive state of a CME is quite generally located above converging footpoint motions of the pre-existing sheared ar- the polarity inversion line (PIL) at the photospheric level and cade. The restraining overarching magnetic field is converted show the strongly sheared magnetic field along this inversion by the flux cancellation into a flux rope (removal of magnetic line separating magnetic flux of opposite sign. Magnetic shear tethers) and from a certain moment a loss of equilibrium can indicates that the field has a large component parallel to the occur when a certain threshold is reached. Roussev et al. (2004) PIL and has a significant amount of free magnetic energy above demonstrated the flux cancellation in a more realistic three- the minimum magnetic energy of a potential field. A theory for dimensional solar corona by combining the solar wind with explaining sheared fields at PILs was put forth by Manchester synoptic map information. In the flux cancellation mechanism &Low(2000). the reconnection takes place at the photospheric level. In the Many of the models for CME initiation can be considered magnetic breakout mechanism (Antiochos 1998; Antiochos et as storage and release models (Klimchuk 2001). Additional al. 1999), the cutting of magnetic tethers happens on the leading free magnetic energy can be stored into the corona as a edge of the pre-eruptive arcade. This is possible for config- result of photospheric surface flows as well as emergence and urations where the restraining overarching field is oppositely cancellation of magnetic flux. The excess of energy can result directed to the arcade field, so that a magnetic null appears in in a loss of stable equilibrium, potentially giving rise to a the corona. Due to photospheric shear flows, the arcade gets en- CME eruption. For reviews on CMEs and CME models, see ergized and as a result expands. This sheared arcade reconnects for instance Low (2001) and Forbes et al. (2006). with the overlying unsheared magnetic field and thereby cre- One class of CME models assume a pre-existing fluxrope. ates a passage during the eruption phase. The breakout model The analytical force-free flux tube model of Titov & Demoulin´ was further analyzed by MacNeice et al. (2004); Phillips et (1999), which has been proposed to explain CMEs and solar al. (2005); DeVore & Antiochos (2005); van der Holst et al. flares in twisted configurations, was used as a starting point for (2007). The first three-dimensional breakout scenario was pre- the numerical experiments by Roussev et al. (2003). In their sim- sented by Lynch et al. (2005, 2008). DeVore & Antiochos (2008) ulations, they demonstrated that to achieve a CME eruption the 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 mag- The solar wind model is constructed in the framework of netic flux at the photosphere. They showed that this is possible an ideal MHD on a spherical grid (r, θ, ϕ). The full system of due to the flare reconnection that follows each eruption reforms equations are in conservative form given by the magnetic null in the corona. ∂ρ Both the flux cancellation and magnetic breakout mechanism + ∇·(ρu) = 0, (1) are possible in simple bipolar active regions. In the case of ∂t the breakout configuration, the direction of the overarching 2 magnetic field must be opposite to that of the pre-eruptive ∂(ρu) B BB GM arcade. The combination of both initiation mechanisms in a + ∇· ρuu + p + I − =−ρ er , ∂t 8π 4π r2 quadrupolar active region configuration was analyzed by Amari et al. (2007). They showed that the presence of the magnetic null (2) above the arcade makes the eruption faster due to the weaker restraining of the overarching field. ∂B In this paper, we will analyze the breakout mechanism in + ∇·(uB − Bu) = 0, ∇·B = 0, (3) the solar wind. The overarching magnetic field that is needed ∂t for the magnetic breakout is arranged by placing the bipolar active region at the base of the helmet streamer. In van der ∂E B2 u · BB + ∇· u E + p + − = 0, (4) Holst et al. (2007) a similar approach was followed and shown ∂t 8π 4π that, if the breakout CME was launched in the midplane of a helmet streamer, the breakout reconnection failed to continue where ρ, u, B, p are the mass density, the velocity, the magnetic all the way to the top of the helmet streamer. Instead, the field, and the plasma pressure, respectively. Other symbols in- breakout plasmoid reconnected on the sides with the magnetic dicate the gravitational constant G, the solar mass M, a unit field of the helmet streamer. This resulted in a disconnection vector in the radial direction er , and the polytropic index γ . of the helmet top and retarded the breakout process. We will The latter is chosen to be spatially varying to account for the demonstrate in this paper that under certain circumstances the unknown coronal heating mechanisms as outlined by Cohen breakout reconnection can succeed. The important condition is et al. (2007). The total energy expressed in terms of this poly- that the streamer swells before the breakout CME reconnects tropic index with the overarching helmet field. The pre-event swelling of 2 2 ρu p B ρGM streamers was observed by Hundhausen (1993). E = + + − (5) The paper is organized as follows: in Section 2, we describe 2 γ − 1 8π r the initial steady state solar wind configuration containing a helmet streamer and breakout arcade. In Section 3, we describe is conserved. This solar wind model uses the Wang–Sheeley– two CME scenarios for the breakout eruption. The first one is a Arge (WSA) model (Arge & Pizzo 2000;Argeetal.2004) and breakout CME, where the breakout reconnection succeeded to the Bernoulli integral along the magnetic field lines to deter- remove all the overarching magnetic field of the helmet streamer. mine γ . These equations are solved in slightly reorganized form In the second scenario, the breakout reconnection fails to remove (gravitational energy is implemented as a source term), using the all the overarching helmet field and instead of a breakout CME a block-adaptive tree solar wind Roe-type upwind scheme (BATS- streamer blowout will happen.
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