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The Role of Magnetic Reconnection in Solar Activity

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The Role of Magnetic Reconnection in Solar Activity View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by CERN Document Server 1 The Role of Magnetic Reconnection in Solar Activity Spiro K. Antio chos and C. Richard DeVore Naval Research Lab oratory,Washington, D. C. Short title: MAGNETIC RECONNECTION AND SOLAR ACTIVITY 2 Abstract. We argue that magnetic reconnection plays the determining role in many of the various manifestations of solar activity. In particular, it is the trigger mechanism for the most energetic of solar events, coronal mass ejections and eruptive ares. We prop ose that in order to obtain explosive eruptions, magnetic reconnection in the corona must have an \on-o " nature, and show that reconnection in a sheared multi-p olar eld con guration do es have this prop erty. Numerical simulation results which supp ort this mo del are presented, and implications for coronal mass ejections/eruptive are prediction are discussed. 3 1. INTRODUCTION Magnetic reconnection is widely b elieved to be the most imp ortant pro cess by which magnetic elds transfer energy to plasmas and, consequently, has b een invoked as the explanation for a multitude of phenomena observed in space, astrophysical, and lab oratory plasmas. Reconnection is the most likely mechanism by which the Sun's magnetic eld p owers the solar atmosphere, the chromosphere and corona. Reconnection is also the pro cess by which the solar wind ow couples to planetary magnetospheres, and is the key pro cess b ehind the eventual transfer of this energy to the Earth's ionosphere and lower atmosphere. Hence, it would not be unjusti ed to describ e the ISTP as a program to study the e ects of reconnection on the Sun-Earth Connection. Reconnection is particularly attractive as an explanation for solar variability and activity b ecause of the p otential for b oth rapid dissipation of magnetic energy and the accompanying strong plasma dynamics. In general, one can categorize the energy release due to reconnection as taking four distinct forms, all of which are b elieved to o ccur commonly in the solar atmosphere. First, reconnection can result in direct plasma heating. This has b een prop osed as the pro cess that heats b oth the non- are [e.g., Parker, 1972, 1979, 1983; Sturrock and Uchida, 1981; van Bal legooijen, 1986; Antiochos, 1990] and are corona [e.g., Sweet, 1958; Parker, 1963; Petschek, 1964; Carmichael, 1964; Sturrock, 1966] Second, reconnection can pro duce strong mass motions, the so-called reconnection jets. These have b een prop osed [Karpen, Antiochos, and DeVore, 1995, 1996, 1998] as the explanation for a large variety of transient dynamic phenomena ranging from spicules [e.g., Blake and Sturrock, 1985] to explosive events [Dere et al., 1991] to large surges and sprays [e.g., Rust, 1968; Herant et al., 1991; Schmeider et al., 1994; Yokohama and Shibata, 1996] and X-ray jets [Shibata et al., 1992]. Third, reconnection can lead to the acceleration of non-thermal particles. This pro cess has b een prop osed as the explanation for the electron b eams that give rise to the hard X-ray emission in ares [e.g., Sturrock, 1980 and references therein]. Finally, reconnection can 4 pro duce MHD waves. This pro cess has b een prop osed as the explanation for coronal heating [e.g., Falconer et al., 1997] and for accelerating the solar wind [e.g., Parker, 1988; Mul lan, 1990]. In fact, it is dicult to nd a coronal phenomena that has not b een attributed to magnetic reconnection! Rather than considering one of these direct e ects of reconnection, we discuss in this pap er one of the most interesting and more subtle roles of reconnection | the initiation of ma jor disruptions of the Sun's magnetic eld. Large magnetic disruptions are generally observed as a coronal mass ejection (CME) and a are asso ciated prominence/ lament eruption. We will refer to them as CME/eruptive are (EF). These 33 events are the most energetic (up to 10 ergs) and destructive of solar disturbances. They are the main drivers of space weather and are a fo cus of study of the ISTP program. The May 2 1998 event, for example, was observed by SOHO/ISTP to consist of a lament eruption, fast CME (1,000 km/s) and an X-class are that pro duced a strong proton storm at Earth and the strongest geomagnetic activity in recent years (K = 9). CME/EFs will b e of particular interest in the up coming maximum. p In addition to their key role in driving space weather, CME/EFs are imp ortant from the viewp oint of understanding basic space-plasma physics. They are the classic manifestation of solar activity, and the mechanism for their initiation and evolution has long b een a central issue of solar physics research [e.g., Sturrock, 1980]. Space observations such as those from Yohkoh and SOHO have led to much progress in understanding magnetic disruptions, and a general framework for these events has emerged. A CME/EF is b elieved to consist of four steps: (1.) Stressed magnetic eld slowly emerges from b elow the photosphere building up the free energy in the corona quasi-statically. The emerged eld may be further stressed by the convective and rotational photospheric motions. (2.) Some triggering mechanism destroys the magnetic equilibrium of the corona, resulting in an explosive instability or loss of equilibrium. (3.) The magnetic eld erupts outward, accelerating and ejecting plasma into the 5 heliosphere. (4.) The eld closes back down via reconnection to a less stressed state, heating the plasma and pro ducing the intense X-ray burst. In this four-step picture, a CME/EF can be thought of as simply the metho d by which the Sun rids itself of magnetic stress. Since the coronal conductivity is so high, the Sun has no choice but to eject any large-scale magnetic stress into the heliosphere. Note also that in this picture, a CME and an eruptive are are part of a single phenomenon. The CME and prominence/ lament eruption constitute step (3.), the op ening of the eld. It should be emphasized that by op ening we don't mean that the eld actually disconnects from the Sun, only that the eld b ecomes dynamic and expands outward without limit. During this step the free magnetic energy is used primarily to accelerate plasma and lift it against gravity. The X-ray or H are constitutes step (4.), the closing of the eld. During this step the energy is used primarily to heat magnetically con ned plasma and to accelerate nonthermal particles. As with any astrophysical phenomenon, one can always nd CMEs and ares that app ear to be exceptions to the standard picture describ ed ab ove. Many CMEs, esp ecially high-latitude slow ones, have no detectable X-ray emission. In this case the reconnection during step (4.) is presumably very slow, resulting in minimal heating. Much more dicult to reconcile, however, is the observation that intense X-ray ares sometimes o ccur with no apparent magnetic eld op ening i.e., no CME or prominence/ lament eruption. Strong are heating is widely b elieved to be due to fast reconnection, which generally involves the formation of a current sheet. It is the op ening of the eld during (3.) that forms a large current sheet, thereby, allowing rapid reconnection to o ccur during (4.). In order to account for non-eruptive ares, it must be that either the eld do es erupt but pro duces undetectable plasma motions | this seems unlikely | or else large current sheets can form and fast reconnection can o ccur even in fully closed bip olar con gurations. There are go o d arguments, however, against the formation of current sheets in such a magnetic top ology [e.g., van Bal lejooigen, 6 1985; Antiochos, 1987], furthermore reconnection is generally b elieved to be inhibited by line-tying e ects for such top ologies [Dahlburg, Antiochos, and Zang, 1991; Vel li, Einaudi, and Hood, 1993]. More observational and theoretical work is clearly needed on the issue of apparently non-eruptive ares. In this pap er we adopt the standard 4-step picture ab ove for a CME/EF and fo cus on Step (2.), the triggering mechanism. Determining the are trigger has long b een one of the outstanding problems in solar physics [e.g., Sturrock, 1980]. In addition, it is clearly necessary to understand the triggering pro cess if we are ever to develop physics-based CME and eruptive are prediction schemes. We argue b elow that magnetic reconnection in a multi- ux magnetic top ology is this long-sought-after triggering mechanism. 2. THEORETICAL RESULTS Over the years many mo dels have b een prop osed for the initiation of CMEs and EFs, the large ma jority of which are magnetically driven [e.g., Sturrock, 1989; Moore and Roumeliotis, 1992; Mikic and Linker, 1994; Antiochos, DeVore, and Klimchuk, 1998], although mo dels in which gas buoyancy e ects are imp ortant [e.g., Low, 1994; Wolfson and Dlamini, 1997] have also b een discussed. Since the solar corona is generally 2 measured to have alow plasma b eta < 10 , it is almost certain that the energy for the eruption is stored in the magnetic eld. This is esp ecially true for the very energetic, fast eruptions which can have sp eeds exceeding 2000 km/s { of order the Alfven sp eed, far larger than coronal sound sp eeds. In recentyears, however, a ma jor obstacle to the magnetic mo dels has b een p ointed out by Aly and Sturro ck.
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