Coronal mass ejections (CMEs) consist of large structures containing plasma and magnetic fields that are expelled from the Sun into the heliosphere. They are of interest for both scientific and technological reasons. Scientifically they are of interest because they remove built-up magnetic energy and plasma from the solar corona (Low, 1996), and technologically they are of interest because they are responsible for the most extreme space weather effects at Earth (Baker et al., 2008), as well as at other planets and spacecraft throughout the heliosphere. Most of the ejected material comes from the low corona, although cooler, denser material probably of chromospheric or photospheric origin is also sometimes involved. The CME plasma is entrained on an expanding magnetic field, which commonly has the form of helical field lines with changing pitch angles, i.e., a flux rope. This paper reviews the best-determined coronal properties of CMEs and what we know about their source regions, and some key signatures of CMEs in the solar wind. Observations of Earth-directed CMEs, often observed as halos surrounding the occulting disk of near-Earth coronagraphs, are important for space weather studies.
Until the early years of this century, images of CMEs had been made near the Sun primarily by coronagraphs on board spacecraft. Coronagraphs view the outward flow of density structures emanating from the Sun by observing Thomson-scattered sunlight from the free electrons in coronal and heliospheric plasma. This emission has an angular dependence which must be accounted for in the measured brightness (e.g., Billings, 1966; Vourlidas and Howard, 2006; Howard and Tappin, 2009). They are faint relative to the background corona, but much more transient, so some form of background subtraction is typically applied to identify them. CME-related phenomena such as flares and prominence eruptions have been known since the late 19th century, and energetic particles (Forbush, 1946), type II and IV radio bursts (Wild et al., 1954), and interplanetary shocks (Sonnett et al., 1964) have been observed since the 1940s, 50s and 60s. The first spacecraft coronagraph observations of CMEs were made by the OSO-7 coronagraph in the early 1970s (Tousey, 1973). These were followed by better quality and longer periods of CME observations using Skylab (1973 – 1974; MacQueen et al., 1980), P78-1 (Solwind) (1979 – 1985; Sheeley Jr et al., 1980), and SMM (1980; 1984 – 1989; Hundhausen, 1999). In late 1995, SOHO was launched and two of its three LASCO coronagraphs still operate today (Brueckner et al., 1995). Finally late in 2006, LASCO was joined by the STEREO CORs (Howard et al., 2008a). These early observations were complemented by white light data from the ground-based Mauna Loa Solar Observatory (MLSO) K-coronameter viewing from (Fisher et al., 1981; Koomen et al., 1974) and green line observations from the coronagraphs at Sacramento Peak, New Mexico (Demastus et al., 1973) and Norikura, Japan (Hirayama and Nakagomi, 1974).
Throughout the early years also, at larger distances from the Sun, interplanetary transients were observed using interplanetary radio scintillation (1964 – present; Hewish et al., 1964; Houminer and Hewish, 1974; Vlasov, 1981) and from the zodiacal light photometers on the twin Helios spacecraft (1975 – 1983; Richter et al., 1982; Jackson, 1985). The Helios photometers observed regions in the inner heliosphere from 0.3 – 1.0 AU but with an extremely limited field of view. The new millennium witnessed the arrival of a new class of detector, the heliospheric imager, with the Solar Mass Ejection Imager (SMEI) launched on board the Coriolis spacecraft early in 2003 and the Heliospheric Imagers (HIs) launched on the twin STEREO spacecraft in late 2006. LASCO has detected well over 104 CMEs during its lifetime (Yashiro et al., 2004; Gopalswamy et al., 2009b; http://cdaw.gsfc.nasa.gov/CME_list/). SMEI observed nearly 400 transients during its 8.5 year lifetime (Webb, 2004; Webb et al., 2006; Howard and Simnett, 2008); it was switched off in September 2011. The number of “events” reported using the HIs is now over 1340 (http://www.stereo.rl.ac.uk/HIEventList.html), although less than 100 have been discussed so far in the scientific literature.
These mostly white light observations have been accompanied by those of the solar disk at coronal wavelengths including the SOHO Extreme Ultraviolet Imaging Telescope (EIT), SOHO Coronal Diagnostic Spectrometer (CDS) imagers, STEREO Extreme-UltraViolet Imager (EUVI), and instruments on board the Yohkoh, TRACE, RHESSI, and Hinode spacecraft (see Hudson and Cliver, 2001), as well as near and beyond 1 AU by in-situ experiments on spacecraft including the Voyagers, Ulysses, Helios, Wind, ACE, and STEREO. In February 2010 the Solar Dynamics Observatory (SDO) spacecraft joined the solar disk imaging ensemble, while EIT, Yohkoh, TRACE, SMEI and Ulysses no longer return scientific data. In addition, important new plasma diagnostics of CMEs have been obtained from ultraviolet spectroscopy from SOHO (CDS, SUMER, and UVCS, Hinode (EIS) and SDO (EVE). The UVCS instrument, in particular, which overlaps the same height range as LASCO C2, has provided a wealth of data on the evolution of hundreds of CMEs (see review by Kohl et al., 2006). Figure 1 shows a timeline of the launches of spacecraft relevant to CME study.
White light observations of CMEs reveal that, even near the sun, the CME can dwarf the solar disk (see Figure 2). Coronal images of CMEs have also been obtained at radio frequencies, beginning with the pioneering work at the Culgoora (Australia) Radioheliograph in the 1970s. Much of this involved the tracking of shocks (via type II bursts) through the corona and into the heliosphere, but both thermal (Gopalswamy and Kundu, 1992) and non-thermal CME radio emission (such as type IV bursts) have also been imaged. Figure 3 shows a rare image of a radio CME from the Nançay (France) Radioheliograph. The onset of CMEs has been associated with many solar disk phenomena such as flares (e.g., Feynman and Hundhausen, 1994), prominence eruptions (e.g., Hundhausen, 1999), coronal dimming (e.g., Sterling and Hudson, 1997; Thompson et al., 1999), arcade formation (e.g., Hanaoka et al., 1994; Hudson and Webb, 1997), and X-ray sigmoids (e.g., Canfield et al., 1999). However, the vast majority of the ejected energy assumes the form of mechanical energy carried by the CME and not the associated solar flare, even in the most energetic cases (Emslie et al., 2004). Many CMEs have also been observed to be unassociated with any obvious solar surface activity (Howard and Tappin, 2008; Robbrecht et al., 2009a). Most flares occur independently of CME eruptions and it now seems likely that any flare accompanying a CME is part of an underlying magnetic process rather than being a direct cause of the CME launch (Kahler, 1992; Gosling, 1993). Recent models describing the onset and early evolution of CMEs (e.g., Moore and Roumeliotis, 1992; Antiochos et al., 1999; Fan and Gibson, 2003; Lynch et al., 2005) provide a variety of mechanisms by which this may be accomplished.
We refer the reader to reviews of these models by, for example, Forbes et al. (2006), rather than discuss them at length here in this review of CME observations. We also draw the reader’s attention to other reviews of solar eruptive phenomena and CMEs, including Kahler (1992, 2006), Webb et al. (1996), Hundhausen (1997, 1999), Low (1997), St Cyr et al. (2000), Webb (2002, 2004), Gopalswamy (2004), Gopalswamy et al. (2006b), and Aschwanden (2006). Several recent journal special issue volumes are devoted to CMEs: LASCO-era CMEs (Kunow et al., 2006), CME and energetic particles (Gopalswamy et al., 2006a), STEREO results (Christian et al., 2009), and 3-D measurements (Mierla et al., 2011). In addition, see also the Living Reviews by Schwenn (2006) and Chen (2011), and other Living Reviews in Solar Physics articles on prominences, flares, space weather, and other related phenomena. One of us has also recently published an introductory text on CMEs (Howard, 2011b).
Living Rev. Solar Phys. 9, (2012), 3
This work is licensed under a Creative Commons License.