3.2 Flares

Throughout most of the history of detailed solar observation (i.e., since ∼ 1850) it was generally accepted that the solar flare was the cause of interplanetary disturbances and major space weather effects on Earth. So when interplanetary shocks were discovered by Mariner 2 at the dawn of the space age (Sonnett et al., 1964), most believed them to be blast waves from solar flares. Likewise, when CMEs were discovered in 1973, many thought they were also flare-driven. Careful work through the 1970s and 1980s established that the CME is a separate and, in fact, the central phenomena responsible for both interplanetary shocks and geomagnetic storms. This was finally established in the controversial paper by Gosling (1993Jump To The Next Citation Point) which, however, was disputed in several other papers, best summarized in Hudson et al. (1995). Workers now typically regard CMEs and flares as sometimes related phenomena and not as one being the cause for the other.UpdateJump To The Next Update Information

There is no one-to-one relationship between CMEs and flares. Many CMEs are associated with solar flares but many are not, just as most flares are not associated with mass ejection. When CMEs and flares occur together, the CME onsets seem to precede the flares in many cases, and the CMEs contain far more total energy than that radiated by the flare itself (Section 2.6). It is now generally accepted that CMEs and flares are part of a single magnetically-driven “event” and, therefore, it is more appropriate to consider a unified model that accounts for both. A schematic of one such unified model is shown in Figure 21View Image (Lin, 2004Jump To The Next Citation Point). This “standard” flare model has been developed and refined over the last few decades and has become known as Flux Cancellation or the Catastrophe model (e.g., Švestka and Cliver, 1992Jump To The Next Citation Point; Shibata et al., 1995; Lin and Forbes, 2000Jump To The Next Citation Point; Lin, 2004Jump To The Next Citation Point). In this model a stressed magnetic arcade that may contain a magnetic flux rope at its core begins to rise. A current sheet develops beneath it as external pressure causes oppositely directed magnetic field lines to converge and reconnect. Some of the liberated energy heats the CME plasma and adds mass and magnetic flux to it. Other energy is directed downward in the form of shock waves, energetic particles, and/or rapidly moving plasma. This energy can heat the low-lying or reconnecting magnetic loops and travel down the loops to the chromosphere, producing the flare. In some cases, especially if a prominence lifts off slowly, there may be too little energy deposited in underlying structures to produce a detectable surface brightening, or flare. Typical flares are “confined” or “compact” and do not have sufficient energy or magnetic topology to open up the ambient field and produce an eruption or ejection. However, Shibata and colleagues have argued that impulsive, compact flares might also have narrow, plasma ejections yielding small CMEs.

Other models have been developed to describe the relationship between flares and CMEs. The so-called Breakout model of Antiochos et al. (1999Jump To The Next Citation Point), for example, involves the launch of the CME via magnetic reconnection between a core and the surrounding strapping magnetic field, which produces underlying magnetic reconnection (that may give rise to a flare) later in the process. It also allows for the passage of the core field past the strapping field, which is an essential process for ensuring that the net energy throughout the CME eruption is reduced.

View Image

Figure 21: Schematic diagram of a disrupted magnetic field that forms in an eruptive process (Lin, 2004Jump To The Next Citation Point). Catastrophic loss of equilibrium, occurring in a magnetic configuration including a flux rope, stretches the closed magnetic field and creates a Kopp–Pneuman-type structure. This diagram is created by incorporating the traditional two-ribbon flare model (bottom), from Forbes and Acton (1996) with the CME model (top) of Lin and Forbes (2000Jump To The Next Citation Point). Colors denote the different hierarchies of plasma in the configuration. Image reproduced with permission from Lin (2004Jump To The Next Citation Point), copyright by Springer.

Comparisons of low coronal soft X-ray, EUV and radio data with the white light observations provide many insights into the source regions of CMEs. Previous statistical association studies indicated that erupting prominences (EPs) and X-ray events, especially of long duration, were the most common near-surface activity associated with CMEs. Gopalswamy et al. (2003b) showed that 73% of microwave EPs, and nearly all those attaining high heights, were associated with CMEs, confirming results first found during Skylab (e.g., Munro et al., 1979Jump To The Next Citation Point). There is a strong correspondence between X-ray ejecta and CMEs. Nitta and Akiyama (1999) found that flares with X-ray ejecta were always associated with CMEs and the X-ray ejecta corresponded with CME cores, likely dense, heated prominence material (also see Rust and Webb, 1977).

Although most flares occur independently of CMEs, the fastest, most energetic CMEs do tend to be associated with bright flares, and reported flares are associated with most frontside, full halo CMEs (e.g., Webb, 2002Jump To The Next Citation Point; Gopalswamy et al., 2007). This rate may be high because the surface sources associated with halo CMEs can be clearly viewed near sun center and halo CMEs appear to be faster and more energetic than average CMEs. Thus, either or both mass motion or ejection speed seem to be critical for the association of a flare with a CME. This may be because there is a larger net energy reservoir available for both phenomena.

Sheeley Jr et al. (1983) first showed that the probability of associating a CME with a soft X-ray flare increased linearly with the flare duration, reaching 100% for flare events of duration > 6 hours. Confirming previous results with lower statistical validity, Yashiro et al. (2005) found that the LASCO CME association rate with X-ray flares also increased linearly with the peak X-ray intensity. Thus, the more energetic the flare, the more likely it was to be associated with mass ejection. When longitudinal visibility effects were accounted for, Yashiro et al. found that nearly all flares above the M5 level were associated with CMEs. The SMM CME observations indicated that the estimated departure time of flare-associated CMEs typically preceded the flare onsets. Harrison (1986) found that such CMEs were initiated along with weaker soft X-ray bursts that preceded any subsequent main flare by tens of minutes, and that the main flares were often spatially offset to one side of the CME. Also, the location of flares is more closely associated with the footpoint, rather than the center, of the CME (Simnett and Harrison, 1984, 1985). We note however, that more recent results using the LASCO data reported by Yashiro et al. (2008aJump To The Next Citation Point) showed more variation between flare location and CME span, with X-flares usually centered under the CME. More details about solar flares appear in the Living Review by Benz (2008).


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