The early acceleration for most CMEs must occur low in the corona (). Despite its increased field of view, only 17% of all LASCO CMEs exhibit acceleration out to (St Cyr et al., 2000). St Cyr et al. (1999) compared ground-based Mauna Loa, HI MK3 and SMM observations of CMEs above . These had either constant speed or constant acceleration profiles. The average acceleration of the events was found to be +264 m s–2, clearly much faster than the near-zero values of acceleration for LASCO CMEs (Yashiro et al., 2004, and our Table 1). Those features associated with active regions were found to be more likely to have constant speeds and those associated with prominence eruptions to have constant accelerations. Using observations of flare-associated CMEs close to the limb in the LASCO C1 field of view (), Zhang et al. (2001, 2004) found a three-phase kinematic profile: a slow rise (< 80 km s–1) over tens of minutes; a second phase with a rapid acceleration of 100 – 500 m s–2 in the height range during the flare rise phase; and a final phase with propagation at a constant or declining speed. Gallagher et al. (2003) and others have narrowed the strong (> 200 km s–1) acceleration region of impulsive CMEs to . Using LASCO data, Sheeley Jr et al. (1999) and Srivastava et al. (1999) found that gradually accelerating CMEs were balloon-like in coronagraph images, whereas fast CMEs moved at constant speed even as far out as . However, when viewed well out of the sky plane, gradual CMEs looked like smooth halos which accelerated to a limiting value then faded, while fast CMEs had ragged structure and decelerate (Sheeley Jr et al., 1999). Yashiro et al. (2004) found that slow CMEs tend to accelerate and fast CMEs decelerated through the LASCO field of view, with those around the solar wind speed having constant speeds. Thus, CMEs attain fast acceleration low in the corona until gravity and other drag forces slow them further out. This process continues into the interplanetary medium. More recently, the high temporal and spatial resolution STEREO COR and EUVI and SDO AIA imagery has been used to investigate the initial formation and kinematics of CMEs erupting from active regions (see, e.g., papers by Zhang et al., 2012; Liu et al., 2011; Patsourakos et al., 2010a,b; Temmer et al., 2010).
Sheeley Jr et al. (1999) used LASCO data to suggest that there were two dynamical classes of CMEs: gradual CMEs, which are slower, accelerate in the coronagraph fields of view, and are preferentially associated with prominence eruptions; and impulsive CMEs, which are faster, decelerate in the coronagraph fields of view, and are preferentially associated with solar flares. This appeared to confirm the flare-prominence eruption distinction found by MacQueen and Fisher (1983) using Mauna Loa, Skylab and SMM data. The tendency for fast CMEs to be associated with solar flares has been known since the earliest observations of coronagraph CMEs (for example, Gosling et al. (1976), using Skylab observations of CMEs, found a tendency for faster CMEs to be associated with solar flares and slower ones to be associated with prominences). However, prominence eruptions are often associated with two-ribbon flares and flares can be also accompanied by prominence eruptions, especially in active regions. The basic question then is whether there are two physically different processes that launch CMEs or whether all CMEs belong to a dynamical continuum with a single physical initiation process. This issue was revisited at several SHINE workshops (e.g., Crooker, 2002), with no definitive answer. In addition, Low and Zhang (2002) proposed a model of two kinds of erupting prominence-CMEs depending on whether they had normal or inverse magnetic geometries. They found that CMEs arising in normal polarity eruptions have more energy and higher speeds. To the contrary, in a comparison of flare-associated and non-flare CMEs, Vršnak et al. (2005) found considerable overlap of accelerations and speeds between the two CME groups. While flare-associated CMEs are generally faster than those without flares, there is also a correlation between CME speeds and flare X-ray peak fluxes, in which CMEs associated with the smaller flares are similar to CMEs with filament eruptions. This argues for a CME continuum and against the two-class concept. Yurchyshyn et al. (2005) found that the speeds of both accelerating and decelerating LASCO CMEs are distributed lognormally, implying that the speeds of both groups result from many simultaneous processes or from a sequential series of processes. Recently, Howard and Harrison (2012), using historical observations, argue in favor of a single launch mechanism and a continuum of energies.
Living Rev. Solar Phys. 9, (2012), 3
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