2.5 Kinematics

Estimates of the apparent speeds of the leading edges of CMEs range from about 20 to > 2500 km s–1, or from well below the sound speed in the corona to well above the Alfvén speed (Figures 8View Image and 12View Image). The annual average speeds of Solwind and SMM CMEs varied over the solar cycle from about 150 – 475 km s–1, but their relationship to sunspot number was unclear (Howard et al., 1986; Hundhausen et al., 1994Jump To The Next Citation Point). However, LASCO CME speeds did generally track sunspot number in Solar Cycle 23 (Yashiro et al., 2004Jump To The Next Citation Point; Gopalswamy, 2010bJump To The Next Citation Point), from 280 to ∼ 550 km s–1 at the maximum and following it in 2003 (Figure 15View Image). Above a height of about 2 R⊙ the speeds of typical CMEs are relatively constant in the field of view of coronagraphs, although the slowest CMEs tend to show acceleration while the fastest tend to decelerate (St Cyr et al., 2000Jump To The Next Citation Point; Yashiro et al., 2004Jump To The Next Citation Point; Gopalswamy et al., 2006bJump To The Next Citation Point). This may be expected, given that CMEs must push through the surrounding solar wind, believed to have a speed of around 400 km s–1 in the outer corona.
View Image

Figure 15: Annual mean and median speeds of LASCO CMEs from 1996 – 2010. There are two peaks, the first near solar activity maximum and the second in 2003. Thus, high speeds were still prevalent during the early declining phase. Image adapted from Gopalswamy (2004Jump To The Next Citation Point), updated by S. Yashiro (2011).

The early acceleration for most CMEs must occur low in the corona (< 2R ⊙). Despite its increased field of view, only 17% of all LASCO CMEs exhibit acceleration out to 30 R ⊙ (St Cyr et al., 2000Jump To The Next Citation Point). St Cyr et al. (1999) compared ground-based Mauna Loa, HI MK3 and SMM observations of CMEs above 1.15 R⊙. 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., 2004Jump To The Next Citation Point, 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 (1.1– 3.0R ⊙), 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 1.4– 4.5R ⊙ 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 ∼ 1.5 –3.0 R⊙. Using LASCO data, Sheeley Jr et al. (1999Jump To The Next Citation Point) 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 30R ⊙. 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., 1999Jump To The Next Citation Point). 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., 2010aJump To The Next Citation Point,bJump To The Next Citation Point; 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 (2012Jump To The Next Citation Point), using historical observations, argue in favor of a single launch mechanism and a continuum of energies.

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