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; St Cyr et al., 2000; Gopalswamy, 2004). The apparent angular width of CMEs ranges from a
few degrees to more than 120∘, with an average value of about 50∘ (partial and full halos excluded). Note
that width and source latitude are severely affected by projection effects (Burkepile et al., 2004; Cremades
and Bothmer, 2004).
The total mass ejected in CMEs ranges from some 1013 g to a few 1016 g, the total energy (kinetic plus
potential energy) from 1027 erg to some 1033 erg, with averages of 1.4 × 1015 g and 2.6 × 1030 erg,
respectively (Vourlidas et al., 2002; Gopalswamy, 2004).
Yashiro et al. (2004) have assembled a huge catalog of more than 10,000 CMEs (by now, still counting)
ever registered by the SOHO instruments over the last 10 years. For each event they present important
information (e.g., images taken by the EIT and LASCO instruments, animations, difference images, and
some important characteristic properties), and make it easily accessible to the science community
through the public website 
http://cdaw.gsfc.nasa.gov/CME_list/. The catalog contains also
time-height diagrams which are evaluated for the fastest section of the outermost front. Thus, values
for speed and acceleration of these plane-of-sky projections are derived. The diagram for the
light bulb CME is shown in Figure 30
, right panel. In this case all points can be fitted pretty
well by a simple straight line, indicating a constant speed value of about 570 km/s. In other
cases, there is acceleration seen in the inner range, usually up to about 10 Rs, often followed by
deceleration in the outer range, from about 15 Rs on (see example in Figure 30, left panel).
The single acceleration values listed in the catalog by Yashiro et al. (2004) are thus at least
doubtful.
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The apparent 3-part structure of the light bulb CME (a bright outer loop, followed by a dark
void and finally by a bright kernel) is particularly noteworthy. This kind of topology is very
common (Hundhausen, 1987). The large number of CMEs observed since their discovery in 1973 has
allowed statistical analyses of their properties. The results are sometimes controversial, because of different
evaluation schemes that often are biased by subjective judgments. I refer the reader to papers
by Hundhausen et al. (1984); Howard et al. (1985); St Cyr et al. (2000); Gopalswamy (2004).
Some authors claim that there are two (or more) kinds of coronal mass ejections (e.g. Sheeley Jr et al., 1999; Srivastava et al., 1999a,b; Švestka, 2001; Moon et al., 2002): (1) Gradual CMEs, with balloon-like shapes, accelerating slowly and over large distances to speeds in the range 300 to 600 km/s, and (2) Impulsive CMEs, often associated with flares, accelerated already low down to extreme speeds (sometimes more than 2000 km/s). It is not clear yet whether these are really fundamentally different processes or whether they represent just the extrema of an otherwise continuous spectrum of CME properties.
Zhang et al. (2001, 2004) described the initiation of CMEs in a three-phase scenario: the initiation phase, the impulsive acceleration phase and the propagation phase. The initiation phase (taking some tens of minutes) always occurs before the onset of an associated flare, and the impulsive phase coincides well with the flare’s rise phase. The acceleration ceases with the peak of soft X-ray flares. It is interesting to notice that some of the theoretical CME models begin to postulate different phases of acceleration (see, e.g. Chen and Krall, 2003).
Right at the launch time of several CMEs, Kaufmann et al. (2003) discovered rapid solar spikes at radio submillimeter wavelengths that might be representative of an early signature of CME onset. The role of some other observed processes is also still unclear: coronal “dimmings” (Hudson et al., 2003), Moreton waves (named after their discoverer, see Moreton (1960), also Thompson (2000)), EIT waves (named after the EIT instrument on SOHO that made them visible, see Thompson et al. (1998)), the various types of radio bursts (Reiner et al., 2001), coronal inflow (Wang et al., 1999; Sheeley Jr and Wang, 2002; Tripathi et al., 2005).
Most CMEs are originating near the heliographic equator (Howard et al., 1985; St Cyr et al., 2000).
Sometimes CMEs are seen at very high latitudes (for example the light bulb CME shown in Figure 25
).
Usually these are CMEs originating at mid latitudes but directed near the Sun–Earth line, such that in
projection they look as if they were poleward pointed (Burkepile et al., 2004). Cremades and
Bothmer (2004) corrected this projection effect for some 200 CMEs observed between 1996 and 2002 and
determined the “true” center latitudes at several solar radii from the Sun. At solar minimum, they clearly
peak at the solar equator. But their source regions as determined near the surface from EIT images
are centered in two belts at around 25∘ northern or southern latitude. That means that these
CMEs must have been deflected from their mid-latitude sources towards the solar equator.
Cremades and Bothmer (2004) found that the deflection is proportional to both: the proximity and
the size of coronal holes. At times of high solar activity, in absence of the big polar coronal
holes, there was no net deflection found. The latitudinal distribution of source regions and CME
center latitudes were generally broader, but high latitude CMEs were observed rarely even
then.
The shapes of the vast majority of CMEs appear to be consistent with a nearly perfect
cross section. Indeed, halo CMEs moving exactly along the Earth–Sun line exhibit generally a
circular and smooth shape (see examples in Figures 31, 32, 33). This observation is rather
surprising in that CMEs result from the eruption of basically 2D elongated filament structures (for
further discussion see the review by Schwenn (1986) and Cremades and Bothmer (2004)).
Thus, the apparent lateral expansion of CMEs can be considered independent of the viewing
direction.
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Making use of that, Schwenn et al. (2005) point out that the speed of lateral CME expansion is the only
CME parameter that can be uniquely determined for any CME, be it on the limb or pointed along the
Earth–Sun line, on the front as well as on the back side. They selected a representative subset of 57 unique
limb CMEs. For those the true radial speed Vrad and the expansion speed Vexp measured across the full
CME width in the direction perpendicular to Vrad were determined. A striking correlation was
found:
This correlation holds for the slow CMEs as well as for the fast ones, for the narrow ones as well as the
wide ones. This means that the quantity Vexp can actually be used as proxy for the frontal speed that is
most often inaccessible because of projection effects. Schwenn et al. (2005) demonstrated that the
expansion speed Vexp can be used as a rather simple though empirical tool for predicting the travel times of
CME disturbances to the Earth (see Section 5.3).
After all, we have to admit that some fundamental questions about CMEs are still unsolved. Most importantly: What causes a CME to erupt in the first place? The situation is similarly embarrassing as for flares. Many researchers around the world are intensely tackling this problem. However, the essential ingredients for CME onset are not yet identified. Some candidates: the proximity of a CME site to coronal holes (Bravo et al., 1999), magnetic shear (Mikić and Linker, 1997), filament helicity (Martin, 2003; Rust, 2003), sigmoids (Rust and Kumar, 1996; Moore et al., 2001).
In order to disentangle the various processes around CME initiation new observations with significantly better resolution (spatially and in time) and even supported by spectroscopic diagnostics are needed, as was demonstrated by Innes et al. (2001) and Balmaceda et al. (2003).
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