5.1 CME properties

CMEs are characterized by speed, angular width, and a central position angle in the sky plane. Measured front speeds range from a few km/s (near the Sun) to nearly 3000 km/s (Howard et al., 1985Jump To The Next Citation PointSt Cyr et al., 2000Jump To The Next Citation PointGopalswamy, 2004Jump To The Next Citation Point). 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., 2004Jump To The Next Citation PointCremades and Bothmer, 2004Jump To The Next Citation Point).

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., 2002Gopalswamy, 2004Jump To The Next Citation Point).

Yashiro et al. (2004Jump To The Next Citation Point) have assembled a huge catalog of more than 10,000 CMEs (status 2006, still counting), all discovered upon visual inspection of data from the SOHO instruments. 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 External Linkhttp://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 30View Image, 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 30View Image, left panel). The single acceleration values listed in the catalog by Yashiro et al. (2004Jump To The Next Citation Point) are thus at least doubtful.

There are other attemts to identify CMEs. Note, e.g., the automated CME detection scheme as described by Robbrecht et al. (2009), cf. External Linkhttp://sidc.oma.be/cactus/. UpdateJump To The Next Update Information

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Figure 30: Left panel: Time-height diagram for the CME of June 2, 1998. Right panel: Time-height diagram for the light bulb CME, as derived by Yashiro et al. (2004), see External Linkhttp://cdaw.gsfc.nasa.gov/CME_list/)

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. (1985Jump To The Next Citation Point); St Cyr et al. (2000Jump To The Next Citation Point); Gopalswamy (2004Jump To The Next Citation Point).

Some authors claim that there are two (or more) kinds of coronal mass ejections (e.g. Sheeley Jr et al., 1999Srivastava et al., 1999a,bŠvestka, 2001Moon 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. (20012004) 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., 1999Sheeley Jr and Wang, 2002Tripathi et al., 2005).

Most CMEs are originating near the heliographic equator (Howard et al., 1985Jump To The Next Citation PointSt Cyr et al., 2000Jump To The Next Citation Point). Sometimes CMEs are seen at very high latitudes (for example the light bulb CME shown in Figure 25Watch/download Movie). 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 (2004Jump To The Next Citation Point) 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 (2004Jump To The Next Citation Point) 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 31View Image, 32Watch/download Movie, 33Watch/download Movie). 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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Figure 31: Examples of halo CMEs as observed by the LASCO C2 and C3 coronagraphs on SOHO.

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Figure 32: mpg-Movie (3798 KB) A series of halo CMEs, observed by the LASCO C3 coronagraph on SOHO from January 14 to 18, 2005.

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Figure 33: mpg-Movie (1320 KB) The halo CME of November 4, 2001, as seen by the LASCO C3 coronagraph on SOHO. The loudness of the sound represents the intensity of SEPs during the event. This and other sonifications were prepared by a group of talented musician scientist at SSL in Berkeley External Linkhttp://cse.ssl.berkeley.edu/impact/sounds.html

Making use of that, Schwenn et al. (2005Jump To The Next Citation Point) 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:

Vrad = 0.88 Vexp,
with a correlation factor of 0.86.

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. (2005Jump To The Next Citation Point) 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, 2003Rust, 2003), sigmoids (Rust and Kumar, 1996Moore 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). There is hope that the new Hinode and SDO missions with their excellent instruments will allow major progress in the field.

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