It is important to keep in mind that the sources of magnetic field deflections in the sheath plasma and the ejecta are of fundamentally different origin:
The enormous bandwidth of CME properties is of course reflected in the properties of the related ICMEs and their effects. Tsurutani et al. (2004b) analyzed particularly slow magnetic clouds and found them to be surprisingly geo-effective. A good example is the famous event on January 6, 1997: A comparatively slow, unsuspiciously looking, faint partial halo CME caused a problem storm at the Earth 85 hours later, with enormous effects, as described in a series of papers (Zhao and Hoeksema, 1997; Burlaga et al., 1998; Webb et al., 1998). On the other hand, the very fast ICMEs are often found to be responsible for the most intense geomagnetic storms (Srivastava and Venkatakrishnan, 2002; Gonzalez et al., 2004; Yurchyshyn et al., 2004), apparently because they build up extreme ram pressure on the Earths magnetosphere.
The number of CMEs observed at the Sun is about 3 per day at maximum solar activity (St Cyr et al., 2000). Note though that Gopalswamy (2004) found a higher rate since their count includes the extremely faint, narrow and slow CMEs that become only visible due to the very high sensitivity and the enormous dynamic range of the LASCO instruments. At solar maximum, the number of shocks passing an observer located, say, in front of the Earth, is about 0.3 per day (Webb and Howard, 1994). Both rates taken together let us conclude that an in situ observer is hit by only one out of ten ICME shocks released at the Sun. Thus, the average shock shell covers about one tenth of the full solid angle 4π, i.e., the average shock cone angle as seen from the Suns center amounts to about 100∘. This value exceeds significantly the one of the average angular size of the CMEs of about 45∘ (Howard et al., 1985; St Cyr et al., 2000). The conclusion is that shock fronts extend much further out in space than their drivers, the ejecta clouds, as had been suggested earlier by Borrini et al. (1982). This explains why an in situ observer finds large numbers of shocks followed by sheath plasma only, with no associated ejecta cloud.
With respect to space weather near the Earth, the crucial question is: How can we forecast the arrival of geoeffective ICMEs? For a CME observed near the Sun, it takes about 1 to 4 days for the associated ICME to reach 1 AU, which is a fairly long time for working out a good forecast. However, those CMEs menacing the Earth are usually of the halo type for which the Earth-directed speed component cannot be measured. Several authors tried to use the plane-of-sky speed as listed in the CME catalog as basis for forecasting (e.g. Gopalswamy et al., 2001b; Michałek et al., 2003; Cane and Richardson, 2003). However, the plane-of-sky speed is oriented perpendicularly to the Sun–Earth line and, thus, is not at all representative of the line-of sight speed. Schwenn et al. (2005) found that the lateral expansion speed of a CME which can uniquely be determined for all types of CMEs (for limb CMEs as well as halos) can serve as a proxy value for the inaccessible line-of-sight speed. They associated some 300 candidate CMEs with their ICME signatures observed by in situ spacecraft upstream the Earth. For the 80 uniquely associated event pairs they determined both the CME expansion speed and the travel time to 1 AU as is shown in Figure 35. A fairly good correlation was found. Their empirical formula can be used for forecasting further ICME arrivals, with a 95% error margin of about 24 hours.
There is no doubt that the scatter in Figure 35 is rather substantial. But the scatter was found to be similarly large even in ideal cases when both the true radial CME speed and the travel times to an in situ observer can precisely be determined (Schwenn et al., 2005). One reason for the scatter is probably the fact that CMEs, on their way through space, travel through very different types of ambient solar wind and have to undergo deceleration or acceleration, depending on the relative speeds. Thus, major deviations from simple kinematic models commonly used for forecasting, may result.
There has been quite some discussion in the literature about the optimum CME acceleration/deceleration model (for details and references see Schwenn et al., 2005). Most authors cross-checked their empirical models with measurements of CME plane-of-sky speeds, which are highly corrupted by projection effects as explained above. Anyway, we should not assign too much meaning to those models, unless we know better the actual propagation environment of the individual CMEs. For the time being, I would rather prefer to join in with Reiner et al. (2003) who, in the spirit of Galileo Galilei, proposed that a number of CMEs be dropped from La Torre di Pisa and their drag force be directly measured.
Of course, the initiation and evolution of CMEs and the resulting propagation of the ejecta clouds through the heliosphere have been a key subject for theoreticians and modelers ever since. There is vast literature on various models and numerical codes, some of them being quite controversial. The statement by Riley and Crooker (2004) describes the present status quite well: These models include a rich variety of physics and have been quite successful in reproducing a wide range of observational signatures. However, as the level of sophistication increases, so does the difficulty in interpreting the results. In fact, it is fair to say that at present there is not yet a unified understanding of all processes involved, and we are still searching for the decisive observational facts.
After all, space weather forecasters will have to cope with that 24 hour uncertainty of CME arrival, once they have observed and evaluated a front-side halo CME. The situation is even worse: in 15% of comparable cases a full or partial halo CME does not cause any ICME signature at Earth at all; every forth partial halo CME and every sixth limb halo CME does not hit the Earth. That would lead to false alarms. Further, every fifth transient shock or ICME or isolated geomagnetic storm is not caused by an identifiable partial or full halo CME on the front side. That would result in missing alarms. We have to admit: our capabilities of forecasting geoeffects based on solar observations is still embarrassingly poor. It seems that the ongoing STEREO mission is about to improve the situation.
For the response of the Earths system to those interplanetary processes the situation is not really better. While the details of their interaction with the magnetosphere are still under study, some empirical relationships continue to be of great use, for example, the famous Burton formula (Burton et al., 1975, see also Lindsay et al., 1999 and Huttunen et al., 2005). It allows to predict the ground-based Dst index solely from a knowledge of the velocity and density of the solar wind and the north-south solar magnetospheric component of the interplanetary magnetic field and seems capable of predicting geomagnetic response during even the largest of storms. The article “Space Weather: Terrestrial Perspective” by Pulkkinen (2007) in Living Reviews in Solar Physics deals with these issues in much more detail.
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