Coronal mass ejections

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Coronal mass ejections are spectacular events when seen in white light (e.g., with the LASCO coronagraph on SoHO). But the identification of their interplanetary counterparts, termed ICMEs, is less than a trivial matter and "is still something of an art'' (Gosling, 1997). Zurbuchen and Richardson (2006) have compiled a comprehensive table of 23 ICME signatures subdivided into 5 classes: magnetic field, plasma dynamics, plasma composition, plasma waves, and energetic particle signatures; an image of a typical ICME is given in Figure 3.14.

Figure 3.14. Basic geometry of an ICME indicating and relating its principal signatures (from Zurbuchen and Richardson, 2006).

Ulysses' contributions to ICME studies are twofold: mapping ICME occurrence rates as a function of latitude, on the one hand, and adding new plasma composition parameters for ICME identification, on the other.

Plasma composition parameters as ICME signatures

It was long known that a high alpha-to-proton ratio (> 8%, say) is a signature of an ICME (Hirshberg, Bame, and Robbins, 1972, what we call ICME today was called driver plasma back then). This signature is easy to spot and never occurs outside ICMEs, but it is only present in a fraction of very roughly 50% of all events. Ulysses (and later missions carrying composition instrumentation) has now added new composition signatures that come fairly close to ideal ICME identifiers (an ideal signature would detect all ICMEs with no false identifications).

One such signature is the average charge state of iron, (QFe). Under quasi-stationary solar wind conditions Fe is found distributed over several charge states with a broad maximum around Fe10+ (Ipavich et al., 1992; von Steiger, Geiss, and Gloeckler, 1997); the distribution is not very different between fast streams and slow solar wind (see also Figure 3.9). But this changes, sometimes drastically, during the passage of an ICME. Lepri et al. (2001) found that (QFe) is strongly enhanced, so Fe16+ becomes the dominant charge state (even higher charge states are rarely observed because Fe16+ has an Ne-like configuration that is hard to ionize further). In a follow-up paper, Lepri and Zurbuchen (2004) showed that (QFe) > 12 is a very strong ICME identifier that is present in a large fraction of (but not all) ICMEs, and has a negligible probability for false positive ICME identification. It is further shown that the presence of high Fe charge states is correlated with the magnetic connectivity to the flare site from where the CME has originated in the corona (Reinard, 2005), thus putting into perspective "the solar flare myth'' (Gosling, 1993) by showing that flares and ICMEs are not entirely unrelated after all. The latter conclusion is made from the observation that ICMEs with a high (QFe) are becoming rarer at high latitudes, which have less magnetic connectivity to the active regions at low to mid-latitudes. We will return to the latitude distribution of ICMEs below.

Another ICME signature is the O7+/O6+ charge-state ratio, which has already been used for the separation of the two quasi-stationary solar wind types (although, as argued above, the C6+/C5+ ratio would be superior for that purpose). Of course, in the light of the above paragraph it comes as no surprise that O7+/O6+ is often enhanced in ICMEs. This was already found by Neukomm (1998) and by Henke et al. (2001), who found a good positive correlation of the presence of a magnetic cloud, a sure ICME identifier, with high O7+/O6+. However, the definition of a clear-cut threshold value is much less evident in this case than it is for (QFe). Richardson and Cane (2004) overcame this difficulty by defining a solar wind speed-dependent threshold value instead of a constant one. From observations with ACE-SWICS they determined a correlation between O7+/O6+ and the solar wind speed (in units of km/s) (O7+/O6+)ACE03 = 3.004 exp(—V/173) to hold in the ambient solar wind away from all ICMEs, and define as ICME threshold if O7+/O6+ exceeds twice that value. Although this definition is still somewhat arbitrary it does a significantly better job at picking ICMEs than a constant threshold value. Kilchenmann (2007) has performed the same analysis with data from Ulysses-SWICS and finds an ambient (non-ICME) solar wind correlation of (O7+/O6+)Uly — 3.776exp(—V/128), which is about a factor of 2 lower than the relation of Richardson and Cane (2004) at typical solar wind speeds. However, this apparent discrepancy is not physical because ACE-SWICS was recalibrated after 2004. Using current ACE Level 2 data of the same time period Kilchenmann (2007) finds a relation of (O7+/O6+)ACE07 — 1.210exp(—V/200), which agrees with the Ulysses relation to within 15% at typical solar wind speeds. Note that this discrepancy does in no way invalidate the work of Richardson and Cane (2004) because it is internally consistent, but when comparisons are made the recalibrated, not the published, ACE-SWICS data must be used.

ICMEs at high latitudes

ICMEs are obviously associated with active regions on the Sun, which are found at mid- to low latitudes in the solar corona but never within a coronal hole. It was therefore not a small surprise when Gosling et al. (1994) discovered a new class of ICMEs that are fully embedded in the fast solar wind stream from the polar coronal hole at solar minimum. These ICMEs are characterized by a forward-reverse shock pair driven into the ambient fast solar wind by virtue of their high internal pressure and are therefore termed overexpanding ICMEs (see Figure 3.15). To be sure, such ICMEs are rare events, with only six of them observed with Ulysses during its entire solar-minimum orbit. One of them was even observed simultaneously both at low and at high latitudes (Gosling et al., 1995c). Interestingly, these ICMEs do not show any of the compositional signatures discussed above, but are indistinguishable from the ambient fast solar wind regarding their composition (Neukomm, 1998). It is therefore conceivable that overexpanding ICMEs are not strictly speaking ICMEs, but rather the wake of a solar ejection ICME passing by at lower latitudes, as recently modeled by Manchester and Zurbuchen (2006).

As with the quasi-stationary solar wind the rate of CMEs changes drastically from solar minimum to maximum. CMEs, which at solar minimum are confined to low latitudes almost exclusively, are distributed nearly uniformly over all position angles at solar maximum (Gopalswamy et al., 2006). Likewise, we might expect to observe ICMEs equally uniformly at all heliolatitudes, and indeed we can find ICMEs even at the highest latitudes reached by Ulysses (e.g., von Steiger, Zurbuchen, and Kilchenmann, 2005). However, their rate of occurrence surprisingly seems to decrease with increasing heliolatitude even at a time of increasing and high solar activity, as was already apparent from Figure 3.3. This has been demonstrated quantitatively by Lepri and Zurbuchen (2004) by comparing simultaneous observations of the ICME rate on ACE and Ulysses. Von Steiger, Zurbuchen, and Kilchen-mann (2005) have compiled the latitude distribution of all ICMEs observed on Ulysses in 1998-2001 (i.e., during the rise and maximum phases of cycle 23, see Figure 3.16). Evidently, there is an anisotropy of the monthly ICME rate with a strong preference for ICMEs near the equator. Note that the plotted ICME rate has

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