2.1 CME identification and measurement

Traditionally CME observations were obtained by visual inspection of coronagraph images, and many of these “manual” catalogs of CMEs observed by the P78/Solwind (External Linkhttp://lasco-www.nrl.navy.mil/solwind_transient.list), SMM C/P (External Linkhttp://smm.hao.ucar.edu/smm/smmcp_catalog.html), and LASCO C2 and C3 coronagraphs (External Linkhttp://cdaw.gsfc.nasa.gov/CME_list/index.html) are now on-line (Boursier et al., 2009Jump To The Next Citation Point; Gopalswamy et al., 2009b). These catalogs have in recent times been augmented by additional on-line catalogs of CMEs detected by automatic methods. One is the CACTus CME catalog (Robbrecht et al., 2009aJump To The Next Citation Point), which uses the Hough transform to detect motion of the brightest structures of CMEs. The SEEDS (Olmedo et al., 2008) and ARTEMIS (Boursier et al., 2009Jump To The Next Citation Point) catalogs are based on automated detection of CMEs in the LASCO C2 coronagraph observed at ∼ 2– 6R ⊙. ARTEMIS detects CMEs on synoptic Carrington maps. CACTus also catalogs CMEs detected by the STEREO COR2 coronagraphs, which are in near-1 AU solar orbits.

Comparisons among the LASCO catalogs have shown significant differences. For example, Robbrecht et al. (2009aJump To The Next Citation Point) found that CACTus automatically identified many more events than in the CDAW (manual) catalog but half of them were narrow (< 20° of apparent angular width). In addition, as shown in Figure 6View Image, the shapes of the CME rate curves were quite different with the CACTus and sunspot curves similar, but the CDAW curve flattened out during the cycle decline. This and other comparisons suggest that the CDAW catalog is affected by observer bias since it has been compiled by at least four different observers throughout the lifetime of the SOHO mission. One example of this bias appears in the occurrence rate in later years of the SOHO mission. Occurrence rate increased greatly after 2004, not because more CMEs were physically erupting, but rather because a decision was made to categorize very narrow LASCO features, which were previously disregarded, as CMEs. Another comparison of CME properties of the four LASCO catalogs shows best agreement between the ARTEMIS and SEEDS catalogs (Boursier et al., 2009), which better reflect the early stages of CMEs (i.e., within the LASCO C2 field of view). A recent analysis of LASCO CMEs based on a multiscale method convolving high and low-pass filters with CME images (Byrne et al., 2009) has shown that multiscale values agree much better with the generally smaller SEEDS CME widths than with the larger CACTus and CDAW values. A comparison of the LASCO fast (v > 1000 km s–1) CMEs between the CDAW and CACTus catalogs shows that the CDAW fast CME widths are considerably wider (Yashiro et al., 2008bJump To The Next Citation Point). The CACTus CME width distribution is essentially scale invariant in angular span over a range of scales from 20 – 120° while previous catalogs present a broad maximum around 30°. Yashiro et al. (2008bJump To The Next Citation Point) found that the CACTus catalog has a larger number of narrow CMEs than CDAW, and that the CDAW catalog missed many narrow CMEs during solar maximum. Another significant discrepancy was that the majority of the fast CDAW CMEs are wide and originate from low latitudes, while the fast CACTus CMEs are narrow and originate from all latitudes. In general, automatic catalogs do not always identify wide CMEs, including halos which are the most important ones for space weather applications when observing from near the Earth (e.g., see Figure 8 in Gopalswamy et al., 2010bJump To The Next Citation Point).

View Image

Figure 6: Daily SOHO LASCO CME rates for Cycle 23 (thin curves: smoothed per month, thick curves: smoothed over 13 months) from 1997 – 2006. These have been extracted using CACTus (red) and the CDAW CME Catalog (blue). As reference, the daily and smoothed monthly sunspot number have been overplotted in gray (produced using the SIDC-Royal Observatory of Belgium). The CME rates have been adjusted to accommodate for duty cycle. Image reproduced with permission from Robbrecht et al. (2009aJump To The Next Citation Point), copyright by IOP.

The above discussion demonstrates that CME identification and measurement remain somewhat subjective and no consensus has yet been achieved regarding the establishment of a standard definition of a CME or of the components within. The original definition of a CME as a new, discrete brightening in the field of view over a time-scale of tens of minutes which is always observed to move outward (e.g., Webb and Hundhausen, 1987Jump To The Next Citation Point) is still generally accepted. However, some workers tend to regard any eruption from the Sun observed in the corona, no matter how faint or narrow, as a CME while others regard an eruption as a CME only if it has a certain size or structure. Although a “typical” CME is now thought to involve the eruption of a magnetic flux rope, the structure and magnitude of any CME magnetic field near the Sun can only be inferred, since we cannot directly measure coronal magnetic fields. Efforts to make the connection between magnetic flux ropes measured in-situ with CME structure observed by coronagraphs have been made, most recently by Howard and DeForest (2012aJump To The Next Citation Point).

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