From an operational standpoint, the rapid and clear transfer of space environment information from a forecaster to individuals who must take action based on that information (such as satellite operators, airline industry, and power transmission industry) is paramount. One method to facilitate this process is the creation of simple-to-understand categories, such as the NOAA Space Weather Prediction Center scales for geomagnetic storms, solar radiation storms, and radio blackouts (http://www.swpc.noaa.gov/NOAAscales/). Another long-standing example in solar physics is the solar flare classification system, which is used in operations, basic research, and educational activities (http://www.swpc.noaa.gov/info/glossary.html). The nomenclature of the scale is easy to learn and remember—for example, the strongest flares are X-class (extreme), and the next is M-class (moderate). However, no scale analogous to the flare system has been defined for coronal mass ejections (CMEs): eruptions with speeds less than 500–800 km/s are commonly referred to as “slow,” and the rest are called “fast.” The lack of more specific terminology has hindered the effective communication of space weather events. In order to address this issue, the Space Weather Research Center (SWRC) at the NASA Goddard Space Flight Center (GSFC) has created the CME SCORE scale.
The speed of a CME can be estimated from successive coronagraph images of the Sun's atmosphere using a variety of methods. Figure 1 shows how often CMEs with a certain speed occur per year. The data are derived from two sources: circles represent the manually identified detection entries in the Coordinated Data Analysis Workshop (CDAW) catalog from 11 January 1996 to 12 February 2013 (http://cdaw.gsfc.nasa.gov/CME_list/UNIVERSAL/text_ver/univ_all.txt), and the “x” marks represent the SWRC's real-time CME analysis from 24 February 2010 to 26 March 2013 (http://swrc.gsfc.nasa.gov/main/score). The CDAW value is the average (linear) plane-of-sky speed while in the coronagraph field of view. The SWRC CME speed is determined, in almost all cases, with images from two or more coronagraphs combined to determine the true direction and speed through triangulation developed at the SWRC [Mays et al., manuscript in preparation] when the CME is near the edge of the coronagraph field of view. As the primary mission of the SWRC is to monitor the space environment near NASA spacecraft, at this time, we analyze but do not document CMEs that are directed at high (greater than about |60°|) latitudes. High-latitude CMEs are included in the CDAW catalog.
The relationship between the frequency of occurrence and speed of a CME in Figure 1 naturally led to the creation of five CME types, or scores (http://swrc.gsfc.nasa.gov/main/score). We selected the word “type” to complement, and avoid confusion with, X-ray “classes” of solar flares. In the CDAW catalog, the most numerous CMEs are those with a speed less than 500 km/s. On average, CMEs with these speeds occur more than 100 times per year. We call this group of slowest CMEs “S-type.” CMEs with speeds between 500 and 999 km/s define the second type, “C-type,” or a score of Common. On average, CMEs with these speeds are seen less than 100 but more than 10 times per year. CMEs with speeds between 1000 and 1999 km/s define the “O-type,” with a score of Occasional. CMEs with speeds in this range take place a few times per year. CMEs with a speed between 2000 and 2999 km/s define the “R-type,” with a score of Rare. On average, CMEs with these speeds occur less often than once per year. CMEs with a speed greater than 3000 km/s are scored Extremely Rare. ER-type CMEs are seen on average less than once in 10 years. In May 2012, the SCORE scale was introduced into SWRC operations in both notifications of ongoing events and Weekly Space Weather Summary Analysis provided to NASA robotic mission operators.
The connection between solar events and the resulting space weather effects is complex. We understand that classifying CMEs could be rather complicated, but there is a need for a simple yet informative classification system. The CME SCORE scale serves as an initial step toward possible future improvement. Although the original purpose has been to facilitate a rapid and clear transfer of information to NASA robotic mission operators, this scale might also serve the space weather community and help to quantify the commonly asked question “What is meant by fast coronal mass ejection?”