2.2 Frequency of occurrence

The frequency of occurrence of CMEs observed in white light tends to follow the solar cycle in both phase and amplitude, which varies by an order of magnitude over the cycle (Webb and Howard, 1994Jump To The Next Citation Point). LASCO has now observed the entire Solar Cycle 23 (1996 – 2008) (Figure 7View Image) and continues to observe through this current rising phase of Cycle 24. It has detected CMEs at a rate slightly higher than earlier observations, varying from around one per day around solar minimum to nearly five per day at solar maximum (St Cyr et al., 2000Jump To The Next Citation Point; Gopalswamy et al., 2005Jump To The Next Citation Point, 2006bJump To The Next Citation Point). This has been attributed to the improved sensitivity of LASCO as opposed to any physical difference between CME activity in Cycle 23 and that in prior cycles. LASCO, for example, observes halo CMEs (Section  2.3) regularly whereas no prior coronagraph observed more than a few (Howard et al., 1985Jump To The Next Citation Point, for example, only identified 20 halos out of 998 CMEs observed with Solwind)]. This demonstrates that a fraction of CMEs were undetectable by coronagraphs prior to LASCO. A 13-month running average of the LASCO CME rate vs. sunspot number shows that both have double peaks, but that the CME peak lagged sunspots by many months (Figure 8View Image). This lag has also been seen in previous cycles and is related to observations that high latitude CMEs arise from polar crown filaments which have a “rush to the poles” near maximum and disappear (erupt) with a frequency that slightly lags sunspot numbers at low latitudes (Cliver and Webb, 1998; Gopalswamy et al., 2003bJump To The Next Citation Point).
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Figure 7: LASCO CME occurrence rate (left) and mean speed (right) from 1996 to 2011 averaged over Carrington rotations. The large spike in CME speed is due to highly energetic CMEs that erupted in the late 2003 period. Image adapted from Gopalswamy (2010bJump To The Next Citation Point), updated by S. Yashiro (2011).
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Figure 8: The LASCO CME rate smoothed over 13 Carrington rotations and compared with the solar sunspot number. Arrows indicate the two maxima in CME rate and sunspot number. Large data gaps occurred during June 1998 to February 1999. Image reproduced with permission from Gopalswamy (2004Jump To The Next Citation Point), copyright by Springer.

As has been well documented, Solar Cycle 23 had an unusually long decline and flat minimum, extending the cycle to ∼ 13 years, with the “true” minimum in late 2008 or early 2009 (Hathaway, 2010). Referring back to the updated LASCO CDAW CME rate in Figure 6View Image, Figure 9View Image tracks the CME rate from the CDAW, SEEDS and CACTus catalogs from 2007 into 2011 along with the current and predicted SWPC sunspot number. Despite differences in amplitude, it is clear that the CME rate continues to be correlated with the sunspot number through its minimum and initial rise of Cycle 24, with the CME rate minimum in late 2008 or early 2009. The linear relationship between CME rate and sunspot numbers was first shown by Webb and Howard (1994Jump To The Next Citation Point) and recently confirmed for Cycle 23 by Robbrecht et al. (2009aJump To The Next Citation Point) (Figure 10View Image), although some variation over a solar cycle has been found (Gopalswamy et al., 2010aJump To The Next Citation Point). In Figure 9View Image we have added the counting rate from the STEREO COR1 coronagraphs, demonstrating that the CME rate is relatively constant despite the increasing longitudinal angle between the STEREO spacecraft and Earth from 0 – 90° during this period.

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Figure 9: Daily CME rate for 2010 – 2011 in the context of the rate through recent solar minimum. CME data sources: LASCO = manual online CDAW catalog (black - NRL, CUA) and our counts since January 2010 (light blue); SEEDS = automatic catalog (dotted red) courtesy J. Zhang and J. Bannick (GMU); CACTus = automatic catalog courtesy E. Robbrecht & B. Bourgoignie (SIDC); STEREO COR1 = manual catalog (green) courtesy C. St. Cyr (NASA) and H. Xie (CUA); Sunspot number (SSN – dark blue is current, dotted blue is predicted) from NOAA SWPC. CDAW and SEEDS rates are for CME widths > 20. CDAW, SEEDS, and SSN plots are 13-month, COR1 6-month, and 2010 LASCO counts 6-week running averages. Image courtesy T. Kuchar.
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Figure 10: Daily CME rate vs. SSN both averaged per year. The asterisks refer to rates for Cycle 23 derived from CACTUS (see Table 1). Its absolute scale is shown on the right y-axis. The daily CME rates derived by Webb and Howard (1994) are plotted with diamonds. Its absolute scale is shown on the left y-axis. A scaling factor of ∼ 4.7 applies between the CACTus and the Webb and Howard rates. Image reproduced with permission from Robbrecht et al. (2009aJump To The Next Citation Point), copyright by IOP.

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