Occurrence rate

2.3 Occurrence rate

During the solar cycle 23, the Large Angle and Spectrometric Coronagraph (LASCO) on board the SOHO satellite provided unprecedented observations of CMEs. The occurrence rate of CMEs was found to basically track the solar activity cycle, but with a peak delay of 6- 12 months (Raychaudhuri, 2005 ; Robbrecht et al., 2009

). Before the SOHO era, the averaged occurrence rate was found to increase from 0.2 per day at solar minimum to 3.5 per day at solar maximum (Webb and Howard, 1994 ). With the increased sensitivity and wider field of view, the SOHO/LASCO coronagraph assembly, including C1, C2, and C3 components with different fields of view, detected CMEs more frequently. The CME catalog² in the NASA CDAW data center, where CMEs are identified by eye, shows that the CME occurrence rate increases from $∼$ 0.5 per day near solar minimum to $∼$ 6 near solar maximum, summing up to more than 13000 CMEs during the solar cycle 23 (Gopalswamy et al., 2003

; Yashiro et al., 2004

). However, for the same observational period, the automated software, CACTus³ , identified much more events, with the occurrence rate increasing from < 2 per day near solar minimum to $∼$ 8 per day near solar maximum (Robbrecht et al., 2009

). Figure 2

shows the comparison of the CME daily occurrence rate detected by the two methods, along with the sunspot number.

Figure 2: The CME daily occurrence rate detected by the CACTus archive (red) and the CDAW archive (blue) compared with the daily sunspot number (gray) during solar cycle 23. Thin curves: smoothed per month, thick curves: smoothed over 13 months (from Robbrecht et al., 2009 ).