A statistical study on the relationship between surface field variation and CME initiation

In order to understand whether coronal mass ejections (CMEs) can be related to changes in the longitudinal photospheric magnetic field, we study 51 CME-producing active regions (ARs) which are associated with 77 halo CMEs during the period of their solar disk passage. We look at the magnetic field evolution during the short-term of 12 h before CME eruption and the long-term of 6-9 days of AR evolution. In order to quantitatively study these variations, we calculate three physical quantities: average total magnetic flux (ATF), flux variation rate (FVR) and normalized flux variation rate (NFVR). We find that 58% of CMEs are associated with magnetic flux increases during the short-term of 12 h before CME eruption and 42% are associated with magnetic flux decreases. NFVR of two time periods are found to be statistically identical. Furthermore, small-scale flux emergence and cancellation can be identified easily within the flaring region of the CME-associated flares. However, we can always identify one or more small-scale flux emergence or cancellation in the studied ARs even though not any obvious surface activities. Our study suggests that the relationship between small-scale flux emergence and CME eruption is complex and that the appearance of flux emergence alone is not sufficient for the initiation of CME eruption. Further studies are necessary in order to establish a unique relationship between flux emergence and CME eruption for space weather purpose.