Abstract Interaction between two coronal mass ejections (CMEs) affects their kinematics, morphology, space-weather effects, etc. For the first time, we report the interaction of four homologous CMEs in the field of view of Large Angle and Spectrometric Coronagraph (LASCO) on board Solar and Heliospheric Observatory (SOHO). A new active region (NOAA 11303) started to emerge on 2011 September 23 and developed quickly in the first 2 days. Subsequently, four CMEs (labeled as CME1, CME2, CME3, and CME4 sequentially) from this active region were recorded by LASCO/C2 in 10 hr on September 25. Interestingly, CME2 (677 km s −1 ) and CME4 (921 km s −1 ) are faster and catch up with CME1 (502 km s −1 ) and CME3 (516 km s −1 ) at ∼4.6 and ∼5.0 R ⊙ , respectively. Both CME1 and CME3 are compressed obviously, while the morphology of CME2 and CME4 does not show substantial changes. The aspect ratio of CME3 increases from ∼1.1 to ∼3.2 in 3 hr, indicating that the magnetic field strength inside CME3 increases by a factor of 3 quickly due to the interaction. Very interestingly, CME3 would normally be unable to catch up with CME2. However, the prior interactions accelerated CME3 and decelerated CME2. As a result, the composite of CME3 and CME4 moves faster (624 km s −1 ) and catches up with the composite of CME1 and CME2 (492 km s −1 ) at ∼14.3 R ⊙ . The four separately erupted homologous CMEs eventually combined and formed a large composite near the Sun, which can potentially cause intense geomagnetic storms if it reaches the Earth.
Song et al. (Fri,) studied this question.