Assessing the Nature of Collisions of Coronal Mass Ejections in the Inner Heliosphere

There have been several attempts in the past to understand the nature of the collision of individual cases of interacting coronal mass ejections (CMEs). We selected eight cases of interacting CMEs and estimated their propagation and expansion speeds, and direction of impact and masses, by exploiting coronagraphic and heliospheric imaging observations. Using these estimates while ignoring the errors therein, we find that the nature of collisions is perfectly inelastic for two cases (i.e., 2012 March and November), inelastic for two cases (i.e., 2012 June and 2011 August), elastic for one case (i.e., 2013 October), and super-elastic for three cases (i.e., 2011 February, 2010 May, and 2012 September). Including the large uncertainties in the estimated directions, angular widths, and pre-collision speeds, the probability of a perfectly inelastic collision for the 2012 March and November cases drops from 98% to 60% and 100% to 40%, respectively, increasing the probability for other types of collision. Similarly, the probability of an inelastic collision drops from 95% to 50% for the 2012 June case, 85% to 50% for the 2011 August case, and 75% to 15% for the 2013 October case. We note that the probability of a superelastic collision for the 2011 February, 2010 May, and 2012 September CMEs drops from 90% to 75%, 60% to 45%, and 90% to 50%, respectively. Although the sample size is small, we find good dependence of the nature of collision on the CME parameters. The crucial pre-collision parameters of the CMEs responsible for increasing the probability of a super-elastic collision are, in descending order of priority, their lower approaching speed, expansion speed of the following CME higher than the preceding one, and a longer duration of the collision phase.