Predicting the arrival time and transit speed of coronal mass ejections (CMEs) near the Earth is critical to understanding the solar-terrestrial relationship. Even though STEREO observations now provide multiple views of CMEs in the heliosphere, the true speeds derived from stereoscopic reconstruction of SECCHI coronagraph data are not quite sufficient for accurate forecasting of the arrival time at Earth of a majority of CMEs. This uncertainty is due to many factors that change CME kinematics, such as the interaction of two or more CMEs or the interaction of CMEs with the pervading solar wind. In order to understand the propagation of CMEs, we have used the three-dimensional triangulation method on SECCHI coronagraph (COR2) images and geometric triangulation on the J-maps constructed from Heliospheric Imagers HI1 and HI2 data for eight Earth-directed CMEs observed during 2008-2010. Based on the reconstruction, and implementing the drag-based model for the distance where the CMEs could not be tracked unambiguously in the interplanetary (IP) medium, the arrival time of these CMEs have been estimated. These arrival times have also been compared with the actual arrival times as observed by in situ instruments. The analysis reveals the importance of heliospheric imaging for improved forecasting of the arrival time and direction of propagation of CMEs in the IP medium.

Earlier studies on coronal mass ejections (CMEs), using remote sensing and in situ observations, have attempted to determine some internal properties of CMEs, which were limited to a certain position or a certain time. To understand the evolution of the internal thermodynamic state of CMEs during their heliospheric propagation, we improve the self-similar flux-rope internal state model, which is constrained by the measured propagation and expansion speed profiles of a CME. We implement the model in a CME that erupted on 2008 December 12 and probe the internal state of the CME. It is found that the polytropic index of the CME plasma decreased continuously from 1.8 to 1.35 as the CME moved away from the Sun, implying that the CME released heat before it reached an adiabatic state and then absorbed heat. We further estimate the entropy changing and heating rate of the CME. We also find that the thermal force inside the CME is the internal driver of CME expansion while the Lorentz force prevented the CME from expanding. It is noted that the centrifugal force due to poloidal motion decreased at the fastest rate, and the Lorentz force decreased slightly faster than the thermal pressure force as the CME moved away from the Sun. We also discuss the limitations of the model and approximations made in the study.

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 super-elastic 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.

We have studied two coronal mass ejections (CMEs) that occurred on 25 and 28 September 2012 and interacted near the Earth. By fitting the Graduated Cylindrical Shell model on the SECCHI/COR2 images and applying the Stereoscopic Self‐Similar Expansion method on the SECCHI/HI images, the initial direction of both the CMEs is estimated to be west of the Sun‐Earth line. Further, the three‐dimensional (3‐D) heliospheric kinematics of these CMEs have been estimated using Self‐Similar Expansion (SSE) reconstruction method. We show that the use of SSE method with different values of angular extent of the CMEs leads to significantly different kinematics estimates for the CMEs propagating away from the observer. Using the estimated kinematics and true masses of the CMEs, we have derived the coefficient of restitution for the collision which is found to be close to elastic. The in situ measurements at 1 AU show two distinct structures of interplanetary CMEs, heating of the following CME, and ongoing interaction between the preceding and the following CME. We highlight the signatures of interaction in remote and in situ observations of these CMEs and the role of interaction in producing a major geomagnetic storm.