Multi-spacecraft Observations of interacting CME flux ropes

Interactions between coronal mass ejections (CMEs) in interplanetary space are a highly important aspect for understanding their physical dynamics and evolution as well as their space weather consequences. Here we present an analysis of three CMEs that erupted from the Sun on June 12-14, 2012 using almost radially aligned spacecraft at Venus and Earth, complemented by heliospheric imaging and modelling with EUHFORIA. These multi-spacecraft observations were critical for interpreting the event correctly, in particular regarding the last two CMEs in the series (June 13 and June 14). At the orbit of Venus these CMEs were mostly separate with the June 14 CME just about to reach the previous CME. A significant interaction occurred before the CMEs reached the Earth. The shock of the June 14 CME had propagated through the June 13 CME and the two CMEs had coalesced into a single large flux rope structure before they reached the Earth. This merged flux rope had one of the largest magnetic field magnitudes observed in the near-Earth solar wind during Solar Cycle 24. We discuss also the general importance of multi-spacecraft observations and modelling using them in analyzing solar eruptions.