CME-CME Interactions: Preconditioning, Collisions and Flux Erosion: How to Create a Perfect Storm?

Coronal mass ejections (CMEs) are the largest type of eruption seen in the solar atmosphere and the primary cause of severe geomagnetic disturbances when they arrive at the Earth. While most geomagnetic storms are created by the impact of a single CME, over a quarter are caused by the interaction of multiple CMEs and the majority of fast complex ejecta in the solar wind have indeed been implicated as resulting from multiple eruptions. As we see increasing solar activity within solar cycle 25, we thus seek to understand how CMEs can interact with one another to produce an exceptionally geo-effective event, i.e. "a perfect storm" at the Earth. In this paper we implement a spheromak CME description within a 3-D heliospheric MHD model and parametrically examine their interactions and assess their resultant geo-effectiveness through the use of solar wind-magnetosphere coupling functions. The myriad of potential space weather impacts at the Earth are reflected in the complexities of CME-CME interactions, with different solar wind variables peaking for differing scenarios of solar wind preconditioning, shock-CME collisions and prolonged interactions of their internal magnetic fields. In particular, we reveal how the orientation and handedness of successive CMEs can have a major impact on the large-scale conservation and loss of magnetic flux within these magnetic clouds, and consequently Bz, due to magnetic reconnection with the interplanetary magnetic field.