Comparing CME models to increase our understanding of 3D CME structure

Coronal mass ejections are the main drivers of extreme space weather events, so it is essential to model these transients accurately and with enough lead time to be able to forecast severe geomagnetic storms. Many different analytic and numerical models are currently employed with different structures, from analytic flux ropes with drag based propagation and hydrodynamic pulses in 3D MHD, to magnetised flux ropes and spheromaks. Tools such as the CCMC CME scoreboard are currently used to compare space weather forecasts, however this only compares a few parameters and a more detailed evaluation of when different models replicate CME structures accurately is important to further our understanding. We compare the structure and in-situ signatures of 3 different magnetised CME models: the 3DCORE analytic flux rope, Spheromak and Gibson-Low flux rope models. There is a large amount of uncertainty when extrapolating from single point measurements to infer 3D structure, so we also explore whether galactic cosmic ray (GCR) particles could be used as another in situ measurement for determining the structure of a CME. GCRs show transient decreases in flux due to the passage of CMEs, and we model this using test particle simulations in conjunction with the CME models, reproducing GCR modulations known as a Forbush decreases, and explore how the Forbush decrease signature varies with CME model and parameters. Through these simulations, we aim to gain a greater understanding of what a CME `looks like’ and how to more accurately reproduce geoeffective CME structure using magnetised models.