Authors: Helen Norman (University of Warwick), Ravindra Desai (University of Warwick), Tony Arber (University of Warwick), Keith Bennet (University of Warwick), Hannah Rüdisser (Austrian Space Weather Office), Emma Davies (Austrian Space Weather Office)

Coronal Mass Ejections (CMEs) are the main drivers of geomagnetic storms at Earth, so understanding their structure is essential for improving space weather forecasting models. The global structure of a CME is difficult to recover from a single in-situ measurement, and multi-point observations of CMEs are rare. In addition, interaction with fast solar wind streams and other CMEs can increase the complexity of the magnetic structure of a CME. Signatures of CMEs can also be seen in-situ through decreases of the galactic cosmic ray background, known as Forbush decreases (Fds). Forbush decreases also have varied profiles, and are known to exhibit a 2-step decrease corresponding to the sheath region and flux rope of a CME. However, the mechanisms of interaction with CMEs that produce these decreases are not completely understood, and previous Fd modelling efforts have primarily focused on modelling only a flux rope or shock. We present simulated Fds modelled using full-orbit test particles and full 3D magnetic fields from analytic and MHD models of CMEs. Through comparing these CME models, and their particle data, we aim to gain a greater understanding of how to more accurately reproduce observed CME structure using magnetised models, and whether in-situ particle data could be used to diagnose CME structure.