The tilting spheromak: implications for coronal mass ejection modelling

Modelling coronal mass ejections (CMEs) as magnetised flux ropes has been of great importance when assessing their impact at the near-Earth environment. Many of the magnetised CME implementations in magnetohydrodynamic (MHD) models are of spheromak type, either remaining attached at the inner boundary of the modelling domain after insertion or getting fully detached. Despite its increasing application not much attention is given on how the orientation of the ambient magnetic field affects the evolution (propagation and orientation) of the spheromak. When a spheromak is inserted in the ambient magnetic field with its axis of symmetry at an angle with respect to the ambient field direction then the spheromak experiences a torque and a force that causes it to rotate and drift. Therefore, the final orientation of the spheromak can differ from the one it was inserted with. This has implications when using the spheromak to model an observed CME event. In our work we investigated qualitatively the rotation and drift of the spheromak using EUHFORIA. We generated simple ambient magnetic fields in the MHD heliospheric domain of EUHFORIA, which at the inner boundary were predominantly radial (varying strength and orientation) and inserted spheromaks with different initiation parameters (insertion speed, tilt, magnetic helicity). Our results validate that the spheromak rotates altering its orientation so that it aligns to that of the ambient magnetic field.