Tracking the evolution of spheromak flux ropes in ambient interplanetary magnetic field and plasma environments.

Interplanetary magnetic clouds with flux rope structures are an essential ingredient of space weather models. The main aim is to reconstruct their magnetic field topology and plasma properties and track their evolution in space and time. This has led to the implementation of a variety of flux rope configurations in magnetohydrodynamic models, with spheromak, modified spheromak, and more general toroidal flux ropes being commonly used.  

The spheromak implementation in EUHFORIA (EUropean Heliospheric FORecasting Information Asset) brought to light different manifestations in the simulation domain of a phenomenon called the spheromak tilting (instability). The latter is caused by a torque that is exerted on the spheromak when its magnetic moment forms an angle with the ambient field. The torque forces the spheromak to rotate until it reaches a state of reduced magnetic potential energy. This is a simplified description of the fact that the Lorentz force exerted by the ambient magnetic field on the toroidal currents in the spheromak has in general a rotational component, resulting in a net-torque. As not only spheromaks but also other types of flux ropes carry toroidal currents, these should experience a torque as well. To what extent it affects their evolution is a matter of a game of forces. Being thus able to track the evolution (the position, orientation, etc.) of flux ropes is crucial. 

We have developed a tool to perform such a tracking for the spheromak implementation in EUHFORIA. The tool uses magnetic field and plasma threshold criteria to locate the spheromak and estimate its magnetic moment. It was originally developed and applied to spheromaks inserted in synthetic uniform ambient plasma and unipolar ambient fields that are realistic only locally along the spheromak trajectories. Since its initial development, the tool has been further improved and made capable of dealing with more realistic ambient field scenarios, containing current sheets and high-speed streams.