Studying the spheromak rotation for realistic CME modelling with EUHFORIA and its dependency on initial model parameters

Coronal mass ejections (CMEs) are one of the major sources for space weather disturbances. If the magnetic field inside an Earth-directed CME, or its associated sheath region, has a southward-directed north-south magnetic field component (Bz), then it interacts effectively with the Earth’s magnetosphere, leading to severe geomagnetic storms. Therefore, it is crucial to predict the strength and direction of Bz inside Earth-impacting interplanetary CMEs (ICMEs) in order to forecast their geo-effectiveness. Since the magnetic field of solar eruptions cannot reliably be measured via remote means, and direct continuous measurements of the Earth impacting solar transients are routinely available only very close to our planet, modelling of magnetic properties is paramount. The state-of-the art global heliospheric MHD models typically use the spheromak to characterize the magnetic structure of a CME and simulate its evolution from Sun-to-Earth. However, recent studies (Asvestari et al. 2021) have reported that the spheromak tends to rotate due to its interaction with the ambient medium, posing a great challenge in space weather forecasting.  

In this work, we study the spheromak rotation by modelling a realistic CME event on 2013 April 11 using the “European heliospheric forecasting information asset” (EUHFORIA). We found that when using the default density value in EUHFORIA, the axis of symmetry of the spheromak undergoes approximately 90 degrees of rotation and nearly aligns to the propagation direction of the CME. However, if we constrain the spheromak density using the observational data, we find an order of magnitude higher density value as compared to the default one. Interestingly, the spheromak rotation is observed to be reduced for higher densities. However, we note that the high-density spheromaks undergo significant compression as compared to the low-density ones. Using the observationally constrained density values, we obtain good agreement between the model result and in-situ observation. The simulation is also able to capture the overall magnetic structure of the associated sheath region ahead of the CME flux rope. These results are promising towards forecasting of Bz in near real time inside both ICME and sheath regions.  

This research has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 870405 (EUHFORIA 2.0).