| Abstract : | 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 component (Bz), then it interacts effectively with the Earth’s magnetosphere, leading to severe geomagnetic storms. Therefore, it is crucial to model the strength and direction of Bz inside Earth-impacting interplanetary CMEs (ICMEs) in order to forecast their geo-effectiveness. The state-of-the art global MHD models use spheromak as the flux-rope model to mimic the magnetic structure of a CME and simulate its evolution from Sun-to-Earth. However, recent studies by Asvestari et al. 2021 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 tilting instability by modelling a realistic CME event on 2013 April 11 using the “European heliospheric forecasting information asset” (EUHFORIA). We found that, 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 less in case of higher densities. However, the nature of ICME expansion becomes no more self-similar for such a scenario. Using the observationally constrained density values, we obtain good agreement between the model result and in-situ observation. From the simulation results, we could also capture the overall magnetic structure of the associated sheath region ahead of the CME flux rope. These results are encouraging towards forecasting of Bz in near real time inside both ICME and sheath regions. |
