Dorsch, Brenda ; Rodriguez, Luciano ; Magdalenić, Jasmina

Talk presented at XVIIIth Hvar Astrophysical Colloquium, Hvar, Croatia on 2025-09-16

Abstract: Magnetic flux ropes are ubiquitous features observed in the low corona and propagating through the solar atmosphere. They are formed by combined action of the magnetic field of the Sun and its internal processes, observed remotely and in situ. Flux ropes are one of the main components of Coronal Mass Ejections, drivers of major geomagnetic storms, and they are therefore of high importance in space weather. One of the intrinsic properties of the magnetic flux rope structure is a twist which quantifies the rotation of the magnetic field lines around its axis. Twist is also one of the key input parameters for some of the most advanced heliospheric MHD models simulating propagation of CMEs. Despite its importance, since its estimation is not a simple task, in most studies by default, only an average value of this parameter is used. The twist parameter has a significant impact on the CME modelling results, in particular on the CME speed and the intensity of its internal magnetic field. It is therefore important to understand what the optimal method for the estimation of twist is, and what are the impacts of its variations in the context of space weather forecasting. In this work, we validate a straightforward method for deriving the twist parameter based on observations, for its use in the forecasting workflow. We consider the relationship between the twist and the ratio of axial length to minor radius of the flux rope and adapt it to be implemented based on a selected EUV (Extreme Ultraviolet) image. We apply the adapted method to 43 flux rope events observed simultaneously in the EUV data by one or more spacecraft. Each analysed event is associated with a CME, for which we estimated the kinematics employing a 3D reconstruction method based on the extended geometry considered for the FRi3D (Flux-Rope in 3D) CME model. We study the influence of the projection effect on the estimation of the parameters of solar magnetic flux ropes and found its consistent impact on the estimation of the twist simultaneously observed by different spacecraft. The de-projected twist obtained for the selected events is found to be between 1.8 and 3.3 turns. We also inspected a relationship between the twist and the associated CME speed and flare energy. It was found that the twist value follows a polynomic trend with respect to the CME speed, regardless the flare energy, for C-class and M-class events. However, for events associated with more energetic flares, such trend becomes more diffuse.