The twist of solar magnetic-flux ropes

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Context. Magnetic flux ropes (MFRs) are ubiquitous structures observed in the solar corona and one of the main components of coronal mass ejections (CMEs). A key input parameter for advanced heliospheric magnetohydrodynamic models of CME propagation is the twist, which quantifies the rotation of magnetic field lines around their axis. As CMEs are the drivers of the strongest geomagnetic storms, their accurate modelling, including twist estimation, has a large impact on space weather (SW). Aims. The estimation of MFR twist requires time-consuming processes and/or high-quality cadence data, which presents constraints for the SW forecasting workflow. We developed a method to estimate the twist that is easily integrated in SW operations. The method is tested in a comprehensive statistical analysis. Methods. The twist is estimated by the ratio of axial length and minor radius of the MFR from a selected extreme-ultraviolet (EUV) image. We applied the method, for which we developed a tool, and estimated the twist of 43 MFR events observed simultaneously in EUV data by one or more spacecraft. Each MFR is associated with a CME and the majority with flaring activity. Results. We found a clear and consistent dependence of the estimated twist on the viewpoint. We observe a systematic decrease of the twist value when the MFR structure is seen closer to the solar limb. We thus derive a regression model to estimate the twist at a desired longitude. In addition, our work revealed a strong correlation between the twist of a MFR and the CME speed, in particular for events associated with low to moderate flare energies. This has a strong influence on the simulation of the propagation of magnetised CMEs. We provide an empirical relation that offers a diagnostic proxy of the twist for post-event analyses and real-time SW forecasting framework.

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