Quantifying when and where strong magnetic skew forms in a data-driven global non-potential model of the solar corona I: production and persistence of skew

Strong magnetic skew characterizes several non-potential structures in the solar corona, including filaments, which can erupt, producing major geomagnetic effects. Magnetic skew is thus a useful property in understanding locations of free magnetic energy in, and making forecasts from, global coronal magnetic field models. We develop a new technique for analysing the formation, evolution, and persistence of magnetic skew systematically in such models. This technique is applied to data-constrained magnetofrictional simulations, which include flux emergence in magnetic bipolar regions as observed on the Sun. Magnetic skew in the model is analysed, both globally and within the Earth-bound field of view. The latter is considered as these simulations have an observational bias: magnetogram observations can only be made along the Sun–Earth line. Results show that small patches of strong skew form incrementally, especially around recently emerged bipolar regions. However, large areas of strong skew, of sizes typical of filaments, take approximately two weeks to form after the emergence of new flux. Because of this observational bias in including new bipolar regions, most areas of strong skew in the model form on the far side of the Sun. Once formed, these areas may persist over long periods of time, comparable with the time-scale for solar rotation, and reproduce long-lived solar phenomena. Therefore, most strongly skewed regions in the model on the Earth-facing side of the Sun rotate into the field of view and do not form there. This illustrates a limitation of any global data-driven model that dynamically builds non-potentiality and skew through surface motions and flux emergence, with emergence based solely on observations from the Sun–Earth line.