Long-standing observations reveal a global organisation of the magnetic field directionin solar prominences (aka filaments), large clouds of cool dense plasma suspended in the Sun'shot corona. However, theorists have thus far been unable to explain the origin of this hemisphericpattern. In particular, simple shearing by large-scale surface motions would appear to lead to thewrong magnetic field direction. To explain the observations, we have developed a new model of theglobal magnetic field evolution in the solar corona over six months. For the first time our model canfollow the build-up of magnetic helicity and shear on a global scale, driven by flux emergence andsurface motions. The model is successful in predicting the correct magnetic field direction in thevast majority of prominences tested, and has enabled us to determine the key physical mechanismsbehind the mysterious hemispheric pattern.
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