Prominences (called filaments when they are observed on the solar disk) often lie along polarity inversion lines (PILs - lines separating positive and negative polarity regions). A well-known feature of the corona is that a large-scale shear and twist of the magnetic field is often present at these locations (referred to as filament channels). This energization of the magnetic field can structurally change the shape of the overlying streamers and the connectivity of the underlying fields. It also increases the free magnetic energy in the corona.

Our procedure for energizing the field is described in our paper about the 2017 Eclipse prediction. For this final prediction, the handedness of the shear in filament channels was estimated primarily using the hemispheric rule for those located in the quiet Sun, and by visual inspection of AIA images for those within active regions. We gratefully acknowledge the help of Shuo Wang, James McAteer, Gordon Macdonald of New Mexico State University and Valentin M. Pillet of the National Solar Observatory for using observations from the Dunn Solar Telescope to identify the chirality of the filament in the decayed active region near longitude 275.

The PIL segments where we emerged shear and twist are illustrated on the two maps below. The PIL segments (thick colored lines) were selected by examining H-alpha and AIA observations for evidence of filaments, starting from about 45 days before the prediction.

The movies below illustrate the energization process. An initially current-free potential field is energized using time-dependent electric fields at the inner boundary. The electric field profile is set by the PIL selections and surrounding magnetic field properties. It is designed to end just below the eruption threshold for each PIL.

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