Unknown source surface radius for stellar potential field extrapolation

Ascertain the source surface radius—the distance at which the coronal magnetic field becomes open and radial—for EK Draconis by developing a stellar wind–constrained model, since for stars this value is unknown and depends on wind properties, rotation, and magnetic field strength.

Background

To visualize the coronal magnetic topology, the authors perform a potential field source surface (PFSS) extrapolation of EK Draconis’s radial surface field and must assume a source surface radius beyond which the field lines are open. While the Sun commonly uses ~2.5 R⊙, the appropriate value for other stars varies with stellar wind conditions and global properties.

Because this parameter critically affects the inferred open/closed field regions but is not empirically known for EK Draconis, the authors adopt a value of 3.4 R⋆ following prior stellar studies, explicitly noting that the true value is unknown and likely depends on the star’s wind and magneto-rotational properties.