PFSS

PFSS, or the Potential Field Source Surface model, is a computational method used in space physics to model the magnetic field of the Sun. It approximates the Sun's magnetic field as a potential field, meaning it assumes no electric currents are present in the region being modeled. This simplification allows for a relatively straightforward calculation of the magnetic field lines.

The key concept in PFSS is the "source surface." This is an imaginary spherical boundary, typically located at about 2.5 solar radii from the Sun's center. At this boundary, the magnetic field is assumed to be purely radial, meaning all field lines emerge perpendicularly from the surface. The PFSS model uses observed magnetic field data at the Sun's photosphere (its visible surface) as input. It then extrapolates this field outward through the corona to the source surface, where the radial condition is imposed.

By solving Laplace's equation with this boundary condition, the PFSS model generates a representation of the Sun's global magnetic field. This includes the structure of open field lines, which extend far out into interplanetary space, and closed field lines, which loop back to the Sun. Understanding this structure is crucial for studying phenomena like the solar wind, coronal mass ejections, and the propagation of energetic particles. While it is a simplification of the complex, current-carrying solar corona, the PFSS model provides a valuable first-order approximation and a widely used tool for analyzing solar magnetic activity.