poloidalfield

Poloidal field refers to that portion of a magnetic field whose lines lie in meridional planes of a roughly spherical geometry, i.e., in the radial and polar (r and θ) directions. It is contrasted with the toroidal component, whose lines encircle the axis in the azimuthal (φ) direction. In many contexts the field is described by a poloidal–toroidal decomposition, B = B_p + B_t, where B_p carries the radial and latitudinal components.

In solar and planetary dynamos, the poloidal field plays a central role. The solar magnetic cycle involves exchanges between poloidal and toroidal fields: differential rotation converts poloidal into toroidal (the Omega effect), while helical turbulence or other processes regenerate poloidal from toroidal (the alpha effect). Observations show a global poloidal field that reverses polarity every cycle, with a dipole-like structure at activity minima, evolving and migrating over the solar surface and interior.

In fusion devices such as tokamaks, the poloidal field is essential for confinement. The toroidal field, produced by external coils, provides the main confinement direction, while the poloidal field, generated by plasma current and poloidal coils, creates closed magnetic surfaces and a helical field that helps stabilize and shape the plasma.

Mathematically, poloidal and toroidal fields can be described by a poloidal–toroidal decomposition, B = ∇ × ∇ × (P r_hat) + ∇ × (T r_hat). This formulation is used in geodynamo, solar dynamo, and fusion modeling. Observationally, poloidal structure is inferred from magnetograms and helioseismology for the Sun, and from diagnostic measurements in experiments for confined plasmas.