Alfvén's theorem

Alfvén's theorem states that ideal magnetohydrodynamic flow conserves magnetic flux through every surface moving with the fluid, so magnetic field lines are carried with it.

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Ideal magnetohydrodynamics: The approximation that treats a conducting fluid as perfectly conducting and neglects resistive effects. Its assumptions make the magnetic flux conservation in Alfvén's theorem exact.
Magnetohydrodynamics: The study of electrically conducting fluids interacting with magnetic fields. Alfvén's theorem is a conservation result within this broader framework.
Hannes Alfvén: A Swedish physicist who helped found magnetohydrodynamics and received the 1970 Nobel Prize in Physics. His name is attached to the flux-conservation theorem and related plasma phenomena.
Solar wind: The continuous outflow of charged particles from the Sun into interplanetary space. Its large-scale magnetic field is often modeled as carried by the flowing plasma.
Magnetic helicity: A measure of magnetic-field linkage and twist in a specified volume. In ideal evolution, conserved flux constrains changes to magnetic linkage and helicity.
Induction equation: An equation describing how fluid motion and electrical resistivity change a magnetic field over time. In the ideal limit, its advection form yields Alfvén's theorem.
Ideal conductor: A material model with zero electrical resistivity. Perfect conductivity removes the resistive diffusion term assumed absent by the theorem.
Magnetohydrodynamic waves: Waves in conducting fluids whose dynamics couple fluid motion to magnetic fields. Alfvén's early work on magnetic-field dynamics also led to the wave concept bearing his name.
Interplanetary magnetic field: The magnetic field extending through space and carried outward by the solar wind. Its spiral structure is commonly interpreted using field transport by the solar wind.
Magnetic topology: The arrangement and connectivity of magnetic field lines. Flux freezing preserves field-line connectivity under smooth ideal fluid motion.