title: Ball lightning as a force-free magnetic knot
abstract: The stability of fireballs in a recent model of ball lightning is studied. It is shown that the balls shine while relaxing in an almost quiescent expansion, and that three effects contribute to their stability: (i) the formation in each one during a process of Taylor relaxation of a force-free magnetic field, a concept introduced in 1954 in order to explain the existence of large magnetic fields and currents in stable configurations of astrophysical plasmas; (ii) the so called Alfven conditions in magnetohydrodynamics; and (iii) the approximate conservation of the helicity integral. The force-free fields that appear are termed "knots" because their magnetic lines are closed and linked.
authors: Departamento de Electricidad y Electromica, Universidad Complutense, 28040 Madrid, Spain.
authors: AF
authors: Ranada
authors: M
authors: Soler
authors: JL
authors: Trueba