Ball lightning as a force-free magnetic knot

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The stability of fireballs in a recent model of ball lightning is studied. It is shown that the balls shine while relaxing in:in 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 plas,nas; (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.
© 2000 The American Physical Society. We are grateful to M. V. Berry, F. Blanco, D. Bouwmeester, D. K. Callebaut, A. Ibort, J. M. Montesinos, S. Singer, A. Tiemblo, and T. Tuomi for suggestions and encouragement.
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