Abstract
Force-free electrodynamics (FFE) is a closed set of equations for the electromagnetic field of a magnetically dominated plasma. There are strong arguments for the existence of force-free plasmas near pulsars and active black holes, but FFE alone cannot account for the observational signatures, such as coherent radio emission and relativistic jets and winds. We reformulate FFE as the effective field theory of a cold string fluid and initiate a systematic study of corrections in a derivative expansion. At leading order the effective theory is equivalent to (generalized) FFE, with the strings comprised by magnetic field line worldsheets. Higher-order corrections generically give rise to non-zero accelerating electric fields ($\mathbf{E}\cdot \mathbf{B}\neq 0$). We discuss potential observable consequences and comment on an intriguing numerical coincidence.
Highlights
The astronomical universe abounds with spectacular phenomena that defy explanation years or even decades after discovery
There are strong arguments for the existence of force-free plasmas near pulsars and active black holes, but Force-free electrodynamics (FFE) alone cannot account for the observational signatures, such as coherent radio emission and relativistic jets and winds
Among these persistent puzzles are the origin of ultra high-energy cosmic rays, the mechanism of coherent radio emission, and the formation of relativistic particle jets and winds [1,2,3,4]
Summary
The astronomical universe abounds with spectacular phenomena that defy explanation years or even decades after discovery. The foundations are secure [5,6]: rapid rotation and strong magnetic fields give rise to diffuse plasma that efficiently carries energy away from the central object. There are strong theoretical arguments that active pulsars and black holes possess force-free plasmas (see [5,6] and many later references) This enables efficient extraction of the rotational energy of the compact object. [13], a reformulation of magnetohydrodynamics (MHD) was presented where it was argued that the theory could be efficiently reorganized from a strictly symmetry-based point of view, taking as fundamental starting principles not Maxwell’s equations but rather the conservation of stress-energy and magnetic flux,. Force-free fields arise in a wide variety of physical circumstances: Besides the compact object magnetospheres considered here, they occur in the solar corona [16] and even in relaxed laboratory plasmas [17].
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