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Title: DYNAMICS OF STRONGLY TWISTED RELATIVISTIC MAGNETOSPHERES

Magnetar magnetospheres are believed to be strongly twisted due to shearing of the stellar crust by internal magnetic stresses. We present time-dependent axisymmetric simulations showing in detail the evolution of relativistic force-free magnetospheres subjected to slow twisting through large angles. When the twist amplitude is small, the magnetosphere moves quasi-statically through a sequence of equilibria of increasing free energy. At some twist amplitude the magnetosphere becomes tearing-mode unstable to forming a resistive current sheet, initiating large-scale magnetic reconnection in which a significant fraction of the magnetic free energy can be dissipated. This ''critical'' twist angle is insensitive to the resistive length scale. Rapid shearing temporarily stabilizes the magnetosphere beyond the critical angle, allowing the magnetosphere of a rapidly differentially rotating star to store and dissipate more free energy. In addition to these effects, shearing the surface of a rotating star increases the spindown torque applied to the star. If shearing is much slower than rotation, the resulting spikes in spindown rate can occur on timescales anywhere from the long twisting timescale to the stellar spin period or shorter, depending both on the stellar shear distribution and the existing distribution of magnetospheric twists. A model in which energy is stored inmore » the magnetosphere and released by a magnetospheric instability therefore predicts large changes in the measured spindown rate before soft gamma repeater giant flares.« less
Authors:
 [1] ; ;  [2]
  1. Astronomy Department, Columbia University, 550 West 120th Street, New York, NY 10027 (United States)
  2. Physics Department and Columbia Astrophysics Laboratory, Columbia University, 538 West 120th Street, New York, NY 10027 (United States)
Publication Date:
OSTI Identifier:
22133909
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 774; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; 70 PLASMA PHYSICS AND FUSION TECHNOLOGY; AMPLITUDES; AXIAL SYMMETRY; EARTH MAGNETOSPHERE; FREE ENERGY; MAGNETIC RECONNECTION; MAGNETOHYDRODYNAMICS; RELATIVISTIC RANGE; SHEAR; SIMULATION; STAR EVOLUTION; STARS; STRESSES; TEARING INSTABILITY; TIME DEPENDENCE; TORQUE