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Asymptotic domination of cold relativistic MHD winds by kinetic energy fluxWe study the conditions which lead to the conversion of most Poynting flux into kinetic energy flux in cold, relativistic hydromagnetic winds. It is shown that plasma acceleration along a precisely radial flow is extremely inefficient due to the near cancellation of the toroidal magnetic pressure and tension forces. However, if the flux tubes in a flow diverge even slightly faster than radially, the fast magnetosonic point moves inward from infinity to a few times the light cylinder radius. Once the flow becomes supermagnetosonic, further divergence of the flux tubes beyond the fast point can accelerate the flow via the 'magnetic nozzle' effect, thereby further converting Poynting flux to kinetic energy flux. We show that the Grad-Shafranov equation admits a generic family of kinetic energy-dominated asymptotic wind solutions with finite total magnetic flux. The Poynting flux in these solutions vanishes logarithmically with distance. The way in which the flux surfaces are nested within the flow depends only on the ratio of angular velocity to poliodal 4-velocity as a function of magnetic flux. Radial variations in flow structure can be expressed in terms of a pressure boundary condition on the outermost flux surface, provided that no external toriodal field surrounds the flow. For a special case, we show explicitly how the flux surfaces merge gradually to their asymptotes. For flows confined by an external medium of pressure decreasing to zero at infinity we show that, depending on how fast the ambient pressure declines, the final flow state could be either a collimated jet or a wind that fills the entire space. We discuss the astrophysical implications of our results for jets from active galactic nuclei and for free pulsar winds such as that believed to power the Crab Nebula.
Document ID
19950038780
Acquisition Source
Legacy CDMS
Document Type
Reprint (Version printed in journal)
External Source(s)
Authors
Begelman, Mitchell C.
(Univ. of Colorado, Boulder, CO United States)
Li, Zhi-Yun
(Univ. of Colorado, Boulder, CO United States)
Date Acquired
August 16, 2013
Publication Date
May 1, 1994
Publication Information
Publication: Astrophysical Journal, Part 1
Volume: 426
Issue: 1
ISSN: 0004-637X
Subject Category
Astrophysics
Accession Number
95A70379
Funding Number(s)
CONTRACT_GRANT: NSF AST-91-20599
CONTRACT_GRANT: NSF AST-88-16140
CONTRACT_GRANT: NAGW-766
Distribution Limits
Public
Copyright
Other

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