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Abstract: Recent observations of astrophysical jets emanating from various galacticnuclei strongly suggest that a double layered structure, or a spine-sheathstructure, is likely to be their common feature. We propose that such a sheathjet structure can be formed magnetohydrodynamically within a valley of themagnetic pressures, which is formed between the peaks due to the poloidal andtoroidal components, with the centrifugal force acting on the rotating sheathplasma is balanced by the hoop stress of the toroidal field. The poloidal fieldconcentrated near the polar axis is maintained by a converging plasma flowtoward the jet region, and the toroidal field is developed outside the jet coneowing to the poloidal current circulating through the jet. Under suchsituations, the set of magnetohydrodynamic MHD equations allows two maintypes of solutions, at least, in the region far from the footpoint. The firsttype solution describes the jets of marginally bound nature. This type isrealized when the jet temperature decreases like viral one, and neither thepressure-gradient nor the MHD forces, which are both determined consistently,cannot completely overcome the gravity even at infinity. The second type isrealized under an isothermal situation, and the gravity is cancelled exactly bythe pressure-gradient force. Hence, the jets of this type are acceleratedpurely by the MHD force. It is suggested also that these two types correspond,respectively, to the jets from type I and II radio galaxies in theFanaroff-Riley classification.



Author: Osamu Kaburaki

Source: https://arxiv.org/







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