Two Types of Magnetohydrodynamic Sheath Jets
  Recent observations of astrophysical jets emanating from various galactic
nuclei strongly suggest that a double layered structure, or a spine-sheath
structure, is likely to be their common feature. We propose that such a sheath
jet structure can be formed magnetohydrodynamically within a valley of the
magnetic pressures, which is formed between the peaks due to the poloidal and
toroidal components, with the centrifugal force acting on the rotating sheath
plasma is balanced by the hoop stress of the toroidal field. The poloidal field
concentrated near the polar axis is maintained by a converging plasma flow
toward the jet region, and the toroidal field is developed outside the jet cone
owing to the poloidal current circulating through the jet. Under such
situations, the set of magnetohydrodynamic (MHD) equations allows two main
types of solutions, at least, in the region far from the footpoint. The first
type solution describes the jets of marginally bound nature. This type is
realized when the jet temperature decreases like viral one, and neither the
pressure-gradient nor the MHD forces, which are both determined consistently,
cannot completely overcome the gravity even at infinity. The second type is
realized under an isothermal situation, and the gravity is cancelled exactly by
the pressure-gradient force. Hence, the jets of this type are accelerated
purely 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 the
Fanaroff-Riley classification.
