The stability of current-carrying jets.

The stability of supermagnetosonic current-carrying jets is examined on the basis of ideal magnetohydrodynamics. The stability properties are compared to hydrodynamic and magnetohydrodynamic flows that carry no poloidal current. This allows to separate the effects of a purely poloidal magnetic field and a helical configuration corresponding to an electric current. Two equilibria with different current profile representative of an early propagation phase and of a kpc-jet surrounded by a cocoon are considered. The currents are in both cases balanced by a return current of equal size. A normal mode analysis is performed and results are presented for the pinch and kink modes. Current-carrying jets prove to be more stable than their current-free counterparts. Short wavelength instabilities are suppressed for both the current-carrying and current-free magnetically dominated jet, in contrast to the purely hydrodynamic one. This permits an effective energy transport in accordance with the observed low surface brightness of jets in FR 2 radiosources. The jets in powerful radiosources are likely to be supermagnetosonic and carry large electric currents. The domain of maximal instability of current-carrying jets is shifted towards longer wavelengths compared to flows without currents. This provides them with an enhanced stiffness. The results give strong support to the hydromagnetic origin of jets.