THE STABILITY OF CURRENT-CARRYING JETS

The stability of supermagnetosonic current-carrying jets is examined o n the basis of ideal magnetohydrodynamics. The stability properties ar e compared to hydrodynamic and magnetohydrodynamic flows that carry no poloidal current. This allows to separate the effects of a purely pol oidal magnetic field and a helical configuration corresponding to an e lectric current. Two equilibria with different current profile represe ntative 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 r eturn current of equal size. A normal mode analysis is performed and r esults are presented for the pinch and kink modes. Current-carrying je ts 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 purel y hydrodynamic one. This permits an effective energy transport in acco rdance with the observed low surface brightness of jets in FR 2 radios ources. The jets in powerful radiosources are likely to be supermagnet osonic and carry large electric currents. The domain of maximal instab ility of current-carrying jets is shifted towards longer wavelengths c ompared to flows without currents. This provides them with an enhanced stiffness. The results give strong support to the hydromagnetic origi n of jets.