A magnetodynamical mechanism for the formation of jets in astrophysical situations is presented. The effect of the relaxation of the accumulated nonlinear magnetic twist is calculated in an axisymmetric quasithree dimensional simulation, and it is shown that a jet is formed in the direction of the axis parallel to the large-scale magnetic field along which the packet of the magnetic twist relaxes. The driving force of the jet is due both to the pinching front progressing along the axis and to the j × B force in the front of the unwinding twist in the off-axis region. The effect is strengthened in the stratified atmosphere in which the relaxing packet of the magnetic twist emerges into the low-β (≡ pg/pm) region. The jet has a high contrast in density and has a conspicuously high velocity, because the velocity of the relaxing front which carries the material is determined by the Alfvén velocity which is supersonic in the low-β region. One of the characteristic features of the jet produced in the present mechanism is its helical motion which is favorable in explaining some of the observations of astrophysical jets. As to the creation of the magnetic twist, which is to be discussed in detail in paper II of the series, it is argued to be due to the winding-up effect of the rotational velocity component in the stellar surface convection in the case of the substellar jets, and of the rotational velocity in the contracting accretion disk in the cases of bipolar flows from young stellar objects and of bipolar lobes ejected from active galaxies, respectively.
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Shibata et al. (1985) studied this question.