The mechanism by which astrophysical jets form is an important factor
in understanding the nature and evolution of phenomena such as active
galactic nuclei and quasars, Galactic superluminal X-ray sources and y
oung stellar objects. Of the many schemes proposed for jet production,
only the magnetized accretion disk model of Blandford and Payne(1) se
ems to be applicable to all of these systems, and also offers the pote
ntial for generating the highly relativistic flows observed in some qu
asars(2). But the source of variation in jet morphology observed for d
ifferent sources remains unclear. Here we report time-dependent numeri
cal simulations of jet formation which show that the character and spe
ed of the jets produced depend dramatically on whether magnetic forces
dominate over gravity in the accretion disk corona. This 'magnetic sw
itch' is not predicted by steady-state, self-similar disk models, or b
y relativistic wind theory (which generally ignores the gravitational
field). The effect provides a natural explanation for the existence of
two known classes of extragalactic radio source and for the variation
of their properties with radio luminosity. It also provides insight i
nto protostellar and galactic microquasar systems.