Magnetic energy dissipation in force-free jets

It is shown that a magnetic-pressure-dominated, supersonic jet which expands (or contracts) in response to variations in the confining external pressure can dissipate magnetic energy through field-line reconnection as it relaxes to a minimum-energy configuration. In order for a continuous dissipation to take place, the effective reconnection time must be a fraction ɛ ⪉ 1 of the expansion time. The amount of energy dissipation is calculated, and it is concluded that magnetic energy dissipation could, in principle, power the observed synchrotron emission in extragalactic radio jets such as NGC 6251. However, this mechanism is only viable if the reconnection time is substantially shorter than the nominal resistive tearing time in the jet.