U UD • The Astrophysical Journal, 310:96-103,1986 November 1 ,—i © 1986. The American Astronomical Society. All rights reserved. Printed in U.S.A. oo KO oo 2 MAGNETIC ENERGY DISSIPATION IN FORCE-FREE JETS Arnab Rai Choudhuri High Altitude Observatory, National Center for Atmospheric Research 1 AND Arieh Könige 2 Department of Astronomy and Astrophysics, University of Chicago Received 1985 December 19 ; accepted 1986 April 16 ABSTRACT 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 € < 1 of the expansion time. For a force-free jet (with a magnetic field satisfying \ x B = ¡xB) in the axisymmetric (m = 0) minimum-energy state, the energy per unit length that is dissipated on the characteristic reconnection time scale (assuming conservation of magnetic helicity) is given by (l/1152R 2 )(T / /47r) 2 e 2 (/iR) 6 (where R is the jet radius and T 1 is the axial magnetic flux), to lowest order in e and piR. On the basis of this result, it is concluded that magnetic energy dissipation could, in principle, power the observed synchrotron emission in extragalactic radio jets such as NGC 6251. However, just as in the case of analogous coronal heating models, this mechanism is only viable if the reconnection time is substantially shorter than the nominal resistive tearing time in the jet. Subject headings: galaxies: jets — hydromagnetics — radiation mechanisms I. INTRODUCTION It has long been recognized that the synchrotron-radiating plasma in extragalactic radio jets must undergo continuous particle reacceleration in order to overcome the adiabatic and radiative losses sustained by the expanding flow (see, e.g., the review by Begelman, Blandford, and Rees 1984). The energy for the acceleration process has customarily been assumed to come from the bulk kinetic motion of the jet. Specifically, it has been suggested that a fraction of the bulk kinetic energy might be tapped through surface shear instabilities which could lead to a turbulent energy cascade in the jet and to the formation of internal shocks (e.g., Ferrari, Trussoni, and Zaninetti 1979; Smarr, Norman, and Winkler 1984). It has also been argued that the dissipation of the energy derived in this way could lead to efficient particle acceleration through the Fermi mechanism or by means of resonant interactions with MHD waves (e.g., Bicknell and Melrose 1982; Eilek and Henriksen 1984). Recently, however, an alternative energy source for particle acceleration has been proposed in the context of the force-free-field model for magnetized supersonic jets (Königl and Choudhuri 1985; hereafter Paper I). According to this model, once a jet becomes magnetic-pressure dominated, it tends to settle down to that equilibrium field configuration which has the lowest magnetic energy for the given magnetic helicity. This unique configuration can be expressed as a superposition of the first two modes (m = 0 and m = 1) in the Chandrasekhar-Kendall representation of linear force-free fields (Taylor 1974). Any element of the jet which propa- gates through a region of varying external pressure has to undergo continuous field redistribution in order to satisfy the pressure- balance condition at the boundary while maintaining a minimum-energy configuration. It was suggested that this rearrangement process might be accompanied by field-line reconnection, and that the energy liberated in this fashion could then power the synchrotron emission from the jet. The energy-dissipation scheme just described bears a strong analogy to certain recent proposals regarding the heating mechanism in the solar corona (Norman and Heyvaerts 1983; Heyvaerts and Priest 1984; Browning and Priest 1986). According to these models, the energy for the heating is derived from photospheric fluid motions which build up stresses within magnetic flux tubes that extend into the corona; some of this energy can then be released in the magnetically dominated region above the photosphere as the flux tubes relax to the lowest accessible energy state that is compatible with conservation of magnetic helicity. In much the same vein, one can envision the magnetic field lines in a jet being braided and twisted at the base of the flow (which is in fact, how they, acquire a nonvanishing helicity) and subsequently releasing the accumulated stresses through reconnection at large distances from the source. The main difference between these two scenarios lies in the fact that the field relaxation process in a jet is triggered by the changes in the external pressure which confines the super-Alfvénic flow, whereas in the solar case it is initiated by the footpoint motions of the quasistatic flux tubes. As a consequence of this, the evolution of a jet can be modeled as a steady state process in which the magnetic helicity per unit length remains constant along the flow (see Paper I), whereas the simplest footpoint motions considered in the coronal heating models generally involve a change in the magnetic helicity of the associated flux tubes. The basic principle underlying the energy dissipation mechanism remains, however, the same in both cases. 1 The National Center for Atmospheric Research is sponsored by the National Science Foundation. 2 Presidential Young Investigator. 96 © American Astronomical Society • Provided by the NASA Astrophysics Data System