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We determine the magnetic field strength in the OMC 1 region of the Orion Afilament via a new implementation of the Chandrasekhar-Fermi method usingobservations performed as part of the James Clerk Maxwell Telescope (JCMT)B-Fields In Star-Forming Region Observations (BISTRO) survey with the POL-2instrument. We combine BISTRO data with archival SCUBA-2 and HARP observationsto find a plane-of-sky magnetic field strength in OMC 1 of $B_{\rmpos}=6.6\pm4.7$ mG, where $\delta B_{\rm pos}=4.7$ mG represents apredominantly systematic uncertainty. We develop a new method for measuringangular dispersion, analogous to unsharp masking. We find a magnetic energydensity of $\sim1.7\times 10^{-7}$ Jm$^{-3}$ in OMC 1, comparable both to thegravitational potential energy density of OMC 1 ($\sim 10^{-7}$ Jm$^{-3}$), andto the energy density in the Orion BN/KL outflow ($\sim 10^{-7}$ Jm$^{-3}$). Wefind that neither the Alfv\'{e}n velocity in OMC 1 nor the velocity of thesuper-Alfv\'{e}nic outflow ejecta is sufficiently large for the BN/KL outflowto have caused large-scale distortion of the local magnetic field in the$\sim$500-year lifetime of the outflow. Hence, we propose that the hour-glassfield morphology in OMC 1 is caused by the distortion of a primordialcylindrically-symmetric magnetic field by the gravitational fragmentation ofthe filament and/or the gravitational interaction of the BN/KL and S clumps. Wefind that OMC 1 is currently in or near magnetically-supported equilibrium, andthat the current large-scale morphology of the BN/KL outflow is regulated bythe geometry of the magnetic field in OMC 1, and not vice versa.

The JCMT BISTRO Survey: The magnetic field strength in the Orion A filament