FORMATION OF GIANT MOLECULAR CLOUDS AND HELICAL MAGNETIC-FIELDS BY THE PARKER INSTABILITY

USING the Nagoya telescope 1, Uchida et al. 2 found an unusual helical filamentary structure, spinning about its long axis, in the L1641 cloud in the Orion cloud complex. Noting that this structure is consistent with a helically twisted magnetic field inferred from optical polarization observations 3,4, they argued that the helical filament is a manifestation of torsional magnetohydrodynamic (Alfven) waves draining angular momentum from a nearby massive cloud, thus promoting collapse and star formation. Here we present an alternative interpretation. We suggest that the Orion molecular cloud complex formed through the Parker instability 5 (the buoyancy of a magnetic field entrained in matter), and that the helical filament is the result of spinning gas falling along the magnetic field and twisting it. The twisted magnetic field, unlike a purely planar field, suppresses the Parker instability on small scales, allowing the generation of finite clouds rather than general turbulence.