Mechanism of Magnetic Flux Loss in Molecular Clouds

By T. Nakano, R. Nishi, T. Umebayashi. 2002

172 citations recorded by the source corpus.

We investigate the detailed processes at work in the drift of magnetic fields in molecular clouds. To the frictional force, whereby the magnetic force is transmitted to neutral molecules, ions contribute more than half only at cloud densities nH ≲ 104 cm-3, and charged grains contribute more than about 90% at nH ≳ 106 cm-3. Thus, grains play a decisive role in the process of magnetic flux loss. Approximating the flux loss time tB by a power law tB ∝ B-γ, where B is the mean field strength in the cloud, we find γ ≈ 2, characteristic of ambipolar diffusion, only at nH ≲ 107 cm-3, at which ions and the smallest grains are pretty well frozen to the magnetic fields. At nH > 107 cm-3, γ decreases steeply with nH, and finally at nH ≈ ndec ≈ a few × 1011 cm-3, at which the magnetic fields effectively decouple from the gas, γ ≪ 1 is attained, reminiscent of Ohmic dissipation, although flux loss occurs about 10 times faster than by pure Ohmic dissipation. Because even ions are not very well frozen at nH > 107 cm-3, ions and grains drift slower than the magnetic fields. This insufficient freezing makes tB more and more insensitive to B as nH increases. Ohmic dissipation is dominant only at nH ≳ 1 × 1012 cm-3. While ions and electrons drift in the direction of the magnetic force at all densities, grains of opposite charges drift in opposite directions at high densities, at which grains are major contributors to the frictional force. Although magnetic flux loss occurs significantly faster than by Ohmic dissipation even at very high densities, such as nH ≈ ndec, the process going on at high densities is quite different from ambipolar diffusion, in which particles of opposite charges are supposed to drift as one unit.

Subfields: Astronomy and Astrophysics, Atomic and Molecular Physics, and Optics

View Paper: https://www.semanticscholar.org/paper/7db13acf201f4f9c65a19494c665a3872acd81f2

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Citations in the corpus

High- and Low-Velocity Magnetized Outflows in the Star Formation Process in a Gravitationally Collapsing Cloud (2008, methodology): “Note that ohmic dissipation is effective for 1012 cm 3P ncP 1015 cm 3 (Nakano et al. 2002).”
Ambipolar diffusion in low-mass star formation - I. General comparison with the ideal magnetohydrodynamic case (2015, unknown): “When considering multiple species (see Kunz & Mouschovias 2009; Nakano et al. 2002), Ohm’s law can be written as ∂B ∂t =∇ × [ vn × B −ηΩ(∇ × B) −ηH { (∇ × B) × B||B|| } −ηAD B ||B|| × { (∇ × B) × B||B|| } ] , (5) with || || standing for the L2 norm, and the Ohmic, Hall, and ambipolar diffusivities…”
Collapse, outflows and fragmentation of massive, turbulent and magnetized prestellar barotropic cores (2011, background): “In reality, it is expected that significant flux leakages either induced by ambipolar diffusion and/or ohmic dissipation (Nakano et al. 2002, Tassis & Mouschovias 2005, Kunz & Mouschovias 2010) possibly enhanced by turbulence (Santos-Lima et al. 2010), occur.”
Effect of Magnetic Braking on Circumstellar Disk Formation in a Strongly Magnetized Cloud (2010, background): “In reality, however, the magnetic field dissipates in a high-density gas region by Ohmic dissipation and ambipolar diffusion, because the degree of ionization is considerably low (e.g., Nakano et al. 2002).”
RADIATION MAGNETOHYDRODYNAMIC SIMULATIONS OF PROTOSTELLAR COLLAPSE: PROTOSTELLAR CORE FORMATION (2012, background): “Ohmic dissipation becomes dominant in the high-density region (typically ρ 10−11 g cm−3) and ambipolar diffusion dominates in the low-density region (Nakano et al. 2002; Kunz & Mouschovias 2009, 2010), but in this work we focus on Ohmic dissipation as the first step because it is supposed to be…”
NON-IDEAL MHD EFFECTS AND MAGNETIC BRAKING CATASTROPHE IN PROTOSTELLAR DISK FORMATION (2011, methodology): “Since dense cores are known to be lightly ionized (Bergin & Tafalla 2007), non-ideal MHD effects (including ambipolar diffusion, Hall effect and Ohmic dissipation, e.g., Nakano et al. 2002) are to be expected.”
RADIATION MAGNETOHYDRODYNAMIC SIMULATIONS OF PROTOSTELLAR COLLAPSE: NONIDEAL MAGNETOHYDRODYNAMIC EFFECTS AND EARLY FORMATION OF CIRCUMSTELLAR DISKS (2015, methodology): “The ohmic resistivity and ambipolar diffusion rate are calculated based on the method described in Nakano et al. (2002) and Okuzumi (2009).”
Magnetic processes in a collapsing dense core II. Fragmentation. Is there a fragmentation crisis (2007, background): “On the other hand, Machida et al. (2007a) recently explored the magnetized second collapse taking into account the large Ohmic dissipation which is predicted to occur in the first Larson core by various models (e.g. Nakano et al. 2002).”