Planetary Sciences American and Soviet Research/Proceedings from the U.S.-U.S.S.R. Workshop on Planetary Sciences (1991) / Chapter Skim
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6. Magnetohydrodynamic Puzzles in the Protoplanetary Nebula
6. Magnetohydrodynamic Puzzles in the Protoplanetary Nebula
Pages 70-81
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From page 70... ...
Many large-scale astrophysical systems are strongly magnetized and exhibit phenomena that are shaped by the dynamical behaviors of magnetic fields. Evidence and theoretical ideas point to the possibility that the protoplanetary nebula also might have had a strong magnetic field.
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From page 71... ...
These latter ambiguities are especially confusing to neat interpretations of the absolute intensities of the magnetic fields in which the meteorites acquired their remanence. A further complication arises because the meteorites exhibit a diversity of magnetizations, blocked at different temperatures and in different directions on different scales.
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From page 72... ...
However, the purpose here is not to rule out completely the possibility of an important nebular field arising from the Sun; rather it is to indicate that such an assumption does not lead to an easy and obvious solution of the problem of a nebular magnetic field. Here we will presume a turbulent nebula in which the short mixing times and low electrical conductivity, and the consequent rapid dissipation of magnetic fields, quickly erase any memory of the nebular fluid's previous magnetization.
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From page 73... ...
Now, taking both the scale of the largest eddies, 1, and the scale of the magnetic field, a, to be of the order of the scale height of the disk gas, ~ 5 x 10~2 cm, we find that N ~ 1.3 x 1035 9-2, when r ~ 3 AU. Recent detailed numerical calculations of magnetic field generation in Keplerian disks indicate (Stepinski and Levy 1988)
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From page 74... ...
Here we will focus on the possibility that chondrules melted as a result of being exposed to energetic particles from magnetic nebular flares (Levy and Araki 1988~. In astrophysical systems, explosive restructuring of magnetic fields, associated with instabilities that relax the ideal hydromagnetic constraints and allow changes in field topology, seems to be among the most prevalent of phenomena responsible for energetic transient events.
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From page 75... ...
From equation (5) it is found that the above cited conditions in the flare site, pa 10-~8 gm cm~3 and B ~ 5-7 Gauss, yield flare energy outflows sufficient to melt chondrules; substantially weaker magnetic fields or higher ambient mass densities yield energy fluxes too low to account for chondrule melting.
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From page 76... ...
is an especially sensitive function of the magnetic field strength: a 10 Gauss magnetic field would multiply all of the above energies by a factor of 32. Now it is interesting to compare these results with the observations of flaring T Tauri stars.
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From page 77... ...
Thus, to make a crude estimate, if the volume of coronal space filled with disk-generated magnetic field is, say, five times larger than the volume of the disk itself, suggested as a possibility in this discussion, then the effective expansive stress communicated to the disk gas is some five times larger. In that case the magnetic field becomes a major factor in the structure and dynamical balance of the disk especially with respect to the vertical direction.
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From page 78... ...
Second, detailed observations of the Sun show us that intense magnetic fields may be confined to thin flux ropes, with the intervening field strength being much weaker. Such a situation in the nebula, with a spatially intermittent magnetic field, might admit the most intense magnetic fields inferred from meteorite magnetization, while still producing an overall rate of angular momentum transfer much lower than that estimated above.
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From page 79... ...
This picture has potentially significant implications for our understanding of the dynamical behavior and evolution of the protoplanetary nebula because a magnetic field having the implied strength and character discussed here would have exerted considerable stress on the system. The primary unresolved question involves the electrical conductivity of the nebular gas.
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From page 80... ...
1988. Magnetic reconnection flares in the protoplanetary nebula and the possible origin of meteorite chondrules.
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From page 81... ...
1988. Generation of dynamo magnetic fields in protoplanetary and other astrophysical accretion disks.
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