Chondrule formation by lightning in the Protosolar Nebula?
W. Pilipp 1,
T.W. Hartquist 2,
G.E. Morfill 3 and
E.H. Levy 4
Received 15 November 1996 / Accepted 14 July 1997
We present results for the electric fields generated in steady one dimensional flows in media having properties that obtained in the protosolar nebula. The mean neutral velocity was taken to be constant and antiparallel to an effective gravitational field, whereas the mean gas phase ion and electron velocities as well as the velocities of entrained 0.01 cm to 1 cm radius grains and submicron- to micron-radius grains were calculated. The gas phase ionization structure was followed as were the charges carried by the grains. Grains were taken to be charged in collisions with gas phase ions and electrons; grain-grain charge transfer due to the Elster-Geitel mechanism as well as to an unspecified additional process at a parameterized rate was included.
Electric fields of sufficient strength to induce discharges, in which chondrule formation has previously been supposed to occur, were found to obtain only if the gas phase ionization rate in the nebula was almost as low as that resulting from the decay of and a grain-grain charge transfer mechanism in addition to the Elster-Geitel process operated. We conclude that the existence of lightning in the protosolar nebula has to depend on the operation of unknown grain-grain charge transfer processes whereas the Elster-Geitel mechanism has proved to be completely inadequate for producing lightning there. For all but a small range of model parameters the attainment of breakdown electric field strengths requires that local ionization equilibrium has not been reached. In addition, it was found that breakdown electric field strengths were more likely to be generated if the grains had an average specific mass density not much lower than of order unity.
Bearing in mind these severe constraints, we conclude that nebular lightning is possible, but requires special conditions. From considerations of the global energetics, the time scales for establishing the high field strengths plus the volume affected by discharges, we conclude that if these conditions exist lightning may have heated a large fraction of the protosolar nebula at one time or another to temperatures high enough to induce the melting of solid material.
Key words: meteors solar system: formation solar system: general dust
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© European Southern Observatory (ESO) 1998
Online publication: February 4, 1998