Chondrule formation by lightning in the Protosolar Nebula?

W. Pilipp, T.W. Hartquist, G.E. Morfill and E.H. Levy

Chondrules are millimeter sized beads of glassy rock which are ubiquitious in the primitive meteorites. The chondrules have apparently been flash- heated and cooled again within time scales of seconds to minutes in the protosolar nebula. According to a hypothesis by Whipple (1966) the flash- heating events were lightning discharges.

We investigated whether electric fields strong enough to induce lightning could have occurred in the protosolar nebula. Global space charges and the resulting large-scale electric fields could have been built up when mm-sized and micron to submicron sized dust grains of different electrical charges as well as gas phase ions and electrons are convected with different velocities in a flow along a gravitational field.

We conclude that lightning is possible, but requires special conditions. In particular, it is required that charge transfer processes operated in grain - grain collisions which were much more effective than grain charging by the Elster-Geitel mechanism (a charge transfer process occurring when the grains' surface charge distributions are polarized by a large-scale electric field) and by impact of gas phase ions and electrons. Also the gas phase ionization rate is required to be almost as low as that resulting from decay of long-lived radioactive elements trapped in the grains alone and cosmic ray ionization has to be at most of this same magnitude.

The upper curve in the figure below shows the evolution of the electric field with distance for a steady one-dimensional flow of gas and dust due to turbulent motion in the protosolar nebular for conditions favourable to produce lightning. It is assumed that discharge occurs if the electric field strength is one to several Volts per electron mean free path. The lower curves in the figure show the electric field for mostly the same conditions except for the assumption that no other grain charging mechanisms but only the Elster- Geitel mechanism and charging by impact of ions and electrons on the grains operate and for various boundary conditions and various assumptions for the electron sticking coefficients. These curves show electric field strengths much too low for inducing lightning.


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