Magnetic Fields Recorded by Chondrules Formed in Nebular Shocks

TL;DR

This study investigates whether chondrules record the background magnetic field of the solar nebula or an amplified field from shock events, using models of nebular and planetary bow shocks to analyze magnetic field amplification and relaxation.

Contribution

It provides a detailed analysis of magnetic field amplification in nebular and planetary bow shocks and concludes that chondrules likely record the background nebular magnetic field.

Findings

Magnetic fields are amplified by factors of 10-30 behind large-scale shocks.

Magnetic diffusivity causes fields to relax to background levels behind planetary bow shocks.

Chondrules formed in planetary bow shocks probably record the background nebular magnetic field.

Abstract

Recent laboratory efforts (Fu et al., 2014) have constrained the remanent magnetizations of chondrules and the magnetic field strengths at which the chondrules were exposed to as they cooled below their Curie points. An outstanding question is whether the inferred paleofields represent the background magnetic field of the solar nebula or were unique to the chondrule-forming environment. We investigate the amplification of the magnetic field above background values for two proposed chondrule formation mechanisms, large-scale nebular shocks and planetary bow shocks. Behind large-scale shocks, the magnetic field parallel to the shock front is amplified by factors $\sim 10-30$, regardless of the magnetic diffusivity. Therefore, chondrules melted in these shocks probably recorded an amplified magnetic field. Behind planetary bow shocks, the field amplification is sensitive to the magnetic diffusivity. We compute the gas properties behind a bow shock around a 3000 km-radius planetary embryo, with and without atmospheres, using hydrodynamics models. We calculate the ionization state of the hot, shocked gas, including thermionic emission from dust, and thermal ionization of gas-phase potassium atoms, and the magnetic diffusivity due to Ohmic dissipation and ambipolar diffusion. We find that the diffusivity is sufficiently large that magnetic fields have already relaxed to background values in the shock downstream where chondrules acquire magnetizations, and that these locations are sufficiently far from the planetary embryos that chondrules should not have recorded a significant putative dynamo field generated on these bodies. We conclude that, if melted in planetary bow shocks, chondrules probably recorded the background nebular field.