Magnetic Aggregation I: Aggregation Dynamics and Numerical Modelling

Focussing on preplanetary grains growth, we discuss the properties of dust aggregation driven by magnetic dipole forces. While there is no direct evidence for the existence of magnetic grains present in the solar nebula, there are reasons to assume they may have been present. We derive analytical expressions for the cross-section of two interacting dipoles. The effective cross section depends upon the strength of the magnetic dipoles and the initial velocities. For typical conditions the magnetic cross section is between 2 and 3 orders of magnitude larger than the geometric cross section. We study the growth dynamics of magnetic grains and find that the mass of the aggregates should increase with time like t3.2 whereas Brownian motion growth behaves like t2. A numerical tool is introduced which can be used to model dust aggregation in great detail, including the treatment of contact forces, aggregate restructuring processes and long-range forces. This tool is used to simulate collisions between magnetic grains or clusters and to validate the analytical cross-sections. The numerically derived cross section is in excellent agreement with the analytical expression. The numerical tool is also used to demonstrate that structural changes in the aggregates during collisions can be significant.