Importance of Polarization of Bound ProjectilesElectrons for Transient Magnetic Fields

$K$-vacancy fractions have been measured for Si ions moving in nickel at velocities between $3.9v_0$ and $9.0v_0$. The observed fractions, 2-23%, provide strong evidence that transient magnetic fields for ions moving in magnetized materials are caused mainly by bound, polarized electrons. A polarization close to 0.13 suffices to explain measured transient fields, and it is argued that the polarization can be due to a combination of spin-exchange interactions and capture-loss processes.