Where the report describes how far a field reaches, it measures out to the distance at which the field falls to 5 Gauss.
Where it appears in the report: Penetration Depth Analysis
The evidence
The 5 Gauss edge is a steady measurement reference, roughly ten times Earth's background field. We picked it so a reported reach reflects field that stands clearly above the background. It's a reporting convention, not a biological threshold, and we don't claim the field stops mattering below it.
The field is strongest right at the coil and weakens with distance in every direction. On the on-axis dipole model the report uses, it reaches half its peak strength at about three quarters of a coil radius from center and one tenth at about two radii. A larger coil holds the field up farther out.
