Mechanism and limits of magnetar magnetic fields without dynamo action

Ascertain how magnetars or other compact objects lacking dynamo action can build and sustain their extremely strong magnetic fields, and determine whether the same upper limit on magnetic field strength that applies to current-driven systems also constrains such compact objects.

Background

In discussing magnet requirements for bending ultra-energetic tau beams, the paper surveys physical limits on magnetic field strengths. It references arguments tying field strengths to electric current flow and then raises the case of magnetars, whose fields are thought to arise from stellar collapse and differential rotation rather than conventional dynamos.

This leads to an explicit open question about both the generation mechanism in compact objects without dynamo action and whether the same theoretical upper bounds apply to such astrophysical fields.