2025ApJ...981...72R

The origin of the magnetic fields in white dwarfs (WDs) remains mysterious. Magnetic WDs are traditionally associated with field strengths ≳1 MG, set by the sensitivity of typical spectroscopic magnetic field measurements. Informed by recent developments in red giant magnetoasteroseismology, we revisit the use of WD pulsations as a seismic magnetometer. WD pulsations primarily probe near-surface magnetic fields, whose effect on oscillation mode frequencies is to asymmetrize rotational multiplets and, if strong enough, suppress gravity-mode propagation altogether. The sensitivity of seismology to magnetic fields increases strongly with mode period and decreases quickly with the depth of the partial ionization-driven surface convective zone. We place upper limits for the magnetic fields in 24 pulsating WDs: 20 hydrogen-atmosphere (DAV) and three helium-atmosphere (DBV) carbon-oxygen WDs, and one extremely low-mass (helium-core) pulsator. These bounds are typically ∼1-10 kG, although they can reach down to ∼10-100 G for DAVs and helium-core WDs in which lower-frequency modes are excited. Seismic magnetometry may enable new insights into the formation and evolution of WD magnetism.