In solar physics, Hale's law, also known as Hale's polarity law or the Hale–Nicholson law, is an empirical law for the orientation of magnetic fields in solar active regions.
It applies to simple active regions that have bipolar magnetic field configurations where one magnetic polarity is leading with respect to the direction of solar rotation. Hale's law states that, in the same northern or southern solar hemisphere, such active regions have the same leading magnetic polarity; that, in opposite hemispheres, such active regions have the opposite leading polarity; and that, from one sunspot cycle to the next, these polarities reverse. It is named after George Ellery Hale and Seth Barnes Nicholson, whose observations of active-region magnetic fields led to the law's formulation in the early 20th century.
Hale's law, along with Joy's law and Spörer's law, provides observational constraints for models of the solar dynamo, which generates the Sun's magnetic field. Hale's law suggests that active regions originate from a highly organized toroidal magnetic field in the Sun's interior that reverses polarity across the equator and alternates polarity between sunspot cycles.