Hale's law

The solar magnetic field was first detected in 1909 by George Ellery Hale when he showed observationally that sunspots had strong, bipolar magnetic fields. With these observations, Hale also noted how the majority of sunspot groups within the same heliographic northern or southern hemisphere shared the same leading polarity and that this pattern reversed across the equator. As solar cycle 14 transitioned into solar cycle 15, further observations were carried out by Hale and his collaborators revealing in 1919 that the magnetic polarity of sunspot pairs within both hemispheres reversed from one 11-year sunspot cycle to the next. These patterns became collectively known as Hale's polarity law, or simply Hale's law.[1][2]

Solar cycle 24

Solar cycle 25

Photospheric magnetograms taken during solar cycles 24 and 25 demonstrate Hale's law. Here, white (black) patches indicate magnetic field lines pointing out of (into) the page, and bipolar active regions appear as pairs of these black and white patches. In these magnetograms, the leading polarity of a given active region is the rightmost polarity.

Hale's law describes the magnetic polarity associated with solar active regions. The magnetic field of most active regions can usually be approximated by a pair of magnetic monopoles with opposite polarity, in which case the region is referred to as a bipolar active region. These poles are generally oriented so that one pole is leading with respect to the direction of solar rotation, and one pole is trailing.

Hale's law states that bipolar active regions have the following properties depending on whether the region is located in the northern or southern heliographic hemisphere:[1]

• In the same hemisphere, regions tend to have the same leading polarity.
• In the opposite hemisphere, regions tend to have the opposite leading polarity.
• Leading polarities in both hemispheres reverse from one sunspot cycle to the next.

Bipolar active regions that violate Hale's law are known as anti-Hale regions. The estimated relative number of anti-hale regions varies from 2 to 9%, due to how often anti-Hale regions are misidentified.[3]

Small, weak, ephemeral active regions violate Hale's law more frequently than average with a relative number around 40%. In contrast, only 4% of medium to large sized active regions violate Hale's law.[4][5] Additionally, anti-Hale regions, and small regions in general, tend to have an orientation angle, or tilt, that does not follow Joy's law.[6]

• Spörer's law
• Gnevyshev–Ohl rule
• Sunspot
• List of solar cycles

• "The magnetic nature of sunspots | High Altitude Observatory". www2.hao.ucar.edu.