solar physics Solar physics is the branch of astrophysics that specializes in the study of the Sun. It deals with detailed measurements that are possible only for our closest star. It intersects with many disciplines of pure physics, astrophysics, and comput ...

, Hale's law, also known as Hale's polarity law or the Hale–Nicholson law, describes the tendency for bipolar

active region An active region is a temporary region in the Sun's atmosphere characterized by a strong and complex magnetic field. They are often associated with sunspots and are commonly the source of violent eruptions such as coronal mass ejections and solar ...

s within the same northern or southern hemisphere to have the same leading magnetic polarity with respect to the

solar rotation Solar rotation varies with latitude. The Sun is not a solid body, but is composed of a gaseous plasma. Different latitudes rotate at different periods. The source of this differential rotation is an area of current research in solar astronomy. ...

, and for those in the opposite hemisphere to have the opposite leading polarity. It further describes how this pattern reverses from one

sunspot cycle The solar cycle, also known as the solar magnetic activity cycle, sunspot cycle, or Schwabe cycle, is a nearly periodic 11-year change in the Sun's activity measured in terms of variations in the number of observed sunspots on the Sun's surfa ...

to the next. It is named after

George Ellery Hale George Ellery Hale (June 29, 1868 – February 21, 1938) was an American solar astronomer, best known for his discovery of magnetic fields in sunspots, and as the leader or key figure in the planning or construction of several world-lea ...

and Seth Barnes Nicholson. Hale's law, along with Joy's law, provides observational evidence for the

solar dynamo The solar dynamo is a physical process that generates the Sun's magnetic field. It is explained with a variant of the dynamo theory. A naturally occurring electric generator in the Sun's interior produces electric currents and a magnetic field, ...

History

The

solar magnetic field The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared ra ...

was first detected in 1909 by

when he showed observationally that

sunspots Sunspots are phenomena on the Sun's photosphere that appear as temporary spots that are darker than the surrounding areas. They are regions of reduced surface temperature caused by concentrations of magnetic flux that inhibit convection. Sun ...

had strong, bipolar magnetic fields. With these observations, Hale also noted how the majority of sunspot groups within the same northern or southern hemisphere shared the same leading polarity and that this pattern reversed across the equator. As

solar cycle 14 Solar cycle 14 was the fourteenth solar cycle since 1755, when extensive recording of solar sunspot activity began. The solar cycle lasted 11.5 years, beginning in January 1902 and ending in July 1913. The maximum smoothed sunspot number observ ...

transitioned into

solar cycle 15 Solar cycle 15 was the fifteenth solar cycle since 1755, when extensive recording of solar sunspot activity began. The solar cycle lasted 10.1 years, beginning in July 1913 and ending in August 1923. The maximum smoothed sunspot number observed du ...

, 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.

Definition

Hale's law describes the magnetic polarity associated with solar

s. The magnetic field of most active regions can usually be approximated by a pair of

magnetic monopole In particle physics, a magnetic monopole is a hypothetical elementary particle that is an isolated magnet with only one magnetic pole (a north pole without a south pole or vice versa). A magnetic monopole would have a net north or south "magneti ...

s 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

, 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: * 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

to the next.

Anti-Hale regions

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. 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. Additionally, anti-Hale regions, and small regions in general, tend to have an orientation angle, or tilt, that does not follow Joy's law.

History

Definition

Anti-Hale regions

See also

References

Further reading