Hale's law
In
It applies to simple active regions that have
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.
History
The
Definition
Hale's law describes the magnetic polarity associated with solar active regions. The magnetic field of most active regions can be approximated by a pair of magnetic monopoles of opposing 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 the other is trailing.[4]
Hale's law states that bipolar active regions have the following properties depending on whether the region is located in the northern or southern solar hemisphere:[2]
- 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 cycleto the next.
Anti-Hale regions
Bipolar active regions that violate Hale's law are known as anti-Hale regions. Estimates of the percentage of bipolar active regions that violate Hale's law have ranged from 2 to 9%.[5][6] 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.[7][8] Furthermore, anti-Hale regions—and small regions in general—tend to have an orientation angle, or tilt, that does not follow Joy's law and have been found to be more prevalent during solar minima.[9][10][11][12]
Hale cycle
Since Hale's law states that the leading magnetic polarities in each hemisphere alternate between sunspot cycles, it takes two full cycles for the leading polarities to return to their original pattern. This indicates that the approximately 11-year sunspot cycle is one-half of a 22-year magnetic cycle, which is sometimes referred to as a Hale cycle.[13]
Solar dynamo
Hale's law has important implications for the Sun's internal magnetic field and the dynamo that drives it. Namely, the observation that active regions in a given north-south hemisphere all have the same leading magnetic polarity suggests that their emergence is the manifestation of a highly organized east-west-aligned, or toroidal, magnetic field in the Sun's interior. Additionally, the observations that the polarity of the leading magnetic field reverses across the equator and alternates between successive sunspot cycles further suggests that such a toroidal field also reverses polarity across the equator and alternates polarities between cycles.[14][15][16][17]
Hale's law, along with Joy's law for the tilt of sunspot groups and
See also
References
- doi:10.1086/141602.
- ^ doi:10.1086/142452.
- ^ Charbonneau, P.; White, O. R. (18 April 1995). "Hale's Sunspot Polarity Law". www2.hao.ucar.edu. High Altitude Observatory. Archived from the original on 2021-08-19. Retrieved 2021-08-20.
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