Sunspots with the Strongest Magnetic Fields |
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| Authors: | W Livingston J W Harvey O V Malanushenko L Webster |
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| Institution: | (1) National Solar Observatory, P.O. Box 26732, Tucson, AZ 85726, USA;(2) Division of Astronomy and Astrophysics, University of California, Los Angeles, CA 90095-1562, USA |
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| Abstract: | The strongest observed solar magnetic fields are found in sunspot umbrae and associated light bridges. We investigate systematic
measurements of approximately 32 000 sunspot groups observed from 1917 through 2004 using data from Mt. Wilson, Potsdam, Rome
and Crimea observatories. Isolated observations from other observatories are also included. Corrections to Mt. Wilson measurements
are required and applied. We found 55 groups (0.2%) with at least one sunspot with one magnetic field measurement of at least
4000 G including five measurements of at least 5000 G and one spot with a record field of 6100 G. Although typical strong-field
spots are large and show complex structure in white light, others are simple in form. Sometimes the strongest fields are in
light bridges that separate opposite polarity umbras. The distribution of strongest measured fields above 3 kG appears to
be continuous, following a steep power law with exponent about −9.5. The observed upper limit of 5 – 6 kG is consistent with
the idea that an umbral field has a more or less coherent structure down to some depth and then fragments. We find that odd-numbered
sunspot cycles usually contain about 30% more total sunspot groups but 60% fewer >3 kG spots than preceding even-numbered
cycles. |
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