The penumbral photosphere is dominated by a nearly horizontal radial outflow ( Evershed, 1909). The largest sunspot recorded in the past century had an area of about 6000 millionths of a hemisphere (observed in Mar. The strongest field in sunspots recorded during 1917–20 G ( Livingston et al., 2006), whereas Okamoto and Sakurai (2018) reported an even stronger magnetic field of 6250 G in a light bridge in a large delta-type sunspot.
The magnetic field of sunspots tends to be stronger in larger sunspots and in the darker part of the umbra. About a half of the total magnetic flux emanates from the penumbra. More interesting, the umbra–penumbra boundary in stable sunspots is characterized by an invariant value of the vertical magnetic field component of 1849–1885 G, with the most probable value of 1867 G ( Jurcak et al., 2018). Remarkably the radial profiles of the field strength and elevation show a smooth transition from the umbra to the penumbra, which is in contrast to the sharp boundary between them, as seen in the continuum. Towards the outer part of the sunspot, the field strength declines and becomes more horizontal in a radial direction it reaches an approximate elevation of 10–20 degrees from the solar surface at the edge of the sunspot. In an isolated and round sunspot, the magnetic field is nearly vertical and has a maximum strength of 2000–4000 G at the center. The global structure of the magnetic field of sunspots was extensively studied in the 20th century under relatively low spatial resolution. Sunspots, the most conspicuous solar manifestation of magnetic fields, yielded the first detection of a cosmic magnetic field, carried out by Hale (1908). Kiyoshi Ichimoto, in The Sun as a Guide to Stellar Physics, 2019 3 Sunspots and Active Regions Current theoretical models explain the interlocking comb structure of the filamentary penumbra with outward submerged field lines that are pumped down by turbulent, compressible convection of strong descending plumes. The magnetic field in the umbra is mostly vertically oriented, but it is strongly inclined over the penumbra, nearly horizontally. Their diameters range from 3600 to 50,000 km, and their lifetime ranges from a week to several months. The darkness of sunspots is attributed to the inhibition of convective transport of heat, emitting only about 20% of the average solar heat flux in the umbra and being significantly cooler (∼4500 K) than the surroundings (∼6000 K). Hathaway and NASA/MSFC.Īn individual sunspot consists of a very dark central umbra, surrounded by a brighter, radially striated penumbra. Bottom: Sunspot area as a function of time, which is a similar measure of the solar cycle activity as the sunspot number. Top: Butterfly diagram of sunspot appearance, which marks the heliographic latitude of sunspot locations as a function of time, during the solar cycles 12–23 (covering the years 1880–2000). Sunspot numbers are not to be confused with eruptive solar activity, since solar flares and other activity on the solar surface may occur at any time, and not all sunspots give rise to eruptive solar activity.įIGURE 11.5. The 11-year solar cycle and the approximately 27-day solar rotation period of the Sun are useful for the prediction of geomagnetic disturbances on the Earth, which tend to repeat with the same periodicities. Differential solar rotation refers to an increase in the solar rotation period from 24.47 sidereal days at the solar equator to approximately 38 sidereal days at the poles. The solar rotation period as determined from the observation of sunspots is 27.27 days as seen from the Earth. Most solar flares and coronal mass ejections originate in the magnetically active regions associated with sunspot groups. Sunspots are accompanied by secondary phenomena including coronal loops, prominences and reconnection events. Individual sunspots may persist from a few days to months. The number of sunspots has a cyclical increase and decrease over an approximately 11-year period known as the solar cycle. They usually appear in pairs of opposite magnetic polarity. Sunspots are regions on the Sun's photosphere that appear darker than the surrounding areas on the visible solar disk due to reduced surface temperature associated with concentrations of magnetic field flux and intense magnetic activity on the Sun. Sunspots provide the first indications of the possibility of solar eruptions that may precede geomagnetic storms on the Earth. Cilliers, in Classical and Recent Aspects of Power System Optimization, 2018 2.1 Sunspots
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