DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

the sun. Sunspots tend to occur in groups, are
relatively short-lived, usually less than a day,
but some sunspots can live as long as a month,
and some special sunspots can live even to half
a year. Sunspots occur in cycles of 11 or 12
years. They are restricted to regions (solar active
regions) of 5 ◦ N −40 ◦ N and 5 ◦ S −40 ◦ S,
moving from higher to lower latitudes during a
sunspot cycle. At the beginning of a sunspot
cycle, sunspots appear near 40 ◦ N and 40 ◦ S and
move to lower latitude. Their number increase
and then decrease during the moving processes,
and reach a minimum at about 5 ◦ N and 5 ◦ S regions
at the end of the sunspot cycle. Sunspots
are characterized by strong magnetic fields (up
to several thousand tesla), being concentrations
of magnetic flux, typically in bipolar clusters or
groups and are associated with magnetic storms
on the Earth. Sunspots are the most obvious
manifestation of solar activity. See sunspot cycle,
sunspot number.
sunspot classification A - A small single
unipolar sunspot or very small group of spots
without penumbra. B - Bipolar sunspot group
with no penumbra. C - An elongated bipolar
sunspot group. One sunspot must have penumbra.
D - An elongated bipolar sunspot group
with penumbra on both ends of the group. E - An
elongated bipolar sunspot group with penumbra
on both ends. Longitudinal extent of penumbra
exceeds 10 ◦ but not 15 ◦ . F - An elongated bipolar
sunspot group with penumbra on both ends.
Longitudinal extent of penumbra exceeds 15 ◦ .
H - A unipolar sunspot group with penumbra.
sunspot cycle An approximately 11-year
quasi-periodic variation in the twelve-month
smoothed sunspot number. Other solar phenomena,
such as the 10.7-cm solar radio emission,
show similar cyclical behavior. The polarities of
solar magnetic fields are now known to reverse
with each cycle leading to a roughly 22-year cycle.
This is sometimes called the Hale cycle.
sunspot number The number of sunspots
apparent on the sun at different times. At the
beginning of a sunspot cycle, sunspots appear
near 40 ◦ N and 40 ◦ S and move to lower latitude.
Their numbers increase and then decrease during
the moving processes and reach to the min-
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supercell storm
imum at about 5 ◦ N and 5 ◦ S regions at the end
of the sunspot cycle.
sunspot number (daily) A daily index of
sunspot activity. The sunspot number is computed
according to the Wolf (1849) sunspot
number R = k(10g + s), where g is the number
of sunspot groups (regions), s is the total number
of individual spots in all the groups, and k is
a variable scaling factor (usually