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Sunspot
Numbers
Scientists track
solar cycles by counting
sunspots -- cool planet-sized areas on the Sun where intense
magnetic loops poke through the star's visible surface.
Counting sunspots
is not as straightforward as it sounds. Suppose you looked at the
Sun through a pair of (properly filtered) low power binoculars --
you might be able to see two or three large spots. An observer
peering through a high-powered telescope might see 10 or 20. A
powerful space-based observatory could see even more -- say, 50
to 100. Which is the correct sunspot number?
Above:
Yearly-averaged
international sunspot numbers from 1610-2000. [more]
There are two official sunspot
numbers in common use. The first, the daily "Boulder Sunspot
Number," is computed by the NOAA Space Environment Center
using a formula devised
by Rudolph Wolf in
1848: R=k
(10g+s),
where R is
the sunspot number; g
is the number of sunspot groups on
the solar disk; s
is the total number of individual
spots in all the groups; and k
is a variable scaling factor
(usually <1) that accounts for observing conditions and the
type of telescope (binoculars, space telescopes, etc.).
Scientists combine data from lots of observatories -- each with
its own k
factor -- to arrive at a daily
value.
 The
Boulder number (reported daily on Scientific Frontline) is
usually about 25% higher than the second official index, the
"International Sunspot Number," published daily by the
Sunspot
Index Data Center in
Belgium. Both the Boulder and the International numbers are
calculated from the same basic formula, but they incorporate data
from different observatories.
Right:
Rudolf Wolf devised the basic
formula for calculating sunspots in 1848. Today, Wolf sunspot
counts continue, since no other index of the sun's activity
reaches into the past as far and as continuously. An avid
astronomical historian and an unrivaled expert on sunspot lore,
Wolf confirmed the existence of a cycle in sunspot numbers. He
also more accurately determined the cycle's length to be 11.1
years by using early historical records. [more]
As a rule of
thumb, if you divide either of the official sunspot numbers by
15, you'll get the approximate number of individual sunspots
visible on the solar disk if you look at the Sun by projecting
its image on a paper plate with a small telescope.
Source:
NASA
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