Unlike the Earth, no surface feature on the sun lasts for any appreciable
length of time. The most easily observed feature on the solar
disk is the sunspot group, and most of these last for only
a few days. The longest lasting sunspot groups are observable
for no more than about 100 days, but
these are very few and far between. Other solar features,
such as prominences, have similar
lifetimes to sunspot groups.
The graph below was drawn from Royal Greenwich Observatory
observations of over 3000 sunspot groups in the first half of this
century.
The graph may be interpreted by noting that the vertical
direction shows the percentage of sunspot groups that have
lifetimes equal to or in excess of the interval
shown on the horizontal axis. We can therefore see that fully
50% of all groups have lifetimes of less than 2 days, and only 10%
of all groups last for more than 11 days.
It is interesting to note that on the average, larger
sunspot groups last longer. The famous solar
scientist Waldmeir derived a simple rule of thumb relating the
lifetime of a sunspot group to the maximum area that it attains
during its life:
T = 0.1 Amax
where T is the group lifetime in days and Amax is the maximum
area in millionths of the solar hemisphere (typically areas range
from 1 to 5000 millionths). This can be useful in
predicting the lifetime of a sunspot group as it will typically
grow to maximum area much faster than it
decays into oblivion. The formula, however, is only
a rough guide, as wide individual variations
occur.
Educational