Geoscience Reference
In-Depth Information
intercepts both the normal electromagnetic
radiation and energetic particles emitted during
solar flares.
The sun behaves virtually as a
black body;
i.e.,
it absorbs all energy received and in turn radiates
energy at the maximum rate possible for a given
temperature. The energy emitted at a particular
wavelength by a perfect radiator of given temper-
ature is described by a relationship due to Max
Planck. The black-body curves in
Figure 3.1
illustrate this relationship. The area under each
curve gives the total energy emitted by a black
body (
F
); its value is found by integration of
Planck's equation, known as Stefan's Law:
Figure 3.1
shows the absorption bands in the
atmosphere, which cause its emission to be much
less than that from an equivalent black body. The
wavelength of maximum emission (
max) varies
inversely with the absolute temperature of the
radiating body:
λ
2897
λmax = --—— 10
-6
m (Wien's Law)
T
Thus solar radiation is very intense and is mainly
shortwave between about 0.2 and 4.0
m, with
a maximum (per unit wavelength) at 0.5
μ
m
because
T
~ 6000K. The much weaker terrestrial
radiation with
T
μ
280K has a peak intensity at
about 10μm and a range from about 4 to 100μm.
The solar constant undergoes small periodic
variations of just over 1W m
-2
related to sunspot
activity. Sunspot number and positions change in
a regular manner, known as sunspot cycles.
Satellite measurements during the latest cycle
show a small decrease in solar output as sunspot
number approached its
minimum
, and a sub-
sequent recovery.
Sunspots
are dark (i.e., cooler)
areas visible on the sun's surface. Although
sunspots are cool, bright areas of activity known
as
faculae
(or
plages
), that have higher tempera-
tures, surround them (
Plate 3.1
). The net effect
is for solar output to vary in parallel with the
number of sunspots. Thus, the solar 'irradiance'
decreases by about 1.1W m
-2
from sunspot
maximum to minimum. Sunspot cycles have
wavelengths averaging 11 years (the Schwabe
cycle, varying between 8 and 13 years), the 22-year
(Hale) magnetic cycle, much less importantly 37.2
years (18.6 years - the luni-solar oscillation), and
88 years (Gleissberg).
Figure 3.2
shows the
estimated variation of sunspot activity since
1610. Between the thirteenth and eighteenth
centuries sunspot activity was generally low,
except during
AD
1350-1400 and 1600-1645.
Output within the ultraviolet part of the spectrum
shows considerable variability, with up to 20 times
more ultraviolet radiation emitted at certain
wavelengths during a sunspot maximum than a
minimum.
≈
T
4
F
=
where
10
-8
W m
-2
K
-4
(the
Stefan-Boltzmann constant), i.e., the energy
emitted is proportional to the fourth power of the
absolute temperature of the body (
T
).
The total solar output to space, assuming a
temperature of 5760K for the sun, is 3.84
= 5.67
×
10
26
W,
but only a tiny fraction of this is intercepted by the
earth, because the energy received is inversely
proportional to the square of the solar distance
(150 million km). The energy received at the top
of the atmosphere on a surface perpendicular
to the solar beam for mean solar distance is
termed the
solar constant
(see Note 1). Satellite
measurements since 1980 indicate a value of about
1366W m
-2
, with an absolute uncertainty of about
±2W m
-2
.
Figure 3.1
shows the wavelength range
of solar (shortwave) radiation and the infrared
(longwave) radiation emitted by the earth and
atmosphere. For solar radiation, about 8 percent
is ultraviolet (0.2-0.4
×
μ
m), 40 percent visible
light (0.4-0.7
μ
m) and 52 percent near-infrared
(>0.7
m = 1 micrometer = 10
-6
m). The
figure illustrates the black-body radiation curves
for 6000K at the top of the atmosphere (which
slightly exceeds the observed extraterrestrial
radiation), for 300K, and for 263K. The mean
temperature of the earth's surface is about 288K
(15
μ
m); (1
μ
°
C) and of the atmosphere about 250K
(-23
°
C). Gases do not behave as black bodies, and
