magnetic field. These disturbances are tube-shaped and sometimes pop above the
photosphere. Sunspots occur where the magnetic field lines leave and enter the
photosphere. These areas can grow until some reach sizes exceeding 30,000 km with
individual sunspots usually lasting a few days to a few months. Numbers of sunspots
cycle over 11–13 years, and increased numbers of sunspots are associated with
increased solar activity. It has been conjectured that solar cycles influence weather
and climate but this relationship is unclear because of all the other factors influencing
Earth’satmosphere. Prominences, coronal mass ejections, and flares are other
output variations related to sunspots. However, the measured year-to-year solar
variability at the top of Earth’s atmosphere is about 0.1 percent of the average.
The sun is able to emit huge amounts of energy through space because its gases
are heated as a result of fusion. The hotter a substance, the more energy it radiates
because of the greater intensity of molecular vibration. The vibrating electrons of
the gases’ molecules emit electromagnetic energy, and this energy is propagated
through the void of space and through our atmosphere. In space, the speed is about
300,000 km per second with the speed of transmission through our atmosphere
being negligibly slower. Solar energy can penetrate some Earth materials.
Solar output is a staggering 3.31× 1031 joules per day. Of course, Earth is small
and 150 million km distant and the sun radiates in all directions, so we receive
only about one two-billionths of the total solar output. The mean instantaneous
solar input is 1.74× 1017 joules per second at the top of the atmosphere; compare
this figure with the “measly”1.5× 1013 joules per second consumed by human
activity on the planet.
Solar energy can be understood as having two physical realities. On one hand,
radiant energy is composed of streams of particles called photons. Photons are the
best way to explain the manner in which energy interacts with plants to cause photo-
synthesis. Photons incident on plant leaves energize the photosynthetic process.
On the other hand, streams of photons are arranged in waves, and the waves can
be described by the frequencies by which their crests pass or by the lengths of the
waves; most geographers use the latter. There is an exceedingly wide range of wave-
lengths in the universe; however, solar energy is fairly well focused around its most
plentiful wavelengths near 0.5 micrometers (1 micrometer = 1 millionth of a meter).
This peak energy is visible blue light, and it is not surprising that photosynthesis has
evolved to capture the solar wavelengths most available. Various wavelengths
of solar energy act quite differently when encountering matter (seeHeating and
Cooling). Of immense importance in the Earth system is the propensity for the
particular mix of Earth’s atmospheric gases to be relatively transparent to solar
energy and allow about half of that energy to penetrate directly to Earth’s surface.
The solar beam reaching Earth is well described as “sunlight.” Because the
sun’s radiating photosphere is at approximately 6,000°C, visible wavelengths are
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