The Sun, the Earth, and the Evolution of the Earth’s Atmosphere - Air Pollution and Global Warming: History, Science, and Solutions - page 31

atemperature ranging from 8 million K at its base to
about 5 million K at its top. Energy transfer through the
intermediate interior is also radiative. Ionized H and He
atoms emit photons that are quickly absorbed by other
ionized H and He atoms, which subsequently radiate
photons themselves.
The next layer is the hydrogen convection zone
(HCZ) ,aregion between 0.14 R p and 0.3 R p in which
convection of hydrogen atoms due to buoyancy takes
over from radiation as the predominant mechanism of
transferring energy toward the sun's surface. Tempera-
tures at the base of the HCZ are around 5 million K;
temperatures at its top are near 6,400 K. In the HCZ,
astrong temperature gradient exists and the mean free
path (average distance between collisions) between
photons and hydrogen or helium atoms decreases with
increasing distance from the core. A photon of radiation
emitted from the core of the sun takes about 10 million
years to reach the top of the HCZ.
Above the HCZ is the photosphere (“light sphere”),
which is a relatively thin (500-km-thick) transition
region between the sun's interior and its atmosphere.
Temperatures in the photosphere range from 6,400 K
at its base to 4,000 K at its top and average 5,785 K.
The photosphere is the source of most solar energy that
reaches the planets, including the Earth. Although the
sun's interior is much hotter than is its photosphere,
most energy produced in its interior is confined by
the HCZ.
Above the photosphere lies the chromosphere
(“color sphere”), which is a 2,500-km-thick region of
hot gases. Temperatures at the base of the chromo-
sphere are around 4,000 K. Those at the top are up to
1million K. The name chromosphere arises because, at
the high temperatures found in this region, hydrogen is
energized and decays back to its ground state, emitting
wavelengths of radiation in the visible part of the solar
spectrum. For example, hydrogen decay results in radi-
ation emission at 0.6563

Figure 2.2. Aurora Australis, as seen from Kangaroo
Island, southern Australia. Photo by David Miller,
National Geophysical Data Center, available from
NOAA Central Library; www.photolib.noaa.gov.

The solar wind is the outer boundary of the corona
and extends from the chromosphere to the outermost
reaches of the solar system.
The Earth-sun distance ( R es )isabout 150 million
km. At the Earth, the solar wind temperature is about
200,000 K, and the number concentration of solar wind
ions is a few to tens per cubic centimeter of space. As the
solar wind approaches the Earth, the Earth's magnetic
fields bend the path of the wind toward the poles. In
the atmosphere above these regions, the ionized gases
collide with air molecules, creating luminous bands
of streaming, colored lights. In the Northern Hemi-
sphere, these lights are called the Northern Lights or
Aurora Borealis (“northern dawn” in Latin), and in
the Southern Hemisphere, they are called the Southern
Lights or Aurora Australis (“southern dawn”). Green
or brownish-red colors are due to collisions of the solar
wind with oxygen in the atmosphere. Red or blue colors
are due to collisions with nitrogen. These lights, one of
the seven natural wonders of the world, can be seen
at high latitudes, such as in northern Scotland, Scandi-
navia, and parts of Canada in the Northern Hemisphere
and in southern Australia and Argentina in the Southern
Hemisphere (e.g., Figure 2.2).

m, which is in the red part of
the spectrum, giving the chromosphere a characteristic
red coloration observed during solar eclipses.
The corona is the outer shell of the solar atmosphere
and has an average temperature of about 1 to 2 mil-
lion K. Because of the high temperature, all gases in
the corona, particularly hydrogen and helium, are ion-
ized. A low-concentration steady stream of these ions
as well as electrons escapes the corona and the sun's
gravitational field and propagates through space, inter-
cepting the planets with speeds ranging from 300 to
1,000 km s − 1 .Thisstream is called the solar wind .

2.2. Spectra of the Radiation of the Sun
and the Earth
Solar radiation provides essential energy for heating the
Earth and for driving the chemistry of the Earth's atmo-
sphere. The Earth's temperatures are also controlled by