AY 101 Lecture Notes - Lecture 15: Van Allen Radiation Belt, Solar Wind, Northern Hemisphere
Earth's Atmosphere
Earth's atmosphere is divided into four regions. The lowest of these regions is
the troposphere. Here exists turbulence and atmospheric mixing due to convection
currents (weather, for example). The atmospheric density shows a rapid decrease with
height. The dominant source of energy in this lowest region is conductive heating near
the ground, and the dominant energy losses are radiation and convective mixing. The
net result is a rapid temperature decrease with altitude. At a greater height is
the stratosphere, where there exist strong horizontal wind currents (the jet stream)
that affect ground level weather. Going higher, the solar ultraviolet radiation that oxygen
and the ozone absorb is the dominant source of energy gain, and radiation and
convective mixing are the dominant means of energy loss. The balance between energy
gain and loss results in a local temperature maximum at a height of 50 kilometers at the
base of a region termed the mesosphere. Even higher is the thermosphere, where
absorption of very short wave solar radiation is balanced by thermal radiation. Because
there is very little material at this height to absorb this solar energy, the result is a high
temperature. The lowest layer of the thermosphere (also called the ionosphere, 80
kilometers to 400 kilometers high) reflects radio waves because of ionization, and it is
here that meteors burn up. Farther out, the extremely tenuous outermost part of the
thermosphere (or exosphere) fades away until it is indistinguishable from the
interplanetary material.
Global atmospheric conditions, such as weather, are also affected by the Coriolis
Effect. Earth is a solid globe, and hence every latitude moves once around the rotational
axis in the same period of time. The distance of travel around the rotational axis,
however, depends on the latitude, with equatorial distance being the largest. The
eastward velocity of Earth's surface, therefore, is greatest at the equator and least at the
poles. Air masses moving north or south, however, share the eastward motion of that
part of Earth's surface from where the air began moving and hence drift eastward or
westward relative to Earth's surface. Air moving from all directions into a region of low
pressure shows, relative to the surface, a counterclockwise motion in the Northern
Hemisphere (hurricanes are the extreme examples), but clockwise in the Southern
Hemisphere. Air moving in all directions outward from high pressure regions circulates ‐
clockwise in the Northern Hemisphere.
The atmosphere further produces a thermal moderation of temperature over the whole
Earth (resulting in less extreme temperatures both geographically and seasonally),
shields the surface from life destroying ultraviolet, and is the source of necessary gases‐
for life.
The magnetosphere
The magnetosphere is that region surrounding Earth that is influenced by the planet's
magnetic field (see Figure 2). Within it are two doughnut shaped regions, the‐ Van Allen
belts, high above the equator, in which are trapped charged particles from the solar
wind. These particles drift north and south along the magnetic field lines, enter the
atmosphere near the magnetic poles, and produce the aurora, or the luminous irregular
or streamer like phenomena visible at night in a zone surrounding the magnetic poles. ‐
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Document Summary
The lowest of these regions is the troposphere. Here exists turbulence and atmospheric mixing due to convection currents (weather, for example). The atmospheric density shows a rapid decrease with height. The dominant source of energy in this lowest region is conductive heating near the ground, and the dominant energy losses are radiation and convective mixing. The net result is a rapid temperature decrease with altitude. At a greater height is the stratosphere, where there exist strong horizontal wind currents (the jet stream) that affect ground level weather. Going higher, the solar ultraviolet radiation that oxygen and the ozone absorb is the dominant source of energy gain, and radiation and convective mixing are the dominant means of energy loss. The balance between energy gain and loss results in a local temperature maximum at a height of 50 kilometers at the base of a region termed the mesosphere.
