Structure and Composition of the Atmosphere, Physics tutorial

Layers of Atmosphere:

Atmosphere of earth is made up of different layers. It begins from surface of earth and thins out with height until it merges with space. In terms of variation of temperature with height, the layers of atmosphere are:

i) The troposphere

ii) The stratosphere

iii) The mesosphere, and

iv) The thermosphere.

The layers are listed in order of increasing height. Therefore, troposphere is lowest layer followed by stratosphere, mesosphere and thermosphere. Thermosphere is highest layer of earth's atmosphere (in terms of temperature). In terms of composition, atmosphere is categorized into two:

a) Homosphere (up to 85km height), and

b) Heterosphere (above 85km height).

In homosphere, mean molecular mass is constant because of mixing. In heterosphere, mean molecular mass differs due to diffusive separation. In terms of escape properties of neutral particles, terrestrial atmosphere is known as exosphere.

Troposphere:

Troposphere is first layer above surface of the earth. It stretches from surface of the earth to the altitude of approx 10 kilometers. Gases in troposphere are predominantly molecular nitrogen (N2) and molecular oxygen (O2). Troposphere has 90% of mass of earth's atmosphere. It also has 99% of all water vapor present in atmosphere. Within troposphere, temperature deceases with height at rate of about 6.5oC per kilometer. This rapid decrease in temperature with height is approximately linear. It continues until temperature reaches the local minimum value that defines upper boundary of troposphere called as tropopause. At tropopause, the temperature stays constant. Tropopause may lie somewhere between 10km and 15km above surface of the earth.

Stratosphere:

Stratosphere lies above the tropopause. In the stratosphere, temperature increases with altitude. Increase in temperature with height continues until the local maximum is reached at the height of approx 50 kilometers. This local maximum states stratopause. Stratopause is upper boundary region of stratosphere. The stratosphere is extremely stable. For that reason, several jet planes fly within stratosphere. Though, stratospheric air stays adequately dense to allow hot air balloons to ascend to altitudes of between 15km and 20km. Helium balloons are able to rise to the height of approx 35 kilometers. Ozone layer exists in stratosphere.

Stratospheric region is comparatively calm. Variation of stratospheric temperature with height is not as rapid as variation of tropospheric temperature with height. Stratosphere is comparatively warm as sun's ultraviolet radiation is absorbed by oxygen and ozone.

Mesosphere:

Mesosphere lies above stratopause. In mesosphere, temperature decreases with altitude up to local minimum. This local minimum known as mesopause states upper boundary of mesosphere. At mesopause, the temperature can be as low as -90oC. This is lowest temperature anywhere in complete terrestrial atmosphere. Mesopause is situated at altitude of approx 80 kilometers. Meteors are found in neighborhood of mesopause.

Thermosphere:

Thermosphere lies above menopause. It extends to the altitude of approx 500 kilometers. Air in thermosphere is very thin. Large amounts of nitrogen and oxygen atoms are generated through procedure of photo-dissociation of the dominant N2 and O2 molecules. Above mesopause, temperature increases radically to the overall maximum value which may exceed 1000K. Beyond this point, thermospheric temperature stays almost constant with altitude. Thermosphere comprises ionized portion of atmosphere known as ionosphere.

Exosphere:

The exosphere continues beyond the thermosphere. Transition region between thermosphere and exosphere is called as exobase. Region above exobase is still known as exosphere. Exobase is located at a height of approximately 600km. In this portion of exosphere, neutral densities are so low that collisions become insignificant. For this reason, upper atmosphere can no longer be classified as fluid. Space shuttle orbits around the earth within region of exosphere.

Ionosphere:

Layers of the Ionosphere:

The ionosphere is that part of planetary atmosphere where ions and electrons are present in quantities adequate to affect propagation of radio waves. Ionosphere extends from the altitude of approximately 60 kilometers to altitude of about 2000 kilometers. Composition of atmosphere changes with height. Consequently, ion production rate also changes with height. This leads to formation of given three distinct ionization peaks:

a) D layer

b) E layer, and

c) F layer.

The ionosphere is a very significant region of atmosphere due to its influence on radio waves.

D layer:

The D layer is mostly liable for absorption of high frequency (HF) radio waves. Absorption is strongest for radio frequencies below 10MHz. Absorption become progressively smaller as frequency increases. Absorption is higher in day and lower at night. The ionization is because of Lyman series-alpha hydrogen radiation at a wavelength of 121.5 nanometer (nm) which ionizes nitric oxide (NO).

Therefore, dominant ions are NO+ and O2+. D layer extends in height from approximately 40km to 90km. Electron density of D layer is approximately 2.5x109m-3in day. Electron density diminishes to negligible value at night.

E layer:

E layer lies above D layer. It is the region of ionosphere between the altitude of approximately 90km and an altitude of about 160km. In relation to solar zenith angle and solar activity, electron density in E layer behaves in the regular manner. In daytime, electron density may reach approximately 2x1011m-3. This is high enough to reflect radio waves with frequencies of many megahertz. At night, electron density of E region may decrease to about 1010m-3. The E region can only reflect radio waves which have frequencies lower that 10MHz. Though, sporadic E propagation (Es) is also possible. It is characterized by small clouds of intense ionization. Such clouds can reflect radio waves ranging in frequency from approx 25MHz to about 225MHz. Sporadic-E events take place most frequently during hot season. Occasionally, the sporadic-E event may also take place during cold season. Due to sporadic-E events, some propagation channels (that are usually unreachable) can open up.

602_E Layer.jpg

F layer:

The F layer lies above the E layer. Its behaviour is quite irregular. It is generally categorized into number of anomalies like equatorial anomaly and seasonal anomaly. Day-time electron density is fairly different from night-time electron density. In day, average peak value of electron density is approximately 2x1012m-3. At night, average peak value of electron density falls to approx 2x1011m-3. F region is liable for reflection of radio waves.

Aurora:

Auroras are found in thermosphere. Auroras are light produced when protons and electrons from sun travel along earth's magnetic field lines above North Pole (and above the South Pole) and stimulate neutral atoms and molecules. Aurora related with North Pole is at times known as the northern lights; aurora related with South Pole is at times known as southern lights. Aurora is spectacular display of lights requiring tens of thousands of volts to generate. This fascinating display of energy occurs within ionosphere (60-200km above surface of earth). Specially, aurora borealis over northern Canada takes place at a height of approx 100 kilometers.

Solar Wind:

Surface of sun is like pot of boiling water. Bubbles of hot, electrified gas circulate from interior to surface of sun. This gas is composed of electrons and protons in fourth state of matter called as plasma. As circulation continues, plasma continuously bursts out in space. This steady stream of ionized plasma, that escapes from solar corona and pervades whole interplanetary space, is called as solar wind.

Solar wind is primarily made up of electrons and protons. Though, approx 10% of solar wind are helium ions. Solar wind induces geomagnetic activity by variation of pressure and magnetic field. At orbit of the earth, solar wind has the density of approx 10 particles per cubic centimeters, temperature of approx 10,000,000 K (equal to about 1 keV) and average speed of approx 500 km/sec. The solar wind blows with speeds ranging from 400km/sec to 2,500km/sec. It carries million tones of matter in space every second.

Electromagnetic Coupling:

Electromagnetic coupling arises due to interaction of magnetized solar wind with geomagnetic field of earth. When the supersonic solar wind first encounters geomagnetic field, freestanding bow shock is formed. Thus, solar wind is deflected around earth in the region called as magnetosheath. Subsequent interaction of magnetosheath flow with geomagnetic field leads to formation of magnetopause.

Geomagnetic Storms and Substorms::

Magnetosphere is near-earth region of space where dynamics are governed by internal geomagnetic field. Solar wind is highly conducting, collisionless, magnetized plasma. Its behaviour is administered by ideal magneto hydrodynamic (MHD) equations. Solar wind magnetic field (or interplanetary magnetic field IMF) penetrates magnetopause and connects with geomagnetic field. Aurora oval has dipolar magnetic field lines. Field lines are closed. Though, as solar wind blows across earth, dipolar magnetic field lines are distorted. On dayside, field lines are compressed. On night side, field lines are lengthened to create very long geomagnetic tail. Geomagnetic tail plays dominant role in dynamic procedures related with magnetosphere. For example, geomagnetic tail serves as energy reservoir which powers magnetospheric dynamic procedures.

Impact of Space Weather on Human Activity:

Solar flares generate high energy particles and radiation which are dangerous to living organisms. Though, at surface of Earth we are well protected from effects of solar flares and other solar activity by Earth's magnetic field and atmosphere. Most dangerous emissions from flares are energetic charged particles (mainly high-energy protons) and electromagnetic radiation (primarily x-rays).

X-rays from flares are stopped by atmosphere well above the Earth's surface. They do disturb Earth's ionosphere, though, which in turn disturbs some radio communications. Along with energetic ultraviolet radiation, they heat Earth's outer atmosphere, causing it to expand. This increases drag on Earth-orbiting satellites, decreasing their lifetime in orbit. Also, both intense radio emission from flares and the changes in atmosphere can degrade precision of Global Positioning System (GPS) measurements.

The energetic particles generated at Sun in flares seldom reach Earth. When they do, Earth's magnetic field prevents almost all of them from reaching Earth's surface. Small number of very high energy particles which reaches surface doesn't considerably increase level of radiation which we experience every day.

The most serious effects on human activity take place during major geomagnetic storms. It is now understood that major geomagnetic storms are induced by coronal mass ejections (CMEs). Coronal mass ejections are generally related with flares, but at times no flare is observed when they take place. Like flares, CMEs are more frequent during active phase of Sun's about 11 year cycle. Last maximum in solar activity was in year 2000.

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