ESS 7 Lectures 15 and 16 November 3 and 5, 2008 The Atmosphere and Ionosphere The Earth’s AtmosphereThe Structure of the AtmosphereSlide Number 4Why does the Earth have an Atmosphere?Why the Earth has an AtmosphereA Model of the Earth’s AtmosphereThe IonosphereSlide Number 9The Ionosphere During the Day and at Night The Extent of the IonosphereSlide Number 12Slide Number 13Slide Number 14Slide Number 15Slide Number 16A Simple Model of the Ionosphere – Where the Bumps Come FromThe Decrease in PhotonsForming a Chapman LayerSlide Number 20Slide Number 21Slide Number 22Slide Number 23Region 1 and Region 2 CurrentsSlide Number 25Slide Number 26ESS 7 Lectures 15 and 16 November 3 and 5, 2008 The Atmosphere and Ionosphere ESS 7ESS 7 Lectures 15 and 16Lectures 15 and 16 November 3 and 5, 2008November 3 and 5, 2008 The Atmosphere and IonosphereThe Atmosphere and IonosphereThe Earth’s Atmosphere• The Earth’s upper atmosphere is important for ground- based and satellite radio communication and navigation.• Its density determines the lifetime of satellites in low- Earth orbit. • It is important for aurora and magnetospheric convection.• The upper atmosphere is called the thermosphere. It is composed mostly of neutral atoms and molecules.• Within the thermosphere the amount of ionized gas becomes important and forms a region called the ionosphere. • These two co-located regions are coupled through particle collisions (neutral – ion).The Structure of the AtmosphereThe Different Regions• Troposphere (water vapor, convection due to contact with surface, expansion of air)• Stratosphere (ozone layer)• Mesosphere (radiative cooling)• Thermosphere (X-ray, particle energy input heats this layer)• Ionosphere (region with appreciable ionized component - balance of production and loss)Why does the Earth have an Atmosphere?• Why does the Earth have an atmosphere while other planets like Mercury or the Moon have none?• The pressure gradient in the atmosphere points toward the Earth.• That means that the force is outward toward space• The ideal gas law gives PV=nRT where P is the pressure, V is the volume, n is the number of moles of gas, R is the gas constant (8.3145 J K-1mol-1) and T is the temperature.• This is equivalent to P=nkT where n is the number density, k is the Boltzman constant and T is the temperature. PFP−∇=rWhy the Earth has an Atmosphere• Compared to space the pressure is much greater in the atmosphere.• Why then doesn’t the atmosphere simple go out into space?• The answer is that the atmosphere is roughly in hydrostatic equilibrium with gravity – the pressure force is balanced by the force of gravitywhere ρ is the mass density (mass/volume) and g is the gravitational acceleration (equal to 9.8 m s-2 at the Earth’s surface. gPρ−=∇A Model of the Earth’s Atmosphere• The density decreases as a function of heightwhere height is the altitude above the Earth and H is a “scale height” and n0 is the density at the surface.• The scale height is given by H=kT/mg k is the Boltzman constant, T is the temperature, m is the average mass of the atmospheric constituents, and g is gravity.• The density of the atmosphere falls off rapidly with height.• Mercury and the Moon don’t have atmospheres because they are not massive enough to hold them. ()⎟⎠⎞⎜⎝⎛−=Hheightnheightn exp0The Ionosphere• Ions exist everywhere in the atmosphere but they are most important in the thermosphere.• We call that ionized part of the thermosphere the ionosphere.• The ions come from neutral atoms or molecules that have been ionized either by high energy photons (UV or X-rays- short wave lengths) from the Sun or energetic particles from the magnetosphere that precipitate into the atmosphere and collide with the surrounding gas. • The number of ions in the thermosphere peaks at about 300Km height – the region about this peak is the ionosphere.• Guglielmo Marconi’s demonstration of long distance radio communication in 1901 started studies of the ionosphere.• Arthur Kennelly and Oliver Heaviside independently in 1902 postulated an ionized atmosphere to account for radio transmissions. (Kennelly-Heavyside layer is now called the E-layer).• Larmor (1924) developed a theory of reflection of radio waves from an ionized region.• Breit and Tuve in 1926 developed a method for probing the ionosphere by measuring the round-trip for reflected radio waves.The Discovery of the IonosphereThe Ionosphere During the Day and at Night • The main ionization mechanism is photoionization therefore the highest densities in the ionosphere are on the sunlit side of the Earth.• The ionosphere does not go away at night – the recombination time (time for an electron and ion to come back together) is comparable to the rotation period of the Earth.• In the auroral zone precipitating particle (particles whose mirror altitude is in the atmosphere) also ionize particles.The Extent of the Ionosphere• There are ions and electrons at all altitudes in the atmosphere.• Below about 60km the charged particles do not play an important part in determining the chemical or physical properties of the atmosphere.• Identification of ionospheric layers is related to inflection points in the vertical density profile.Primary Ionospheric RegionsRegion Altitude Peak DensityD 60-90 km 90 km 108 –1010 m-3E 90-140 km 110 km Several x 1011 m-3F1 140-200 km 200 km Several 1011-1012 m-3F2 200-500 km 300 km Several x 1012 m-3Topside above F2• In general densities are larger during solar maximum than during solar minimum.• The D and F1 regions disappear at night.• The E and F2 regions become much weaker.• The topside ionosphere is basically an extension of the magnetosphere.Diurnal and Solar Cycle Variations• At low altitudes the major ions are O2+ and NO+• Near the F2 peak it changes to O+• The topside ionosphere is H+ dominant.Composition of the Dayside Ionosphere Under Solar Minimum Conditions++• For practical purposes the ionosphere can be thought of as quasi-neutral (the net charge is practically zero in each volume element with enough particles).• The ionosphere is formed by ionization of the three main atmospheric constituents N2 , O2 , and O. – The primary ionization mechanism is photoionization by extreme ultraviolet (EUV) and X-ray radiation.– In some areas ionization by particle