ESS 200CLecture 13The Earth’s IonosphereThe extent of the ionosphereIonization profileESS 200CESS 200CLecture 13Lecture 13The Earth’s IonosphereThe Earth’s Ionosphere• Ionospheric studies– The radiation from the Sun at short wave lengths causes photo ionization of the atmosphere resulting in a partially ionized region called 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 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-3Several 1011-1012m-3Several x 1012 m-3F1 140-200 km 200 kmF2 200-500 km 300 kmTopside above F2• Diurnal and solar cycle variation in the ionospheric density profile.– In general densities are larger during solar maximum than during solar minimum.– The D and F1regions disappear at night.– The E and F2regions become much weaker.– The topside ionosphere is basically an extension of the magnetosphere.• Composition of the dayside ionosphere under solar minimum conditions.– At low altitudes the major ions are O2+and NO+– Near the F2peak it changes to O+– The topside ionosphere becomes H+ dominant.• 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 precipitation is also important. – The ionization process is followed by a series of chemical reactions which produce other ions.– Recombination removes free charges and transforms the ions to neutral particles.• Neutral density exceeds the ion density below about 500 km.• Ionization profile– Let the photon flux per unit frequency be– The change in the flux due to absorption by the neutral gas in a distance ds is where n(z) is the neutral gas density, is the frequency dependent photo absorption cross section (m2), and ds is the path length element in the direction of the optical radiation. (Assuming there are no local sources or sinks of ionizing radiation.)– (where is the zenith angle of the incoming solar radiation. – The altitude dependence of the solar radiation flux becomes– is called the optical depth.– There is usually more than one atmospheric constituent attenuating the photons each of which has its own cross section. νΦdsndνννσΦ−=Φνσdzdsχsec=χ⎟⎟⎠⎞⎜⎜⎝⎛−Φ=Φ∫∞∞zdzznz ')'(secexp)(ννσχν∞Φν')'(sec dzznz∫∞=ννσχτ')'(sec dzzntzt∑∫∞=ννσχτ)exp()(ννντ−Φ=Φ∞z– The density (ns) of the neutral upper atmosphere usually obeys a hydrostatic equationwhere m is the molecular or atomic mass, g is the acceleration due to gravity, z is the altitude and p=nkTis the thermal pressure. – If the temperature T is assumed independent of z, this equation has the exponential solutionwhere is the scale height of the gas, and n0is the density at the reference altitude z0.– For this case – For multiple species – The optical depth increases exponentially with decreasing altitude.– In the thermosphere solar radiation is absorbed mainly via ionization processes. Let us assume that – Each absorbed photon creates a new electron-ion pair therefore the electron production iswhere Siis the total electron production rate (particles cm-3s -1).dznkTddzdpnmg)(−=−=Hzznn)(exp00−−=mgkTH =HzndzHzznz)(sec''expsec00νννχσσχτ=⎥⎦⎤⎢⎣⎡−−=∫∞ttttHzn )(sec∑=ννσχτiννσσ=dszndsSvii)(νσΦ=– Substituting for n and gives – The altitude of maximum ionization can be obtained by looking for extremes in this equation by calculating– This gives– Choose z0as the altitude of maximum ionization for perpendicular solar radiation – This giveswhere – This is the Chapman ionization function.– The maximum rate of ionization is given by – If we further assume that the main loss process is ion-electron recombination with a coefficient α and assume that the recombination rate is– Finally if we assume local equilibrium between production and loss we get)(zνΦ⎟⎟⎠⎞⎜⎜⎝⎛⎥⎦⎤⎢⎣⎡−−−−−Φ=∞HzzHnHzznSiii0000expsecexpνννχσσ0=dzdSi1)()(secmaxmax== zzHniτχσν0max0==χzz⎟⎟⎠⎞⎜⎜⎝⎛⎥⎦⎤⎢⎣⎡−−−=HzHzSSiexpsec1exp0χ10−∞Φ= eHSνχcos0maxSS=2enα2einSα=– The vertical profile in a simple Chapman layer is– The E and F1 regions are essentially Chapman layers– Additional production, transport and loss processes are necessary to understand the D and F2 regions.⎟⎟⎠⎞⎜⎜⎝⎛⎥⎦⎤⎢⎣⎡−−−=HzHzSneexp2sec221exp0χα• The D Region– The most complex and least understood layer in the ionosphere.– The primary source of ionization in the D region is ionization by solar X-rays which ionize both N2and O2–Lyman-α ionization of the NO molecule.– Precipitating magnetospheric electrons may also be important.– Initial positive ions are N2+, O2+and NO+– The primary positive ions are O2+and NO+– The most common negative ion is NO3- The first step in making a negative ion is 2222NOON +→+++MOMOe +→++−22• The E Region–Essentially a Chapman layer formed by EUV ionization – The main ions are O2+and NO+– Although nitrogen (N2) molecules are the most common in the atmosphere N2+is not common because it is unstable to charge exchange. For example–