Atmospheric and Ionospheric Basics ATMOSPHERICIONOSPHERIC BASICS 1 Earths

Atmospheric and Ionospheric Basics ATMOSPHERIC/IONOSPHERIC BASICS 1

Earth’s Atmosphere • The Earth’s neutral atmosphere is horizontally stratified • The stratification due to the effects of gravity • The neutral atmosphere can best be organized by a temperature profile • The temperature increase in the stratosphere is due to absorption of UV radiation • The temperature then decreases again in the mesosphere, due to radiative cooling • In thermosphere, the temperature increases dramatically, due to absorption of UV and EUV radiation ATMOSPHERIC/IONOSPHERIC BASICS Kelley [2009] 2

The Ionosphere • Kelley [2009] ATMOSPHERIC/IONOSPHERIC BASICS 3

Particle Precipitation • Precipitating charged particles (as in the auroral regions) are another source of ionization • Precipitating particles are responsible for brilliant displays of light known as aurora • Ionization due to particle precipitation occurs when a particle of sufficient energy collides with a neutral and frees an electron • The freed electron may have enough energy to ionize neutrals itself • “cascading” • Production of ion-electron pairs must be modelled differently than production due to photoionization Kelley [2009] ATMOSPHERIC/IONOSPHERIC BASICS 4

Auroral Precipitation The figure to the right is an image of the auroral oval taken by the Dynamics Explorer 1 (DE 1) satellite. Inset is a plot of simultaneous electron density measurements. Note the correlation between where auroral precipitation is strongest and where electron density is greatest Kelley [2009] ATMOSPHERIC/IONOSPHERIC BASICS 5

Losses/Recombination • ATMOSPHERIC/IONOSPHERIC BASICS 6

Airglow seen at the limb of the earth due to recombination of ions and electrons. news. discovery. com ATMOSPHERIC/IONOSPHERIC BASICS 7

Transport • ATMOSPHERIC/IONOSPHERIC BASICS 8

Putting it all together • ATMOSPHERIC/IONOSPHERIC BASICS 9

Putting it all together 2 • The density at a given instant of time is the sum of all processes in the past which have affected the local rate • Different component of the atmosphere experience different processes of production, loss and transport, leading to quite different density profiles for different species at different altitudes • The ionosphere is dynamic • Rates of production, loss and transport are highly variable ATMOSPHERIC/IONOSPHERIC BASICS 10

Atmospheric Composition Kelley [2009] ATMOSPHERIC/IONOSPHERIC BASICS 11

D Region • The D region is the lower layer of the ionosphere and extends from about 60 -90 km altitude • Photochemistry (photoionization and recombination) is dominant in the D region • The dominant source of ionization is high -energy X-rays • At these altitudes, the motions of the plasma are strongly constrained by the neutral particles ATMOSPHERIC/IONOSPHERIC BASICS 12

E Region • ATMOSPHERIC/IONOSPHERIC BASICS 13

F Region • The F region is the layer above about 130 km altitude • The F region is typically split into the F 1 and F 2 regions • The dominant production mechanism in the F 1 region is photoionization of O+ by EUV radiation • The dominant loss process is dissociative recombination • The F 1 region is well-modelled as a Chapman Layer • The F 2 density peak is also the density peak of the entire ionosphere • Transport processes become important in the F 2 region • Vertical transport allows the F 2 region to be a source of plasma for the plasmasphere during the day, and allows plasma of plasmaspheric origins to flow in at night, maintaining the density ATMOSPHERIC/IONOSPHERIC BASICS 14

References • Rishbeth, H. , and O. K. Garriott (1969), Introduction to Ionospheric Physics, Academic Press, New York. • Schunk, R. , and A. Nagy (2009), Ionospheres, Cambridge University Press, Cambridge, UK. • Kelley, M. C. (2009), The Earth’s Ionosphere, Academic Press, Burlington, MA. • Davies, K. (1990), Ionospheric Radio, Institution of Engineering and Technology, London, UK. ATMOSPHERIC/IONOSPHERIC BASICS 15