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Next: The Ambipolar Electric Field Up: Ionospheric Physics Previous: Impact Ionization and Losses

Basic Theoretical Formalism

The basic approach is to use coupled fluid equations for the electrons and the multiple participating neutral and ion species, with appropriate loss terms. Number conservation, momentum conservation and energy conservation equations are used. Below these equations are written in terms of number densities tex2html_wrap_inline642 rather than mass densities tex2html_wrap_inline644 .

equation172

equation183

and

equation210

etc. The last two sets of terms in the momentum equations relate to ion-electron interactions and neutral-electron and neutral-ion interactions. These equations should then be solved simultaneously with appropriate boundary conditions. Figure 16.3 used this type of calculation.

Rather than address details here, let us focus on the important qualitative points. The first and most major point is that the electric field tex2html_wrap_inline646 in (16.14 - 15) cannot, in general, be ignored when gravity is retained ( tex2html_wrap_inline648 ). Instead, ionospheres tend to have significant polarization electric fields. Physically the reason is as follows: due to their much smaller masses, electrons will be able to reach much greater altitudes than ions with the same temperature, thereby causing a steady-state charge separation. This charge-separation sets up an ``ambipolar'' electric field which pulls ions up to higher altitudes and resists the motion of electrons to higher altitudes, setting up a situation in which approximate charge-neutrality exists. This electric field has a number of consequences that are described below.



Iver Cairns
Thu Sep 23 17:08:59 EST 1999