Appendix B

The Space Environment

This appendix is intended to supplement the material presented in Chapter 2. It presents in more detail, for the interested reader, many of the concepts introduced in Chapter 2.

Earth's plasma is described properly in terms of phase space density or distribution function. Space plasmas can be described most simply in terms of the Maxwell–Boltzmann distribution. As this representation lends itself to efficient manipulation when carrying out charging calculations, it is often the preferred way of describing plasmas. The Maxwell–Boltzmann distribution *F*_{i} is given by

B.1

where

n_{i} = number density of species *i*

m_{i} = mass of species *i*

k = Boltzmann constant

T_{i} = characteristic temperature of species *i*

v = velocity

F_{i} = distribution function of species *i*

Unfortunately, the space plasma environment is seldom a Maxwell–Boltzmann distribution. However, given the actual plasma distribution function, it is possible to define (irrespective of whether the plasma is Maxwell–Boltzmann or not) moments of the particle distribution that reveal characteristics of its shape. In most cases, these moments can then be used to determine an approximate Maxwell–Boltzmann distribution. The first four of these characteristic moments are

B.2

B.3

B.4

B.5

where

< ND_{i} > = number ...

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