Next: Parallel Wave Propagation Up: Waves in Warm Plasmas Previous: Ion Acoustic Waves

# Waves in Magnetized Plasmas

Consider small amplitude waves propagating through a plasma placed in a uniform magnetic field, . Let us take the perturbed magnetic field into account in our calculations, in order to allow for electromagnetic, as well as electrostatic, waves. The linearized Vlasov equation takes the form

 (8.53)

for both ions and electrons, where and are the perturbed electric and magnetic fields, respectively. Likewise, is the perturbed distribution function, and the equilibrium distribution function.

In order to have an equilibrium state at all, we require that

 (8.54)

Writing the velocity, , in cylindrical polar coordinates, , aligned with the equilibrium magnetic field, the previous expression can easily be shown to imply that : that is, is a function only of and .

Let the trajectory of a particle be , . In the unperturbed state,

 (8.55) (8.56)

It follows that Equation (8.53) can be written

 (8.57)

where is the total rate of change of , following the unperturbed trajectories. Under the assumption that vanishes as , the solution to Equation (8.57) can be written

 (8.58)

where , is the unperturbed trajectory that passes through the point , when .

It should be noted that the previous method of solution is valid for any set of equilibrium electromagnetic fields, not just a uniform magnetic field. However, in a uniform magnetic field, the unperturbed trajectories are merely helices, whereas in a general field configuration it is difficult to find a closed form for the particle trajectories that is sufficiently simple to allow further progress to be made.

Let us write the velocity in terms of its Cartesian components:

 (8.59)

It follows that

 (8.60)

where is the gyofrequency. The previous expression can be integrated in time to give

 (8.61) (8.62) (8.63)

Note that both and are constants of the motion. This implies that , because is only a function of and . Given that , we can write

 (8.64) (8.65) (8.66)

Let us assume an dependence of all perturbed quantities, with lying in the - plane. Equation (8.58) yields

 (8.67)

Making use of Equations (8.60)-(8.66), as well as the identity (Abramowitz and Stegun 1965c)

 (8.68)

where the are Bessel functions (Abramowitz and Stegun 1965c), Equation (8.67) gives

where

 (8.70)

Maxwell's equations yield

 (8.71) (8.72)

where is the perturbed current, and is the dielectric permittivity tensor introduced in Section 5.2. It follows that

 (8.73)

where is the species- perturbed distribution function.

After a great deal of rather tedious analysis, Equations (8.69) and (8.73) reduce to the following expression for the dielectric permittivity tensor (Harris 1970: Cairns 1985):

 (8.74)

where

 (8.75)

and

 (8.76) (8.77) (8.78)

The argument of the Bessel functions is . In the previous formulae, denotes differentiation with respect to argument, and .

The warm-plasma dielectric tensor, (8.74), can be used to investigate the properties of waves in just the same manner as the cold-plasma dielectric tensor, (5.37), was employed in Chapter 5. Note that our expression for the dielectric tensor involves singular integrals of a type similar to those encountered in Section 8.2. In principle, this means that we ought to treat the problem as an initial value problem. Fortunately, we can use the insights gained in our investigation of the simpler unmagnetized electrostatic wave problem to recognize that the appropriate way to treat the singular integrals is to evaluate them as written for , and by analytic continuation for .

For Maxwellian distribution functions, of the form

 (8.79)

we can explicitly perform the velocity-space integral in Equation (8.74), making use of the identities (Watson 1995)

 (8.80) (8.81)

where is a modified Bessel function (Abramowitz and Stegun 1965c). We obtain

 (8.82)

where , , and (Harris 1970; Cairns 1985)

 (8.83)

Here, , which is the argument of the modified Bessel functions, is written

 (8.84)

whereas and represent the plasma dispersion function and its derivative, both functions being evaluated with the argument

 (8.85)

Let us consider the cold-plasma limit, . It follows from Equations (8.84) and (8.85) that this limit corresponds to and . According to Equation (8.47),

 (8.86) (8.87)

as . Moreover, (Abramowitz and Stegun 1965c)

 (8.88)

as . It can be demonstrated that the only non-zero contributions to , in this limit, come from and . In fact,

 (8.89) (8.90) (8.91)

and . It is easily seen, from Section 5.3, that the previous expressions are identical to those found using the cold-plasma fluid equations. Thus, in the zero temperature limit, the kinetic dispersion relation obtained in this section reverts to the fluid dispersion relation derived in Chapter 5.

Next: Parallel Wave Propagation Up: Waves in Warm Plasmas Previous: Ion Acoustic Waves
Richard Fitzpatrick 2016-01-23