Guiding of electromagnetic waves along a magnetic field in a plasma

Using ray theory and assuming longitudinal propagation conditions, we investigated the circumstances in which curvature of the ray is identical with curvature of the magnetic field. In general the two curvatures are only identical if there exists an appropriate transverse gradient of the plasma density. For a given ratio of the wave frequency to the gyrofrequency, and of the plasma frequency to the gyrofrequency, the necessary transverse gradient of plasma density may be calculated as a ratio to the curvature of the magnetic field. Curves have been prepared showing, for a wide range of values of the parameters, the transverse gradient of plasma density required to make the ray curvature identical with the field curvature. Of particular interest are the conditions in which no transverse gradient of plasma density is required to make the ray curvature identical with the field curvature. In these circumstances waves are guided by the magnetic field even in a homogeneous plasma. It is found that at hydromagnetic frequencies (wave frequency less than the ionic gyrofrequency) such guiding never occurs. Indeed, at frequencies small compared with the ionic gyrofrequency the ray curvature is equal to the field curvature in magnitude but opposite in sign. Above about twice the ionic gyrofrequency, field guiding is possible in a homogeneous plasma for the left-hand wave (O wave) but not for the right-hand wave (X wave), provided that the homogeneous plasma density is adjusted to an appropriate value.