Active Wave Experiments in Space Plasmas: The Z Mode 

R.F. Benson, P.A. Webb, J.L. Green, D.L. Carpenter, V.S. Sonwalkar, H.G. James, 
and B.W. Reinisch 


The term Z mode is space physics notation for the low-frequency branch of the
extraordinary (X) mode. It is an internal, or trapped, mode of the plasma
confined in frequency between the cutoff frequency fz and the upper-hybrid 
frequency fuh which is related to the electron plasma frequency fpe and the
electron cyclotron frequency fce by the expression fuh^2 = fpe^2 + fce^2 ; fz is
a function of fpe and fce . These characteristic frequencies are directly
related to the electron number density Ne and the magnetic field strength |B|,
i.e., fpe (kHz)^2 ~ 80.6Ne (cm^3) and fce (kHz)^2 ~ 0.028|B|(nT). The Z mode is
further classified as slow or fast depending on whether the phase velocity is
lower or higher than the speed of light in vacuum. The Z mode provides a link
between the short wavelength lambda (large wave number k = 2¹/lambda)
electrostatic (es) domain and the long lambda (small k) electromagnetic (em)
domain. An understanding of the generation, propagation and reception of Z-mode
waves in space plasma leads to fundamental information on wave/particle interac-
tions, Ne , and field-aligned Ne irregularities (FAI) in both active and passive
wave experiments. Here we review Z-mode observations and their interpretations
from both radio sounders on rockets and satellites and from plasma-wave
receivers on satellites. The emphasis will be on the scattering and ducting of
sounder-generated Z-mode waves by FAI and on the passive reception of Z-mode
waves generated by natural processes such as Cherenkov and cyclotron emission.
The diagnostic applica- tions of the observations to understanding ionospheric
and magnetospheric plasma processes and structures benefit from the
complementary nature of passive and active plasma-wave experiments.

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Lect. Notes Phys., 687, 3-35, 2006