Cold plasma transverse-wave cutoff
In a cold collisionless unmagnetized electron plasma with fixed neutralizing ions, transverse waves obey ω²=ω_p²+c²k² and ω_p²=n_e e²/(m_eε₀).
Key knowledge
In a cold collisionless unmagnetized electron plasma with fixed neutralizing ions, transverse waves obey ω²=ω_p²+c²k² and ω_p²=n_e e²/(m_eε₀). Above ω_p the wavenumber is real; below it the source-free spatial solution is evanescent. For propagating waves v_ph=ω/k>c while v_g=dω/dk=c²k/ω<c, with v_ph v_g=c².
Values and equations
ω²=ω_p²+c²k²; ω_p²=n_e e²/(m_eε₀); v_ph=ω/k; v_g=c²k/ω
- ω,ω_p
- Wave and electron plasma angular frequencies rad s⁻¹
- k
- Propagating wavenumber above cutoff m⁻¹
- n_e
- Free electron number density m⁻³
- e
- Positive elementary charge C
- m_e
- Electron mass kg
- ε₀
- Vacuum permittivity F m⁻¹
- c,v_ph,v_g
- Vacuum light, phase and group speeds m s⁻¹
Conditions and scope
- Homogeneous linear small-amplitude transverse wave, negligible collisions and thermal pressure, immobile ions, no background magnetic field, electron number density n_e>0 and ω>0.
- This does not describe longitudinal plasma oscillations, magnetized branches, Landau damping, density-gradient reflection geometry or actual terrestrial radio propagation. Phase velocity above c does not define signal/front speed.
Sources · 1
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Classical Electromagnetism (2014): High frequency limit: Equations 810–812 and the free-electron negligible-damping paragraph: refractive response n²=1−ω_p²/ω² and the plasma continuation below cutoff. Relabel N Z as n_e; differentiate the dispersion for group speed. Excludes the reference to nonexistent equation 813 and the oversimplified ionospheric day/night narrative.
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Entry details
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