Plasma Frequency
The natural characteristic frequency of collective electrostatic electron oscillations in a plasma when perturbed from thermal equilibrium.
Governing formula
ω_pe = √(n_e e² / (m_e ε₀))
SI unit
rad/s (Angular Frequency)
In depth
When electrons are displaced relative to stationary background ions, electrostatic restoring forces pull them back. Electron inertia causes them to overshoot and oscillate at frequency ω_pe, defining the cutoff frequency below which electromagnetic waves are reflected.
Examples in the real world
Ionospheric reflection of shortwave AM radio broadcasts and solar coronal radio emission diagnostics.