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Plasma density must specify what is being counted or weighed. For species $s$, its number density $n_s$ counts particles per unit volume. Electron density $n_e$, ion density $n_i$ and total mass density $\rho ={\sum }_s{m}_s{n}_s$ need not be interchangeable. In a quasineutral mixture with ion charges $Z_{i}e$,For singly ionized hydrogen, $n_e\simeq n_i$, while the ions provide nearly all the mass. Adding heavier ions changes the relation between electron density and mass density.

An electron plasma frequency measurement offers a density diagnostic. In the cold, unmagnetized approximation with effectively fixed ions, displacement of electrons creates an electric restoring force, givingThe first formula uses SI units; the numerical version expresses electron density in particles per cubic centimetre.^[ref]

If an identified electron plasma oscillation has frequency $2000\mathrm{Hz}$, this model givesThis is an inference from an identified wave mode, not a particle counter reading. Magnetization, wave propagation, multiple species and instrument effects can change which spectral feature is observed, so interpreting a frequency requires the relevant plasma wave model.

A plasma wave is a collective disturbance of charged particles and electromagnetic fields in a plasma. Restoring forces can involve charge separation, magnetic tension or pressure, producing different branches rather than one universal wave speed.

For a cold unmagnetized electron fluid with effectively stationary neutralizing ions, electrostatic density oscillations have electron plasma frequency ${\omega }_{pe}=\sqrt{n_e{e}^2/({ϵ}_0{m}_e)}$. Displacing electrons creates an electric field that pulls them back; inertia produces oscillation. Measuring that frequency therefore constrains electron density. Thermal motion, magnetic fields and collisions modify the dispersion and damping, and must be included when the cold-fluid assumptions fail.

Voyager 1 is a spacecraft launched on 5 September 1977 to investigate the outer Solar system. It flew past Jupiter in March 1979 and Saturn in November 1980, then continued outward on an extended mission to study the environment beyond the planets.^[ref]

On 25 August 2012 it crossed the heliopause, the boundary between the solar wind-dominated heliosphere and the surrounding interstellar medium. This boundary concerns the plasma environment; it is not a boundary beyond which the Sun's gravity vanishes. Its scientific instruments include detectors of charged particles, a magnetometer and a Voyager plasma-wave instrument. Together, measurements of particles, magnetic fields and plasma waves allow a moving probe to investigate a remote environment that cannot be sampled in a terrestrial laboratory.^[ref]

The Voyager plasma-wave instrument measures fluctuating electric fields associated with plasma waves. Its original design combines a multichannel spectrum analyser with a wideband waveform receiver, making both frequency content and selected time-domain waveforms available.^[ref]

When an observed feature is identified with electron plasma oscillations, its frequency constrains electron number density through $f_p=(2\pi {)}^{-1}\sqrt{n_e{e}^2/({\epsilon }_0{m}_e)}$. Thus a fourfold density increase doubles the characteristic frequency. Wave identification, instrument calibration and the surrounding plasma conditions enter the inference. On Voyager 1, such measurements have provided local information about plasma beyond the heliopause.^[ref]