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John D. Richardson studies space plasmas, including the solar wind and its interaction with the interstellar medium. MIT identifies him as a principal investigator of the Voyager plasma experiment and describes earlier work on plasma transport in planetary magnetospheres.^[ref]

His 2008 Voyager 2 study analyzed plasma measurements across the termination shock. The thermal ions were cooler than a simple picture of bulk-flow energy becoming thermal-ion heat would suggest. The result made the partition of energy among particle populations central to understanding the shock.^[ref]

The experiment illustrates how a remote boundary becomes a local measurement. Four Faraday cups sampled particle currents over energy-per-charge ranges; combining differently oriented detectors allowed the team to infer plasma density, flow and temperature. These inferred properties depend on the measured spectra and distribution model, rather than being direct readings of a thermometer.^[ref]

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.