probe used to measure the electron temperature of ionized plasmas. Langmuir probes mounted on spacecraft are often used to measure the electron temperature of the earth's ionosphere.

A Langmuir probe is a device named after Nobel Prize winning physicist Irving Langmuir, used to determine the electron temperature, electron density, and plasma potential. It works by inserting one or more electrodes into a plasma, with a constant or time-varying electric potential between the various electrodes or between them and the surrounding vessel. The measured currents and potentials in this system allow the determination of the physical properties of the plasma.