Bulk measurement of the plasma is critical for our understanding the ionized space environment. As an example, the electron temperature drives many reaction rates in the thermosphere and ionosphere regions. Therefore, electron temperature data are important for any attempt to model the chemistry or predict the dynamics in this region. A key in situ instrument that can provide this information is the Langmuir Probe. However, interpreting the measurements from a Langmuir Probe requires a scientific understanding of how the local plasma environment interacts with the probe and the spacecraft. With more and more Langmuir Probes flying, there is a need to have a common scientific understanding of how these interactions impact the measurements. The literature on how to perform and interpret Langmuir Probe measurements in space is incomplete, scattered, and often very focused on one particular spacecraft in one particular environment.

Pioneering work was done by Irving Langmuir and Katherine Blodgett in 1923-1924. This was followed by a seminal 1926 theory paper by Mott-Smith & Langmuir on how the current to a probe in a tenuous plasma depends on the electron density and temperature. This paper is still frequently cited in space physics applications. This work was done well before the advent of the space age. A century after Langmuir’s work, the Langmuir Probe (LP) is still indispensable for measuring the core electron population characteristics of any plasma.

Interpreting spacecraft-borne measurements is often more complex than in a well-controlled laboratory environment. Without the experience from previous satellite missions and without corroborating plasma measurements made simultaneously on the same platform by other types of instruments, there will be limitations on how well the LP data can be interpreted, and thereby the quality of the data from those missions. Since LP instruments can be built reasonably easy and require limited resources they are frequently flown in the new era of small spacecraft. These instrument teams often do not appreciate all the complexity of designing, accommodating on a SC and flying LP’s and more importantly the difficulty of interpreting the data. Understanding LP measurements on spacecraft requires good understanding of the interaction between probe, spacecraft and plasma.

This ISSI team will to collect and organize existing knowledge on topics such as: how do plasma flows change the potential around a spacecraft/sensor and thereby the current to the probe; can Langmuir probe measurements truly provide the spacecraft potential; how fast can the spacecraft/probe reach equilibrium with the surrounding plasma; the effect of magnetic fields, and what limits the lower and upper temperature range measured by a Langmuir probe. These topics are critical for interpreting the Langmuir probe measurements and the team is selected to cover experience from many different high quality Langmuir Probe instruments. This international ISSI team brings together key people from some of the latest Langmuir probe instruments in space to provide information and guidance for future LP instruments and missions through review articles.