Since their discovery in laboratory plasmas in the 1920s, Langmuir waves have been observed to be ubiquitous in plasma environments, particularly in space plasmas. From the greater solar wind to planetary foreshocks and the auroral ionosphere, Langmuir waves are a key factor mediating electron temperature, and controlling electron beam propagation and beam-plasma energy transfer. Because they are so important, Langmuir waves in the space environment have been intensively investigated; however, there remain two challenging types of experiments that are relatively lacking: three-dimensional measurements of Langmuir-wave fields, and measurements of Langmuir wave-electron correlations. This thesis works on filling these two gaps, plus development of new Langmuir-wave instrumentation.

The CHARM-II wave-particle Correlator instrument was designed to study the energy transfer between electron beams and plasmas via the sorting of incoming particles by concurrent Langmuir-wave phase, allowing for direct observation of electron bunching. Data from the CHARM-II sounding rocket comprises the first such observations with statistical levels of events, revealing an association between the polarity of the resistive component of the electron phase-bunching and changes in the electron flux at the associated energy, such that a negative resistive component goes with an increase in electron flux, and vice versa, effectively showing energy flow from the beam to the waves, and subsequent enhancements of wave damping. Surprisingly, the results also show comparable amounts of resistive and reactive activity. A test-particle simulation was developed to confirm the details of the theoretical explanation for the observed effect.

A three-dimensional Langmuir-wave receiver flown on the TRICE sounding rocket mission reveals the beat signature of the amplitude-modulated `bursty' form of Langmuir waves which has been observed in many environments. An analysis of the three-dimensional data shows agreement with expected signatures of beating between pure, field-aligned, linearly polarized Langmuir waves and obliquely propagating, elliptically polarized, hybrid whistler-Langmuir waves.

Finally, an autonomous digital signal processor/receiver has been refined and augmented to achieve high time- and frequency-resolution radio observations, synchronized sampling between multiple receivers, and on-board processing of data. This system was deployed on the CHARM-II rocket, resulting in measurements of the polarization of fine structures in auroral roar emissions.