Abstract Artificially enhancing the pitch angle diffusion of the relativistic electrons trapped in the radiation belts with transmission of very‐low frequency (VLF) electromagnetic waves is the primary science objective of the wave‐particle interaction experiments (WPIx) on the Demonstration Science Experiments (DSX) spacecraft. The pitch angle diffusion occurs due to the interaction of stimulated whistler‐mode waves with these extremely high energy electrons. The primary payload of the DSX spacecraft is a high‐power VLF transmitter paired with a matched tuner, a two‐channel narrowband receiver, and an 81.6 m tip‐to‐tip carbon fiber longeron dipole antenna, collectively known as Transmitter, Narrowband receiver, and Tuner (TNT) instrument. TNT can execute a wide variety of programmable WPIx experiments and provide supporting in situ and remote plasma characterizations. In its main high‐power mode, the whistler stimulation on selected frequencies between 2.5 and 34 kHz is done by applying a high‐voltage signal, up to nominally 5 kV, at each transmit antenna monopole feed. Maximum power is radiated when the antenna‐plasma circuit reactance is tuned out, leading to resonance conditions at the selected frequency. In the radiation belts, the unknown and varying contributions of the ambient plasma to the total reactance have to be determined automatically and tuned out adaptively, steering the overall VLF circuit within the resonance state as DSX orbits the Earth. Auxiliary to the main WPIx mode, the low‐power relaxation sounding, remote plasma sensing, and listen‐only VLF‐HF emission spectrographic operations are used to probe the surrounding and remote plasma density and the local magnetic field strength.
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