= Voyager plasma-wave instrument
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The <Voyager plasma-wave instrument> measures fluctuating electric fields associated with <plasma waves>. Its original design combines a multichannel spectrum analyser with a wideband waveform receiver, making both frequency content and selected time-domain waveforms available.<A Plasma Wave Investigation for the Voyager Mission (1977)>[Original1977 instrument paper, system overview]{ref}
When an observed feature is identified with electron plasma oscillations, its frequency constrains electron number density through $f_p=(2\pi)^{-1}\sqrt{n_e e^2/(\varepsilon_0m_e)}$. Thus a fourfold density increase doubles the characteristic frequency. Wave identification, instrument calibration and the surrounding plasma conditions enter the inference. On <Voyager 1>, such measurements have provided local information about plasma beyond the <heliopause>.https://space.physics.uiowa.edu/~dag/publications/2013_InSituObservationsOfInterstellarPlasmaWaveVoyager_Science.pdf[Primary interstellar-plasma observations, frequency-derived density]{ref}
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