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Incoherent scatter radar observations of ionospheric plasmas rely on echoes from electron density fluctuations with properties governed by the dispersion relations for ion acoustic and Langmuir waves. Radar observations of echoes associated with Langmuir waves (plasma lines) from thermal plasma are weak, and only a few near-thermal level measurements have been reported. Plasma line echoes are typically only observed with existing radars only when the Langmuir waves are enhanced by suprathermal electrons. A new observation technique has been developed which is sensitive enough to allow observations of these echoes without the presence of suprathermal electrons up to at least 1000km. This paper presents recent observations from the Arecibo Observatory 430MHz incoherent scatter radar which show plasma line echoes during the night when no suprathermal enhancement is expected to be present. The observations are compared with theory, and the results are found to be in agreement with classical incoherent scatter theory for thermal plasmas. The theoretical ratio of the ion line and plasma line power spectral density is within approximately 3dB of the predicted value. The finding adds a new observational capability, allowing electron density to also be observed at night using the plasma line well into the top side of the ionosphere, increasing the accuracy of the electron density measurement.

Plain Language Summary Our results address a fundamental theory of scattering of electromagnetic waves from thermal plasma, which was formulated in the 1960s. The theory has so far been validated only partially, for the ion line component of the theory, which describes scattering from ion acoustic waves. Our new measurements confirm for the first time that the second part of the theory, which describes scattering of electromagnetic waves from thermal plasma waves, also known as Langmuir waves, is also accurate to with a factor of 2, when comparing the relative power of the ion line and the plasma line.