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The formation of an anomalous, high‐altitude plasma cavity was observed during the 11–15 June 2019 ionospheric heating campaign at Arecibo Observatory. The cavity was induced by interactions of the transmitted 5.125‐MHz heater wave with the ionospheric plasma. Simultaneous ion line and plasma line measurements were taken by the Arecibo Incoherent Scatter Radar (ISR). We used a novel experimental technique, enabling rapid pulsing of the heater, to collect ISR spectral data of the heated ionosphere without high frequency (HF)‐induced enhanced ion line (HFIL), simplifying the analysis. ISR data were analyzed using Tikhonov regularization, revealing that the cavity was confined to a very narrow altitude range around 350 km and constituted a deep depletion of the electron density, along with strong electron temperature enhancement, and ion temperature enhancement. We report on the details of these observations, and the possibility for unique combinations of physical interactions to explain the underlying mechanisms. Key Points: Formation of a strong plasma cavity occurred during a recent ionospheric heating experimentCavity formation was simultaneous with disappearance of incoherent scatter radar enhanced ion and plasma linesIncoherent scatter radar ion line spectra indicate strong enhancement of ion temperaturein the plasma cavity [ABSTRACT FROM AUTHOR]