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Electric field measurements of the Time Domain Sampler (TDS) receiver, partof the Radio and Plasma Waves (RPW) instrument on board Solar Orbiter, oftenexhibit very intense broadband wave emissions at frequencies below 20~kHz inthe spacecraft frame. In this paper, we present a year-long study ofelectrostatic fluctuations observed in the solar wind at an interval ofheliocentric distances from 0.5 to 1~AU. The RPW/TDS observations provide anearly continuous data set for a statistical study of intense waves below thelocal plasma frequency. The on-board and continuously collected and processedproperties of waveform snapshots allow for the mapping plasma waves atfrequencies between 200~Hz and 20~kHz. We used the triggered waveform snapshotsand a Doppler-shifted solution of the dispersion relation for wave modeidentification in order to carry out a detailed spectral and polarizationanalysis. Electrostatic ion-acoustic waves are the common wave emissionsobserved between the local electron and proton plasma frequency in the solerwind. The occurrence rate of ion-acoustic waves peaks around perihelion atdistances of 0.5~AU and decreases with increasing distances, with only a fewwaves detected per day at 0.9~AU. Waves are more likely to be observed when thelocal proton moments and magnetic field are highly variable. A more detailedanalysis of more than 10000 triggered waveform snapshots shows the mean wavefrequency at about 3 kHz and wave amplitude about 2.5 mV/m. The wave amplitudevaries as 1/R^(1.38) with the heliocentric distance. The relative phasedistribution between two components of the E-field shows a mostly linear wavepolarization. Electric field fluctuations are closely aligned with thedirections of the ambient field lines. Only a small number (3%) of ion-acousticwaves are observed at larger magnetic discontinuities.

First-year ion-acoustic wave observations in the solar wind by the RPW/TDS instrument onboard Solar Orbiter