  This work reports a novel plasma wave observation in the near-Sun solar wind:
frequency-dispersed ion acoustic waves. Similar waves were previously reported
in association with interplanetary shocks or planetary bow shocks, but the
waves reported here occur throughout the solar wind sunward of $\sim 60$ solar
radii, far from any identified shocks. The waves reported here vary their
central frequency by factors of 3 to 10 over tens of milliseconds, with
frequencies that chirp up or down in time. Using a semi-automated
identification algorithm, thousands of wave instances are recorded during each
near-Sun orbit of the Parker Solar Probe spacecraft. Wave statistical
properties are determined and used to estimate their plasma frame frequency and
the energies of protons most likely to be resonant with these waves. Proton
velocity distribution functions are explored for one wave interval, and proton
enhancements that may be consistent with proton beams are observed. A
conclusion from this analysis is that properties of the observed
frequency-dispersed ion acoustic waves are consistent with driving by cold,
impulsively accelerated proton beams near the ambient proton thermal speed.
Based on the large number of observed waves and their properties, it is likely
that the impulsive proton beam acceleration mechanism generating these waves is
active throughout the inner heliosphere. This may have implications for
acceleration of the solar wind.
