  Determination of the wave mode of short-wavelength electrostatic waves along
with their generation mechanism requires reliable measurement of the wave
electric field. We investigate the reliability of the electric field
measurement for short-wavelength waves observed by MMS. We develop a method,
based on spin-plane interferometry, to reliably determine the full 3D wave
vector of the observed waves. We test the method on synthetic data and then
apply it to ion acoustic wave bursts measured in situ in the solar wind. By
studying the statistical properties of ion acoustic waves in the solar wind we
retrieve the known results that the wave propagation is predominantly
field-aligned. We also determine the wavelength of the waves. We find that the
distribution peaks at around 100 m, which when normalized to the Debye length
corresponds to scales between 10 and 20 Debye lengths.

Space Physics