This letter describes a new phenomenon on the Parker Solar Probe of recurring
plasma density enhancements that have Δn/n ~10% and that occur at a
repetition rate of ~5 Hz. They were observed sporadically for about five hours
between 14 and 15 solar radii on Parker Solar Probe orbit 12 and they were also
seen in the same radial range on both the inbound and outbound orbits 11. Their
apparently steady-state existence suggests that their pressure gradient was
balanced by the electric field. The EX electric field component produced from
this requirement is in good agreement with that measured. This provides strong
evidence for the measurement accuracy of the density fluctuations and the X-
and Y-components of the electric field (the Z-component was not measured). The
electrostatic density waves were accompanied by an electromagnetic low
frequency wave which occurred with the electrostatic harmonics. The amplitudes
of these electrostatic and electromagnetic waves at ≥ 1 Hz were greater
than the amplitude of the Alfvenic turbulence in their vicinity so they can be
important for the heating, scattering, and acceleration of the plasma. The
existence of this pair of waves is consistent with the observed plasma
distributions and is explained by a magneto-acoustic wave theory that produces
a low frequency electromagnetic wave and electrostatic harmonics.Comment: 9 pages including 5 figure