Density Enhancement Streams in The Solar Wind

TL;DR

This paper reports the discovery of recurring plasma density enhancements and associated waves in the solar wind near 14-15 solar radii, observed by Parker Solar Probe, with implications for plasma heating and acceleration.

Contribution

It introduces a new phenomenon of density enhancements and coupled electrostatic and electromagnetic waves in the solar wind, supported by measurements and wave theory.

Findings

Density enhancements occur at ~5 Hz with ~10% amplitude.

Electrostatic and electromagnetic waves exceed local Alfvenic turbulence.

Wave phenomena are consistent with magneto-acoustic wave theory.

Abstract

This letter describes a new phenomenon on the Parker Solar Probe of recurring plasma density enhancements that have $\Delta$n/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 $\ge$ 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.