Radial evolution of intermittency of density fluctuations in the fast solar wind

TL;DR

This study analyzes how the intermittency of density fluctuations in the fast solar wind evolves with distance from the Sun, revealing decreasing intermittency with heliocentric distance and suggesting parametric instability as a key mechanism.

Contribution

It provides the first detailed analysis of the radial evolution of density fluctuation intermittency in the fast solar wind using multiple diagnostic tools.

Findings

Intermittency decreases with distance from the Sun.

Density fluctuations are strongly temporally clustered.

Parametric instability may drive density fluctuation behavior.

Abstract

We study the radial evolution of intermittency of density fluctuations in the fast solar wind. The study is performed analyzing the plasma density measurements provided by Helios 2 in the inner heliosphere between $0.3$ and $0.9$ AU. The analysis is carried out by means of a complete set of diagnostic tools, including the flatness factor at different time scales to estimate intermittency, the Kolmogorov-Smirnov test to estimate the degree of intermittency, and the Fourier transform to estimate the power spectral densities of these fluctuations. Density fluctuations within fast wind are rather intermittent and their level of intermittency, together with the amplitude of intermittent events, decreases with distance from the Sun, at odds with intermittency of both magnetic field and all the other plasma parameters. Furthermore, the intermittent events are strongly correlated, exhibiting temporal clustering. This indicates that the mechanism underlying their generation departs from a time-varying Poisson process. A remarkable, qualitative similarity with the behavior of plasma density fluctuations obtained from a numerical study of the nonlinear evolution of parametric instability in the solar wind supports the idea that this mechanism has an important role in governing density fluctuations in the inner heliosphere.