Sunward-propagating Alfv\'enic fluctuations observed in the heliosphere

TL;DR

This study identifies and analyzes sunward-propagating Alfvénic fluctuations in the heliosphere, revealing their increasing prevalence with distance from the Sun, which offers insights into their generation mechanisms.

Contribution

First observational evidence of SAFs inside 6 au and their statistical increase with heliocentric distance, enhancing understanding of solar wind turbulence.

Findings

SAFs observed in the heliosphere inside 6 au.

Percentage of SAFs increases from 2.7% at 1 au to 8.7% at 5.5 au.

Two SAF events show distinct magnetic field-velocity correlations.

Abstract

The mixture/interaction of anti-sunward-propagating Alfv\'enic fluctuations (AFs) and sunward-propagating Alfv\'enic fluctuations (SAFs) is believed to result in the decrease of the Alfv\'enicity of solar wind fluctuations with increasing heliocentric distance. However, SAFs are rarely observed at 1 au and solar wind AFs are found to be generally outward. Using the measurements from Voyager 2 and Wind, we perform a statistical survey of SAFs in the heliosphere inside 6 au. We first report two SAF events observed by Voyager 2. One is in the anti-sunward magnetic sector with a strong positive correlation between the fluctuations of magnetic field and solar wind velocity. The other one is in the sunward magnetic sector with a strong negative magnetic field-velocity correlation. Statistically, the percentage of SAFs increases gradually with heliocentric distance, from about 2.7% at 1.0 au to about 8.7% at 5.5 au. These results provide new clues for understanding the generation mechanism of SAFs.